Old: A Direct-Coupled Input-Capacitorless Active Mic Preamp

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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

ilya -

Let me look this over but it looks close. The one thing I do see is that the servo injection resistors, R1 and R2 need to be a lot lower in value to correct the two major sources of offset. Remember that the servo circuit normally works against an input coupling capacitor. In this circuit it's working against R8||Rsource and a significant amount gets converted from differential to common mode by the low differential resistance.

I would also ground the phantom switch when off rather than have the 100k do it. This keeps the resistances that the servo works against constant.

C10 and C11 should go to "real" ground. C10 since it offers the lowest impedance at RF and C11 since it's detecting the common mode potential relative to actual ground. Be sure to add a couple of diode rail clamps at the junction of C12 and R14.

I would experiment with values of R1 as low as 10k since a lot of DC offset may come from the microphone rather than the 1510/1512. I would make R2 a jumper to defeat the servo for adjustment. If R1 is made 10k then an additional resistance of 10K should connect to FLY_GND. This keeps the resistive load balanced per leg. Another option is a differential output servo. You can invert the OP07 output with a -1 and then feed that output through a 10k to the opposing input. This doubles the ability of the servo to correct error. Take a look at the 1570/5171 servo connections in the 5171 datasheet.

bruno2000 - Yes, the pictures... :D Thanks for building the servo circuit up and the rack of preamps. Have you tried a flying rail build yet?
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by ilya »

Wayne,

Thank you very much for the comments. I'll make the corrections you pointed out.
Meanwhile, here's the quick preview of the PSU board. I think it's tougher than the preamp itself...

Image
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

On the PSU I suggested originally I would change the pre-loading resistors (on the output) so that they go from each output to ground rather than the supply below them. The values and wattages will need to be resized. The objective is to meet the minimum Iout requirements of the regulators. So the 72V preload should not connect to +48V. The 72V should go to ground, the 48V to ground etc.

I built it the original way and it worked fine but I think it's bad practice.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by ilya »

Wayne, thanks for pointing that out.
I've checked the specs for regulators and they show a typical load of 3.5mA for regulation. I've adjusted the circuit as follows:
Image

Here's the PCB layout if anybody's interested ;)
http://www.dontsov.ru/DIY/DCPreamp/DC_P ... PCB_V2.pdf

I've got no time to go through the preamp circuit itself yet, was busy with cleaning up the PSU section...
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

Thanks Ilya! The layout is nice and tight.

Yes, moving the resistors to ground is the way to go. It worked the other way but there may be startup issues with some regs. I used the stacked supply approach so we could use conventional transformers and regs without resorting to HV windings, TL783s etc.

Wayne
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by ilya »

Wayne, thanks for comments.

I've uploaded an updated circuit with a differential servo. The image is too big to post in the forum, so I linked to the PDF instead:

http://www.dontsov.ru/DIY/DCPreamp/Main ... ircuit.pdf

It just occured to me, that maybe I should take the signal to the inverter AFTER the R10, not before? How do the servo resistor values look now?
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

Since the servo has no gone fully diff maybe it's better to just link open R9 and R14 to defeat it during low gain Vos adjustment. You could eliminate R10 that way too.

The values of R9 and R14 look about right based on my prototype. I made some assumptions about how much DC offset mics might produce. In practice, once experience is gained with a large number of mics, they might be made larger. R9, R1||Rsource and R14 do form a voltage divider which affects servo gain. It also helps reduce the noise contributions of U2 and U3.

Be sure to add those rail clamps at the node of C6 and R17.

If you want an exact 2K input impedance you might raise R1 slightly since it's in parallel with R3 + R4.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by ilya »

Sorry for the delay. I've got sidetracked by my CNC machine project...

Here we go: http://www.dontsov.ru/DIY/DCPreamp/Main ... ircuit.pdf

Changes:
Removed R10
Adjusted jumpers
Adusted impedance resistor


I'll add protection diode clamps in the following circuit section (which I yet have to draw :geek: )
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

Ilya -

Thanks for sending the new pdf!
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by ilya »

Wayne,

I've got some question regarding the second part of the circuit:

1. There's a 1M resistor right after the 1uF/250V cap. In the other circuit it is 100K instead. What value is correct?
2. In the filter section there are two optional caps. If they have to be omitted, should they be short-circuited?
3. I've noticed that the trim stage is inverting. Does it mean we get the inverted signal at the output (I don't see any other inverting stages)?

I was looking for a phantom power switches, and most miniature switches are rated for 5A @ 120V AC or 28V DC. I wonder if it's safe to use them @ 48V and small phantom current?
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