Opamp Mic Pre Design (Transformer In/Out)

Hi !

Here is the schematic from the XXX desk found in the street
The parts in grey are added or modified.

http://img.photobucket.com/albums/v439/jazzy_pi/Schema.gif
http://www.groupdiy.com/index.php?topic=4112&start=0

There?s something wrong to you ? any improvements ? any non-sense ?
I think i wanna build two channels, one with the (original) melodium 1:8
And the other with a Lundahl 1538 1:5, is there a modification to do ?

Any informations are welcome !!
(impedance, gain, capacitors values, resistors values?)

Here are some photos of the trannies?
http://img.photobucket.com/albums/v439/jazzy_pi/outputtrannie.jpg
http://img.photobucket.com/albums/v439/jazzy_pi/trannies.jpg

Really Phat ? No ? :twisted:

I have make some tests with a testing board, the sound is great !!
I have enough gain to overload the A/D with the potentiometer in the middle. too much ?
Really phat & vintage sound, but I think if the treble could be a little more clear
It would be my favourite preamp. Any idea to clear it ?

Thanks ! :thumb:

I have no advice on the schem... but man... that outut tranny is huge! :twisted: :twisted:

Justin.

The first OPA op-amp has no DC path anywhere from its inverting input... you might want to fix that.

You're going to try and drive that transformer with just the second OPA? -Usually you see some more "beefy" drive capability, like complimentary a transistor pair following the op-amp, so that there's enough "brute force" to subdue and control any misbehaviour reflected back through the transformer.

Keith

:roll:

wow, this is hot technical english...
i try to understand, but if you can repeat slowly

What must i do exactly ?
1...
2...
3... ?

thanks very much :thumb:

> The first OPA op-amp has no DC path anywhere from its inverting input...

Sure it does. It is through the potentiometer.

> You're going to try and drive that transformer with just the second OPA?

Why not? The OPA604 is very strong.

For a National Network Distribution Center, I would want something stronger. But for a small studio, it should be fine.

And we won't have to fight output transformer saturation.... that output core is bigger than some loudspeaker amplifiers.

> I have enough gain to overload the A/D with the potentiometer in the middle. too much?

With the 2-gang 100K Audio pot all the way up, gain is 75dB. In the middle, about 46dB.

With dynamic mikes on small choir, I would want more than 45dB. But I don't think I have ever wanted 75dB. I once modified a mixer with 90dB total gain because I could not turn the channel and master more than 1/4 up from zero. So I wonder if you really need two op-amps, or if the first one alone would be enough. That would give 37db in the middle, 55dB maximum. Change the 1.5K resistor to 1K or 470 for a little more maximum gain.

Ok thank you for this first informations
about the Gain etc...

Anybody can tell me more about
the possible improvements of this design ?

Any idea to have a bit more treble ?

thanks :thumb:

[quote author="PRR"]Sure it does. It is through the potentiometer.[/quote]

duh. I thought all paths were cap-blocked... Don't ask me to read a map for you!

Keith

I'll argue with PRR here. Two opamps at 20db gain sound better than one at 40.

That's why I like that old design with the pot in the feedback path of both.

But, not a pot, switches.

[quote author="Dan Kennedy"]Two opamps at 20db gain sound better than one at 40.
[/quote]

For example Jensen Twin 990 versus single 990 opamp pre.

chrissugar

Ok !

thanks very much
but why don't use a pot

i have an alps 2x100k Log
and it work fine, with a good resistance
under the finger.

what the problem with the pots ?

Any others informations about the design
is welcome, i'm waiting to fix the pcb.

thanks :thumb:

ok, let's start with clear questions

- what are the first 100k & 47pf here for ?

- what is the role of the lytic & the second 47pf ?

thanks very much :thumb:

You don't have to be a genius to realize that the 100k and 47pF is there to set the best square wave response from the transformer and the electrolytic cap and 47pF forms a kind of frequency controlled feedback network to provide more LF feedback and less HF feedback to improve the transformer's HF roll off.

Analag

thanks !!

i don't know nothing in opamp design.
so i'm not really a genius :roll:

so.. if i wanna more HF
i just have to remove the
lytics and the 47pf ?

:grin:

if the feedback in the minus input
have more LF, so the output have more HF !!

right ?

if i change the values, can i have more HF ?

thanks

Exactly because the schematic shows you that the transformer is suffering from high frequency loss. So the feedback is tuned to compensate for this problem.

Analag