Inline microphone preamps

[michaelyallop]

Hey,

 

I need to replace some of our old Shure preamps for our SM91A (yes really old I know) mics. Does anyone have any that they recommend? The replacement Shure one, which has now be superseded, is £128. There is a stageline 'Phantom Power' supply inline adaptor, bu there are no specs for it. I need a mini XLR to XLR version.

 

Cheers Mic

[paulears]

The Shure needs an in-line preamp, the stageline phantom power unit provides phantom where none is normally available? A verty different beast. The Shure one is matched to the microphone section - and is just a way of reducing the physical size of the mic end? I doubt you'd be able to get the spec for the Shure in any meaningful way, without pulling the mic and psu apart?

[michaelyallop]

I had thought of this, but just wanted to find out if anyone had found a replacement that is not the matched Shure one as it is very expensive.

 

Thanks Paul

[Bobbsy]

AFAIK, the Shure unit is not simply a standard pre-amp. It's the electronics that would normally be in a microphone moved to a separate unit--but still specifically matched to the mic. As such, you're unlikely to find a 3rd part alternative.

 

In view of the cost of the replacement, why not consider the CPC boundary mics? I've not used them but, in many BR threads, they're very well thought of as an alternative to more expensive units. From memory, you could get two entire mics for the cost of the Shure box.

[mervaka]

I think the new B91A has its electronics contained? Lists for £294 mind!

[mutley]

That is a condenser microphone so requires a preamp and a low voltage supply to the capsule's built-in electret capsule FET.

 

Basically, it is a capacitor where one plate is the "diaphragm", so sound pressure waves change the capacitance. The built-in FET amplifier translates that into conductance, which acts upon the low voltage supply to generate a low level output. This output has an extremely high impedance, which the in-line preamp module also transforms to a low impedance output (eg 600 ohms).

 

You may be able to use a standard pre-amp as long as it works with phantom power (at your own risk).

 

Personally, I would look at using a standard preamp, but make a simple resistor-capacitor filter to allow a lower voltage than the 48v phantom power into the positive terminal of the capsule. Normally even 1.5v from a single AA cell will work, with a 100k series resistor, and last for at least 10 hours continuously (depending on the capsule itself).

 

The circuit is pretty simple:

 

Use two fairly high value resistors as a voltage divider to put about 1-2 V of the 48v phantom power, onto the positive terminal of the capsule. This same terminal will then have a capacitor of between 1uF to 10uF to pin 2 of the XLR, with pin 3 being signal ground that goes straight to the capsule. (Suggested values to try first: 100k, 3.3k, 4.7uF)

[Chappie]

How about one of these?

[Mixermend]

Well found Phil - it looks as this one:

 

Canford: 20-765 4-pin Tiny QG to suit Shure and Audio-Technica - should be just the job!

 

There is a PDF document on the Canford page - check this to confirm compatibility.