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Brouilly 11th March 2009 08:00 PM

My unbalanced preamp problem
Hello all,

I am currently building my first ever project, it is a valve microphone preamp based on the Brimar SP-55 design. The problem I am facing is that the circuit is not balanced.
Am i right in saying that if i am to use a good quality microphone with this circuit I will need to unbalance the signal going into the circuit, and then balance the signal going out of the circuit? The only thing I can think of is to have a simple op-amp circuit at either end to do this.

I am afraid it is too late in the day to change the preamp circuit to a more suitable one because it is for my dissertation.....

If anyone out there has any suggestions or help that would be greatly appreciated.



grhughes 12th March 2009 12:36 AM

Most mic preamps are not balanced or push pull
The easiest way to convert from balanced to unbalanced and vice versa is with an audio transformer. Most professional mic preamps have an input and an output transformer that performs this function but most of the internal circuitry is unbalanced for noise considerations. A balanced all the way through circuit can be more noisy.

Eli Duttman 12th March 2009 02:00 AM


You have a terrific resouce for both magnetics and technical assistance, right in the UK. Sowter, a firm with many decades in the business, offers some highly suitable pro audio trafos. In addition, Brian Sowter [], an heir to the firm's founder, has proved quite helpful about technical matters, many times over.

Brouilly 12th March 2009 02:03 PM

Thanks guys, youve been very helpful



grhughes 12th March 2009 03:39 PM

If you're gona use op amps then why use a tube at all?
There are many discreet transistor designs augmented with opamp mic preamps on the web. But they invaribly involve a lot of cheap electrolytic caps in the signal path. Personally I would rather listen to a "good" transformer than cheap electrolytic caps. Brian Sowter at Sowter can help you alot. He makes replacement transformers for all the antique and highly sought after mic preamps. And most of the sought after ones are NOT solid state! Ray Hughes

ruffrecords 19th April 2009 12:20 PM

I would just like to add my vote for Sowter. Brian Sowter is very approachable and very knowledgeable. There are several 'standard' tube mic transformers of theirs you could use. I use them in my own tube mic pre designs.

By the way, I searched the net for a circuit of the Brimar SP-55 preamp but could not find it anywhere. Do you have a link to it or some such??



Brouilly 19th April 2009 02:09 PM

Hi Ian,

I have been in contact with Brian Sowter, he has been more than helpful. He suggested some really nice 4:1 mic transformers for my circuit.

With regard to the Brimar, it is found in 'Valve and Transistor Audio Amplifiers' by John Hood. Its one half of a stereo preamp, which I have tried to adapt into a microphone preamp for my dissertation. Its not really suitable though, I wish I researched more into different schematics.

I have built it now, but I am planning a rebuild when i finish uni. Mainly simplifying the circuit, and adding a cathode follower. I can post a schematic if you want to see?

You are right tho, Sowter are very helpful and I also would recommend them to anyone. I personally just dont want to buy the transformers until I have a preamp circuit that deserves them.



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