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rruss 15th October 2004 11:34 PM

Building a pre-amp output isolation buffer?
I've searched the forums and found many similar circuits and conversations, but nothing that I believe specifically addresses my question.

I have a Denon Receiver that does not provide pre-amp outputs. I want to add a pre-amp output for the center channel and though I might as well build 5 of whatever circuit I use and port out the other 4 channels as well (5.1 system... sub is already ported out).

I have isolated the pre-amp board and clearly see the signals carrying the "pre-amp" output signal to the amplifier stage of the receiver. I'm hesitant to simply connect to these signals in parallel with an out-board amplifier.

Is there a simple circuit that I can place in between the pre-amp signal inside the receiver and a pre-amp out port that I add to the back of the receiver to isolate the load of the external amp from the internal pre-amp circuitry?

I'm picturing some kind of resistively loaded single transistor amplifier with an AC coupling input cap and a small DC bias network on the input. Unfortunately, I'm not familiar with the details of audio specifications and in the interest of getting this done quickly I was hoping to steal a schematic :)


AndrewT 16th October 2004 02:46 PM

you could use an opamp configured for a gain of one.
AN5534a or its 2channel twin an5532. 5534 must have a comp cap added for gains of less than 3 if I remember correctly,
or try TL072, a fet input 2 channel opamp. If you want to drive very low impedance lines then a true buffer opamp is better but more expensive.
Look up ESP web pages for lots of good design info. The 553 options are quieter than 074 but others with less noise are available at higher cost. You'll need 1 channel of opamp for each output of your 5.1 configuration. 5534 draws about 4mA per channel plus current to output load say 10mA each. 553 will drive 1k0 successfully but cable capacitance increases the loading, I suggest 10k as your lowest load & 100r as the output current limiter. 6 channels will consume about 100mA. I suggest a 500mA PSU with plus and minus rails at 1 or 2 volts below max for chosen opamp. i.e. +-20 volts for 5534 & +-15volts for 072. You should find enough space inside the casing to fit this in along with a pcb mounted 12va tx
hope this gets you started.
regards Andrew T.

rruss 16th October 2004 05:38 PM

Thanks for the details andrew!

In looking over a cheap-o AudioSource amp I have for inspiration I see that they also use the 5532s in the input stage of the amplifier.

I'm not sure what you are referring to by the "ESP webpages" so it would be great if you could clarify that.



AndrewT 16th October 2004 09:09 PM

Hi Russ,
ESP = elliot sound projects. You'll find them at

rruss 18th October 2004 03:52 PM

Excellent resource! Thanks Andrew!

It looks like his pre-amp, set to unity gain will be perfect for my application.


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