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miallen 17th October 2012 11:37 PM

INA134 vs Transformer?
What would be quieter - INA134 or a non-super-expensive transformer like Hammond 109T or Triad Magnetics SP66 with shield (or some other transformer - I know nothing about them)?

This would be for the XLR input of a high-gain guitar preamp so my focus is noise and not frequency response or THD. If the difference in noise floor is 10db, that is going to be amplified 10000x so I'm wondering if a transformer would actually be noticeably quieter in this scenario. Or would the EMF pickup actually contribute more noise than the INA134 input resistors?

Any help would be appreciated.


miallen 18th October 2012 09:35 PM

To answer my own question - from reading Small Signal Audio Design by Douglas Self page 345 (and section 14.1), it sounds like INA134 would actually add significant noise. So I bought some old BeyerDynamics 1:1 600 metal can transformers for about $25 USD each. Now I just have to figure out the best way to wire them up.

SY 18th October 2012 09:39 PM

Why not use a step up transformer for even lower noise? With the high and reactive source impedance of a guitar pickup, you might get an interesting sound.

miallen 19th October 2012 02:09 AM

I'm not running guitar direct into the XLR. The source is line-level so the input circuit should be like that of a standard mixer desk.

There will be an alternate high impedance input to directly connect a guitar that will also disconnect the balanced input.

SY 19th October 2012 09:58 AM

In that case, noise won't be much of an issue for the IA.

miallen 19th October 2012 10:59 AM

Even after amplifying it 10,000 times? In a high gain guitar preamp there is noise. That is unavoidable. The question is, will I hear less noise using a studio grade input transformer or the INA134?

I suppose the only way to really know is to test. I have the chip. When I get the transformers I'll do a comparison.

SY 19th October 2012 11:07 AM

I thought you said "line level." Now I'm totally confused.

miallen 19th October 2012 02:32 PM

I'm taking a line level signal from another piece of equipment like a compressor or some effect or whatever and then running that over a balanced line into a high gain preamp unit I'm building. That line-level input would come in through the transformer, buffered and fed into the high gain preamp. The signal gets amplified by 3 tube stages with a gain of about 20 or so each which is 20 x 20 x 20 or about 8000 times. Then it get's attenuated and routed back out at line level using DRV135.

The point is to add tube character, overdrive or outright distortion to the signal. Two channels for a stereo effect. And there will be a phase switch on one channel.

SY 19th October 2012 02:46 PM

Well, if your signal is line level, it's on the order of 1V. So your unit will try to put out 8000V, but in reality will clip heavily at any signal more than a millivolt or so.

miallen 19th October 2012 11:52 PM

Right. High gain guitar amps clip. Actually there are frequency dependent RC networks and attenuation that goes on between the stages so it doesn't clip 8000 x. But it does clip or at least compress a lot. That's the point.

But it doesn't clip noise. It just amplifies the hell out of it. So I'm assuming that any reduction in noise at the input will be noticeable. Thus the question.

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