Discrete Transformerless Preamp?

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Dylan W

Well-known member
Joined
Apr 18, 2012
Messages
349
Location
Boston
Hi all,

I need 8 channels of good, relatively inexpensive mic pres for recording acoustic ensembles.

I'm kicking around the idea of cobbling together a discrete transformerless design to fulfill my requirements. This is a first attempt.

Design Goals
-28 dbm into 600 ohms without crapping out
- -129 EIN or better
-No electrolytic caps in audio path
-Low enough per/channel cost that I can build 8 and later 16 channels affordably
-Subjectively, more "musical"-sounding than the THAT 1510 pre I've built

References
http://groupdiy.com/index.php?topic=1503.msg19054#msg19054
http://groupdiy.com/index.php?topic=1503.msg19125#msg19125
http://groupdiy.com/index.php?topic=2043.msg25439#msg25439

http://www.forsselltech.com/media/attachments/JFETMP1.PDF
http://www.forsselltech.com/media/attachments/SMP2a_Manual_2.pdf

https://seventhcircleaudio.com/assets/C84/C84R20/docs/c84_sch-b209514dbb74ee164b11fd0d30a0f9b2.pdf
https://seventhcircleaudio.com/assets/T15/T15R15/docs/t15_sch-c400cbbe662173dcbe6005982cb3d4cb.pdf
 
Starting Point
-The Forssell SMP-2 seems to be a 4-op amp Cohen design with discrete JFET op amps and a discrete JFET front end.

-The now-defunct Forssell JMP-1  module was a discrete Cohen JFET front end, 3 discrete JFET op amps, and unbalanced output (presumably basically an instrumentation amp ).

-The GML pre is an instrumentation amp (no Cohen front end?) with unbalanced out.

I'll start with a 3-op amp configuration with discrete JFET front end.
 
Here's the initial schematic. I started with the Forssell "JFET MP1" linked above.

Then:
-Changed the output stage to single-ended to resemble an instrumentation amp (GML design is this way, and Forssell recommend it as well--I trust these guys have done a lot of listening in every direction, so this may be a good starting point)

-Added DC servo found on SCA T15 schematic linked above to eliminate DC across the gain switch, enabling removal of coupling capacitor. Did I do this correctly?

-Added DC servo from SCA T15 on output stage to eliminate DC offset on output

Notes
I haven't added EMI protection or phantom blocking caps yet. I'll use John Hardy 990 or Samuel Groner's SGA-HVA-1 as a starting point for DOAs.

Thoughts? Is this totally stupid?
 

Attachments

  • TFL Mic Pre v1.2.png
    TFL Mic Pre v1.2.png
    71.9 KB · Views: 176
more "musical" and "realistic"/"3D"

This is a repellent for the people you need. Sugar it with some more gearslutz name dropping and you will surely get your answer.
 
FWIW, I have a Millennia M2-B (a Forsell design) which is transformerless and has an amazing sense of depth/space and more musicality than other 'clean' preamps I've used.  Now, it does have 4 tubes per channel (which has a lot to do with the sense of depth and the musical nature) and the noise specs aren't great.  But, it's the first thing that comes to mind when someone mentions a high-end transformerless pre.
 
Kingston said:
more "musical" and "realistic"/"3D"

This is a repellent for the people you need. Sugar it with some more gearslutz name dropping and you will surely get your answer.
Click to expand...

Very much a fan of your contributions and approach. Point well taken, those are soft words

I think the important thing to get at is that this isn't about chasing 0.8db NF or lowest possible THD. There's a subjective element here that's really only going to come out through putting it to work, especially since I don't have an AP. I need something that's cheaper than a Hardy per channel, and better-sounding, subjectively, than the THAT 1510 I've built.
 
abbey road d enfer said:
No phantom power?
Click to expand...

Haven't yet added phantom or EMI stuff to the schem. It will have phantom, common mode choke, and coupling caps.

Thought about trying Wayne Kirkwood's flying rails idea to eliminate coupling caps at the front end, but that may be too much to get working for the first revision.
 
Samuel Groner has a lot of interesting stuff on his website:

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

He also publishes the gerbers for 'Monte Generoso' which meets pretty much all your requirements.
Designs' C and F might also be interesting to you
 
Dylan W said:
abbey road d enfer said:
No phantom power?
Click to expand...

Haven't yet added phantom or EMI stuff to the schem. It will have phantom, common mode choke, and coupling caps.
Click to expand...
You lost me at subjective, but got me back at flying rails. I did a lot of late night scribbling about that.
Thought about trying Wayne Kirkwood's flying rails idea to eliminate coupling caps at the front end, but that may be too much to get working for the first revision.
Click to expand...
In a raise the bridge or lower the water exercise, it might be simpler to leave the mic preamp grounded and tweak the phantom power.  Namely drive the phantom power resistors with modest voltage, then drive pin 1 to a negative voltage using a servo to keep inputs at 0V.

Like I mentioned I have been thinking about this for decades and at the end of the day it is mostly a marketing exercise to satisfy capacitor haters. Modern caps are certainly not a sound problem, but there is something sexy about saying no caps in the path.

I suspect any cap in the gain leg will be working harder so more suspicious. Phantom blocking caps need to be low impedance for input noise considerations while the 2k input termination can be before those caps so they don't have to pass much current and/or exhibit an inband pole.

I wondered why tweaky preamps don't use a switch to just bypass the phantom caps when phantom power is not needed.

After delousing all the caps from the mic preamp, next we need to look inside the microphones at their components.

Perfection is hard work... An exotic marketing hook, but impractical for mass market products.

JR
 
JohnRoberts said:
Dylan W said:
abbey road d enfer said:
No phantom power?
Click to expand...

Haven't yet added phantom or EMI stuff to the schem. It will have phantom, common mode choke, and coupling caps.
Click to expand...
You lost me at subjective, but got me back at flying rails. I did a lot of late night scribbling about that.
Thought about trying Wayne Kirkwood's flying rails idea to eliminate coupling caps at the front end, but that may be too much to get working for the first revision.
Click to expand...
In a raise the bridge or lower the water exercise, it might be simpler to leave the mic preamp grounded and tweak the phantom power.  Namely drive the phantom power resistors with modest voltage, then drive pin 1 to a negative voltage using a servo to keep inputs at 0V.

Like I mentioned I have been thinking about this for decades and at the end of the day it is mostly a marketing exercise to satisfy capacitor haters. Modern caps are certainly not a sound problem, but there is something sexy about saying no caps in the path.

I suspect any cap in the gain leg will be working harder so more suspicious. Phantom blocking caps need to be low impedance for input noise considerations while the 2k input termination can be before those caps so they don't have to pass much current and/or exhibit an inband pole.

I wondered why tweaky preamps don't use a switch to just bypass the phantom caps when phantom power is not needed.

After delousing all the caps from the mic preamp, next we need to look inside the microphones at their components.

Perfection is hard work... An exotic marketing hook, but impractical for mass market products.

JR
Click to expand...

Forgetting about "subjective" for the moment, is the servo IC2G$2 in my attached schematic an OK way to get rid of the big electrolytic in the gain leg?

Samuel's Monte Generoso gets around the cap by using high-precisions ICs and an offset trim, right?

http://www.sg-acoustics.ch/analogue_audio/microphone_preamplifiers/pdf/monte_generoso_r1.pdf
 
Dylan W said:
Forgetting about "subjective" for the moment, is the servo IC2G$2 in my attached schematic an OK way to get rid of the big electrolytic in the gain leg?
Click to expand...
It looks like a good start but build it and see how it works.  I like that it is just servoing the voltage across the pot, rather than servoing the voltage after the gain stage that has that forward gain in the path, making it more twitchy, but may not be completely silent.

Wayne has done a lot of bench work on those kind of servos so check his work.
Samuel's Monte Generoso gets around the cap by using high-precisions ICs and an offset trim, right?

http://www.sg-acoustics.ch/analogue_audio/microphone_preamplifiers/pdf/monte_generoso_r1.pdf
Click to expand...
Yes, he uses precision matched bipolar input devices, and a trim.

JR
 
Nobody mentioned, I don't know the properties of those particular transistors at the input but usually JFETs have rather high optimal input impedance speaking about noise, Usually BJT is much lower, even with higher current noise, at 2KΩ the Ein is usually better. Using 2 in parallel is a good start, make a measurement to see how it goes.

For the input termination, instead of using two 22k resistors is usually better for CMRR using two 1k resistors and then one 10k or 22k making a T between the two inputs and ground. Usually not applicable if you want to put your termination before the decoupling caps, since this network is being used to polarize the input transistors but you already have the 100k resistors for that and for JFETs that should be enough. At this point maybe using two 1k resistors at the phantom and a 2k9? maybe 3k (easier to get I guess) for the phantom in the T configuration I said may be better. The matching between the resistors became less critical in the way for a good CMRR, you could do the math I guess...

JS
 
> those particular transistors at the input but usually JFETs have rather high optimal input impedance speaking about noise

Look them up.

Yes, generally a JFET is not your first choice for lo-hiss mike impedance. But these get within around 1dB of perfection. Unless he thinks the noise figure is sexy, these are inoffensive.

I'm not even sure there is point in paralleling them as shown. But I think he has to find his own path (remembering there is supposed to be music at the end!), and the cost is small.
 
Ok, it did worth looking at the datasheet I guess... I didn't know there where such low voltage noise JFETs there, maybe they are already a bunch in parallel in that die...

JS
 
joaquins said:
Ok, it did worth looking at the datasheet I guess... I didn't know there where such low voltage noise JFETs there, maybe they are already a bunch in parallel in that die...

JS
Click to expand...
There are some very respectable modern JFETs but not sure it's worth the bother (cost) for mic impedances.

JR
 

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