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johanwahlberg 10th January 2013 12:28 PM

Adding a line level output to PGA2311 preamp
 
1 Attachment(s)
I have a chinese PGA2311 based preamp I use to feed my HT stereo signal to Dynaudio active main monitors. I'm planning on feeding a pair of reinforcement speakers in the kitchen (active Audioengine A5 10-15 meter cable run) with a Tape or "Rec out" kind of output. Problem is, there is none on my preamp. Solution: Add one.

The volume pot on the preamp controls output on the Dynaudios while the A5s want a full signal as it has its own volume control, so i cannot use a Y split on the output. I believe the combined impedance is too low as well (47K + 10K = 8.25K)

Attached are the preamp schematics.
I read in some forum examining these questionable Chinese PGA preamps, that their layout is pretty bad, although there are different versions and mine is an upgraded one. I think it sounds good though.
However, output goes from pin 11 and 14 on the PGA2311, via 330R each (what does these resistors anyway?) to P1 output RCAs to Dynaudios.

My thought is to add a simple JFET or OPAMP buffer, "stealing" the signal after the source relays, before reaching the PGA chip.
Is this the way to do it?

I know too little of audio electronics to understand this - Would "stealing" the signal "leave" the original signal proceeding to the PGA chip "alone" and untouched, or might it have any impact on sound quality of the Dynaudios?

I have not yet chosen a buffer but I've been looking at some basic designs by Curl and Borbely. As I understand it, these designs has Vin and ground connections that could go right into my preamp circuit. The supply voltage could be shared with the PGA, right? I also have a spare +-15V from the trafo.

If so I really just need someone to point out where in the circuit to connect the +V in feeding the buffer and where to connect the buffer ground (if it matters).

But i guess there are tons of impedance or feedback issues showing up that I don't understand any of...

Attachment 322912

Esperado 11th January 2013 02:36 PM

If i understand well, you want to add an output after the relays and BEFORE the PGA in order to get a signal not affected by the level controls of the PGA but after sources commutation ?
Just add a dual Operational amplifier feeding their + inputs from the tracks going to pin 9 & 16 of the PGA.
Chose an OPA with enough current capability to load the long line capacitance.
Or a specialized line driver device. If you like to have a symetrical output for the long line to your kitchen; have a look to Analog Device SSM2142, as an example.
The 330 ohm is correct, and protect the PGA output stage from RFI/EMI returns in the feedback loops and from short circuits as well.

Take care the PGA use +-5V for its analog power supply. So you will need +/-12v or 15v for your OPA.
If you use a separate power supply for those +-15V, reference all the new grounds to a single point (PGA analog ground).

johanwahlberg 11th January 2013 02:51 PM

Thanks Christophe!
I don't think I need a balanced output. The A5s in the kitchen doesn't have balanced inputs, which defeats the purpose i guess... I read the specs on the SSM2142 and it definitely sounds as the right kind of product. Do you think there are any similar, suited for distribution amps and such, but dual, and not balanced?
Preferrably one wich can share voltage supply with the +/-9V going to the PGA...

Edit: Yes, you understood me correctly.

Esperado 11th January 2013 03:18 PM

They are hundreds, from old NE5532 to fastest devices 1000V/s (current feedback) i love, but, for a kitchen :-)
Chose an op amp able to drive 40ma, and unity gain stable. You can use fast video line drivers as well, but most of them are triple OPAs
Don't forget to put a 50 or 100 Ohm resistance in serial to the output.


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