Guide to install pre-outs on an AVR

+1....I'd like to know as well....but this might be more appropriate in another forum.

With point to point wiring, yes. With an analog volume control (tap it on the input side) yes. With modern electronics, using wave soldered SMCs and VCA volume controls, not easy at all.

I've worked in electronics for decades and am confident in most gear, but this is not a job I would willingly take on without a schematic and layout and design will vary between brands and models so anything done will be case specific.

I'd look to find the input to each amplifier channel internally, as that would be the easiest place to take signal from and would be the natural pre out point anyway. If you're lucky, that will be a cable or cables that connect the line level section and power amp boards where you can break out the signal.

Best of luck. Considering how easily an amateur can damage something like this, I'd suggest just buying an AVR with pre outs.

In a pinch you could build or buy some level converters from the world of car stereo. A quick google found this russound piece.

The problem with those is they're slaved to the output level of the amp. Depending on the OPs application that may or not be an issue.

If you're lucky, it's quite possible that they use the same pcb, and just change which components get installed - in which case it's just a matter of adding parts.

That said - even being a professional audio engineer, I think it's more work than just buying the right product. If you got a good deal, it should be easy to break even selling your unit and then not a huge cost to step up to the one with preouts.

Have you ever designed a circuit before, or is this your way of trying to learn? I definitely don't want to discount the hobby side of things...

Check out the DI4000 by Behringer. I used this to convert the line out from my avr into a pro amp to drive my mains. Worked flawless and didnt add any additional noise that i noted. For about $100 it was perfect to do what i wanted, it was also able to +20db to the sub output to work perfect to drive a pro amp for that as well.

If it gives you the same result without having the possibility of ruining your AVR then why not?

Easy part done, then. I think you need to get ahold of the schematics unless you can find someone who has made the change you want to the very same AVR. In days long past, what you want to do was easy. All you had to do was pick up the signal at the output of the volume control. Today it is more complicated. CE manufacturers typically use designs which combine rotary encoding for the volume control, source selection, tone control, volume control and other functions into a single LSI chip. You will at least need to identify the chip and its pinouts to be able to do what you want. Failing that, I guess you can always trace back from the output transistors.
Edited by Colm - 7/14/12 at 12:49pm

What power?

The unit takes line-level (speaker out) and converts into line (rca type) output.

aren't there high level to line level convertors for about $20?

would this work:

http://www.manventureoutpost.com/products/FUSION-HL%252d02-High-to-Low-Level-Converter-(HL%252d02).html?google=1

btw, i like your initiative, its just that i've never even heard of somebody wiring up their own line level outs to a factory receiver. if you can do it...great!

I haven't looked at the specs of your AVR, so lets assume 100w/8Ω for a channel which is 28.3Vrms before clipping. The end to end resistance of the voltage divider shown in an earlier post is 4k2Ω. P=V²/R = 28.3²/4200 = 190mW at full power, and lower most of the time.
In a class AB amplifier, which almost all AVRs are, every stage except the output will be class A, so the current draw will remain the same irrespective of signal (or none), so no power saved here, and as shown, only a trivial amount actually dissipated in the output stage and into the voltage divider load.
How does plugging into a pre out (an RCA connector) stop the pre signal from also going to it's respective power amp stage? The pre out and the power amp in are in parallel (in an AVR with pre outs).

Using a voltage divider after the amp is hardly optimal, but it would work and use a trivial amount of power.
There is no guarantee that the chipset doing the VC will be a typical line item like a PGA2310 or CS3318 but if you're lucky it might be. Tapping the signal at the power amp input is what I suggested in post 5.
Edited by A9X-308 - 7/14/12 at 3:34pm

Like this:  http://www.epanorama.net/circuits/speaker_to_line.html

Of course you can reverse engineer if you want.

FWIW a simple voltage divider, like the one Kal provided a link for, and others have suggested, across the speaker outputs is probably quicker and easier, if not as elegant. There will be very little power wasted because of the large (relative to speaker impedance) value resistors used. By using high value resistors, we limit the current, and thus the power loss. The high impedance input of whatever downstream device you add takes almost no current to drive. It just has to be the right voltage.
Edited by Colm - 7/14/12 at 5:21pm

"Like this"

neat! that looks simple enough.

Not if the resistors are a high enough value, which they must be to adequately reduce the voltage. Besides, if they did dissipate a lot of power they wouldn't be half watt resistors.

Did it work? :P

I guess if your time is worth less than $400 then it's worth exploring. It shouldn't take too long to reverse engineer a schematic of what's in your AVR - which is where I'd start. I would be very surprised if you could find a node to just tap off and wire directly to an RCA output and work well enough to sound better than the speaker level converter. Gain structure and output impedance would be two areas of greatest concern...

900ohm output impedance is pretty high for a line level signal - I would try to scale the resistor values back so that the shunt resistor is more like 100ohms. Also, it's not uncommon for speaker amplifier stages to have a little DC offset - I personally would add a series cap to prevent any potential offset from transferring down stream.