Jump to content

Bob Carver Amplifier Design Philosophy. Forum Controversy.What Sounds Good?


Ar9Jim

Recommended Posts

V2, 8.27.22

 

 

Bob Carver Amplifier Design Philosophy — What Sounds Good?

 

Since the early days, after earning my physics degrees, my approach to audio design has created controversy.

 

My unconventional approach has brought both criticism, accolades and world-wide recognition for achieving musical excellence from my wonderful fans. World-wide recognition for offering a more affordable product, compared to most other brands of comparable products. It’s a great pursuit.  Myriad technological advances have emerged from someone doing things differently.

 

My amplifiers have often been smaller, lighter, and less costly than others —while remaining powerful, musical, and accurate. These designs and their musical performance are quite successful.  I do indeed make comparisons between my design practices and those of other designers. This is done not to foster a “Carver against the world” attitude but rather to highlight significant creative differences. Most other designers have chosen a heavily-trodden path; I simply take a fresh route.

 

What makes an amplifier sound good?

 

Dynamic power, low distortion, and wide frequency response. My tube amplifiers have high voltage (B+);  the power supplies are able to “bounce” and increase voltage, closely tracking the musical load with very little distortion. This is an important key to musical performance that cannot be revealed by hooking an amp up to a resistor.

 

Do you design amplifiers using load resistors or speakers?

 

Both. On my bench I start out with resistors, then I use different speakers with a scope and voltmeter connected, while playing music and measuring the amp and speakers reacting together. The back EMF that is present makes speakers slightly easier to drive. Power response, by design, tapers below 80Hz, yet frequency response goes below 20Hz. 

 

My designs will drive difficult loudspeaker loads, playing music far better than the specifications listed, without clipping, and with lots of headroom available. 

 

These long held design targets have served Carver well. The designs have delivered excellent performing, highly musical products that more people could afford, without sacrificing the powerful and musical performance desired when powering loudspeakers. 

 

 

Stay tuned for more of my very latest designs and the on-line store coming soon.

Edited by Ar9Jim
  • Thank You 3
  • Love this! 1
Link to comment
Share on other sites

  • The title was changed to Bob Carver Amplifier Design Philosophy. Forum Controversy.What Sounds Good?

He's always made great power amplifiers. In our collection we have Phase Linear 400, TFM6c, TFM15cb, TFM35, and now Crimson 275. They all sound fantastic. The little 6 is a real sleeper - it's matched to a pair of late 1960s Coral speakers. A 70s pressing of Abbey Road sounded great through this system. 

  • Thank You 5
  • Love this! 1
Link to comment
Share on other sites

On 8/28/2022 at 9:09 AM, Ar9Jim said:

V2, 8.27.22

 

 

Bob Carver Amplifier Design Philosophy — What Sounds Good?

 

Since the early days, after earning my physics degrees, my approach to audio design has created controversy.

 

My unconventional approach has brought both criticism and accolades. World-wide recognition for achieving musical excellence from my wonderful fans. World-wide recognition for offering a more affordable product, compared to most other brands of comparable products. It’s a great pursuit.  Myriad technological advances have emerged from someone doing things differently.

 

My amplifiers have often been smaller, lighter, and less costly than others —while remaining powerful, musical, and accurate. These designs and their musical performance are quite successful.  I do indeed make comparisons between my design practices and those of other designers. This is done not to foster a “Carver against the world” attitude but rather to highlight significant creative differences. Most other designers have chosen a heavily-trodden path; I simply take a fresh route.

 

What makes an amplifier sound good?

 

Dynamic power, low distortion, and wide frequency response. My tube amplifiers have high voltage (B+);  the power supplies are able to “bounce” and increase voltage, closely tracking the musical load with very little distortion. This is an important key to musical performance that cannot be revealed by hooking an amp up to a resistor.

 

Do you design amplifiers using load resistors or speakers?

 

Both. On my bench I start out with resistors, then I use different speakers with a scope and voltmeter connected, while playing music and measuring the amp and speakers reacting together. The back EMF that is present makes speakers slightly easier to drive. Power response, by design, tapers below 80Hz, yet frequency response goes below 20Hz. 

 

My designs will drive difficult loudspeaker loads, playing music far better than the specifications listed, without clipping, and with lots of headroom available. 

 

These long held design targets have served Carver well. The designs have delivered excellent performing, highly musical products that more people could afford, without sacrificing the powerful and musical performance desired when powering loudspeakers. 

 

 

Stay tuned for more of my very latest designs and the on-line store coming soon.

Please feel free to repost this on the hack sites. I considered posting this on ASR, but honestly can't give them my single click count. Anything less than a cowardly, submissive response to their totally deceptive, passive aggressive attacks, is said to be a bad attitude and scarlet letter on their latest target.  That being said, I have a very bad attitude from their point of view and always will. 

 

This is the response Bob wrote for his new website..

Edited by Ar9Jim
  • Thank You 3
Link to comment
Share on other sites

On 8/28/2022 at 10:09 AM, Ar9Jim said:

Do you design amplifiers using load resistors or speakers?

 

Both. On my bench I start out with resistors, then I use different speakers with a scope and voltmeter connected, while playing music and measuring the amp and speakers reacting together.

 

So much can happen in less than a millisecond inside an amplifier when driving real world speaker loads that unless you measuring for it and look for it while it is occurring the conventional standard measurement (like static distortion vs. freq. or wattage) is all but meaningless. This is were Bob gets it. He is aware of things happening in real time with real speaker loads and uses unconventional means to to monitor and correct these anomalies.  Also his knowledge of human perception and how we actually hear things I think also plays a key role in his grasp and mastery of what may go wrong within amplification circuity and how we perceive it. His approach to research is always novel, therefore his designs are too.

 

Edited by straylight
  • Thank You 2
Link to comment
Share on other sites

  • Community Admin

These two pictures epitomize the point of "what sounds good" vs "any other way to design audio..."  they show Bob using the "best test instrument ever designed by a higher power..., the human ear!"  His tried and true "design philosophy."

 

I can't recall/find the thread here on TCS! (or, was it the other site, now all gone?) where these were originally posted.  I archived them for some reason at the time...  they are of Bob, back in 2015 or so, doing the "does it sound good" evaluation of the CarverFest 2015 speaker build that James Sauter et. al., developed, produced, and shipped to attendees and non-attendees.  I presume they are final tuning of the crossovers, evaluation of best height for the stands, etc., etc.  

 

If anyone else recalls the details of the photos, who actually took them (for proper credits), and what Bob is finalizing - please pipe in here.

 

Some old-timers here on C! and CarverFest attendees from 2015 (I was not one but do have two pair of the speaker kits) will remember these snapshots.

 

image.thumb.jpeg.a9a9be667e4819d6bcee2bf13312d827.jpeg

 

image.thumb.jpeg.9fe5f88ff6592093ccd447141d26dc9c.jpeg

Edited by AndrewJohn
  • Thank You 3
  • That Rocks 1
Link to comment
Share on other sites

Yep, I remember those pics. RobertR and Bob hammering out the crossovers. I believe you are correct - that was at the Sauter's residence in southwest Florida. Gotta love the purple Christmas tree !!!

  • Thank You 4
Link to comment
Share on other sites

During a recent call with Bob, I mention how low the distortion numbers are on the latest gear, and how well the real world bench test numbers are matching the simulation data.

 

Tube gear in the 1/10 of 1% distortion range, at full power is not that common.

 

Bob made this comment, "I can design tube amplifiers to have distortion numbers comparably low to solid state without a problem." "There is no need to do it, it adds complexity without sonic benefit."

Edited by Ar9Jim
  • Thank You 5
Link to comment
Share on other sites

I've listened to both transistor and tube amps over the years 

 

It has always been my perception that the distortions produced by tubes are primarily even order harmonics. In the same proportion vacuum tube distortions seem to be much less objectionable than the odd order harmonics present in most transistor designs.

 

That is why tube amps with one percent distortion at rated power often sound much better than transistor amps with .1 % distortion. 

 

Agree with what's been said by Ar9Jim and Bob. Delivering enough power at the frequency extremes is more of a problem than distortion is for tube amps designs.

 

The 180 amps I have sound better on the less negative feedback setting than on the higher feedback setting even though technically you will have slightly more distortion.

 

Less complex tube circuits tend to sound better and are more stable than complex designs...there is less to go wrong when driving real-world speaker loads.

 

 

 

Edited by straylight
  • Thank You 4
Link to comment
Share on other sites

 Some attribute depth of soundstage and holographic imaging to the even order harmonics in tube gear, what happens when you eliminate that? That's my only real criticism of my Black Magic 20 amplifier, the original one from 10+ years ago, while it has a large, very wide soundstage and a nice, warm-ish tone it lacks the depth of the newer 275 and BM 25. Some reviewers call it "new" tube sound vs "old" tube sound newer being more solid-state like.  

  • Thank You 1
Link to comment
Share on other sites

On 8/31/2022 at 1:12 PM, jjptkd said:

 Some attribute depth of soundstage and holographic imaging to the even order harmonics in tube gear, what happens when you eliminate that?

 

There are a few tube circuit designs that will reduce even order harmonics but I don't think they can easily be totally eliminated because this is the native state for tubes. They prefer to distort at even order harmonics. The late Bascom King had a design that significantly reduces even order harmonics. My guess is that it may give a more analytical type presentation, more like a Mosfet than a tube but not as analytical as some transistors sound. 

 

Depth of image or depth of soundstage may, in fact, be a bit more complex and actaully be a result of several different parameters.

 

For example tube brands, bias setting and circuit topology may ALL play a part. Bob told me how to  change the bias voltage on the 180 mono amps and it also shortened the depth of the soundstage....Bob was fascinated by this result as it was quite surprising to both of us. This was using the 180s with Apogee ribbon speakers which are pretty good at depth of field presentation.

 

However not everyone may perceive this depth of soundstage the same way.

 

 

Edited by straylight
  • Thank You 2
Link to comment
Share on other sites

@jjptkd

There is one correction that I should make on the above. Some notes were taken on the 180s when I first installed them and after checking my written notes I realized that it was not the bias but the 2 ohm, 8 ohm or 16 ohm taps that made THE BIGGEST difference in the depth of field. At 2 ohms the soundstage was very wide but not that deep.  At 16 ohm the soudstage had inverted and had great depth but was not as wide. This was an interesting result and is what I shared with Bob. The soundstage  geometry was changed by which impedance tap was used for the speakers. Just wanted to make that correction. 

 

Although I did change the bias it did not seem to be associated with any soundstage characteristics. 

 

Edited by straylight
  • Thank You 1
Link to comment
Share on other sites

Great Conversation,

 

Here is some SIM info on the 285. We also have 3rd party evaluations  of products. So far, simulation and bench testing have amazing levels of measurement correlation using these parameters. 

Note H3 being higher than H2 by design.. Fun stuff. Hugh Dean is a great guy and very brilliant. He is a mentor to one of the PhDs on our team. Mr Viet Nguyen. 

 

 

 

 

ANALYSIS LTSPICE OF CARVER BC285 POWER TUBE AMP
 6/28/2022

 Viet asked me to review the proposed Carver KT120 power tube amplifier, the Crimson, which will be produced for the hifi market.

This power amplifier appears to be a conventional, push pull tube style using KT120 pentodes and producing around 80W* into 8R.

Here is the basic schematic:

I have annotated the voltages and current across the signal chain of this amplifer and redrafted it here for simplicity. It is not as full as the original, production schematic but clarifies analysis.

*See postscript for analysis at max power of 125W into 8ohms.

GAIN RELATIONSHIPS

To begin, we consider the gain relationships at maximum input from 2Vpp to output (secondary transformer, 51Vpp). This is a gain of 25.5 (28.15dB) at

1KHz, which is acceptable in the domestic high fidelity world. It is very close to the THX standard, which is 28.5dB.

My initial examination shows interesting gain relationships. Despite using a 12AX7 U1 first stage with mu of 100, the plate current is low at 460uA, low even for a 12AX7, and the stage gives a gain of only 2.1, 6.4dB. Normally with a very high plate impedance of 515k we would expect at least a gain here of 85, so this reduction (no less than 32dB at 1KHz!) reflects heavy feedback to U1 cathode. This high level of feedback, and across the transformer, is combined; voltage from the OPT secondary, and current feedback from R28. This combination of voltage and current feedback on this amplifier is unusual and shows careful manipulation of bass and equalisation which I have seen only on Greek bazouki power amplifiers and, once again, indicates Mr Carver is aware of tube technology all around the world. Since a single pair of pentode KT120 used with constant screen voltage (not ultralinear) delivers high gain but high impedance, this feedback technique can restore superior bass performance but is not well understood. The equalisation on the feedback and grid drives of this amplifier show careful optimisation of stability issues and THD, and this is necessary because the levels of feedback are high for a tube amplifier.

The second stage uses a 12AT7 twin triode (mu 50) as a long tailed pair, configured as a phase splitter. The plates are operated at 2.6mA. This stage gives a gain 12.1 [21.7dB] yielding 51Vpp from 2Vpp at input. The plate loads are dissimilar, R13 36k and R14 39k, set to reduce the THD of the amplifier at the speaker. This is conventional, well understood design. By changing one of the resistors (or both) we can set the relationships between even and odd harmonic for this amplifier.

HARMONIC PROFILE

I have used the operating parameters as follows:

( I omitted the simulation parameters as proprietary, Jim Clark)

 

 

As mentioned earlier in my first consultation with the preamp, this command set has been used with SS design and uses it to predict sound and electrical performance, in particular the THD and the sum of H2+H3+H4 alone. If H2+H3+H4 is better than 99% of the THD (total) figure and IF THD is less than 0.5%, such a metric indicates very low levels of the objectionable H5, H6 and beyond. This implies that the ear will perceive the sound as ‘natural’. This is based on Jean Hiraga’s work in the sixties in France, and has never failed me.

I have set the FFT simulations at +20dBU output, which is 14.14Vp, 28.28Vpp into 8R. This corresponds to 550mVpeak and delivers 12.5W into 8R, about one third the highest power rated for this amplifier. We could expect the results to accord nicely with a listening session in a domestic situation.

Here is the FFT at 1KHz 40W into 8R:

H2 is at -83dB, H3 is -57dB, H4 is -91dB and H5 is -71dB. THD: 0.168%

Second harmonic is at -72 degrees; optimum should be -90 degrees for best depth of image but this figure is very good.

By using feedback, THD is low for this tube power amplifier, but at the expense of low EVEN order harmonics. Odd orders are relatively high (typically 25dB higher), reflecting the use of KT120 output pentodes in push pull. Generally with tube amplifiers we try to have higher levels of even orders over odd; this shows many audiophiles prefer a single ended output stage rather than a push pull, but SE is very, very inefficient and leads to much less output power, and high quiescent current, typically 100mA plus.

Here is the FFT at 20Hz 40W into 8R:

H2 is at -68dB, H3 is -48dB, H4 is -77dB and H5 is -61dB. THD: 0.434% The very low distortion at this very low frequency shows a very linear design, carefully equalised for low bass. This comes back to the current feedback, based around R28.

Here is the FFT at 20KHz 40W into 8R:

 

H2 is at -68dB, H3 is -51dB, H4 is -85dB and H5 is -80dB. THD: 0.287%

This shows the output at very high frequencies has dropped, from +20dBU to +17.3dBU. This is less than 3dB however, and at 20KHz it is unlikely most listeners would notice, particularly if metal ribbon tweeters are used. THD is excellent at this high frequeny, again due to high feedback. The 1KHz preference of ODDs over EVENs is evidence again, but this is a quality of all push pull amplifiers and yet, because the odds decrease evenly with order anyway, the ear perceives this acceptable although it prefers evens higher than successive odds. It is worth noting that a number of audiophiles actually prefer H3 over H2; Nelson Pass has mentioned this in his writings, so we need to recognise that there will ALWAYS be a market for tube amplifiers, PP and SE.

SIMULATION AND INTERPRETATION

Between the two opposing audio worlds, subjective audiophiles and technical engineers, there is considerable lattitude to interpret simulation results.

At 1kHz, the Carver tube design gives a very high ratio of (H2+H3+H4)/THD of 99.85%. The predominant second harmonic is at -63dB, and H3 a -47dB.

At low frequencies, 20Hz, this ratio is 97.36%, with H2 at -63dB and H3 at - 48dB.

At 20kHz, the Carver tube design ratio is again at 99.85%, with H2 at -65dB down, and H3 at -52dB.

Despite having slightly lower output at 20KHz (down 2.7dB on 1KHz output) the subjective measurements in percentage of (H2+H3+H4)/THD(all) are very high, all over 97.3% for 1KHz and even higher at 20Hz and 20KHz.

These point to a very even, highly linear amplifier with excellent levels of feedback control at all frequencies and outputs. Although the Carver tube power amplifier delivers H3 higher than H2, all levels are very low for a tube amplifier with feedback including the output transformer, no mean feat, and it judiciously apportions feedback in two modes to give a very easy, high resolving and non-fatigueing sound.

THE TOPOLOGY

The Carver topology, like all evolutions, is deceptically simple. First stage as a plate loaded 12AX7 running at very low current (550uA) , low voltage (102V) and a plate load of 515k. The cathode features an unusual complex network accommodating two feedback loops; voltage from the output transformer, and current from a 0.1R sense resistor plumbed into the speaker ground connection.

The second stage is a pair of 12AT7 triodes in a long tailed pair, configured to give gain and two anti-phased outputs for the output pentodes.

The final stage is a push pull pentode pair of KT120s, transformer loaded and biased at -50V to deliver 30mA quiescent to each tube.

With no feedback at all, the gain of the amplifier is 201 (46dB) at 1KHz. With negative voltage gain alone, the gain is reduced drastically to 10 (20dB), giving 26dB of negative feedback at 1KHz and somewhat more and less at other frequencies. With current positive feedback installed, the amplifier has an overall gain of 25.57 (28.1dB), reducing the overall negative feedback from 18dB, about the maximum achieved with a tube power amplifier owing to phase shifts in the output transformer. Manipulating this high level of compound feedback and it with voltage and current drive is an achievement in itself and represents an astonishing level of expertise. Feedback, despite its bad press, is still the dominant technique of refining audio power amplifier design.

There are topologies which can be used for PP tube amplifiers. For example, the first stage could use two triode (one bottle) to form an SRPP or a cascade version; the SRPP would not meet gain levels for this circuit, but the cascode would offer a little more feedback but probably tip the amplifier into instability. The phase splitter can be revised, but here is little advantage because the long tail pair in the tube world is ubiquitous for gain, low impedance drive, and moderate requirements of high voltage. The output stage can be reconfigured as ultra-linear, which would add roughly 40% more power output, but the consensus is that the benefit in sound quality is not justified. No, this configuration is a high quality, economy design of very high performance and while I would not design the amp identically, I’m sure I could not improve on the Crimson.

POWER SUPPLIES

There are five power supplies for this power amplifier. These are; 340V @ 460uA (first stage), 350V @ 1.7mA (pentode outputs screen), 435V @ 5.2mA (phase splitter), 685V @ 64mA (output tubes and transformer) and -47V @ 100uA (pentode output bias voltage).

 

These would not be required regulated, and need not be complex or expensive. Capacitors are always costly for high voltages, but this design would not be more complex than any other design.

CONCLUSION

The Crimson is an exceptional design; simple in schematic, but complex in its feedback regimes, including negative voltage fb and positive current fb. This is almost never implemented in power tube amplifiers, and the result here should deliver sound quality at very high quality, and yielding very good measured performance as well. The frequency response has been tailored to a flat response, and the output impedance has been optimised for low bass signals. It is very difficult to design good tube amplifiers in AB at high power; 80W* is a medium level and this design is outstanding.

Hugh R. Dean. 

Edited by Ar9Jim
  • Thank You 4
Link to comment
Share on other sites

@Ar9Jim

 

This is quite a bit of a download of information on the Crimson but should be welcomed by all since it gives meaningful information about circuit topology, distortion levels and real-world performance that old-school static specifications cannot and do not address. I have a few questions and some comments.

 

4 hours ago, Ar9Jim said:

This power amplifier appears to be a conventional, push pull tube style using KT120 pentodes and producing around 80W* into 8R.

 

Q: Is the spec of 80 watts into 8 ohms an average wattage for all frequencies from 20 Hz to 20kHz or is it a simple 1 KHz frequency power test?  Not stated.

 

Comment: The use of stated percentages of harmonic distortions is very welcome as it drives home some new ideas of the type of distortions that we perceive. Amp designer Nelson Pass (of Threshold fame) also discovered that H2,H3,H4 are not all perceived the same as far as how objectionable we perceived them. His Threshold amplifier designs, although bi-fet transistor in nature, also favored the lower harmonics like H2, H3 and he found that these could be tolerated in much higher amounts. He also obtained his hand selected bi-fets directly from Motorola based on matching up the ones with similar distortion characteristics.  The legendary designer John Curl also plays very close attention to the distortion-quality (harmonic content) of the parts he puts into his amp designs too.

 

As the H2, H3 and H4 approach -80 db levels, however, it is rather doubtful that they would be audible to the human ear to make much of a difference. So the much lower db level harmonics are probably not contributing much to the perceived sound.

 

The other comment I'd like to make is that most tube amplifiers are somewhat deficient at delivering power at the frequency extremes.  My EL-34 Acrosound amps, Tube Dynaco amps of the 50s and many modern tubes designs all lose power at the bass end and at  high frequencies.  A simple power test at the frequency extremes (for example using an Audio Precision tester at low frequency sweep) will show a pronounced drop in power between 20 Hz and 100 Hz and also a drop at the high frequency extremes (15kHz - 20kHz). The fact that the Crimson has only -2.7db drop at 20kHz in amazingly good performance for any tube amplifier of any power level to achieve.

 

Also the use of Fast Fourier transform for your tests is a great idea as the amplifiers performance can change significantly during the several second scan time necessary for frequency swept testing methods.

 

Thanks for posting this great information. Its food for thought about amp design that you don't normally see shared.

 

 

Edited by straylight
  • Thank You 3
Link to comment
Share on other sites

2 hours ago, Ar9Jim said:

Great Conversation,

 

Second harmonic is at -72 degrees; optimum should be -90 degrees for best depth of image but this figure is very good.

 

 

 Intrigued by this statement right here-- curious if you have a number for the 275 or BM-25 for comparison? 

  • Thank You 1
Link to comment
Share on other sites

59 minutes ago, jjptkd said:

Second harmonic is at -72 degrees; optimum should be -90 degrees for best depth of image but this figure is very good.

 

Degree designations of -70 or -90 usually means phase shift or phase distortion produced within the circuit topology itself. You are correct to point out that it is not explained but if it is phase differences (which are a type of distortion too) I have always been told that no phase shift is best (0 degrees) situation since you always want accurate phase for accurate sound and dynamics. But the designers may know something we don't and phase anomalies may be perceived as a difference in depth of soundstage.

 

Never heard this before, so yes very intriguing, unless we are interpreting this incorrectly?

 

@Ar9Jim....can you comment about what the -70 degrees for 2nd order harmonic is? Is this phase shift or something else they are referring to?

 

Edited by straylight
Link to comment
Share on other sites

5 hours ago, Ar9Jim said:

If H2+H3+H4 is better than 99% of the THD (total) figure and IF THD is less than 0.5%, such a metric indicates very low levels of the objectionable H5, H6 and beyond. This implies that the ear will perceive the sound as ‘natural’. This is based on Jean Hiraga’s work in the sixties in France, and has never failed me.

 

Sounds like a solid approach...and it should actually work for most designs too...vacuum tube, solid state even Mosfet or whatever. The fifth, sixth, and higher order harmonics are the 'meanies' and should be avoided at all costs for pleasing, musical sound.  I would call this "combing the distortions"...just like straightening your hair to get kinks out!

Link to comment
Share on other sites

3 hours ago, straylight said:

 

Degree designations of -70 or -90 usually means phase shift or phase distortion produced within the circuit topology itself. You are correct to point out that it is not explained but if it is phase differences (which are a type of distortion too) I have always been told that no phase shift is best (0 degrees) situation since you always want accurate phase for accurate sound and dynamics. But the designers may know something we don't and phase anomalies may be perceived as a difference in depth of soundstage.

 

Never heard this before, so yes very intriguing, unless we are interpreting this incorrectly?

 

@Ar9Jim....can you comment about what the -70 degrees for 2nd order harmonic is? Is this phase shift or something else they are referring to?

 

Sure can. Well, I can't personally, but I'll ask in the morning meeting. Be back shortly. I'm curious as well. 

Link to comment
Share on other sites

4 hours ago, jjptkd said:

 

 Intrigued by this statement right here-- curious if you have a number for the 275 or BM-25 for comparison? 

Hi Jesse,

I'll double check. I'm pretty confidant saying the number is exactly the same. The 275 and Black Magic are same board and the 285 is based on the same design.

 

I tried KT120s in the Black Magic. Considerable difference. I like them both. Do you have a preference EL-34 vs KT120 in the Black Magic 25. Like having the extra power supply capacity to play with tubes. KT120s obviously made more power. 6550s can be great in these as well.

Edited by Ar9Jim
  • Thank You 2
Link to comment
Share on other sites

6 hours ago, straylight said:

@Ar9Jim

 

This is quite a bit of a download of information on the Crimson but should be welcomed by all since it gives meaningful information about circuit topology, distortion levels and real-world performance that old-school static specifications cannot and do not address. I have a few questions and some comments.

 

 

Q: Is the spec of 80 watts into 8 ohms an average wattage for all frequencies from 20 Hz to 20kHz or is it a simple 1 KHz frequency power test?  Not stated.

 

Comment: The use of stated percentages of harmonic distortions is very welcome as it drives home some new ideas of the type of distortions that we perceive. Amp designer Nelson Pass (of Threshold fame) also discovered that H2,H3,H4 are not all perceived the same as far as how objectionable we perceived them. His Threshold amplifier designs, although bi-fet transistor in nature, also favored the lower harmonics like H2, H3 and he found that these could be tolerated in much higher amounts. He also obtained his hand selected bi-fets directly from Motorola based on matching up the ones with similar distortion characteristics.  The legendary designer John Curl also plays very close attention to the distortion-quality (harmonic content) of the parts he puts into his amp designs too.

 

As the H2, H3 and H4 approach -80 db levels, however, it is rather doubtful that they would be audible to the human ear to make much of a difference. So the much lower db level harmonics are probably not contributing much to the perceived sound.

 

The other comment I'd like to make is that most tube amplifiers are somewhat deficient at delivering power at the frequency extremes.  My EL-34 Acrosound amps, Tube Dynaco amps of the 50s and many modern tubes designs all lose power at the bass end and at  high frequencies.  A simple power test at the frequency extremes (for example using an Audio Precision tester at low frequency sweep) will show a pronounced drop in power between 20 Hz and 100 Hz and also a drop at the high frequency extremes (15kHz - 20kHz). The fact that the Crimson has only -2.7db drop at 20kHz in amazingly good performance for any tube amplifier of any power level to achieve.

 

Also the use of Fast Fourier transform for your tests is a great idea as the amplifiers performance can change significantly during the several second scan time necessary for frequency swept testing methods.

 

Thanks for posting this great information. Its food for thought about amp design that you don't normally see shared.

 

 

Speaking of John Curl.. He has some very interesting views on accuracy for measuring audio products vs measuring accuracy needed for measuring differences in individual component parts.

He speaks about designing amps that test great into resistive loads that perform poor running speakers. Things he knows that can't be measured and other things that come with experience.

I have heard some of Johns designs taking criticism from the same crowd. He builds what he believes sounds the best. Building something to test good running a resistor is easy. Running loudspeakers well is another goal to a large extent.

 

 

 

Edited by Ar9Jim
  • Thank You 1
Link to comment
Share on other sites

2 hours ago, Ar9Jim said:

Hi Jesse,

 

I tried KT120s in the Black Magic. Considerable difference. I like them both. Do you have a preference EL-34 vs KT120 in the Black Magic 25. Like having the extra power supply capacity to play with tubes. KT120s obviously made more power. 6550s can be great in these as well.

 

 I have been using the KT-120's since they've arrived-- they run considerably cooler than the 34's do and my AC unit has a hard time keeping up during these hot summer days. First thing I noticed when I received the 25 was that voices and certain guitars seem to be pushed forward compared to the 120's and the 34's seem to be just a tad lighter in the bass. Overall balance is different, I like both too.

 

 That being said almost everything is different in my system now (new DAC, Pre and speakers) and I'm dealing with a broken back so haven't been able to do much right now, part of the reason I've been online so much over the last month. Once my back is a little better and temps drop a bit I plan on playing with the stereo some more. 

Edited by jjptkd
  • Thank You 2
Link to comment
Share on other sites

14 hours ago, Ar9Jim said:

Sure can. Well, I can't personally, but I'll ask in the morning meeting. Be back shortly. I'm curious as well. 

 

Hello Straylight. 

Viet was around last night. Here is some info.. Bob will be around this morning. Bob can talk at length on these topics. 

 

Hope this helps.

 

 

 

Hi Jim,

 

Sure I hope I can help but need more background or context. What figure is this referring to and how was the data taken? I am not sure there is much difference between -90 and -72 deg phase for the second harmonic. 

 

There is a difference at 180deg though.  This is accomplished by flipping phase of the speaker binding posts. That’s different than flipping polarity of the signal before the amp though.  This is the difference between positive phase and negative phase second harmonic distortion. 

 

I know that Nelson Pass has said that there is positive phase vs negative phase second harmonic distortion and they sound different. 

 

Below is from an article that Pass wrote - here is a typical oscilloscope trace of an amp signal in red and the distortion is shown below in blue and here it is dominant second harmonic.  

 

 

 

The red fundamental goes up and down at a 1kHz rate, and we see that the blue wave goes up and down as well at twice the frequency, but aligned with the red wave in a particular way—the blue goes down when the red goes up and the blue also goes down when the red goes down. I refer to this as negative phase 2nd harmonic. 

 

Positive phase 2nd harmonic is the opposite:


With positive phase 2nd harmonic, the blue goes up when the red is either up or down.

 

So why is the phase important? Well, it's a subtle thing. I don't suppose everyone can hear it, and fewer particularly care, but from listening tests we learn that there is a tendency to interpret negative phase 2nd as giving a deeper soundstage and improved localization than otherwise. Positive phase seems to put the instruments and vocals closer and a little more in-your-face with enhanced detail.

 

Hope this helps. 

 

Viet

image0.jpeg

image1.jpeg

Edited by Ar9Jim
  • Thank You 2
Link to comment
Share on other sites

13 hours ago, Ar9Jim said:

Speaking of John Curl.. He has some very interesting views on accuracy for measuring audio products vs measuring accuracy needed for measuring differences in individual component parts.

He speaks about designing amps that test great into resistive loads that perform poor running speakers. Things he knows that can't be measured and other things that come with experience.

I have heard some of Johns designs taking criticism from the same crowd. He builds what he believes sounds the best. Building something to test good running a resistor is easy. Running loudspeakers well is another goal to a large extent.

 

 

 

One of the best points in this video is about test gear accuracy and proper applications of the tech.. The old masters have no limits on their access to state of the art test gear, they have that tech as well. 

Note the distortion analyzers from the 1980s that John is using. Here is one of the same that works well. This one is at the Carver factory in IL. Its one of Bobs.

 

Near the factory, teammate and digital guru, Doug Goldberg has the capacity to test to the extreme precision needed for his DAC development. Doug has been designing digital audio gear, as a hired gun for some of the greatest names in audio, since the birth of digital audio. Doug recently retired as a chief engineer at Lockheed Martin. Bob has the right team in place at every position. 

 

Doug can test our products to next level accuracy when required.

 

Older School. 

 

IMG_1327.jpg

Edited by Ar9Jim
  • Thank You 1
Link to comment
Share on other sites

The scope plots below should help members to visualize and understand what the harmonic distortion products are and how they actually look on an oscilloscope.

We are showing a test signal of 50 Hz (bass frequency).

 

and so the higher order harmonic distortions are easily calculated as follows:

 

H2 = 50 x 2 = 100 Hz

H3 = 50 x 3 = 150 Hz

H4 = 50 x 4 = 200 Hz

H5 = 50 x 5 = 250 Hz

H6 = 50 x 6 = 300 Hz

 

The first plot is of my HP 3580A Spectrum Analyzer shows a clean 50 Hz peak without any measurable distortion. Notice how clean it looks to right of the scope display. A perfect amplifier without any distortion would look like this when reproducing a 50 Hz wave. Of course this is just the test signal on the scope…no amplifier would look this clean in real life.

 

535291402_PlotNo.2-042405-50HzSignal(clean).jpg.4d17b63430b2dab7d9fa5c7f4dae14b1.jpg

 

The second plot shows my Acrosound-330 EL-34 project amps that Bob helped me with at 1 watt into an 8 ohm load. We were also working on how much negative feedback to feed the circuit to clean up the sound. This plot shows the amp with -6db of negative feedback at 1 watt into 8 ohm load. This is the feedback level I finally decided to use in the circuit.

 

Notice the evenly spaced peaks AFTER the first 50 Hz fundamental tone peak on the left…each successive peak is a higher order harmonic going from left to right on the display. The small peaks in between the first few harmonics are probably power supply related and therefore are not associated with the EL-34 output tube distortion tones produced.

 

1669464073_PlotNo.6-0408061watt_edited-2.thumb.jpg.955fe75638034b26b12d5b2da0c4d763.jpg

 

Notice how the distortion products get progressively smaller as you go to the right. This is a natural state of how tubes distort in many circuit topologies. The more offensive sounding fifth, sixth and seventh harmonics (H5 H6 H7) can be seen in the second band of peaks above 250 Hz about mid-scope.

 

According to research by consultants in postings above, H5, H6 and H7 and higher order harmonic peaks are much more objectionable to the human ear and should be suppressed by the amplification circuit in order to get a more pleasing musical sound from the amp. By contract H2 and H3 can be present in much higher proportions without sounding harsh or offensive to the listener.

 

Edited by straylight
  • Thank You 2
Link to comment
Share on other sites

6 hours ago, Ar9Jim said:

I know that Nelson Pass has said that there is positive phase vs negative phase second harmonic distortion and they sound different. 

 

Below is from an article that Pass wrote - here is a typical oscilloscope trace of an amp signal in red and the distortion is shown below in blue and here it is dominant second harmonic.  

 

 

 

The red fundamental goes up and down at a 1kHz rate, and we see that the blue wave goes up and down as well at twice the frequency, but aligned with the red wave in a particular way—the blue goes down when the red goes up and the blue also goes down when the red goes down. I refer to this as negative phase 2nd harmonic. 

 

Positive phase 2nd harmonic is the opposite:


With positive phase 2nd harmonic, the blue goes up when the red is either up or down.

 

So why is the phase important? Well, it's a subtle thing. I don't suppose everyone can hear it, and fewer particularly care, but from listening tests we learn that there is a tendency to interpret negative phase 2nd as giving a deeper soundstage and improved localization than otherwise. Positive phase seems to put the instruments and vocals closer and a little more in-your-face with enhanced detail.

 

@Ar9Jim and Viet,

 

Fascinating!  Thank you for follow up on the question, much appreciated.

 

Whether or not there really is any more detail in the music or not the positive phased 2nd harmonic (relative to the main signal) when present persuades the human ear into thinking that the music is closer and more detailed whereas the negative phased 2nd harmonic lets the listen believe its embedded in a deeper soundstage farther away from the listener.  Wow, that's amazing.

 

The phase difference makes some sense because I think a doppler effect tricks the human ear in a very similar way - perceiving either rising tones or decreasing tones, depending on the  direction of movement of the sound source relative to the listener. But we all know the tone pitch never changes...it is only the ear that is tricked into thinking there is a change. It's a trick.

 

As Bob was once famously quoted as saying, "Its all trickery"

 

This concept also seems to relate directly  to my perception/experience of hearing different levels of soundstage depth on the different speaker taps (2, 8 and 16 ohm) of the Rootbeer 180 mono amps. I am aware that each tap results in measurably different distortion levels and harmonics on each tap. If this also results in a substantially different amount of  phase shift (plus or minus 180 degrees) for the 2nd order harmonic than this could explain it!

 

I can measure the different levels of 2nd order on each impedance tap but determining the phase shift would be something that I would not know how to detect with my existing equipment. Nonetheless this is really interesting and goes beyond just circuit topology. It begins to approach the borders of the ear/brain interface and how it all works together with electronic circuits to actually perceive sound.

 

 

Edited by straylight
  • Thank You 2
Link to comment
Share on other sites

  • 2 months later...

Hello Carver fans,

Owner of several Carver/Sunfire amps for decades, I'm always happy with them. Carver amps have a very specific personality, a way to make music alive, that'what I'm searching for... 

PM series, TFM series, I'm listening them daily, and also repair and recap some. 

 

Could someone explain to me why the carver amps have two different designs :

- the PM1. 5 / TFM42-45 type : amp PCBs are set verticaly on the amp frame, and the power PCB is separated from them 

 

- the PM1. 0 / TFM25 / TFM55 type : the power transistors are on both sides. 

 

Why these deep structural differences on different products at the same period of time? 

 

Regards for any hint

Mathieu

 

  • Thank You 2
Link to comment
Share on other sites

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...