Linear potentiometer into logarithmic

sssboa

Super Member
Hi

I have a NOS stereo linear pot for volume of my vintage 1970s amp, same as original (producer, resistance, dimensions) but 2 things, it's
  1. step
  2. linear
Original is a different variant of the same pot, it's
  1. smooth
  2. logarithmic
I already replaced it and it works as a new pot should work. I wonder though if I could modify it. Most important make it more like logarithmic. I saw 2 articles, they refer to guitar amps I think and propose to solder a resistor of around 20% of the pot's resistance.
https://www.jt30.com/jt30page/potfix/
http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

Does it make sense to you? Would it go between output lug and the ground lug? Won't I mess up output impedance or something?

Second thing the stepping of the pot, less important of course. I guess maybe some corrugated wheel can be removed from inside?

In the picture is the original logarithmic pot, the linear replacement looks the same.

Thx guys,

pot1.jpg
 
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Rod Elliot's site speaks of this when there is no loudness tap to use.
There is no loudness tap in this volume pot. It's Yamaha so there is a separate loudness potentiometer. I used this linear pot that is not even made for audio volume in mind and loudness works ok with it.
 
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Main advantage of a log pot is that it allows the amp to be paired with a wider range of speakers (with different sensitivities). Also provides better/more
listening options/range. Linear pot works as well, probably better matching between the channels. Down side is the volume comes on in a rush.
If you've got speakers below about SPL=90dB then linear is fine. No chance with SPL=106dB altecs.
 
i was referring to the originator of the post, if its working ok why does he want a log response

I would like to use more of the volume scale. At the moment I use less than the first step.

I tried adding resistors to the same pot outside circuit but it doesn't seem to give the result when I measure it for resistance.

The original pot was not great either. Not well matched channels at low volumes and not logarithmic enough.

imagespot.jpg
 
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I tried it before without any success. My volume pot was a linear 50K and I tried 6.8K, 4.7K, and 3.3K. I couldn't tell any difference.
 
I tried it before without any success. My volume pot was a linear 50K and I tried 6.8K, 4.7K, and 3.3K. I couldn't tell any difference.
You probably wired something wrong or your circuit already had some resistors doing this. Adding a resistor in parallel with the output (wiper to ground) do modify the potentiometer response, and is an usual and proven mod. The effect depends of the ratio between the pot and the resistor.
 
You probably wired something wrong or your circuit already had some resistors doing this. Adding a resistor in parallel with the output (wiper to ground) do modify the potentiometer response, and is an usual and proven mod. The effect depends of the ratio between the pot and the resistor.

For such a simple mod, I didn't think I would make such a mistake. Nevertheless, I opened it up to double check again. I was right.

pot.jpg
 
Connecting 4.7K to a 50K linear pot should be noticeable near the half of the rotation. You'll have less than 4.7K at one half, and 25K at the other side (near 1:6 ratio) , instead of 25K and 25K (1:1). But if you don't hear any difference, perhaps the circuit is not so sensitive to those differences.

Anyway, a real Log pot at half rotation will have near 5K at one side, and 45K at the other side (1:9 ratio)
 
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As I said, I tried 6.8K, 4.7K, and 3.3K. All three plus no additional resistor case give me the same result - I can't turn the volume knob over 9 O'clock or the amp becomes very very loud. Any workaround?
 
As I said, I tried 6.8K, 4.7K, and 3.3K. All three plus no additional resistor case give me the same result - I can't turn the volume knob over 9 O'clock or the amp becomes very very loud. Any workaround?

It sounds like you need a smaller signal feeding the amp. A simple workaround is to use some attenuator at the input, there are RCA attenuators or you can solder a voltage divider at the input (one or two resistors, we need a schematic to confirm this).

Anyway check your gear specs. Input level of the amplifier, output level of your source.
 
Here is the background story.

The amp is a Sony TA-1120. Actually, I have two of them. Both have been fully recapped. This one had a bad volume pot which did not track well causing imbalance, so I replaced the pot with a 50K linear pot (original was 30K) and then added those resistors to make it log. In addition to the pot replacement, I also shotgun replaced all its small signal transistors with 2SC2240GR for this TA-1120. The other one's resistors are all original.

With the same input signal feed, the amp with the original pot behaves normally, That is why I doubt the effectiveness of this mod.
 
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