Discrete LineAmp/MicAmp designs that works on +/-12v?

[Ted Krotkiewski]

Hello Friends.

Does anyone know of quality discrete transistor based line amp/mic amp designs that can run happily on 12v?
I want to experiment and look for alternatives to replace IC´s in the Eurorack format that runs on +/-12v.

I am currently experimenting with the Helios 2128 amp card, wich I love and is simple enough for me to put together, but it does not work on 12v. I would love to have something similar for my Eurorack builds.

I´ve read that a 990 opamp can run from +/-12v for example, though, that OpAmp is a wee too clean for my taste. (or at least the John Hardy M1 preamp is.)

Any suggestions for available designs out there that can run on 12v?
It could as well be a mix of IC opamps and transistors.

Thank you for your time! 
//Ted

 

[abbey road d enfer]

12V or +/-12V? It makes a huge difference.
Most circuits that are designed for +/-15 are happy with +/-12, with only a couple of dB less headroom.
Single +12 requires an optimized topology where transistor Vcesat does not tax headroom unduly.
Many circuits designed for +24 can be modded for +12, but the headroom limitation will still be there.
Now you can do worse than checking Nagra schematics.

 

[Ted Krotkiewski]

12V or +/-12V? It makes a huge difference.
Most circuits that are designed for +/-15 are happy with +/-12, with only a couple of dB less headroom.
Single +12 requires an optimized topology where transistor Vcesat does not tax headroom unduly.
Many circuits designed for +24 can be modded for +12, but the headroom limitation will still be there.
Now you can do worse than checking Nagra schematics.

Thank you for your answer!
Yes, 12v VS +/-12v is totally different. I just meant, throw anything at me. Since Eurorack PSU can can deliver either one rail of 12v or dual +/-12v I am interested in any known circuits that are considered "good" to work on eiter single or dual.

RE modding a +24v design into +12v. It would be truly amazing to learn how to do mod a 12v version of the Helios 2134 circuit that I am currently playing around with. or why not a BA283 for that matter.
Perhaps this requires a thread of it´s own, depending on how complex this is?

Is it a matter of changing/adding a few components or is a much more complex road to enter?

With my 0% knowledge I immediately start to wonder: Is there not a way to take advantage of the -12v rail, even though its a single + rail design. Perhaps convert that to bump up the voltage of the +12v single rail somehow?
oh, shut up.. I shouldnt be guessing things I do not know of.  :

Thank you for taking your time.
//Ted

 

[dbelousov]

CFIA, wildly known as "Cohen preamp", despite the fact that this approach was invented significantly earlier, is one of the best solutions available. It's semi-discrete, though, and very "clear". It will be happy with +-12V.

 

[Bo Deadly]

CFIA, wildly known as "Cohen preamp", despite the fact that this approach was invented significantly earlier, is one of the best solutions available. It's semi-discrete, though, and very "clear". It will be happy with +-12V.

Wrapping transistors in op amps is not at all what I would call "discrete". He said he didn't want transparent. Most of the descrete designs are single supply. But with a little extra care and circuit tweaking I see no reason why you couldn't use -12V as 0V and then use any of the usual 24V single supply circuits. OTOH if you really want some dirt, adjust for 12V single supply and deliberately push it.

 

[mjrippe]

Most of the descrete designs are single supply.

I would argue that there are two families of discrete opamp designs, the single rail and the dual rail type.  While many earlier circuits (RCA, Altec, Langevin, Neve, Neumann, etc.) used single +24v, the later types (Melcor, API, Hardy, anything that fits a 2520 socket) all used bipolar supplies.  And yes, with some cleverness or brute force you can run either type on either style of supply.  Look up the Urei 1109 preamp which has options for +24-30v or +/-15v.

 

[Ted Krotkiewski]

Thank you everyone for excellent info. I am learning a lot.

Many circuits designed for +24 can be modded for +12, but the headroom limitation will still be there.

I simply have to just try hooking up +12v to the circuits and see what happends. My guess is that it won´t work just like that. (I cant try it out for yet another week.)

If there are some obvious steps to adress that comes to mind when modding a +24v design  to run on +12v, I would be very grateful to hear your thoughts on where to start and what areas to first look at in the circuit.

I am attaching a Helios Type69 schematic made by @Dogears here on the forum. If we only zoom in on the 2128 Output amp, is there something obvious that needs to be adressed for +12v operation?

I would love to have this design or similar designs run on +12v to make up the insertion loss from the EQ rather than a IC gain buffer.

Thank you for your time.
//Ted Krotkiewski

 

[merlin]

Neve 1073 pre section works fine off a 12V supply, but the output section will only put a maximum of 20dBu into 600 ohms (optimum output section bias is about 40mA at 12V). I've never tried the EQ section but I expect it would also be OK at 12V, headroom notwithstanding.

 

[Jarno]

I built simple a simple 325 type preamp for eurorack and  use my own 2520 clone in that, works fine. I also have a clone of the Melcor 1731 which I like better, and the 990, but as yourself, I think that is too clean for my taste.
I added a FET instrument input which runs on a single ended supply, and left the possibility to add a SMPS PC mount supply to get the 12v to 24v. That's an option as well, given that you are not looking for a crystal clear high end, presumably, you could lop the top off to get rid of the noise.