Turtlerock Forum

Hi, does anyone know about this technique that I swear I read in Colin Abraham's "Studio Connections" website?
It involves adding a small value cap between ground and cold when hooking up balanced outputs to unbalanced inputs.
I did this for my unbalanced outboard when I installed my current console a couple of years ago and it works.
Strangely, I can find no mention of it in that website now...
Cheers,
Paul

This simple circuit ensures the balanced out sees a balanced load and drops the level down to about -10, so that your +4dBu balanced output does not overload the unbalanced input.

https://www.gearslutz.com/board/attachments/newbie-audio-engineering-production-question-zone/349522d1371356656-4dbu-balanced-line-output-unbalanced-instrument-line-input-bal-unbalanced-attenuator.jpg

Hi Junction, thanks for that. What that circuit is, is a pad, which isn't really what I'm talking about. Most of my unbalanced gear runs at +4, ADR Compex, Allison Research Gain Brains etc so no pad is needed. Other unbalanced gear I have are delays and reverbs, as they are usually sent signal from an aux send, no pad is needed as I can just turn down their sends to where they are happy. Could be handy for running my Revoxs with though.
What I'm describing and I THOUGHT I got it from Colin Abraham's site, is a variation on the "tie cold to ground when using a balanced cable to hook up unbalanced gear" rule of thumb. The variation simply puts a low value cap on the ground wire before it is tied to the cold wire. I hope I've described that coherently.
Any thoughts on that, anyone?
Cheers,
Paul

I remember seeing something like that in some JLM Audio circuits.
I think it was his powersupply kits.
Where there is a bus, which only connect to ground via a 10uF cap.

Yep Paul its a pad, providng a unbalanced out. I had assumed you were going into -10dBV consumer gear. Unbalancing a balanced output requires an understanding of the type of balanced output. If it is a transformer output, then you can simply tie the cold leg to ground. If its an opamp output, tieing cold to ground can cause problems, some opamp circuits will become unstable, as you are effectively shorting one opamp output to ground. The safest but not cheapest approach would be to use a transformer. If this is not the route you want to take, then it needs further analysis of the output circuit.

Just thinking out loud here and I haven't tried this but the capacitor got me thinking. The issue is to connect hot and cold signal lines, maintain an earthed shield and keep the unbalanced earth off the balanced earth.



Please excuse the crude sketch, as I said I'm doing this on the fly.

Now, tell me why it won't work.

Hi Bob, so the unbalanced piece of equipment is deriving it's ground reference from the cold or negative wire of the balanced gear via a small cap in your diagram?
From my limited understanding, I don't think this would work.
Paul

Wouldn't the ground reference be modulating with the cold signal from the balanced output?
I would expect that the unbalanced unit would see that as no signal.
Cheers, Paul

Now, tell me why it won't work.

From my limited understanding, it may work in some cases and not in others - it depends on how the balanced source signal is generated and how the unbalanced input is implemented. What you are doing here is tying the ground reference of the unbalanced input to the cold reference of the balanced source (rather than to the ground reference of the balanced source which is left floating at the unbalanced input) and using a capacitor to hopefully decouple any DC difference between the two.

The implicit assumptions seem to be that the unbalanced input ground is tied to earth, that the ground reference of the balanced signal is not itself floating at the signal source and that there is no major DC offset in the balanced signal (in gear running off a single sided supply, for example, the cold reference can be at half the input voltage unless transformer or buffer balanced - which it should be, but who knows what gear is out there?).

These assumptions in various implementations may have impacts ranging from non whatsoever to frying your input. I would want to know the gear involved before trying it.

Later on last night I thought up an example - some equipment puts an inverted signal on the cold wire of a balanced send - so tying the cold to ground effectively halves your signal strength because this inverted signal will be drained to ground rather than lifting ground to it!

This is a very crude rendering of the hook up I have been referring to.

The hot and cold output of an electronically balanced output circuit is essentially two of the same source audio, but they are 180degrees out of phase to each other. It is not advisable with an electronically balanced output to tie the cold leg to ground, the worst case is you could damage the output amp. You can merely connect your unbalanced connection across the hot and the ground of the balanced output and leave the cold not connected.
If its a transformer output, then yes, tie the cold leg to ground.

the drawing up above is correct

as a first rule
hot to hot
cold to cold
ground to ground

with an unbalanced device cold is ground

rule two
lift the ground wire at one end to avoid mains earth loops. Be consistent, lift all outputs or all inputs, not both

rule three
if the receiving device is unbalanced then ( output balanced) cold goes to ( unbalanced ) ground.
there are three different sorts of balanced outputs
transformer - this must have the hot and cold connected at the receiving end - rule three applies
servo balanced amplifier - this will shut down the cold output if it is shorted to ground and jack up the hot output 6dB to compensate - rule three applies
non servo amplifier - rule three will short out the cold output. You won't get the 6dB correction above, so you lose 6dB but rarely will the shorted cold cause any damage or other sonic issues ( sometimes a small increase in distortion at hot levels can be noted )

the cap to ground thing is a finessing of rule two, where rather than leaving one end of the ground floating or lifted it is brought to ground at the far end via cap. Say a 0.1uF ... a low value. This shunts any radio frequencies to ground and prevents the ground wire ( shield of the cable ) acting like an antenna. Call this rule four and only worry about it if you get RF pickup. It has nothing to do with the trick of connection balanced and unbalanced equipment.

rule five
sometimes you have to break all the rules to get something to work correctly and hum free.

http://www.proharmonic.com/articles/AT6 ... _BENCH.pdf

Beautiful, thanks Rob!