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.
Its more effective since the signal is larger, and slave to the head-amp's noise so not to introduce different noise between channels. Rs in 1st is part of its noise outcome.
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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Paralleling the trimmer to a fixed resistor is a good practice indeed. Have used it like that in many occasions, for Rs here also.
🙂 Will do !
Any hint on how to measure gain (other than just by listening to subjactive volume differences).
Any hint on how to measure gain (other than just by listening to subjactive volume differences).
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.
Those channel differences are ~1.8dB to 2.1dB.
Your cartridge balance may have something to do with this.
I suggest you swap the cartridge channels first and check again before making any adjustments.
Your cartridge balance may have something to do with this.
I suggest you swap the cartridge channels first and check again before making any adjustments.
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.
He is comparing like signals to each other. An average reading DVM scaled for sinewave AC is plenty good enough.
That is a good idea... I will swap cart inputs and remeasure.... That way I can also verify if there is an influence due to cart imbalance 🙂
For a certain cart you can even compensate its channel difference in the gains. But not practical if you will move between systems.
