Sledgehammer to crack a Lightspeed walnut

[number7]

I liked the idea of the Lightspeed volume control, but couldn't live with the channel imbalance.

I liked the idea of a stepped attenuator, but couldn't live with all those dirty contacts.

Why choose? I decided to marry the two technologies by making 4 banks of 24 presets, operated by a 24 pole / 4 way switch which could then adjust the LDR voltages precisely, at each stage, whilst monitoring the output. I managed to get perfect channel balance and a perfect log taper. I also like the idea of being able to dial in different tapers.

This is a video of it in action:

https://www.youtube.com/watch?v=0Px0BDsCEL8

[xrk971]

Aren't your "dirty" switch contacts still actuating the LDR? I don't know what people have against a pot for the actual signal anyway? When used in front of a high impedance buffer or input (JFET) works very well. Smooth and infinitely adjustable.

[number7]

Originally Posted by xrk971

Aren't your "dirty" switch contacts still actuating the LDR? I don't know what people have against a pot for the actual signal anyway? When used in front of a high impedance buffer or input (JFET) works very well. Smooth and infinitely adjustable.

Yes, but the signal doesn't pass through them, which is sort of the point of the Lightspeed design.

[DF96]

Some people like a little low order distortion with their music. LDR volume controls suit them. Others use them just because they are different, and more expensive than a pot.

[xrk971]

Originally Posted by number7

Yes, but the signal doesn't pass through them, which is sort of the point of the Lightspeed design.

But the analog resistance of the active part is directly controlled by the stability of the light intensity controlled by the dirty contact. You have just added another layer but have not isolated the effect.

[number7]

Originally Posted by DF96

Some people like a little low order distortion with their music. LDR volume controls suit them. Others use them just because they are different, and more expensive than a pot.

No - I like perfect channel balance, accurate logarithmic tracking, no mechanical parts to deteriorate and a clean signal path - everyone to their own.

[number7]

Originally Posted by xrk971

But the analog resistance of the active part is directly controlled by the stability of the light intensity controlled by the dirty contact. You have just added another layer but have not isolated the effect.

Are you really saying that a signal passing through dirty contacts is equal to the effect of that same dirty contact on a controlling resistance!

[cbdb]

Not the same but similar. If those contacts are dirty and the ldr control voltage is jumping around what do you think will happen to the audio output?

[number7]

Originally Posted by cbdb

Not the same but similar. If those contacts are dirty and the ldr control voltage is jumping around what do you think will happen to the audio output?

'Jumping around' ? I suspect the problem would be more about an increase of capacitance / resistance.

Just to recap then:

All those who think a stepped attenuator is the best solution for a volume control are wrong.

All those who think an LDR is the best solution for a volume control are wrong.

A normal potentiometer is the best solution for a volume control.

Blimey.

(Have you ever looked at how they produce a logarithmic taper in pots?)

[DF96]

A dodgy contact feeding an LED+LDR has a similar effect on the LDR resistance as a dodgy contact has directly. LEDs react very quickly to current changes; LDRs are perhaps a little slower but you will still get LF intermodulation. If you don't trust switches don't use them.

Originally Posted by number7

All those who think an LDR is the best solution for a volume control are wrong.

Definitely. Unless they prefer a little distortion.

The best solution is a good quality pot, used in the correct circuit.