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Old 12th October 2012, 08:25 PM   #1
robmil is offline robmil  United Kingdom
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Default Transformerless input for 150ohm mics ??

Hi all

I'm building a variation of this:

http://www.sg-acoustics.ch/analogue_...ariente_r1.pdf

but I'm using IC chips (LT1115) for the gain stages. The layout so far looks OK. I am on a budget so don't want to go down the input transformer route AT ALL.

Is it possisble to use a bunch of resistors and caps to get the same result as the transformer (going to use 48v phantom low impedance mics)? The transformer route is costly but I still want low noise levels, phase and frequency response using a passive/active version.

If anyone can help...point me in the right direction OR even better - has achieved what I am after PLEASE share )

regards

Rob.
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Old 13th October 2012, 11:18 AM   #2
AndrewT is offline AndrewT  Scotland
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Read Jung and Whitlock and Self and Cordell.
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Old 13th October 2012, 11:38 AM   #3
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This might be of interest. Not much in the way of specification given, but perhaps if you follow up data on the chips referred to you'd get an idea.

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You mentioned that you would be using powered microphones. You may well find that the self noise of the microphone's inbuilt preamplifiers will exceed the input noise of the preamplifier you follow them with.

Terry
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Old 13th October 2012, 11:52 AM   #4
SY is offline SY  United States
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Your friends are THAT1510/1512 and SSM2019- look at their app notes. If you want to get all the way back into the capsule electronics, read Scott Wurcer's two part article in Linear Audio.
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Old 13th October 2012, 12:20 PM   #5
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THAT Corporation Audio Technology Are you interested in balanced input and balanced output? THAT corp currently have the most advanced chips for transformerless balanced inputs and outputs and I suggest you have a good read of the literature on -

Preamplifier, THAT 1510 or 1512

Outsmarts bal line driver THAT 1646

InGenius bal line receivers THAT 1200, 1203 and 1206

Bill Whitlock, who Andrew mentioned, had a hand in the design of these devices. You can read his tutorials on the Jensen transformers site.

These devices are also discussed on the Picocompressor audio design forum.

Sorry if I seem to be mentioning one manufacturer to the exclusion of all the others, but they are currently the best products.

Keith
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Old 13th October 2012, 01:03 PM   #6
EssB is offline EssB  United Kingdom
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It makes no sense to build that design without the transformers. Use his other design:
http://www.sg-acoustics.ch/analogue_...eneroso_r1.pdf

There are similar transformerless in-amp based designs out there, I've lost all my links so you'll have to search for them. e.g.
- Greme Cohen, OM1556 - Phillips Australia 1984
- Anthony Tilbrook, AEM 6012 - 1987
- Neve design? Amek Mozart 9098
- Green mic pre.

Last edited by EssB; 13th October 2012 at 01:05 PM. Reason: unbust link
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Old 13th October 2012, 04:19 PM   #7
Tajzmaj is offline Tajzmaj  Slovenia
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Amek M2500 is also good option. It sounds very good and it's really cheap to build. PM me if you ant p-cad pcb layout.
Cheers
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Old 13th October 2012, 06:26 PM   #8
robmil is offline robmil  United Kingdom
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thanks all.

I quite like the Amek design and I think I can add phantom to it easily enough.
I might change those ZTX214s for 4403s or THAT300 transistors to make the layout easier - seems the way to go )
Essb..that design looks harder to layout but may well sound far better but will take me 3 weeks to lay the thing out

Rob
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Old 13th October 2012, 06:43 PM   #9
Tajzmaj is offline Tajzmaj  Slovenia
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I use BC550/560c and it's fine. And very cheap as well.
Cheers
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Old 13th October 2012, 07:12 PM   #10
EssB is offline EssB  United Kingdom
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Quote:
Originally Posted by robmil View Post
Essb..that design looks harder to layout but may well sound far better but will take me 3 weeks to lay the thing out
Rob
You can easily simplify it. I did a trial layout of my own version (2 channel) and it wasn't that bad. I've yet to built it though.
Of course, if If you keep simplifying it you end up with something like the Amek M2500 that Tajzmaj posted - they're all similar in-amp designs.
(Thanks Tajzmaj, I've added that one to my collection)
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