PLLXO with stepped volume control?

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Hi Guys,

Nice forum!

I'm a bit of a newb with little electronics theory, so I'd really appreciate a little help on this question:

Is is possible to stick a stepped attenuator upstream of a PLLXO for volume control, or would it mess up the PLLXO aligment?

I don't know if it matters, but I was thinking of using a ladder-type attenuator and a combination of RC and LC filters in the PLLXO.

Thanks in advance for any light you can shed on me!

Dart
 
Hi Dart,

I encountered this issue when I was playing with PLLXO for my JBL CS3150 3-way horn speakers. I dealt with this problem by isolating the stepped volume attenuator and the PLLXO with an active buffer stage. I used the BUF03 IC, but I'd assume that any good OpAmp or buffer IC will do. With the active buffer stage, it is no longer an all-passive setup and thus loses some of the "purist" or "minimalist" flavor, but it's still better than using big coils and caps on the speaker drivers.

Kurt
 
Kurt,

Thanks for the information.

Dang. I was hoping to avoid active opampish thingamabobs, mostly because I really don't understand them . . .

So should I forget about it and just use a preamp? I mean, if it has to be active at some point, would I see any real noise reduction with a buffer versus a preamp?

By the way, the preamp I'd be using is a Dynakit PAS-2, which I believe has an output impedance of 1kohm, and it was originally paired with amps with 470kohm input impedance.

Will this work with a perhaps 4-way PLLXO (no impedance compensation planned) feeding amps with input impedances ranging from 10 to 25kohms?
 
Hi Dart,

My view is that a good opamp or buffer is as good or better than most expensive commercial preamps, and it is very easy to implement an opamp/buffer circuit. Also, opamps or buffers tend to have an output impedance much lower than some tube preamps. In the case of your preamp, its output impedance of 1Kohm is kind of on the high side, and you'd have to take that into account when you calculate the values of the components for the PLLXO. I'd go the opamp/buffer route.

Kurt
 
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