A substitute for pots

[GMB]

My modern (as I see it) HiFi which I built from modules in the 70's is getting a bit annoying with pot and switch issues.

I have often wondered what suitable substitutes there are for pots.

I have seen ICs that provide a voltage-controlled gain feature - but I worry that such things are bound to generate distortion as non-linearity is how they must work, and they don't just replace pots in other situations.

Another idea that I have run into is the photo-pot, where a light dependent resistor is the variable resistance element - a direct pot replacement possibility. But are there non-linearity and/or noise issues with these?

[Richardgr]

One solution is a media player and use the volume control on that. Then the pots can be left in fixed positions on the amp, corresponding with the point cracks start to appear in the walls.

[Lucien Nunes]

If you only want manual control, the stepped attenuator is the 'proper' solution.
Excellent channel tracking, linearity, and a life dependent only on how much you are prepared to spend on the switch.

VCAs have gotten quite good and are widely used in professional audio equipment. Even budget ICs available for a few pounds can be trimmed to <0.05% THD or better and >110dB dynamic range. However, in a true hi-fi system the VCA might still be the single biggest source of distortion and noise.

Optical gain controls are widely used in organs, where continuous movement of the swell pedal and the need to control multiple independent signal paths makes pots cumbersome and short-lived. There are other musical applications, such as tremolo in guitar amps. Implementations range from a torch bulb and an ORP12 glued into a piece of plastic pipe, through to commercial modules such as the Raytheon 'Raysistor' and VacTec Vactrol. I've never investigated the upper limit of audio performance available, I think there are significant limitations but like you I would be interested to know whether there have been any successful HiFi versions.

[wd40addict]

Don't forget volume control chips like the PGA2320 and its friends.

[wireman]

Maybe I missed the point but both Farnell and RS sell a wide range of 'digital potentiometers'

[G6Tanuki]

How is the volume-control done in the likes of remote-control TVs? There's no pot here, it's a digital attenuator circuit that gets given a 7- or 8-bit value as a result of pressing the up/down buttons.

Components to implement this are cheap as chips [pun intended].

[-ADBeta-]

I have done a couple things in the past about this -
Like Richardgr said, I have replaced pots with fixed resistors (i prefer to bias the fixed resistors in the 90% range, this is to provide *some* input impedance for ringing or impulse mitigation. Although this is probably me being overly cautious)
In conjunction to this I made a 3D printed box with a single slide pot between my audio source and the amps input, while this is just moving the issue to a new place it was due to the amp being in a very had to reach place.

I have also one of the many flavours of digital volume control modules, I forget which one but it was very generic. It didn't introduce any audible distortion but I'm not an audiophile by any means so I'm not an authority on that. Overall it worked fantastically, but the practical use of it was a little more inconvenient than I had wished.

Hope this helps

[knobtwiddler]

The ALPS RK27 pot (~£12) typically gives better than 0.3dB left / right matching in its top 70dB of operation. The human ear can't detect threshold shift below 0.4dB. And if you're not perfectly between the speakers, it's a non-issue anyway. There is a motorised version of the RK27. They go on for years and are used in stuff like DJ mixers that get a real battering. RK27 = zero distortion (which cannot be said for all pots...strangely enough)

[GMB]

Originally Posted by Lucien Nunes

If you only want manual control, the stepped attenuator is the 'proper' solution.
Excellent channel tracking, linearity, and a life dependent only on how much you are prepared to spend on the switch

OK, but that moves the problem to a switch. I am also having problems with what were quite expensive switches (when new) - but I may start a new thread on that subject.

Originally Posted by Lucien Nunes

Optical gain controls are widely used in organs

...and that is where I have seen them. But organs typically use them for controlling fairly high level signals where noise might be less noticable. Also, organs may not be so bothered by some harmonic distortion. You can still get the old ORP12 which I think was what I found in one organ, but just how good are they?

Originally Posted by wireman

Maybe I missed the point but both Farnell and RS sell a range of 'digital potentiometers'

I had never heard of these but they seem to require a lot to make them work, unless there are some that do the digital work for you. I am also not sure they can be used to replace a pot in the middle of some circuit operating at fairly high voltage.

Originally Posted by knobtwiddler

The ALPS RK27 pot (~£12) typically gives better than 0.3dB left / right matching in its top 70dB of operation.. ... RK27 = zero distortion (which cannot be said for all pots...strangely enough)

I was not aware of these. I had wondered if really good pots were still a thing that one could buy.

[dglcomp]

Voltage controlled amplifiers are what are used in synthesisers and other devices needing an analogue control of gain, Behringer's Coolaudio division have a V2162 dual channel VCA, designed to be controlled by being fed a voltage to one of it's pins.

https://mediadl.musictribe.com/downl..._DATASHEET.pdf

[joebog1]

All the suggestions for "digital " bits n pieces are off topic. Why have a very nice hand made amplifier if you just gunna chuck 8 bit MP3's at it?

LDR's and bulbs work, but in my experience you wont get perfect DC unless you use a battery, so any residual noise in your power supply will be amplified too.
SWitched attenuators can be had for cheap on the auction sites. I have a couple that cost $30 and they are excellent.
To compare them I also bought a $150 US one, its twice the size and does precisly the same thing.
Both models use the same resistor manufacturer. ( Dale ) All models have gold plated switch contacts.
I can supply pics if you wish.

I also use Alps pots, mostly for balance controls, but I have used them for volume as well.
Very well sealed and has been mentioned, very long life.

Joe

[GMB]

Originally Posted by joebog1

LDR's and bulbs work, but in my experience you wont get perfect DC unless you use a battery...]

I can see why that might have to be very good, and using an old filament bulb would probably be better that an LED. The organ that I used to have didn't seem to have any problems with the supply for its filament lamp.

I don't really want to have an MPU around. Maybe these modern (expensive) pots might be worth a go but I do rather like the idea of the photocell approach.
I had added circuits to be able to shift the stereo image around and this has proved very useful since we got a modern TV that has crap sound. So I can set the stereo to be appropriate for the TV wherever we are sitting but there is some control interaction that could be designed out using photopots.

I have never seen a specification sheet that gives any clues about noise and linearity for photo-resistors so still wonder how well they work, and indeed which ones are any good.

[joebog1]

LDR's that have good range ( some are very limited in resistance change ) also have good sensitivity.
If you want to play around I have some circuitry using LDR's as volume controls.
( I will have to find it ).

Seriously though, a new Alps pot is probably cheapest, easiest and least painful. NASTY thing about Alps pots is stupid 6mm diameter shafts, and short shafts. You can cut a piece of food/beer tin to make a shim, but its a pain.

So I have tried to use LED's as the light source, but they are even worse than bulbs because they can operate at very high frequencies ( relativly ). Enough to be a real PITA at audio anyway.

I use all these gizmos for fuzz boxes and wah wah pedals to keep noise down. Batteries are not a problem because these days I use rechargeable lithium and they last ages.
If you play with LDR's you will have to screen them very well as any extraneous light will modulate your volume. I drill a hole through a piece of black plastic rod, and mount the LDR at one end and the bulb/LED at the other. A dab of black Humbrol will fix any leaks. LDR's are very easy to find and are really cheap on the auction sites. 100 off will set you back about two quid.
Today they are not made like the old ORP12 units which had the leads protruding from the back of the LDR. These days they all seem to be situated out the sides which makes it fiddly to mount.
You could probably invent a better mounting system than mine though. Although its nice because its about 1/2" long and 1/2" diameter with two leads out each end. Fits comfortably on a bit of Vero or tagstrip.

Hope that helps.

Joe

[joebog1]

I should have added that Alps pots come with a very nasty cheap zinc diecast nut that will split if you tighten it too much. They rely on a tab that is cast into the frame ( small pots ) or pressed from the sheet metal ( large pots ) but in either case probably won't line up with any original English measurement holes. Mounting is by 8mm diameter too, rather than 9mm or 3/8".
Nice pots, shame about the mechanicals. Also, as far as I have been able to find, 100K is the highest value. More for semiconductor amps than high impedance valve jobbies.

Joe

[knobtwiddler]

The ALPS RK27 can be tough as old boots, as long as you a) make sure to utilise the anti-rotation lug (a lot of people snip them off - bad move); b) use a little low-strength threadlock.

The previous Alps RK40 had a brass bush. I regularly see them from 40-odd years back and they are still quiet as a mouse. The RK27 has the same electrical performance, but the die-cast housing will wear out eventually if subject to daily abuse. But outside of DJ gear, where does that happen?

There are TKD pots at several times the Alps RK27 price. Electrically, they are no better, unless you opt for the 50dB laser-trimmed taper. It won't give you full cut, which can upset some people (they are certainly no use in a mixing bus!). The TKD one is the CP2500 / CP2511. You have been warned about the price! And they have inferior channel drift at high attenuation compared to the vastly cheaper Alps unless you get the 50dB trimmed one.

Here's something you might not know: the discontinued P+G pots (top model was in the hundreds of quid) had terrible image / channel drift at low levels! And they need lots of maintenance. The bearing is a thing of beauty though!

[Craig Sawyers]

Regarding switched volume controls, back in the day I built a preamp using discrete opamps (this was when the only show in town was the 741). I decided to use a rotary switched attenuator.

Now only 12 way ones were easily available then - not enough range. So the lowest setting triggered a flip flop configured as divide by 2 and relay. That removed a short across an additional resistor, so you got a low volume range. Switching back to zero shorted the resistor and you got a high volume range. Worked a treat.

Since 24 way switches are now easy to obtain (Blore Edwards) and cheap, you can end up with 47 attenuator settings. at a reasonable 1.5dB per step that is zero to 70dB of accurate attenuation with 1% metal film resistors.

Craig

[Craig Sawyers]

In the mid 80's Mark Levinson set up a company called Cello. They made a very expensive range of products, the most commonly quoted one was a very sophisticated tone control called the Audio Palette. That was based around discrete buffers and opamps.

To get accuracy of setting, they designed custom switches with 6 degree indexing - so up to 59 positions plus a stop. They were a thing of beauty - ball races - the full 9 yards.

These things were mainly used in recording studios where the $30k (in 1986 money) was easily spent. So they were switched on 24/7, and are now in need of maintenance - including the switches. But once serviced these beasts sell for well north of £10k, and are sold to well-heeled audio (philes/phools).

Now 6 degree indexing switches are commercially available - at a price. Seiden (Japan) do one - either single or double bank (for stereo) at £300 each. So do Shallco (USA) again at the same sort of price.

Using a 24 way switch at 15 degree indexing (about £25) in two switched ranges gives essentially 7.5 degree indexing. About the same price as an Alps RK27.

Craig

[Richardgr]

I restored to working condition a couple of radios from mid 1930's last year, and neither of them had noisy volume and tone controls. Is this a modern phenomenon?

[Nickthedentist]

Don't forget the old chestnut of DC blocking caps failing, which can also cause
pots to operate erratically.

[GMB]

Originally Posted by Richardgr

I restored to working condition a couple of radios from mid 1930's last year, and neither of them had noisy volume and tone controls. Is this a modern phenomenon?

I think it might be. The one surprising thing about my old HiFi is that it now has a motley collection of very different pots and they are all scratchy in much the same way (nothing older than the 60's).

But there may be another issue: I am starting to think that I am getting DC leakage through paths that are supposed to be capacitor blocked. I have been putting off looking at this because I am sure I will find that all the capacitors need changing. The HiFi currently gets used about 12/7 so down-time will be an issue. But one concern with a modular system was always the input+output blocking capacitors. On connecting together we end up with pairs of electrolytics in series. I had wondered about adding in resistors to ground everything but was concerned about extra noise.

Originally Posted by joebog1

Seriously though, a new Alps pot is probably cheapest, easiest and least painful. NASTY thing about Alps pots is stupid 6mm diameter shafts, and short shafts. You can cut a piece of food/beer tin to make a shim, but its a pain.

Yes, I noticed that. So for decades we had pots with a standard size of fitting and shaft and now they have standardised on all being different? The "all being different" standard seems popular these days for many things.

Originally Posted by Nickthedentist

Don't forget the old chestnut of DC blocking caps failing, which can also cause
pots to operate erratically.

Crossed with your post. Yes - maybe.