Amps and Power hungry speakers.

[UncleMeat 1,173]

 

 

 

Loner_t bought several TFM-42's from NASA that had the gain pots modded to allow external adjustment (dual trim pot mounted to the back exterior). That mod to yours would allow gain adjustment similar to the TFM-35 and TFM-55 (non-"X" versions). Reducing the gain will limit the amount of maximum output of the amp.

 

If those pots are a relocation of/addition to the RP2 pots on the amp boards, then they affect the output impedance of the amp. You could change the power seen by the speaker in much the same way by using rheostats(hefty ones!) at the output. But when you get into that territory of low damping factor, interaction with the speaker impedance can cause the "voice" of the system to change... kind of the point of the whole "TFM" circuit. 

 

A long time ago, when she was almost brand new, I added a volume pot to the *input* of my first TFM-45 (profile pic). Crude but effective, it's been very useful over the years. 

 

Yes, I was thinking the same about that Fill and was going post it, but I just didn't want to post a caution about it without knowing for sure (as you do) that the amp's gain control could cause the TFM to change.  

 

Is the gain dial on the TFM-35 and TFM-55 an input level control, and not a gain control?

[fill35U 1,846]

Is the gain dial on the TFM-35 and TFM-55 an input level control, and not a gain control?

 

Yes, they're just at the signal input off the RCA's. 100k for the -35, without actually looking at my -55 and based on the rest of the schematic I'd guess the same. 

[dr_gallup 42]

IMHO, going to the lower power amp for a pair of "power hungry speakers" is completely the wrong thing to do.  Driving a lower powered amp into clipping is the single biggest cause of speaker damage.  If the speakers are rated for 300-500 watts, use the 600 watt TFM-42's, not going to hurt a thing.

[morris 390]

loner_t asked me to look inside my NASA TFM42 to see how the volume pots are wired.

 

Note the ground wire is red. 

 

 

[fill35U 1,846]

 

 

loner_t asked me to look inside my NASA TFM42 to see how the volume pots are wired.

 

Note the ground wire is red. 

 

 

Thanks for the diagram and pic, Morris! 

 

According to your diagram, that's an unusual way of doing it! Typically, the pot wiper is connected to the *downstream* side of the signal path. With the wiper connected to the input side like this, when the volume is turned all the way down, the input is shunted to ground, presenting a dead short to the source. A typical source might have an output impedance of around 200 to 1200 ohms. Hopefully the intended source is designed to not mind having its output shorted for long periods of time... As the amp's level pot approaches the source's output impedance, the voltage from the source will drop, making the level control effectively more nonlinear at the quiet end. At the same time, with the amp's level turned all the way down, the pot presents a high impedance to the amp input, which means it could be noisy when it should be quietest. Are you *sure* that's how they did it? 

 

They moved the shunt cap for the left input to the new pot, and used its board holes for the wires to the level control. 

 

Did anybody notice they put the left level control on the *right* side of the amp, and the right level control on the *left* side? And who uses red for ground, and black for signal?? NASA... 

[morris 390]

 

 

Are you *sure* that's how they did it? 

 

 

Perhaps it was like this?

 

[fill35U 1,846]

 

 

 

Are you *sure* that's how they did it? 

 

 

Perhaps it was like this?

 

 

Yeah, that would be the way the rest of the world does it.   

[morris 390]

I like the volume control on the amp.

 

You can use it to limit the power output if you are worried about harming efficient speakers, and make more usable volume control on the preamp.

 

If you by-amp, it helps balance the two amps. I used the TFM42 for bass, and the M1.0t for highs. I have to turn the volume down a bit on the TFM.

 

It makes it easy to integrate the amp in a home theater set up. Adjust the volume until all the channels are the same. 

[Daddyjt 9,612]

In my experience (limited as it may be), a speaker will usually "tell" you when it's receiving too much/not enough power, and/or reaching its physical limits...

[Papajoe 221]

Morris- Thanks for taking the time to get into your NASA TFM-42 and sending photos. Your schematic helps me to understand what was done and how it works. Do you think that was done by NASA or did Carver specially modify the amps for them? I did look at the connections when I first got the amp; but, do not understand enough to tell where they go on the board.

I located ,on the web, of course, the original tech booklet that was put out by Thiel when they first introduced the CS 3.6s. It states that the speakers are rated at 100-500W @4 Ohms. So, we have decided to not do anything to the TFM-42's output after all. Just hook them up and listen to the Carver magic.

Papajoe

[morris 390]

I believe the work was done by NASA.