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Author Topic: Reduce signal from preamp.  (Read 821 times)

flester

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Reduce signal from preamp.
« on: January 19, 2021, 06:47:34 PM »
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

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Re: Reduce signal from preamp.
« Reply #1 on: January 20, 2021, 12:37:14 AM »
Well how about a schematic?
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flester

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Re: Reduce signal from preamp.
« Reply #2 on: January 20, 2021, 04:02:46 AM »
Sorry, here you go. It was late when I posted
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flester

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Re: Reduce signal from preamp.
« Reply #3 on: January 20, 2021, 04:04:27 AM »
I'm running it at 12 v. The preamp and the power amp work fine when tested seperately.
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flester

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Re: Reduce signal from preamp.
« Reply #4 on: January 20, 2021, 04:09:59 AM »
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Jazz P Bass

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Re: Reduce signal from preamp.
« Reply #5 on: January 20, 2021, 10:38:32 AM »
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.
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flester

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Re: Reduce signal from preamp.
« Reply #6 on: January 20, 2021, 01:41:45 PM »
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
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SerpentRuss

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Re: Reduce signal from preamp.
« Reply #7 on: January 21, 2021, 01:44:16 PM »
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?
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tonyharker

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Re: Reduce signal from preamp.
« Reply #8 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.
« Last Edit: January 21, 2021, 02:24:49 PM by tonyharker »
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SerpentRuss

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Re: Reduce signal from preamp.
« Reply #9 on: January 22, 2021, 08:55:26 AM »
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.
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flester

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Re: Reduce signal from preamp.
« Reply #10 on: January 22, 2021, 02:04:11 PM »
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.

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Re: Reduce signal from preamp.
« Reply #11 on: January 22, 2021, 06:16:17 PM »
Quote from: flester on January 22, 2021, 02:04:11 PM
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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flester

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Re: Reduce signal from preamp.
« Reply #12 on: January 23, 2021, 04:55:13 AM »
https://1drv.ms/u/s!Ahfhrrb-UXCanS2OqSaG-XLKGiCx

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flester

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Re: Reduce signal from preamp.
« Reply #13 on: January 23, 2021, 05:07:14 AM »
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

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Re: Reduce signal from preamp.
« Reply #14 on: January 23, 2021, 05:09:29 AM »
By the way it's all running at 12v dc and I have left out the tone pot.

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