GAIN Mic Pre / Makeup Amp Kit

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GAIN Build Thread

Post by Joe Malone »

GAIN Build Thread
Gain PCB is a gain makeup up amp using a single DOA opamp and output transformer or DIP8 dual opamp for electronic balanced out.
Gain PCB runs on a single +V power rail and makes the opamp run in a vintage 1/2v rail configuration.
Example if running a 2520 opamp on +/-16v = 32vdc total. Set the 10k trimpot for 32v from the BD681 reg at +V opamp socket.

GAIN PCB is a cut-down version of our BA Mic pre PCB leaving off the 48v, pad, phase switches and input transformer
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Joe :-)
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Re: GAIN Mic Pre / Makeup Amp Kit

Post by Joe Malone »

fragletrollet wrote:
Sun Oct 31, 2021 6:12 am
Hello!

See just started above Build thread with schematic above

Any schematic for this one? Is it the RGain or 33R resistor that's supposed to be 1/2w?
33R which drives the output transformer.
Value of CL and CZ? Are RZobel resistor only needed if using an input transformer?
Yes correct
As this board makes a bipolar supply from a 48V source, can I just solder wires to the opamp power pins as to further use the bipolar supply for other boards?

No not easily as it is setup like a vintage gain amp to run on a single +V. Use our Dingo kit or PCB if you want to run on +/-V
Planning to use as makeup gain for some passive pultec boards. They need a high impedance load (at least 470k) and apprx 20-23 db gain. Is the RLoad where I would place my 470k?
Yes but with that high impedance you have to watch the output offset of the opamp due to the input transistor current. I don't know of any passive eq that needs that high impedance as they were all made low impedance to keep noise low.
Just install a trimmer for RGain to trim for unity? Recommended value, 5-10k?
Either 5k or 10k will fine.
Joe :-)
JLM Audio
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fragletrollet
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Re: GAIN Mic Pre / Makeup Amp Kit

Post by fragletrollet »

Thanks for the feedback.

I have various passive EQ's from Ian @ customtubeconsoles (Helios & Pultec), which are re-designs based on the named units. 470K input impedance is specified in all the gain makeup amps, although differing in how much gain they need. So you're saying other/the original passive designs use low impedance for the makeup amp input? I'm pretty confident in Ian's design choices in regards to these circuits, so a little surprised :shock:

Is the JLM Gain pcb running on a reference of 24V, for +/- 24V = 0-48V? So it works fine cascading more Gain pcb's but if you want to add a board that uses the conventional +/- 24V (-24v/0v/24V) this can't be done as the reference is shifted between the two?

I want to add a simplified passive eq (say just two bands, high and low shelf) to my new tube compressor build, and was hoping to add a m/s board using THAT chips, all running from a SMPS instead of a toroid. So I guess I'll have to use a bipolar SMPS like this in addition to the 48V SMPS I allready have installed in anticipation of the GAIN pcb's.

So, how do I "watch the output offset" and make sure it works nicely with such a high input impedance?

And, BOTH 33R resistors should be 1/2W then I presume, both being opamp outputs? The marking on the pcb is a little confusing.

Thanks again and all the best,

Magz

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Re: GAIN Mic Pre / Makeup Amp Kit

Post by Joe Malone »

fragletrollet wrote:
Thu Nov 04, 2021 4:22 pm
Thanks for the feedback.

I have various passive EQ's from Ian @ customtubeconsoles (Helios & Pultec), which are re-designs based on the named units. 470K input impedance is specified in all the gain makeup amps, although differing in how much gain they need. So you're saying other/the original passive designs use low impedance for the makeup amp input? I'm pretty confident in Ian's design choices in regards to these circuits, so a little surprised :shock:
Pultec EQ are actually fairly low output impedance which usually feeds an audio transformer before the high input impedance of the tube. But since you give me no schematic to go on I cannot advise as I do not know if you are including this transformer or not or which circuit we are actually talking about. But you can make RLoad 470k and see if the output of the opamp you choose to use is sitting at 22v to 26vdc as then it is fine. But most bipolar input opamps draw input current so the higher Rload the most the offset from center on output. If there is a transformer and it is floating secondary into the gain then don't use a blocking cap Cdc and the resistance of 470K will mean nothing as the transformer winding resistance becomes the actual input resistance and little if any offset will happen. Or use a fet input opamp so it has a similar input impedance with no current like a tube.
Is the JLM Gain pcb running on a reference of 24V, for +/- 24V = 0-48V? So it works fine cascading more Gain pcb's but if you want to add a board that uses the conventional +/- 24V (-24v/0v/24V) this can't be done as the reference is shifted between the two?
Yes. If you want to do the same as the gain but with +/-V rails then use our dINgO kit which is the same but runs on +/-V.
I want to add a simplified passive eq (say just two bands, high and low shelf) to my new tube compressor build, and was hoping to add a m/s board using THAT chips, all running from a SMPS instead of a toroid. So I guess I'll have to use a bipolar SMPS like this in addition to the 48V SMPS I allready have installed in anticipation of the GAIN pcb's.
I Have run many THatcorp the same way as the GAIN you just have to understand and design for -V to be 0v instead.
So, how do I "watch the output offset" and make sure it works nicely with such a high input impedance?
See above in this reply
And, Both 33R resistors should be 1/2W then I presume, both being opamp outputs? The marking on the pcb is a little confusing.
The other 33R is only if you use a dual DIP8 opamp so running electronic balanced out so both 33R can be anything as they will not see a low impedance output transformer primary.

It is always easier to email me a rough overall schematic first so I can advice the best way to go about making it work as I can always simply most designs and save money for you.
Joe :-)
JLM Audio
Capturing Audio without Injury

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