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Old 20th August 2012, 07:39 AM   #1
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Default trouble connecting a pot (ta2020)

Hi,
I am feeding my TA2020 amplifier with audio from my iphone or amp headphone outputs. This input is too loud for my amp unless I turn the iphone or amp volume to almost zero so I decided to install pot so I control it better. The problem is the following:
when I connect 100k alps pot in the standard way (left pins go to input, mid pins go to board decoupling caps, right is grounded) without any signal connected and the pot all the way down I hear prominent hiss noise. If i rotate the knob to the right, the hiss decreases and I start hearing the buzz caused by the power supply. I have quiet power supply so this will be fixed, but I cannot understand where the hiss comes from. The behavior is the same when I connect input source as well, so the hiss is obviously not originating in the source signal.

I am starting to suspect that this behavior is caused by the pot changing the impedance between the board's input signal path and the ground. The way it is connected, the pot varies this input-ground impedance from zero to 100kOhm and I am getting the hiss when this impedance is largest.

Additionally I have the following question - most headphones have impedance <100 ohm while with the pot my amplifier simulates headphones with 100kOhm impedance - would that cause problems on the source side (the iphone) ?

Cheers,
Georgi
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Old 20th August 2012, 09:29 AM   #2
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With the pot shaft facing you and the pins facing down, it's ground/out/in.

You want a pot with a resistance reasonably lower than the amp input impedance. 100k is about 5x higher in your case. Try a 10k or 20k.
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Old 20th August 2012, 12:19 PM   #3
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Thanks for the reply. I will look into rewiring the pins.

Another experiment I did - put variable resistor in front of the decoupling caps (no pot connected) and when the resistance was several kOhm the hissing decreased to the point it was not audible while the input signal volume was pretty much in the zone where I wanted it. It is like using a pot but not wiring it to the ground. Is such wiring preferable? Could it cause troubles in the amp or in the signal source?
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Old 20th August 2012, 02:12 PM   #4
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With a high value pot you will be picking up radio frequencies and anything else that is floating around.
A lot of amps have a low pass filter on the input to kill RF.
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Old 20th August 2012, 03:02 PM   #5
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Quote:
Originally Posted by gdanov View Post
Another experiment I did - put variable resistor in front of the decoupling caps (no pot connected) and when the resistance was several kOhm the hissing decreased to the point it was not audible while the input signal volume was pretty much in the zone where I wanted it. It is like using a pot but not wiring it to the ground. Is such wiring preferable? Could it cause troubles in the amp or in the signal source?
The tripath chips have inverting input gain configurations. Increasing Rs will effectively lower the gain. You obviously need less gain.

Gain is Av=12(Rf/Ri).

It's best to change the value of Rf instead of adding another resistor in series with the input.

Last edited by theAnonymous1; 20th August 2012 at 03:06 PM.
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Old 20th August 2012, 03:16 PM   #6
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thanks for both suggestions I will experiment tonight

remember some time ago I put variable resistor trying to control the gain the but the result was unexpected, who knows what else I have had wrong at that time
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Old 20th August 2012, 08:56 PM   #7
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Quote:
Originally Posted by nigelwright7557 View Post
With a high value pot you will be picking up radio frequencies and anything else that is floating around.
A lot of amps have a low pass filter on the input to kill RF.
looks like this was spot on.
In the end I decreased the gain by 50% by adding second 20k resistor (dont have 10k) and connecting the input directly to the decoupling caps and now the noise is gone and the gain is "in the zone"

Thanks a lot!

Now have to figure out why the bass guitars sound so anemic...
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Old 21st August 2012, 01:51 PM   #8
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Quote:
Originally Posted by gdanov View Post
Now have to figure out why the bass guitars sound so anemic...
What is the value of the input caps?
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Old 21st August 2012, 03:09 PM   #9
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It was originally 2.2uf, now is with 3.3uf bypassed by 0.1 and 0.01.

I added yesterday second 4700uf bulk capacitor in parallel to the power input and this added some body to the bass. It is still a on the "shy" side though. I read in another article that if I change the two bulk capacitors on pin 25 and 22 to 600+ uf it would further improve. Got any experience with that? What if I add one more 4700uf bulk capacitor?
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