Dumb question

better safe than sorry

Say you have an older piece of outboard gear, like a compressor, that only has lo imp in and out, could you just use some of those radio shack lo-hi converters on the in and out of it into your board’s inserts?



Tom - tell us more about the older device in question (make & model), and about the “low” impedance in’s and outs it has.

If your inserts are balanced then you may not need to convert with the adapters. Just use a cable(s) that is XLR to TRS.

If this is the case you will keep about +3db in headroom.


I used to use those RS transformers all the time (I have about 10 male XLR to 1/4" female phone jacks…umm…I had that many but a bunch seem to have gone on vacation during the move from NC). They are cheap and work really well, though The Golden Ears out there might not like them. I can’t tell the difference in them and Shure brand that costs four or five times as much (had 4 of them). They are great for DI work when you don’t need a pass-thru, such as line out on a guitar/bass/keys amp, coupled with mics for live use. Why spend big bucks when you already have a good line level that could care less about getting loaded down?

Quote (archimedes @ Oct. 28 2004,19:26)
Tom - tell us more about the older device in question (make & model), and about the "low" impedance in's and outs it has.

It's a TOA compressor from the 1970s, taht I mentioned here a while back, while I was digging around trying to find a manual, to see if there was a way to hook it up unbalanced. I couldn't find one. It works fine with the balanced outs from my little Bellari or Art preamps, but I want to use it in the inserts of the Aardvark Q10. I'm no electronics genius, esp. without a manual! :)

I think I meant to say "unblanced" rather than "lo impedence" on the compressor, and unbalanced inserts on the Aardvark. I'm no electronics genius! :) I have to use a Y cable with TRS at the insert point to 2 mono 1/4 inch plugs. So I need to get from xlr ins and outs on the comp to that.

Well, you want to get that right.

If it’s just the balanced/unbalanced difference, here’s the rules as far as I know:

bal -> unbal: OK. Use a 2-conductor cable.
unbal -> bal: Not sure. Consult the manual for the output. If that says it’s OK to use a 2-conductor cable, then you’re fine. (I think this is typical for modern line level gear, but may not apply to older gear or to lo-z gear.)

And for impedance:

- lo-Z -> line: OK (might not sound good, but won’t harm your gear)
- line -> lo-Z: BAD: might damage the line level output.

To answer for sure on the BAD case directly above, you need specs for both input and output. For the output, you need a spec that’s variously called “minimum load impedance”, “maximum load impedance”, or “load impedance”. For the input, you need what’s called the “input impedance”, or often just “impedance”.

The spec for the input must be HIGHER than the spec for the output (regardless of “maximum” or “minimum” terminology – if you want to know why that is, just ask. It’s just semantics.)

If the input impedance is lower than the required load impedance on the output, you can burn out the output line drivers. (Thus the spec.) With lots of modern gear, I don’t even see that specified. I don’t know whether that’s because they’re idiots, or they’ve improved the circuitry with limiters so it’s no longer an issue. But you’re talking about old gear here, so you’d better be careful.

The RS adaptors mentioned above do BOTH conversions: bal/unbal and hi-z/lo-z. If that’s what you need, then that’s the ticket. I agree with Phoo, nuthin wrong with those babies.

But if you don’t need to convert BOTH impedance and balance mode, then you need something different.

Too bad there aren’t simpler answers! I thought there were, but I was assuming the lo-z were balanced and hi-z were unbalanced (or balanced inputs).

Someone please double check my “facts”! If I’m wrong about something,

I was about to do a short essay on my solutions for bal-unbal stuff. But then I remembered the grail - the technotes from RANE, which I think is the absolute last word on the subject.

Thanks Learjeff and Arcimedes - I’m gonna mull over what I’ve learned.

I would google the item and see if you can’t find some specs for it. Amazing what you can find. For example, if it’s this:


then it’s lo-Z input, and I’m not sure what output, but I bet lo-Z. Someone like Mac would probably know.

Actually it’s one of these.


I’ve not been able to find a manual for it, specs, or anything. Perhaps my web skills are no good.

Actually, I did find one fellow who said he had it, but after weeks and weeks of waiting and excuses I got my money back.

Based on what I see there, the inputs are definitely balanced, but the outputs look single-ended. I wonder why there are 4 outputs, though.

Also, the way the inputs are labeled, it looks like they’re meant to be connected to XLR connectors (thus the “2” and “3” labels – pin 1 would go to the ground). So, in addition to being balanced, I’d expect them to be lo-Z – but this kind of gear is a bit outside my domain so who knows.

I can only think of two reasonable possibilities for the outputs.

1) There are 4 unbalanced line outputs. But what are outputs 3 and 4?
2) there are 2 balanced outputs – but why are they labeled that way?

I suspect that the outputs are line outputs, and you can use them balanced or unbalanced or inverted, and with some simple testing you can figure out what each output does. You’d probably want a program that turns your PC into a digital oscilloscope. There are several free ones out there.

A lot of older stuff like that simply had dupe outputs (and inputs as you see) so the signal could be split easier. They are grouped together to show that they are effected as one. They MIGHT be phase reversed so they could create a balanced out, but I seriously doubt it (They’re not labeled as such and that could lead to all kinds of “WTF’s” down the road). There is also an input between the two inputs, marked with a ground label. That MIGHT be a sidechain input (the audio input here affects compression but isn’t audible in the output).

Phoo, that one in the middle is labeled with the ground symbol. I assume the inputs are balanced, with ground (3-conductor).

This unit is “public address” grade gear, which is TOA’s main business area. So it may not be anything special as a mic pre or as a compressor.

I would bet that the 4 output terminals are really one transformer-balanced output with a number of impedance or level taps. It looks to me like those 4 terminals are fed from the large-ish transformer on the upper-left of the PC board, which looks the right size for a multi-tap line out transformer.

To connect the output terminals, I would get the thing running with tone at 0 on the meter (meter set to the +4 range), and the output control set to about 2 o’clock. Pick the pair of output terminals that come closest to the normal input level of the devce you want to feed. Eg if you’re feeding the line input of a soundcard, set the card’s input control to it’s usual setting (eg 75% up) then pick the pair that gives you around -12 to -16 on nTrack input meter.

Might be easier to do this test using a mixer’s line input, if the mixer has VU meters; then you choose the pair of terminals that come closest to 0 on the Mixer’s VU.

Quote (archimedes @ Oct. 31 2004,12:46)
This unit is "public address" grade gear, which is TOA's main business area. So it may not be anything special as a mic pre or as a compressor.

Hey! Every piece of gear is special, in it's own way! :)

Thanks guys, you've given me a number of things ot check out. I will be back with more questions.

Some other info: the compressor had two optional mic pres, the unit I have has neither in there. The inputs are balanced, I hooked up an XLR cable, and fed a signal in from my art mic pre, which has a balanced out, and the comp's meter danced nicely. The comp has two separate input channels, wouldn't it have two separate outs? You'd need two transformers then, however, right? I don't think they were originally thinking just PA market, I honestly don't know what they were thinking, it wasn't a successful prodcut apparently. It does have switchable comp ratios that remind one of those on an 1176 (missing two middle settings).

Even if it sounds like poop, it looks cool in the rack.

I'm purposely avoiding comments about cool racks.

Big DUH on my part. I look dat the picture and was thinking RCA jacks…it’s a terminal strip…never mind… :)

Well, you guys helped me get it. Balanced in, unbalanced out. It sounds pretty neat, not exactly subtle at 20/1, although at that setting a de-esser sounds like it would be necessary on vox, at 4/1 it evens things out very nicely. And it is not very noisey, surprisingly enough. The ART mp is much noisier. WIth an sm57 through the ART into the TOA comp at 4/1 it produces a very rock and roll sound. Pushed too hard it does not distort nicely, however - a really crackly sound, not at all smooth.

Thanks guys, I’ve been noodling with the thing now and then for a couple of months and now can finally use it!

I bet Archimedes is right – up to two inputs but only one channel. Since you’re only using one input, no matter anyway. Tom, that means you might want to try each of the 4 outputs (using the ground as the other conductor in each case) and see what differences there are, if any.