If the variable resistor is genuinely a "trimmer", then precision is not required and the $32 each, I see in Google results won't be necessary.
I believe its very low ppm they sell at that price.
trimmer VR for Rs
Then add the trimmer in parallel to the existing Rs, or in parallel to a slightly higher Rs.
eg.
Existing Rs = 30r
Proposed range of Rs +-10%
Select 36r for Rs and 1k for VR trimmer +110r fixed resistor.
Minimum trimmed Rs = 36//[0k+110r] = 27r1
Maximum trimmed Rs = 36//[1k+110r] = 34r9
Set Vr to 70r for nominal Rs = 36//[70r+110r] = 30r
Now, let's look at the sensitivity of Rs for the VR tolerance
At maximum trimmed Rs:- VR+2% gives 1020r and trimmed Rs becomes 34r89
A 0.05% increase in trimmed Rs for a +2% tolerance on the VR value.
If Rs ~+-20% is required for a gain adjustment range, then use 36r in parallel with 75r +VR= 2k
Then add the trimmer in parallel to the existing Rs, or in parallel to a slightly higher Rs.
eg.
Existing Rs = 30r
Proposed range of Rs +-10%
Select 36r for Rs and 1k for VR trimmer +110r fixed resistor.
Minimum trimmed Rs = 36//[0k+110r] = 27r1
Maximum trimmed Rs = 36//[1k+110r] = 34r9
Set Vr to 70r for nominal Rs = 36//[70r+110r] = 30r
Now, let's look at the sensitivity of Rs for the VR tolerance
At maximum trimmed Rs:- VR+2% gives 1020r and trimmed Rs becomes 34r89
A 0.05% increase in trimmed Rs for a +2% tolerance on the VR value.
If Rs ~+-20% is required for a gain adjustment range, then use 36r in parallel with 75r +VR= 2k
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With a 300Hz test tone I measured the following values:
Tone volume.........Right.....Left
+12dB.................0.58V.....0.71V
+14dB.................0.80V.....0.98V
+16dB.................0.96V.....1.22V
Left plays slightly lowder and so hisses more.
I will now use the trimmers on second stage.
Tone volume.........Right.....Left
+12dB.................0.58V.....0.71V
+14dB.................0.80V.....0.98V
+16dB.................0.96V.....1.22V
Left plays slightly lowder and so hisses more.
I will now use the trimmers on second stage.
With a 300Hz test tone I measured the following values:
Tone volume.........Right.....Left
+12dB.................0.58V.....0.71V
+14dB.................0.80V.....0.98V
+16dB.................0.96V.....1.22V
Left plays slightly lowder and so hisses more.
I will now use the trimmers on second stage.
Wich DVM true RMS are you using Ricardo?
He does not need an rms reading voltmeter.
He is comparing like signals to each other. An average reading DVM scaled for sinewave AC is plenty good enough.
It can also be due to improper azimuth adjustment.
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