Adding balanced insert mod to consumer gear

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stickjam

Well-known member
Joined
Jun 17, 2004
Messages
325
Location
Grand Rapids MI
I'm looking at modifying an AV receiver to add balanced I/O inserts on the 5.1 signals between the preamp and power amp (before volume control)

The signal chain for each channel is conceived (in order) as follows:

- existing receiver preamp switching circuit providing 200mV signal
- *Send Level shift (gain?)
- BurrBrown/TI DRV134 Balanced Driver
- DB25 connector
- Tascam DA88 snake to TRS fan
- 6 balanced console channel line inputs (0dBu=0.775V +30dBu MAX)
- 6 balanced console monitor sends
- another TRS fan-to-DA88 snake
- another DB25 connector
- BurrBrown/TI INA134 Input device
- *Return level shift (attenuation?)
- existing receiver volume control/power amp expecting 200mV

The idea is to interface to the console through true balanced interconnects. Theoretically the receiver could also be essentially restored to its original operation by strapping the two DB25's together.

Knowing all the above, can the required Send and Return level shifts (*) be reasonably calculated? If so, what would they be? If not, what else would one need to know?

Thanks
--Bob
 
Well, it's not certain that I've grasped what you want to do, but 12dB gain and 12dB drop will get you from -10dBV - +4dBm and back again (the apparent 14dB being much closer to 12 due to the 2-point-something dB difference between dBV and dBm)

The receiver is likely to be -10dBV on RCAs presumably.

bear in mind that unless you're really using seperate preamp and amplifier inputs, the receiver may not present decoded line-level signals to the outside world at all in the same way that the old stereo ones did at "tape out" or "Graphic EQ" sockets...

The lack of a 6 or 8-channel domestic recording format from analog inputs, and the largely discredited use of graphic equalization (thank the lord!!!) makes it tedious, expensive and damaging to the sigal to convert dts or Dolby decoded information to analog, then back to digital again for the likely delay correction and other signal processing that a lot of amps have built in these days.

Why you might want to go through a console and back out again is a different question, but whether you can do it is by no means certain unless you can prove it by feeding the signal through six RCA-RCA cables and breaking each channel individually by removing a single connection. -If you can do this, what you're thinking is possible (however unsnusual). If you can't get this simple test to work, then what you're asking is not do-able.

Keith
 
Thanks to a helpful service tech, I found the place to break the signal just before the master volume control; the returns would simply become the inputs to a 7-channel power amplifier.

I'm planning to use the preamp side of the receiver mostly as a dubbing and switching unit to hang a bunch of legacy consumer gear together (turntable, cassette, VCR, DVD player, Canopus ADVC-100): no EQ, no DSP no nothing. I'm planning on using the preamp outs into the console more as a way to monitor or do stereo/surround transfer/restorations from media where digital transfers are impossible. I'm handling surround delay management with my Layla interface, which will feed into the power amp (via the custom receiver return).

Preamp signal path feeding volume control for the Front Left channel shown:
ReceiverMod1.gif


...remove cap and tap in (decoupling moved to balanced I/O cards)

ReceiverMod2.gif


Am I still crazy?

--Bob
 
Regarding the needed 12dB gain/attenuation needed to round-trip from consumer to +4 to consumer, I was looking at the datasheets for the TI chips and noticed that the DRV134 specifies a gain of 2, and the INA134 specifies unity gain.

Does that mean I'll only need another gain stage of Av=2 (6dB) feeding the DRV chip, and -12db attenuation (Av=0.25) coming out of the INA?

What do any of you folks that use DRV134 and INA134 chips usually hang on the balanced ins/outs of them? Coupling caps? protection diodes?

Thanks
 
I've actually abandoned the mod project because one of the reasons for doing it was to get directly at the power amp inputs. Upon closer inspection of the schematic, it turns out this receiver has no bass management on the external 5.1 inputs (or LFE handling for that matter,) just a straight line to the power amp. But...

Now the project is a proper bass management unit. I started a new thread in The Lab with questions about that

Thanks
--Bob
 

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