Switched attenuator/volume control

[joebog1]

A question for the Hi-Fi nuts.
Years ago somebody bought me a switched attenuator as a gift.
I have never used it becuase for the life of me I cannot figure out how to connect it. Its Chinese (beautifully) made and came without a circuit diagram.
Perhaps somebody else here has seen them before and maybe received a wiring diagram with it. ANY information would be appreciated.
As a "final" build I have decided to drag "alla that junk" ( as wifey calls it) out of the shelving, and make a last amplifier for myself. ( I am deaf as a post, and my own tests give me a frequency range about the same as a carbon telephone). I have sort of discussed the idea here before, but will publish as I go so that anybody that wants to watch a real lunatic at work can do so.
Separate power supply chassis and power amp that is push/pull parallel 5B/254 that should produce something over 50 watts.
A pre amp/phone stage also with separate power supply to drive the "mothership".
FIRST I need to get some info on the attenuator!!

Thanks in advance
Joe

[G8HQP Dave]

I would guess that the three wires on each half are in, out and ground. You could experiment with a low voltage DC supply and a decent voltmeter. I woud start by assuming that black is ground - but I could be wrong. Sometimes these are essentially a switched pot, and sometimes some sort of ladder attenuator.

In either case you need to be aware that to avoid switching clicks you need to make sure that no DC gets to the device when in use as a volume control. Coupling caps and ground leak resistors will be needed.

[Craig Sawyers]

Nicely built unit. Those resistors are Dale mil spec ones.

If you google "switched attenuator dale" you get hits like https://bottlehead.com/smf/index.php?topic=6116.0

Which is pretty much identical with the one you have, along with wiring diagrams.

Good luck!

Craig

[dave cox]

I would have thought that the steps might be a bit coarse on its own.
Maybe add another switched attenuator (0, 20db) before / after to give it more range ?

dc

[Herald1360]

Some time back I suggested (wondered about) a simple switched 12 position attenuator with 6dB steps as a cost effective replacement for volume controls made of unobtanium. It's also an easy way to make your own tapped pot for loudness controls etc. The logarithmic nature of the E series makes design relatively trivial. This Dale control looks like a professional implementation- my idea just used a single wafer of 12 way make before break switch with resistors between each tag round the edge. (I reckoned that break before make switching would be a bit noisier!).


A lot of modern gear has relatively coarse (less than 12 steps) step up/down type volume control which is just about acceptable in use.

[Craig Sawyers]

Back in the day I built a preamp based on discrete op-amps (because 741's were the only IC act in town then) and bootstrapped emitter followers.

I built a switched attenuator, but the only available wafer switches at that time were 12 position. So I added a wafer and used that to toggle a flip flop, with a transistor to fire a reed relay. That either added or short circuited a series resistor.

So - reduce volume to zero, relay clicks and you are on the loud range. Reduce to zero, click and you are on the quiet range. So you get 24 steps of volume with a 12 position switch. Power on state was the low volume range.

[Radio Wrangler]

There are various circuits for a simple, single attenuator. Pi, Tee bridged-Tee etc. So a switched step attenuator could be any of them with the appropriate resistor sections switched. You'll only solve this uncertainty by tracing out the circuit.

The BBC used two-track ilver stud faders with a complete attenuator section between the studs of the two tracks, so the full things got switched.

The nice thing about attenuators rather than potentiometers is they can be designed to maintain a constant output impedance. This is probably of no real value at audio, but no doubt someone will claim to be able to hear it.

Back in 1981, I designed myself a preamp around the NE5534 and I used a BBC silver stud fader to make a stereo pair of potentiometers with a peculiar law designed to give equal dB steps when configured around an amplifier to vary the feedback and give variable gain, giving optimised noise floor and overload margin across the range.

I'm planning on replacing this preamp with a Revox B252 (once I get stuck into sorting a CPU problem.) This uses two multiplying ADCs around a discrete opamp to give an active gain control with enough finesse to take on balance duty too.

So your switches could be turned into whatever you want, or you can trace out whatever you already have.

David

[Ed_Dinning]

Hi Joe, I think there is a website that has been mentioned on the forum, that has most of the BBC designs and app notes.
That should give you some ideas for amplifiers and attenuators.

Ed

[joebog1]

Thanks everyone!!
Craig's link is exactly what I have.
These faders are pretty good stuff because they are cheap (about 20 quid to you fellas).
They really are nicely made, have 2 dB steps, and are ladder networks, (this I already knew, BUT no wiring diagram) are constant impedance output, and as an aside there are only two resistors in the audio circuit at any time.

To Dave, mine didnt have any wires, the pic I posted was the nearest thing I could find on.

Thanks again all.

Joe