Mic Pre Design With Lundahl In/Out And Four DOAs

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Samuel Groner

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Joined
Aug 19, 2004
Messages
2,940
Location
Zürich, Switzerland
Hi

As a first project for my discrete opamp ([removed]) I designed a mic preamp: [removed]

I'm looking for a slightly coloured, yet versatile preamp. It was important to me to have similar sound at different gains, hence the two gain stages.

The two gain stages are adjusted such that the first has twice the gain of the second (e.g. 20 dB and 10 dB) and from 6 dB to 60 dB overall. The transformer balanced output and the input transformer each add 6 dB to this.

The values of R103-R105, R109 and R111 are not final yet.

The transformer balanced output is taken from the LL1539 datasheet (1539.pdf).

The input bias cancellation of the first opamp is somewhat overkill but it reduces DC current through the transformer secondaries and perhaps clicks during gain changes as well.

I opted for on-board regulators as the PSRR of the used opamps is not as good as we are used to from ICs and I want it dead silent.

Any comments, suggestions, drawing errors?

Thanks!
Samuel
 
[quote author="Samuel Groner"]
Any comments, suggestions, drawing errors?
[/quote]

Hello Samuel,

Nice design ! Lots of opamps but I understand.
One question about the presence of U104A though:
Just wondering where the colouration will now be coming from. Since as I understood that output-circuit reduces outp-TX influences.

I'm looking for a slightly coloured, yet versatile preamp.

The transformer balanced output is taken from the LL1539 datasheet

Application example: This schema shows the principles of mixed feedback circuitry for eliminating transformerinduced
distortion
and for reducing output impedance

Bye,

Peter


PS
Thanks for the component-info & suggestions you added to the Buy-DOA-PCBs thread ! :thumb:
 
Another just curious, this one w.r.t. the bias-current compensation resistors:

why is R118 6k8, so unlike R121 ? Typo or on purpose ?
R121 has the 'correct' value 3k3, but can't see yet that the used output arrangement changes the requirement for the value of R118.

Bye,

Peter
 
[quote author="Samuel Groner"]Any comments, suggestions, [/quote]

Me again, entering FWIW-suggestions-territory:

How about some fun with providing DC-biasing for the various electrolytics ?
And for those 6800uF caps, how about connecting these to a clean DC-shifted ground ?
 
Thanks for your answer!

Lots of opamps but I understand.
I would have chosen a simpler transformer balanced output if I didn't had these LL1539 laying around. And the datasheet suggests to use them this way only, so I arrived where I am.

Just wondering where the colouration will now be coming from.
I agree with you that the transformer balanced output will be rather transparent. It wasn't my intention to provide two outputs with different colours, rather a second output in case I want to split things (live recording etc.). However, there is still the input transformer giving enough distortion and the DOAs are surely not the ultimate in linearity (though they are not that bad either).

Why is R118 6k8?
Because of C110, the only DC path to U103 inverting input is through R116.

How about some fun with providing DC-biasing for the various electrolytics? And for those 6800 uF caps, how about connecting these to a clean DC-shifted ground?
Actually I'd prefer biased 'lytics, but I decided to give the idea up because of complexity. If I'd want to bias the 2x6800 uF for the first gain stage I'd need 8x6800 uF for the same low-frequency performance. That's too much for my taste... Another solution would be to increase the feedback network impedance, but this would increase noise as well. :roll: Not easy, all these trade-offs...

Samuel
 
[quote author="Samuel Groner"]Thanks for your answer!

Lots of opamps but I understand.
I would have chosen a simpler transformer balanced output if I didn't had these LL1539 laying around. And the datasheet suggests to use them this way only, so I arrived where I am.[/quote]
I see, hmm, a pity if that TX is not to be used otherwise.
Why is R118 6k8?
Because of C110, the only DC path to U103 inverting input is through R116.
Oops ! :oops: :wink: Silly me.

How about some fun with providing DC-biasing for the various electrolytics? And for those 6800 uF caps, how about connecting these to a clean DC-shifted ground?
Actually I'd prefer biased 'lytics, but I decided to give the idea up because of complexity. If I'd want to bias the 2x6800 uF for the first gain stage I'd need 8x6800 uF for the same low-frequency performance. That's too much for my taste... Another solution would be to increase the feedback network impedance, but this would increase noise as well. :roll: Not easy, all these trade-offs...

Samuel

I see, way too much caps then... the space required would be enough for servoing the whole universe.

I'm wondering if still some corners could be cut though, with say an unity gain stage that's buffering a clean off-centre supply tap - to which those big caps could be connected. It'll be at the expensive of complexity, but no big parts.
But OK, might perhaps then directly throw these 6800's out & servo the whole thing :?

Last attempt :wink: if you're willing to accept different headroom in + & - direction then some other things could be done.

Bye,

Peter
 
Since D1 and D2 are going to be essentially across the input xfmr, are you concerned that they will begin to act as hard-clippers when the audio voltage gets above.6v ?
 
I guess the diodes in your opamp itself. But hey, if they're fine in a 5532 they won't do harm here either :thumb:
I don't think they're across the inp-TX-sec
 
Opps, sorry I looked at the opamp schematic itself. the first GIF.
Well, if they don't clip in a 5532, then they wont here.
It just looked like my first fuzz-pedal design.
 
Due to the action of the feedback/open loop gain the voltage difference at the two inputs is very low under normal circumstances (one of the properties of the ideal opamp). The diodes protect the input transistors from reverse biasing which could result in increased noise or even failure.

Samuel
 
Why the pushpull output transformer drive scheme. I think it looks better than how it's going to perform.
 
This very same schematics were used in calrec and NTP mixers and standalone units, and that´s what Lundahl recommends for best ransformer-coupled output performance... It sure is very low distortion. I´ve never heard one working thought...
 

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