Mic output transformers in parallel.....?

Hi guys.....

Can anyone help me with the potential issues of running ribbon mic output transformers in parallel (from a single mic to a single pre-amp)?

I think that with a pair of output transformers in parallel, the output impedance will halfed and the headroom & output would be doubled......

Any thoughts on the matter would be great.....especially questions of loading!

I will try this with a pair of Lundahl sooner or later, but wanted to be aware of the possible issues......

Cheers & regards,

Andy

PS, I wonder if there is any hum-bucking potential here......?

If you want to feed one mic to two preamps, I would use a 150:150+150 transformer after the original ribbon mic transformer.

Or just use two preamps in parallel with one transformer winding, and make sure the preamps input impedancies are over 2k. There are mic preamps these days that use 6k input impedance, like some of AMEK Rupert NEVE stuff, and other transformerless designs go even higher input impedance, like 10k.

I don´t see why one mic with a 150ohms output (or lower, like many 30-50ohms ribbon mics...) wouldn´t drive two or even more of those modern mic preamps.

Wow, quick response, thanks.

But I don't want to feed one mic to two pre-amps.......what I meant was running a pair of transformers in parallel from the same mic and back into parallel feeding into the same pre-amp.....

Andy

Not quite sure what you have in mind here. Are you suggesting wiring the primaries of the mic transformers in parallel and their outputs in series?

Bear in mind the functions of the transformer are:

1. To match the impedance of the ribbon to the input impedance of the preamp. The impedance is transformed by the square of the turns ratio, so if you have a mic with a 2 Ohm output impedance and a 1:50 transformer, the mic will look like 5 KOhm to the pre-amp. If you set the pre-amp input to be 5 KOhm, i.e. matched to the transformed mic, there will be maximum power transfer and best damping factor on the ribbon, but the volts at the pre-amp (and microphone) will be half the unloaded values. If you want to match the mic to a pre-amp with a fixed input impedance, you need a transformer with the correct turns ratio. Once you have that, no adding of extra transformers will improve things.

2. (as a consequence of 1) To step-up the output voltage of the microphone. The volts are multiplied by the turns ratio.

3. To generate a balanced signal. Most ribbons are not centre-tapped, so the transformer gives a way of generating a balanced signal through centre-tapping the secondary. With care, this could be done with multiple transformers if you had free access to the windings, but you would have to think carefully about phasing.

The upshot of all this is that multiple transformers would only benefit you if (a) a single transformer was not the correct turns ratio to match your mic to your preamp or (b) you didn't care about running unmatched and simply wanted more volts at the preamp.

The disadvantages:

1. Liklihood of not presenting the ribbon with a matched load, so compromising the damping and transient performance.

2. Possible deterioration in signal quality because of non-identical characteristics of the transformers.

3. Increase in system noise due to the Johnson noise of the increased source impedance. In theory, this should be compensated by the increased gain, but in practice it rarely is.

4. Danger of increased hum pickup due to distribution of the transformer function over two (or more) physical transformers.

Does that answer your question?

Let us know how your experiments work out. You will need to listen very carefully to subtle changes in the transient response of the microphone as well as the obvious output level changes and any extra noise (and hum).

Sorry for the lengthy delay.....

In short, I am working on a ribbon mic with the potential for such a strong output that I am considering whether the headroom of a single transformer will be adequate.....hence the 'twin transformer' idea.....

This is assuming that the lundahl 'ribbon transformer' was designed with a 'normal' ribbon output in mind - which I believe it was.

I will report back when I have a chance to try it......

Andy

:shock: ribbon with very strong output? Tell us more...

I guess the lundahl will probably take lts of level. All Lundahl does.

If your mic can put out that much level, do you actually need a mic pre?


M

Pray-tell, what ribbon mic be this, noble sir?

[quote author="andy_simpson"]
In short, I am working on a ribbon mic with the potential for such a strong output that I am considering whether the headroom of a single transformer will be adequate.....hence the 'twin transformer' idea.....
[/quote]

I will be adequate.

Also, wiring two transformers for "extra" gain isn't really a good idea.

If you need a bit more gain than the LL2911 gives, then Lundahl makes a new transformer called the LL1927A that is 1:55+55 and can be used 1:55 to get higher output voltage from a given acoustic input. The transformer itself will handle all of the screaming and drum bashing you can muster at any frequency the ribbon will respond to without the need for doubling the transformer.

wow, imagine wiring it as 1:110!!! :shock:

Do you have any info on DC resistance of the secondaries?

Thanks for the replies......

I don't need any extra gain, I was just wondering about the headroom.

The mic is in development, so I can't post any details at the moment....except that it is not exactly an ordinary ribbon mic and that it is showing rather greater bandwidth than any other ribbon I know of......

Andy