PLLXO with stepped volume control? - diyAudio
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Old 5th October 2004, 04:05 PM   #1
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Default PLLXO with stepped volume control?

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
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Old 5th October 2004, 06:44 PM   #2
Pan is offline Pan  Sweden
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Will mess up the x-over function.

/Peter
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Old 5th October 2004, 08:58 PM   #3
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What would it take to make it work? Could you isolate the PLLXO from the stepped attenuator with something like a line transformer?
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Old 5th October 2004, 09:46 PM   #4
KCHANG is offline KCHANG  United States
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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
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Old 6th October 2004, 01:21 PM   #5
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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?
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Old 6th October 2004, 02:46 PM   #6
KCHANG is offline KCHANG  United States
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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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Old 6th October 2004, 03:23 PM   #7
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Kurt,

You're teaching me a lot here--I appreciate it!

Now I guess I'll have to figure out what an opamp stage or buffer is exactly...

Also, what would work better upstream of a buffer/opamp, a 10k stepped atten. or a 100k? I've got a 100k already...

Cheers,

Dart
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Old 6th October 2004, 03:39 PM   #8
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PLLXO? Phase-Locked Loop Xtal Oscillator? Must have been a different world...

Jan Didden
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Old 6th October 2004, 04:17 PM   #9
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PLLXO? Phase-Locked Loop Xtal Oscillator? Must have been a different world...
Sorry--Passive Line-Level Crossover.
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