600 ohm gear in a modern line-level studio...

[no-fi]

so... this poorman vari-mu compressor project is looking like being a reality in my studio relatively soon.

Soon enough, that I'm already thinking about how it will be hooked up.

Which leads to one thing I've noticed, is that its inputs and outputs are 600 ohm.

Now I'm not so worried about the output, as that should work fine whacked onto a balanced TRS or unbalanced TS 1/4" jack, into anything else I have... (but please tell me if you disagree! I have no experience with 600R gear) but I have a feeling the inputs might be interesting, in terms of supplying a way higher load on my other gear's outputs than they normally have to drive.

so... what do real studios do with interfacing modern line level (mostly unbalanced - and sometimes +4dBu but often -10dBV) electronic instruments and recording equipment with old style 600 ohm inputs? (and outputs?) ...and more importantly, how do I DIY it to an adequate level for my hobbyist home studio for CHEAP?
:P



I'm wondering, seeing everything else I have is high impedance, if I should have just got 10k:10k for the input transformers? But unfortunately they're already ordered and paid for...

[no-fi]

wow.... 37 views, and nobody knows the answer?

I'm sure this can't be the first time in Australian home studio history that someone's considered plugging modern line-level gear into old 600R gear....

[rick]

robs away working hard i think but i am sure he has an essay for you , i am going to sit on my hands so i am not making mistakes ... otto howard ..? wanna tell it like it is ..?

[jkhuri44]

canare, XLR impedance convertors...simple enough...i think??

http://www.canford.co.uk/Products/23-091.aspx

[jkhuri44]

woops, thats digi impedance conversion, i think they do analog ones....

[jkhuri44]

haha, as rick said, i should shutup now...hehehe

[no-fi]

aaaah well.. thanks for the attempt guys. I appreciate it.
:-)

But I guess I'll have to wait for someone less scared of being wrong to jump in.
:-P

It's not like I'll have this thing built inside a month anyway.... I'm still needing to order some of its bits, then the wait for them to arrive will begin..... It's just I've never really considered this sort of thing before, and so it's all new to me, and I'm excited to know right now. And web searching didn't really bring up anything useful (or my google-skillz totally suck)

[TimS]

I'm sure Rob will pitch in when he has some free time..

[Howard Jones]

There are several questions buried in what you have asked... it would also be helpful to know what mixer (if any) you will be connecting to... Anyway, let's look at how it might work for you.

I'm assuming from what you have said that you will be using your compressor in a basically -10dBV unbalanced environment. I'm also asuming that the compressor is transformer balanced on its inputs and outputs. Therefore, you will obviously need to connect both the hot and the ground of your unbalanced leads to, say, pins 2 and 3 if the compressor is using XLR connectors. You are best off using balanced leads with the shield not connected to anything at the unbalanced source and destination and letting the compressor extend the shields through as far as the RCAs or whatever connector is at the other end, without actually connecting them.

If your other equipment uses typical -10dBV type circuitry, the inputs will be 10kohm bridging and the output of the compressor won't have any trouble driving those inputs to a satisfactory level. Also, I would expect the compressor outputs to clip at +24dBU or thereabouts which will be hotter than your -10dBV gear. Even loaded, the compressor outputs will be fine.

You are right in theory to wonder if the 600ohm input of the compressor may represent too low a load for a typical op amp output driving into it. However, I would suck it and see. In practice, there is likely to be little or no problem. Modern op amp output topologies are capable of driving 3 or 4 loads in parallel so they won't have any trouble driving the compressor input. If in the end you find that there is some issue, you will need to effect a 10k-600ohm transition i.e. change the transformers that you have or stick in some line transformers.

Some of my comments above may need to be revisited if you aren't driving the compressor from a mixer, but from something like a PC interface.

[Thirteen]

It can be a bit of a "try it and see" dark art. Usually my gear works fine between solid state and transformer gear, as long as everything is properly balanced, but some xformer pieces such as my ADR Vocal Stresser can get grumpy with certain solid state gear driving it. As Howard said, it is best to set up either source or destination or star earthing in your studio if you have the luxury of nice patchbays, otherwise you will have to find some way of marking the XLR leads that you lift the grounds on in case you ever want to use those leads in the future with phantom powered mic's or DI's.

[no-fi]

the compressor is properly transformer balanced I/O. so I'm happy with unbalancing the output with a simple lead. no problems there. and I'm happy with unbalancing the input the same way.

the grounding in this design looks well thought out, and I'm pretty sure I won't need any magic groundlift cables to make it not hum. in fact, if I keep the audio I/O totally unreferenced to the chassis, I think the inputs and outputs can float to whatever they want.

mostly just unsure of putting a 600 ohm impedence on the outputs of my other gear, than anything else.


As for what I might use with it, well I have an allen and heath system 8 mkII as my mixing desk... which has unbalanced outs on XLRs for busses (Though that might go at the end of the year when I move house.) I would most likely run a mixer bus into it, whatever mixer I'll have, but also maybe want to use it directly with my firewire interface... or just drive it with drum machines or synths, as I see fit.

thats the thing - lots of options, means all sorts of different stuff could get connected to the 600 ohm inputs, so it would be nice for me to have a way of making sure there's no problems with any of it.

[Linear]

Hi,

Thirteen is right, it's a bit of a black art as to what can connect to what. Generally I've found that if everything is properly balanced, you don't get too many dramas. You need to be careful when connecting balanced to unbalanced though, as it depends on the input and output topology.

The biggest problem I've encountered are with non ground-sensing transformerless outputs. If they are connected to an unbalanced input, they try and drive the ground (rather than shut off) leading to lots of distortion and crosstalk on the board. This happened when I tried to insert such gear across the (unbalanced) insert points on the board I have. Manufacturers such as Manley allow for this by including an unbalanced output (the Dual Vocal Combo is one such piece). Interconnecting balanced and unbalanced with transformers works fine, you just get the expected 6db drop. Note that simply lifting the negative wire won't necessarily fix it because often the shield isn't 'ground' as such, it's floated with respect to the power supply ground. You might fix one problem but create others (depending on how well the unit is designed).

The other problem I've encountered is transformerless inputs normalled to console outputs - when the gear is switched off it causes major distortion due to the weirass loading created. Took me ages to work that one out, I thought my output amplifiers were toasted. Be careful what your console outputs are normalled to!

So really, it depends on your setup. I'd say that your compressor will be the least of your problems, from my experience transformered-coupled gear is quite forgiving. And don't forget that 600ohm was the old 'standard' so most gear should be designed to capably connect to it.

Chris

[no-fi]

ok, cool!

Well, it sounds like it should be OK for most things, then.


Any suggestions for what people do when they DO need to get a signal with a weak line level output to run into a 600R input?? I guess I could always run it through the desk...


thanks to everyone who contributed so far!
;-)

[rob]

yep, just go for it. Howard has painted the picture for connection

The operational issue you may have is not to do with balanced / unbalanced setup or the 600R loading, but is to do with the -10dBV operating level of your console. ( if indeed that's what it is ). In this case you may find it hard to get enough level into the compressor to yield the full operational range of the threshold control. Indeed you wouldn't be running the compressor in it's sweet spot and there will be tradeoffs on headroom and noise floor.

but i reckon i'd be focusing on building the baby and having it working correctly first up, and then of course you'll be falling over yourself to just plug it it and see what happens!

good luck and enjoy!

Rob

[no-fi]

hmm.. from my manual, it looks like my mixer does +4dBv (and -8dBv if you get into it and set jumpers.... for compatability with some tape machines, apparently)

only issue is the bus outs are not balanced. but again, the inputs on this compressor are just the primary side of an input transformer, so there should be no problem there.

reading further, it specifically says this desk is fine driving 600 ohms, too. and has an output impedence of just 22R. so yay! Even if some of my other gear isn't going to work right going into 600R, I've always got my big old mixer to take a signal and kick it out. (hmm.. might go and download a mackie manual to see what my little mixer can do, if I have to sell this at the end of the year)