Transformerless input for 150ohm mics ??

[robmil]

Hi all

I'm building a variation of this:

http://www.sg-acoustics.ch/analogue_audio/microphone_preamplifiers/pdf/sassariente_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.

 

[AndrewT]

Read Jung and Whitlock and Self and Cordell.

 

[Terry McGee]

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.

eBay Australia: Buy new & used fashion, electronics & home d?r

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

 

[SY]

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.

 

[Keith Taylor]

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

 

[EssB]

It makes no sense to build that design without the transformers. Use his other design:
http://www.sg-acoustics.ch/analogue_audio/microphone_preamplifiers/pdf/monte_generoso_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.

 

[Tajzmaj]

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

 

[robmil]

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

 

[Tajzmaj]

I use BC550/560c and it's fine. And very cheap as well.
Cheers

 

[EssB]

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)

 

[Tajzmaj]

Back at home I have 3 channel preamp. Ch 1 is Neve 1073, ch 2 Neve 1083 and ch 3 M2500. All 3 nice sounding and all 3 different. 1073 and M2500 are similar maybe M2500 more detailed and 1073 more musical...it's hard to describe. 1081 is kind of more saturated similar to API...Amek is the cheapest of course and simple to build. That's why I like it.
Cheers

 

[robmil]

mmm.

I like my 1073 but need somethng that sounds more 'real' and not coloured like the 1073 etc.

I may start the 2500 today. I have loads of 4403 transistors around so might use those. I also have some 5532s and a few LT1115s lying around.

Can the LT1115 be dropped in place easily or does it need compensating?

Rob

 

[EssB]

LT1115 has same pinout as 5534 and also needs compenstation for low gain. The 5534 is already compensated in the M2500 schematic. So you could replace the 5534 with it but will it work ? probably but check for instability with a good scope.

Looks like there's an error in the M2500 shematic: shouldn't there be a cap across R19 ? HF CMR will suffer without it.

 

[Tajzmaj]

Rob you have PM.


OP amps. There might be some improvement if you stick something new and fast inside but make sure it won't oscillate. I've tried with some OPxxx units and it's fine. I don't know haw big the difference was because never tested A/B. It is probably possible to hear something. If you decide to make layout. 100n decupling caps, close as possible to both OP amps are wise idea. And it's not shown in the schematic.
Cheers