Relay based volume controls on ebay

[scottjoplin]

It would help if you can trace the wiring. I think, judging by the number of resistors and the binary coding, and that the input resistance varies considerably that it must be arranged as a shunt volume control. Does the resistance input to output change?

 

[IDM]

Hi, Yes, the series resistance does vary. Again I haven't recorded every position but you will get the picture from this graph. Any thoughts?

 

[scottjoplin]

That's the same as the input resistance graph. What does "inf" mean? Also on the previous graph it shows output at zero for all settings. Your graphs don't make sense, what you wrote makes more sense.

 

[IDM]

Inf was infinity or off the scale for resistance. I agree the values seem really odd. So odd that I repeated the testing and got the same result!

The output resistance was never zero, it was generally about 300 ohms for almost all settings, it just looks like zero due to the scale difference. If you turn the volume way up the output resistance eventually goes up to about 47K. But as in the graph most of the way it is no more than 300R!

 

[scottjoplin]

That is the nature of a logarithmic volume control at the bottom of it's scale. Measure the output resistance at around position 200 for example

 

[IDM]

Right I have measure input, output and series resistance across the entire range by measuring every 5th position. The graphs shown below. Essentially the input resistance drops too low (as far as I understand it). Also when being used only the first 15-20 positions are of much use without it becoming ridiculously loud and the first position is too loud. Resistors need to be changed to spread the range of effective volume control. Any ideas?

 

[scottjoplin]

I think you should trace the circuit, see what's going on exactly, otherwise it's all guess work

 

[kodabmx]

So I finally got around to testing/installing this volume control. At min and max it reads 1k6. I've built a 12SN7 buffer to drive it. Too many steps in mid volume, not enough at the ends. Still it works. I'm not sure if I like it better than a Chinese Alpha pot though. The pot need the buffer.

 

[kodabmx]

I can also confirm that there is bleed through at zero volume unlike a pot. I also noticed that the 50K pot specified was actually 20K, and both gangs are wired in parallel. I'm using an off board 10K single gang pot in lieu of the original pot. I also removed the 7812 and the input cap and drove it directly from 12V.

 

[kodabmx]

As a follow up, I wired a 2K resistor from wiper to ground on the control pot (10K) and made the curve a lot more LOG.

 

[FJC Corpas]

I bought the volume control indicated in the first post and I installed it in my tube preamp. It work correctly except if I connect a phono in any of the inputs. In this case, I get strong spikes from the loudspeakers.

Anyone have any idea about how avoid this problem?

If anyone have the schematics of the volume control it would be really useful.

Regards, Francisco

 

[kodabmx]

By "a phono in" I assume you mean an RIAA preamp? It's probably outputting a bit of DC. Check it's output caps to see if they are leaking. Otherwise, connect the volume control with a capacitor input.

 

[FJC Corpas]

By "a phono in" I assume you mean an RIAA preamp? It's probably outputting a bit of DC. Check it's output caps to see if they are leaking. Otherwise, connect the volume control with a capacitor input.

Thanks for your response. Yes, I mean a "RIAA preamp". It's an EAR 834 that has a 2uF cap at the output. Attached is the schem.

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