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.
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.
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.
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.
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)
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.
.
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.
.
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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You could be correct. The combined gain of your pre-amp and power amp is very high. What are you using as a source? How high is its output?
You know the gain of each stage so you could calculate the output voltages at each stage.
Clearly this thread is headed off into the land of workarounds, so I'm just going to add that:
100 is not an unreasonable amount of gain, mic preamps commonly have a gain of 500 or so (2 milliamps amplified to 1 volt). Additionally, 10k is an OK load for a TL072. All of which leaves us free to consider that:
a. Ground is, by definition, zero volts, and obviously zero volts cannot have a volume of anything but zero volts. And:
b. Terminal c of Pot1 is at ground, therefore it cannot have a volume of anything but zero volts. However:
c. Terminal c of Pot1 does have a volume of other than zero volts. Which tells us that:
d. Terminal c of Pot1 is not at ground.
OK I said it and I feel better.
PS Pot1 doesn't alter gain. It's a level control, not a gain control.
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Sound source is my laptop.Preamp gain 100 (40db) and input sensitivity of the amp with a TDA7360 26dBs.Not sure about the Vrms of the headphones jack but here I got way too much gain.The preamp probably needs to put out just a couple of dbs or even not be used at all.
I need this preamp for my mobile phone where more preamplification is needed.But still too much gain.
Well sure, for that you'd probably want a gain of maybe 7 or so. The whole circuit would be operating as more of a volume control than an amplifier. But still ground = ground = zero volts.
...for that purpose you wouldn't maybe prefer an NE5532...better audio quality...? At that gain level, using the inverting input, an NE5532 really sings.
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Maybe this should be said:
The magical-mystery figure is "line level." This is the voltage around which all audio orbits. All preamps, mixers, you name it, are expected to have a line level output.
Unless otherwise specified, you can consider than any amp needs a line level input to output its rated watts.
The classic line level is one volt. In these multinational times there are actually several "line levels," but you still won't be wrong to just use a working number of line = one volt.
It varies, but you won't be far wrong if you consider that any consumer grade (as opposed to pro) player has an output level of some 3/4 volt. "Player" being a CD player, computer line out, whatever.
Headphone outputs are similar. Nearly always a headphone output doubles successfully as line out (this does not apply to telephones, no telling about them).
So you were right the first time, you don't need much gain. Still, remember that you don't want to run with the volume control at max, you want it somewhere near the middle, which means a little more gain than is absolutely necessary, maybe 7ish.
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Does this mean that now the 10K pot at the output of the preamp controls gain?
I'm thinking about replacing the 100k resistor with a 5K pot or Vresistor for a variable gain of 14dBs.
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the vol pot sets the attenuation.
You can use attenuation to counteract excess gain elsewhere in the system. But that wastes signal after it has been excessively amplified and that in turn means more noise.
Have you read system gain on the front page of this Forum?
http://www.diyaudio.com/forums/diyaudio-com-articles/186018-what-gain-structure.html
You can use attenuation to counteract excess gain elsewhere in the system. But that wastes signal after it has been excessively amplified and that in turn means more noise.
Have you read system gain on the front page of this Forum?
http://www.diyaudio.com/forums/diyaudio-com-articles/186018-what-gain-structure.html
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