Forsell opto sidechain into U-pad with LDR as shunt

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Title says it all. Trying to figure how to lash an opto comp onto the output of a BabyAnimal mic amp for a simple gig bag on-the-go recording channel. Since the output is electronically balanced, it seems desirable to use a U-pad with the vactrol's LDR as the shunt R.Schematic of BabyAnimal with 2604 (PDF)

Looking at the datasheet for the VTL5C2, the actual resistance of the LDR will be somewhere between 2k5 and 100k in practical use. With series Rs of 5k, this should net 14dB gain reduction when the LDR is at 2k5, and 6dB at 10k.VTL5C2 Datasheet

That sounds useful to me, but perhaps the output impedance will be an issue? Only needs to drive ADCs with 10k or 20k inputs. It shouldn't stray much over 10k with the LDR up at 100k, and lower with any amount of compression happening. Maybe I should just make it an H-Pad to keep the Z-out constant.

I welcome and appreciate any input!

Brian

 

[PRR]

> a U-pad with the vactrol's LDR as the shunt R. ... the LDR will be somewhere between 2k5 and 100k in practical use. With series Rs of 5k, this should net 14dB gain reduction when the LDR is at 2k5, and 6dB at 10k. ... to drive ADCs with 10k or 20k inputs.

Well, 2*5K or 10K series, with say 10K loading, is -6dB static loss.

Throw a 2.5K LDR on, the shunt is 10K||2.5K= 2K, and 10K series 2K shunt is 2/12 or -15.56dB loss at full suck-down.

So the max depth is (-6dB)-(-15.6dB)= 9.6dB. Not a lot. I usually like 10dB GR headroom above my nominal ducking, because I've sure had 10dB surprises.

What is the Animal asked to do? Say your ADC input is 0dBu nominal, +16dBu peaks. When not limiting, the Animal must deliver +24dBu peaks to cover the -6dB loss. When limiting we have -15.6dB loss so 31.6dBu is needed from the Animal. That's 30V RMS, which even with push-pull oomph seems awful high.

20K inputs are slightly more favorable, but still inelegant.

If you parallel inputs for safety (I try to never have just one recorder on live musical performance) then 10K 20K 6K7 loading on 10K source makes a mockery of any level-setting.

Not to mention the ~~10K line impedance is far more susceptable to hum, motor-hash, and radio than the Animal's raw 102 ohm output.

Why MAKE POWER and then throw it away?

OR.... capture the audio unmolested. This isn't Dolby B cassette, right? With 16- and 24-bit ADCs, about any acoustic event can be captured raw. Let the meters bop down at -20VU or -30VU. Do the limiting in mix-down, not in the heat of performance.

 

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Thanks for the sanity check!

[quote author="PRR"]
OR.... capture the audio unmolested.[/quote]
Hmmmm... maybe I am overstuffing this box. I'm not sure why I think a hardware comp is worthwhile in this travel rig. Either way, I have the Forsell built on perfboard, so it is sure to see some use in mixing.