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Old 13th June 2012, 10:15 AM   #1
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Default Question regarding volume pot

Just finished my stereo TDA7294 amp. Thanx to the help of some DIY forum members! Sounds just great.

I have 2 questions.

1. I tried various (dual mono) pots to control volume (50k, 100k, 200k, lin and log). None of 'em seem to work properly. Volume is at max (clipping) when the pot is like 10% open. Any solutions/suggestions?

2. I have a 10mV ripple (at all frequencies) when volume is (close to) zero, and some 5mV during normal playback. While this seems quite normal, my Teac shows just 2mV. I like to know where this comes from, and if it's fixable (it's not a problem, just being curious). Top is DIY; bottom is Teac.
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Old 13th June 2012, 10:37 AM   #2
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That is not the fault of the volume pot. You are simply overdriving your amplifier - ie the input signal is too high.

Try putting a large resistor in series between the signal input and the top of the pot.

With your 50K Log pot a 50K (47K) resistor would give you a reduction of 3dB in the input signal. You might need to play with the required value to get the results that you are looking for.
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Last edited by KatieandDad; 13th June 2012 at 10:53 AM.
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Old 13th June 2012, 11:59 AM   #3
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Is there any way to remove a gain stage in the upstream equipment?
Can you reduce the gain of the earlier stages?
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Old 13th June 2012, 03:15 PM   #4
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Post a schematic of the circuit, perhaps changing a R can reduce input sensitivity or gain.
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Old 14th June 2012, 01:45 PM   #5
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Wink Some measurements

@KatieandDad

Did some measuring with different pots and resistors. Pots (indeed) hardly make any difference (25k up to 200k, lin or log). Check attachment. Looks an added resistor somewhere between 470k and 1M should do fine. HOWEVER... Audio starts degrading when using 470k or higher (2.2M is like clearing your throat continuously .

(schematics don't show zobel between -Vs and output, but it is there)
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Old 14th June 2012, 02:14 PM   #6
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What's the value of R102 and R103 (they set the input opamp gain) and R107, R108 (gain of the power amp).
I think you'll find that your gain is way too high; I bet your 'buffer' is more an amp than a buffer.

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Old 14th June 2012, 02:17 PM   #7
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A better way to decrease the gain is to lower the value of R103. This will lower noise as well. Try 1/2 the value for starters.

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Old 14th June 2012, 02:18 PM   #8
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Quote:
Originally Posted by janneman View Post
What's the value of R102 and R103 (they set the input opamp gain) and R107, R108 (gain of the power amp).
I think you'll find that your gain is way too high; I bet your 'buffer' is more an amp than a buffer.

jan didden

Hi Jan, we x-posted :-).

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Old 14th June 2012, 02:30 PM   #9
AndrewT is offline AndrewT  Scotland
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What are the gains of u101, u102 & u201?
What does u201 do?
Back to adjusting the input attenuation.

If you have a 10k vol pot and add a 100k resistor in front of it you reduce the signal by ~20dB.
If you increase the resistor to 1M you reduce the signal by ~40dB. BUT
both resistors add noise to the system that at low volumes could become quite audible.

Instead, add a 20k resistor in series, but this time add a second resistor in parallel to the 10k vol pot.
If the second resistor is 100k then the attenuation is ~10dB.
Change that second resistor to 10k and the attenuation becomes ~14dB
to 5k and you get ~17dB
to 3k and you get ~20dB
to 1k and you get ~27dB
100r and you get ~46dB

this time the added noise is due to the 20k, but you can get a whole range of attenuations and each has no more noise added than the basic 20k gives. This should be tolerable with a range of quiet signal.
But better by far is to remove the excessive gain in the system, not attenuate the signal.
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Last edited by AndrewT; 14th June 2012 at 02:46 PM.
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Old 14th June 2012, 02:44 PM   #10
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