Preamp does not mute. - diyAudio
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Old 4th February 2015, 04:29 PM   #1
mitsof is offline mitsof  Greece
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Default Preamp does not mute.

I've built this small preamp but it does not mute even at 0% volume.One leg of the potentiometer is connected to ground.I've also checked audio input and output of the preamp-every seems ok.What could cause this problem?Can I use some resistor or cap to correct this?
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Old 4th February 2015, 04:54 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by mitsof View Post
I've built this small preamp but it does not mute even at 0% volume.One leg of the potentiometer is connected to ground.
Measure the minimum resistance from wiper to ground, maybe there's some residual amount.
Or, try returning the lower volume control terminal separately to the main ground instead of the input stage ground.
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Old 6th February 2015, 10:43 AM   #3
DF96 is offline DF96  England
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Wiring error. The OP hasn't built what he thinks he has built, or he doesn't understand how volume pots work. A common mistake is to wire the pot as just a series resistor.
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Old 6th February 2015, 11:20 AM   #4
AndrewT is offline AndrewT  Scotland
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I have described a vol pot as both a Receiver for the input signal
and
as a Source for the output signal.
The description goes on to explain why there is a twisted pair of Flow and Return INPUT wires
and
why there is a twisted pair of Flow and Return OUTPUT wires.

That construction avoids virtually ALL the different wiring errors that can be concocted for a vol pot.
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Old 6th February 2015, 11:25 AM   #5
AndrewT is offline AndrewT  Scotland
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Following up on the "does not mute" and the "maybe there's some residual amount" comment:

a 10k vol pot with a residual amount of lower leg resistance of 1r will attenuate to 1/10000 of the input voltage. That 1/10000 is equivalent to -80dB. That is not completely silent.

10r of lower leg resistance at the minimum volume setting achieves a maximum attenuation of only -60dB.
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Old 6th February 2015, 05:29 PM   #6
mitsof is offline mitsof  Greece
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Quote:
Originally Posted by AndrewT View Post
Following up on the "does not mute" and the "maybe there's some residual amount" comment:

a 10k vol pot with a residual amount of lower leg resistance of 1r will attenuate to 1/10000 of the input voltage. That 1/10000 is equivalent to -80dB. That is not completely silent.

10r of lower leg resistance at the minimum volume setting achieves a maximum attenuation of only -60dB.
The pot is at the output of the preamplifier to the amplifier.The preamplifier has too much gain (40db) Maybe this is the problem.The 10k pot is probably too small for this arrangement.Here is the preamp outputing to this amp. DIY stuff - ????????????: TDA7360 bridge amp (22 watts)
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Old 6th February 2015, 05:44 PM   #7
rayma is offline rayma  United States
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Quote:
Originally Posted by mitsof View Post
The pot is at the output of the preamplifier to the amplifier.The preamplifier has too much gain (40db) Maybe this is the problem.
The 10k pot is probably too small for this arrangement.Here is the preamp outputing to this amp.
A 10k pot is usually a good value to use. You could try adding a series 10k resistor before the pot to scale down the preamp output by half.
You would lose 6dB of gain, and you will have to run the control higher than your the normal setting.
If your volume control is wired right, is it really a problem to have a little residual output at the minimum setting?
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Old 6th February 2015, 07:23 PM   #8
mitsof is offline mitsof  Greece
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Quote:
Originally Posted by rayma View Post
A 10k pot is usually a good value to use. You could try adding a series 10k resistor before the pot to scale down the preamp output by half.
You would lose 6dB of gain, and you will have to run the control higher than your the normal setting.
If your volume control is wired right, is it really a problem to have a little residual output at the minimum setting?
The 10k pot is at output of the preamp .If i am not mistaken it already forms a voltage divider with the load.Here is the schematic of the preamp.Click the image to open in full size.
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Old 7th February 2015, 02:42 AM   #9
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.
Volume doesn't reduce to zero, two possibilities stand out.

1. A bad solder joint at terminal c of Pot1, so the wiper is never really at ground.

2. Ground for the shown circuit is at one voltage level, ground for the following stage is at another voltage level. So again the wiper of Pot1 is never at ground as far as the following stage is concerned.

Either way I'd put away my meter and use a test lead with one end grounded. Be sure it's grounded.

Probe the 3 terminals of Pot1, something should mute.

OK so use the test lead to jumper to ground of the following circuit. Something should mute.

Or maybe not. Well, then something else.

ADDENDUM: Which, looking again, is similar to the first thing rayma suggested. Well...good idea.
.
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Last edited by bentsnake; 7th February 2015 at 03:02 AM.
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Old 7th February 2015, 03:45 AM   #10
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3. Ground the test lead not to the circuit shown, but to the following stage. Again probe the 3 terminals of Pot1, looking for something to mute.
.
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