High impedance mics / jack inputs

[Shez]

As high impedance mics seemed to disappear before I got involved in sound, this is a little beyond my area of experience...

 

Situation: I've acquired an old portable mixer amp (Adastra A56 - I'm sure you know the sort of thing) which has three mic input jacks that I believe are high impedance. (Are all old mic jack inputs unbalanced high impedane?) I've asked Skytronic about it (they seem to look after Adastra products now) who were able to send me a spec sheet for a similar, newer model which was useful but that one had normal low impedance XLR mic inputs.

 

Question: How would I best connect a 58 to this? I'm aware of those transformer barrels that I think interface a low Z mic to a high Z input. Would making up a cable with XLR on one end with pins 1 and 3 shorted and a mono jack on the other be workable? I only need a couple of meters of cable so noise pickup shouldn't be an issue.

 

Any advice welcome!

[Sound Man]

After a quick search on the internet I've found this link. I wonder if Farnell can give you any more info as they used to supply them.

 

I would have thought that the mic inputs would have been low impedance. To check that it is balanced, plug an unwired stereo jack plug in the input socket and with the amp switched on check if you get a buzz through the speaker when you touch the ring connection.

 

David

[andy_s]

(Are all old mic jack inputs unbalanced high impedane?)

Any advice welcome!

 

not necessarily - I'm pretty sure I recall Revox A77 and B77 having hi and lo z mic inputs, both appearing on jack sockets.

 

this now exhausts my knowledge of this topic, and I relinquish the thread to someone who knows better!

[duncanborrowman]

Go to www.canford.co.uk and search 37-110

I would try just the cable first rather than a barrel with a transformer in it. They also sell a barrel with am XLR female on one end and a 2 pin jack on the other - essentially this cable in a barrel and they sell barrels with transformers in. When I was freelance I carried 2 of each of these in my kit for connecting my portable mixer into house setups.

[Arthur]

Can you / have you had a look inside?

 

There were old balanced circuits on TRS jacks, is it a 2 pole socket, or three? Does the input feed into an op amp? The inverting input will be lower impedance than the non inverting input.

 

Try an XLR to mono jack mic lead, what does it sound like, what level do you have? To compensate for old tech magnetic material they used more turns, sometimes the Hi imp mics gave more signal -but not a lot!

 

http://www.terralec.co.uk/microphone_acces...ne/17633_p.html

 

Also sometimes available with jack lead

[jamesperrett]

Are all old mic jack inputs unbalanced high impedane?

 

No - low impedance unbalanced mic inputs were very common on cheaper gear from the 70's and 80's. In fact, many microphones from that era would include an XLR to jack lead as standard rather than an XLR to XLR which would be more common today.

 

Cheers

 

James.

[dB or not dB]

Even if input is Hi-Z it may work OK with low-Z mic but sometimes frequency response is odd.

 

 

If you have a passive DI Box (Direct Injection Box) it will contain a Hi-Z to Low-Z microphone transformer. Being passive it can be used backwards, to give you a Hi-Z jack output. (Make sure any PAD / Attenuation is off).

 

 

Only snag is its XLR connection is male not female so you would need a Female XLR – Female XLR adapter barrel so you can plug in the SM58's XLR lead.

 

Good Luck.

[duncanborrowman]

Problems really only arose with high impedance sources into low impedance inputs not vice versa. Think of it as two resistors in series and how it reduces the voltage. You really should be fine with just the lead I pointed you at. As said above mics used to come with one of these leads. I remember buying Shure's own cheaper clone of their own SM57 and it coming in a zip up bag with a clip and am XLR to jack lead

[Pete Alcock]

Hi Shez,

 

I'd try exactly what you suggest first - an XLR-F to mono jack wired as you describe. If you get enough level for your application, fine. If not, one of the many miniature inline mic transformers available from Maplin, CPC etc. would give you a 10-20dB boost.

 

You can get rigid adaptors that have a mono jack at one end, XRL-F at the other end, with a transformer inside. These are best avoided since the whole thing sticks out from the socket about 5" and can strain the socket.

http://images.maplin.co.uk/300/bv52g.jpg

 

The ones with a short cable terminating in a mono jack are better, like this:

http://www.keene.co.uk/pages/cat/jpgs/KAMT.jpg

 

Cheers.

 

Pete.

[Shez]

To check that it is balanced, plug an unwired stereo jack plug in the input socket and with the amp switched on check if you get a buzz through the speaker when you touch the ring connection.

There is a buzz when touching pin 3 (balanced XLR to jack lead) but it's much quieter than when touching pin 2.

 

Can you / have you had a look inside?

 

There were old balanced circuits on TRS jacks, is it a 2 pole socket, or three? Does the input feed into an op amp? The inverting input will be lower impedance than the non inverting input.

I did have a quick peek inside when I first got it; I seem to recall only seeing two wires coming from the jack socket. When I metered the inputs, both the tip and ring quickly rose to >40Meg - either capacitors inline or else something very high impedance?

 

I'd try exactly what you suggest first - an XLR-F to mono jack wired as you describe. If you get enough level for your application, fine.

Tried now with success - loads of level and it sounds about as good as I would expect from an amp like that. Time to dig out the soldering iron to make up a non-bodged version :(