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May 19, 2022, 08:21:37 AM

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Reduce signal from preamp.

Started by flester, January 19, 2021, 06:47:34 PM

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flester

I have built amp with a starved plate 12au7 preamp feeding a solid-state power amp module. There's a 100k audio pot controlling the signal going to the power amp. Whats the best way to pad the input to the power amp? A resistor in series somewhere?

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Jazz P Bass


flester

Sorry, here you go. It was late when I posted

flester

I'm running it at 12 v. The preamp and the power amp work fine when tested seperately.


Jazz P Bass

Not too sure what you want to do.
The input sensitivity of the module is listed as 80mv's.
If your input signal is too hot, knock it down with a voltage divider.
2 resistors. 1 in series. 1 to ground.

flester

Thanks, I was trying that and it was not working but I found a few other issues on my circuit. I'll fix those and try arain

SerpentRuss

I'm curious about a design like this and I have possibly a very stupid question.  If you're running at 12 volts and this is your schematic, what are you doing to attenuate the filament voltage?  It seems like you should have a resistor in that path.  Also, the filament is happy to run on AC and does so in traditional Tube Amp designs.  Is this a normal setup for a "starved plate" design?

tonyharker

#8
12a?7 valves have a 12v6 heater between pins 4 & 5 and a centre tap on pin 9. for 6.3 v common pins 5 & 6 and feed supply between these and pin 9.  AC or DC  doesn't matter.

SerpentRuss

Quote from: tonyharker on January 21, 2021, 02:21:35 PM
12a?7 valves have a 12v6 heater between pins 4 & 5 and a centre tap on pin 9. for 6.3 v common pins 5 & 6 and feed supply between these and pin 9.  AC or DC  doesn't matter.

Thank you very much for answering that question.

flester

Heres the noise which comes in as I increase the volume.  https://drive.google.com/file/d/1KL_uZ8kQ9_aCp2k0lP7rCePaSFM_Zxf0/view?usp=drivesdk

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Den.



flester

Noise was worse when I wiggled the gain pot so I replaced that which seemed to solve the problem. The whole thing did work fine at first and the preamp and power amp each worked fine on their own so interested to know if the noise was due to the power amp getting too much signal or not.

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flester

By the way it's all running at 12v dc and I have left out the tone pot.

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