Stepped Attenuator opinions?

[jrmintz]

I've been using an old conrad-johnson PV1 preamp to connect my console to my power amp, mostly because I needed the switching and tape loop facilities. I rarely use those any more, I can connect anything I need through my patch bays, and I now have a power amp with balanced inputs. I was thinking about replacing the preamp with a 20K balanced stepped attenuator. Is there anything I need to know? The console is an old, discrete custom-made Chaos Audio with transformer outputs, and the attenuator would be feeding a balanced-input White 4700 EQ. Any thoughts?

Thanks. :thumb:

 

[NewYorkDave]

What's the recommended load impedance for the console's monitor output?

What's the input impedance of your power amp?

What's the circuit of the attenuator you intend to use: constant-impedance ladder, stepped potentiometer, etc.?

 

[jrmintz]

Thanks for your help, Dave.

What's the recommended load impedance for the console's monitor output?

The documentation for this console is sketchy - it was a one-off and everything I have is hand written. I don't see any reference to the load impedance, just a +4dB operating level. All of the output transformers including the monitor bus are API 2503s. The specs for them say the output impedance of 1 winding is 75 ohms, two windings in series are 150 ohms, and 3 windings in series are 600 ohms. I don't know how this particular circuit is connected.

What's the input impedance of your power amp?

The next device in the chain would be a White 4700 EQ, which has a 60K ohms impedance for the balanced input.

What's the circuit of the attenuator you intend to use: constant-impedance ladder, stepped potentiometer, etc.?

It is a 46-step series attenuator with metal film resistors and a shallco switch. You can see it here If a series attenuator is not the best choice there are other options - I like this one because it's a kit with everything included.

Thanks again for your help.

 

[NewYorkDave]

Actually, it should work just fine if you keep your connecting cables short or use cable of very low capacitance--see below. (I say that because your attenuator has a fairly high and variable output impedance). It's a fairly safe assumption that your console output transformers are strapped for 600 ohms, as this is common practice, so I would suggest terminating the outputs with 620 ohm resistors, then into the attenuator, and then the EQ and power amp. Low-capacitance shielded pair such as Belden 1801B would be ideal for use between the attenuator and the input to the EQ.

 

[jrmintz]

I should be able to keep the cable between the attenuator and the EQ under 18". If I can put the Attenuator enclosure on top of the EQ with no noise problems, it could be under 12". I'll get some of the low capacitance cable and try it out.

Is it important how physically close the terminating resistor is to the transformer? I could put it inside the output XLR going into the attenuator. Or I could put it in the xlr at the console end if it needed to be closer.

Thanks again for your help, Dave. I'm very interested to hear what kind of sonic difference the change from a tube preamp to a passive attenuor will make, as well as the change from an unbalanced to balanced monitoring system.

:guinness: :sam:

 

[NewYorkDave]

Seth: You don't have to go too nuts with keeping the cable lengths ultra-short; just don't make 'em longer than they need to be. I'd put the termination at the attenuator, just for convenience, but it's not critical. You could build it into the attenuator's box, along with a "600 ohm term. on/off" switch.

 

[jrmintz]

You could build it into the attenuator's box, along with a "600 ohm term. on/off" switch.

That's a great idea - that's what I'll do.