Audio pot with center tap for loudness control?

[tobia]

Hi there

I'm replacing the volume potentiometer (linear 100k, stereo) in an old stereo amplifier (Kenwood KA-54) because one of the channels has become disconnected.

As an aside, I don't understand this failure mode: one of the two carbon tracks is still working perfectly, but the other seems electrically disconnected: even the end-to-end connectors measure open, let alone the wiper. Except that looking at it, it seems intact. It's a mystery.

Anyways, I cannot seem to be able to find a replacement pot on the usual suppliers, because this one has a center tap that is used in the "loudness control" button. Here is an article explaining what the loudness control is.

I have attached the original diagram, somewhat cleaned. Since all the passive filtering go way over my head, my question to you is:

Is there a way to somehow approximate the original response, without using a center tap potentiometer?

Also, does anybody know of any suppliers of this kind of part? I did find something that looks like the pot I need, from the usual anonymous Chinese eBay vendors, but I'm thinking I would rather use a good quality audio pot without the center tap, rather than a shoddy Chinese clone of the original part. I also found references to some ALPS pots with a center tap, but looking at their current catalog they don't seem to make them anymore?

[bob91343]

First, I don't think it's a center tap.  I think it's around 40% if memory serves.

Second, it would seem you could scavenge a suitable part from a defunct stereo receiver.  They give these away often on Craigslist, for instance.  Even one from a boom box.

Third, don't give up on the old part.  Probe the carbon track with an Ohmmeter and find the broken place.  Then fill it in with a soft pencil.  It takes patience but you have nothing to lose but time.

[CaptDon]

My first though is if it is a volume control pot it isn't linear!!! Wouldn't it be an audio taper with a tap?
All of that 'loudness' was sort of a bunch of crap anyway. The ear is sensitive to the mid range (voice) but as volume goes up the ear becomes more linear, so at low volumes the bass and treble are boosted to have 'presence' to the non-linear ear. So expensive audiophool stuff always featured a loudness control. Since everyone seems to have a favorite listening level they always adjusted the normal tone controls to their idea of pleasing at some particular sound level. Know why we like loud music....because at loud levels the ear becomes its own compressor/limiter, the dynamic range of the music is compressed and there is less difference in level between the fundamental tones and the even harmonics making the music appear richer/fuller.

[Gyro]

Yes, it will be an audio taper pot.

The loudness button was very much an '80s thing. Not just expensive stuff - pretty much all amps had them at the time, at all price ranges. It was an era of tone controls of various types, including graphic equalizers. That was before the fashion changed to minimalist, with no frequency response controls at all.

I would simply remove and isolate the loudness tap leads, fit standard audio taper pots and not use the loudness button. Far better to have something that produces pleasing sound than have it just sitting there.

You can always re-connect the taps if you find a tapped pot later.

[srb1954]

Hi there

I'm replacing the volume potentiometer (linear 100k, stereo) in an old stereo amplifier (Kenwood KA-54) because one of the channels has become disconnected.

As an aside, I don't understand this failure mode: one of the two carbon tracks is still working perfectly, but the other seems electrically disconnected: even the end-to-end connectors measure open, let alone the wiper. Except that looking at it, it seems intact. It's a mystery.

Anyways, I cannot seem to be able to find a replacement pot on the usual suppliers, because this one has a center tap that is used in the "loudness control" button. Here is an article explaining what the loudness control is.

I have attached the original diagram, somewhat cleaned. Since all the passive filtering go way over my head, my question to you is:

Is there a way to somehow approximate the original response, without using a center tap potentiometer?

Also, does anybody know of any suppliers of this kind of part? I did find something that looks like the pot I need, from the usual anonymous Chinese eBay vendors, but I'm thinking I would rather use a good quality audio pot without the center tap, rather than a shoddy Chinese clone of the original part. I also found references to some ALPS pots with a center tap, but looking at their current catalog they don't seem to make them anymore?

If you can measure the ratio of the tap on the good section of the pot you can do a reasonably good approximation to the tapped pot by just using a resistive divider across the pot element to derive the tap position.

The approximation will not be perfect in that there will be a different degree of interaction between the position of the volume control and the balance control but it shouldn't be of any major significance.

[TimFox]

The "loudness" control was supposed to have an effect that depends on the actual sound intensity at the listener, following the measurements of Fletcher and Munson on perceived loudness as a function of frequency and actual sound level.  Since the relationship between the pot position and the power output that drives the speaker is not known, as well as the acoustic path to the listener's head, the design was not well-thought-out.  In fact, in my dormitory years it was just used as an excuse to turn up the bass level for young-people's music.

[tobia]

Thank you for all your input! 🙇‍♂️

I had measured the part properly and it was indeed a linear pot, not logarithmic, with the loudness tap exactly at 50%. I suppose the non-linearity needed to get a proper volume control was factored in somewhere else in the preamp / amp circuit?

I tried to salvage the old part with a pencil, as bob91343 suggested, but my efforts were not going anywhere. The pot was very noisy, the left channel was working intermittently, and the audio that did come out was very "muddy" or low-pass filtered, even when the pot was working.

So I decided to throw it out and replace it with a high quality, brand new component with as many of the same mechanical and electrical characteristics as I could match: 100k linear, stereo, "audio" rated, 6 x 20 mm shaft. I got a Bourns PDB182-K220K-104B from a local supplier and replaced it. (Only downside is that this model has an indent at 50%, like a balance pot, but all the other features were a match and so I chose it.)

The results were astounding! Audio quality immediately went back to crystalline, as if the amp had been thrown back in time! I don't quite understand how a worn out pot can cause the audio to become muddy. Could it be spurious capacitance? I did notice the carbon track was cut into smaller concentric tracks by the mechanical action of the wipers, over the course of 35 years of use.

I also managed to restore the Loudness function. I measured the 50% taps on the original pot, with the volume at around 50% (the most likely position in practical use) and they were almost shorted to the respective wipers (makes sense) so I just shorted them permanently. Now the Loud button works and I cannot tell any difference from the original loudness behaviour (LOL) so I'd call that a Winner Winner Chicken Dinner!

Again, thanks to everybody for your help.

[TimFox]

A worn-out pot could have semiconducting oxidized-metal contact from the wiper to the carbon track, or a broken carbon track that affects the resistance or the noise.

[bsfeechannel]

The loudness control circuit is very simple.



In the off position, C1 is not connected and C2 is shorted. This puts R2 in parallel with the lower half of the linear potentiometer R1. R2 is normally chosen around R1/10 so that R1 works effectively as a log pot. In the on position, C2 is in series with R2 and C1 is in parallel with the upper half of R1. So now for the lower frequencies R2 becomes decoupled from R1 and C2 acts as a bypass capacitor for the higher frequencies. When the wiper is in the lower half, lows and highs are therefore enhanced, but as soon as the wiper is taken to the upper half of R1, the impedances represented by C1 and C2 get less and less relevant and so is the frequency enhancement they produce.

Loudness controls can still be found in cell phone apps, as are graphic tone controls.

[ambrosia heart]

just go to aliexpress to search for 

https://www.aliexpress.com/af/potentiometer-center-tapped.html?d=y&origin=n&SearchText=potentiometer+center+tapped&catId=0&initiative_id=SB_20210310232419

or go to taobao search for 

https://s.taobao.com/search?q=%E7%94%B5%E4%BD%8D%E5%99%A8%E4%B8%AD%E5%BF%83%E6%8A%BD%E5%A4%B4&type=p&tmhkh5=&spm=a21wu.241046-hk.a2227oh.d100&from=sea_1_searchbutton&catId=100

3/4/5/6/7/8脚电位器帶中定点/中心抽头

[tobia]

The loudness control circuit is very simple.

Mostly over my head 😢

I wonder if I'll ever find a program or website that can show the workings of arbitrary RC networks in a graphical way, with colors, charts and stuff. Otherwise I'll never get it.
Give me code, in any language existing or invented, and I'm game. But analog stuff, ugh! Can't do.

just go to aliexpress [...] or taobao

Thank you, those links will be helpful for future projects. Taobao I had never heard of.
For this project, being it audio gear, I decided to use a good quality part, ordered from a large supplier and delivered the next day.
But for future projects, those two websites will expand my treasure trove of Wun-hun-lo parts!