Two channel portable balanced mic preamp

[carlmart]

Transformers are a total no-no for this project. I can assemble 4 transformeless-channels for the price of one transformer.

 

[Bill Wilson]

Look at the Rolls 4 input mixers w vu meters. They publish their schematics. The vactrol is in the circuit following the summing amp.
I would recommend using That 1510 or 1512, in place of the 2017 or at least use AD2019 as mike pre.

 

[carlmart]

Bill,

Can you specify which Rolls model?

 

[carlmart]

I did find the circuit.

what I was told is that the Vactrol is being reactivated for having contaminating minerals inside.

 

[ricardo]

I did find the circuit.

Can you post the link to this Rolls?

 

[carlmart]

Sorry to have taken so long. Here's the link:

http://www.rolls.com/pdf/M_MX422.pdf

 

[ricardo]

http://www.rolls.com/pdf/M_MX422.pdf

Ha!  The Rolls has 2x10k in series with the mike input.  If you are happy with these large evil noisy resistors, you can certainly use a Vactrol on your input to good effect.  ;D
_____________________

They have their limiter after the mike preamp.  The secret to making this work is not to have too much gain in your mike preamp stage so it never overloads.  The subsequent Vactrol limiter stage stops the output overloading.

 

[carlmart]

Well, maybe that's the secret to make this work, but I'm not sure that's the secret for better quality audio, is it? Two 10K in series?

You have seen the final design I will be using on my mic preamp, and I'm not sure if a limiter on the second stage wouldn't be any useful.

Unfortunately I can't find a limiter design that could work with the first stage I'm using.

But the two IC mic preamp they are using made me curious to have another balanced design I tried once which sounded quite well. But I can't remember how the gain was handled.

Vactrols AFAIK have to be in parallel with main gain stage, and it's that way that is used on all transformer mic preamps I have seen.

From my talks with THAT, putting an LDR in parallel with the RG pins wouldn't work and would make things worst.

BTW, I owe this forum showing what mic preamp I plan to use for now.

 

[Harpo]

Maybe revisit your dB numbers on your gain switch (they don't compute to 49, 30, 10dB) or change your resulting Rg numbers.
With a THAT1510, left position/highest gain setting is about 59.4dB, center position/lowest gain is 26.9dB, right position/mid gain is 40dB.
With a THAT1512, left position/highest gain setting is about 53.4dB, center position/lowest gain is 20.8dB, right position/mid gain is 34dB.
The single pole on/off/on toggle switch center position will always be the lowest gain setting. Using a rotary switch for an increasing gain setting seems more intuitive. YMMV.

 

[carlmart]

You're absolutely right, and please forgive my distraction.

Those gains were for the +/-9v supply I had used on the original project.

Please have a look at the original preamp I had released in 2000.

http://www.preciseaudio.com/brochure.html

As you can see, Low gain was in the middle.

 

[ricardo]

Well, maybe that's the secret to make this work, but I'm not sure that's the secret for better quality audio, is it? Two 10K in series?
......
From my talks with THAT, putting an LDR in parallel with the RG pins wouldn't work and would make things worst.

I was referring to making a Limiter stage work after the mike preamp.  There are loadsa reasons why this is sensible.  I'll just include one more.

A limiter stage needs to have a rough idea of what signal levels to expect. or at least what the limited signal will be.
______________________

A LDR like the Vactrols goes to LoR very quickly but is VERY slow to recover to HiR.  That's why in Limiters, they are either the bottom leg of an attenuator or across the Feedback resistor.  Across RG is the worst place for them.
______________________

There's component values in your circuit that Wayne in proaudiodesignforum has already commented on which I won't repeat.

 

[carlmart]

Well, I guess you mean the limiter has to be connected with the output of the stage to be limited, and act fast enough before it becomes a problem.

I can't imagine any other way for the limiter to have an idea on what to expect. If it acts fast, you may be able to disguise the clip in editing somehow.

And the decay has to be so as to be as invisible as possible.

What I do find a good option on 4-channel recordings is to have one or two channels attenuated on situations where there might be unexpected shouts or loud noises.

 

[carlmart]

A LDR like the Vactrols goes to LoR very quickly but is VERY slow to recover to HiR.  That's why in Limiters, they are either the bottom leg of an attenuator or across the Feedback resistor.  Across RG is the worst place for them.

If I understand the principle of the LDR, it's a cell with a resistor that is sensitive to light, and resistance gets lower (10Kor less) as it gets more light. Being in parallel with the high leg of the gain chain feedback, it lowers the gain of that stage and so lowers the level. Then goes back to its high value (200K or more), taking some time do so.

There's component values in your circuit that Wayne in proaudiodesignforum has already commented on which I won't repeat.

You mean the resistors in the gain switch? Yes, I will correct them. Particularly the one for low gain.

 

[carlmart]

A LDR like the Vactrols goes to LoR very quickly but is VERY slow to recover to HiR.  That's why in Limiters, they are either the bottom leg of an attenuator or across the Feedback resistor.  Across RG is the worst place for them.

If I understand the principle of the LDR, it's a cell with a resistor that is sensitive to light, and resistance gets lower (10Kor less) as it gets more light. Being in parallel with the high leg of the gain chain feedback, it lowers the gain of that stage and so lowers the level. Then goes back to its high value (200K or more), taking some time do so.

In page 1, I uploaded an example of Vactrol used for gain in the second stage.

There's component values in your circuit that Wayne in proaudiodesignforum has already commented on which I won't repeat.

You mean the resistors in the gain switch? Yes, I will correct them. Particularly the one for low gain.

 

[ricothetroll]

Hi,
BTW, the best solution I could find for a limiter is to put a vactrol after the mic transformer, so that the ON value of the vactrol is multiplied by the squared ratio of the trafo. That's the only way I could get a genuine and clean input limiter, and that's why I forgot the 1512 for this use. Please note that I'm not an electronic guru of any kind, so solutions might exist, but as no one here pointed it out, I kinda doubt it You could put several vactrols in parallel but I think you'll end with the same budget as with a Lundahl.
Regards

 

[ricardo]

the best solution I could find for a limiter is to put a vactrol after the mic transformer, so that the ON value of the vactrol is multiplied by the squared ratio of the trafo. That's the only way I could get a genuine and clean input limiter,

Rico, can you post your final circuit somewhere?

 

[Spino]

kinda off topic, sorry, but i recently worked with a field mic pre by sound devices. that thing worked on two 1.5 batteries, pretty good mic pre and pretty good headphone amp with it. how can they pull that off on 3V?

 

[carlmart]

With a very good DC-DC supply, of course.

But AFAIK batteries run-out pretty fast.

I think you have to use an external battery, where current demands are smaller.

 

[ricothetroll]

The last version I posted in my thread is final as far as the input stage is concerned :
http://img59.imageshack.us/img59/3407/lcod.png
I don't plan to sell that design, that's why I can spend a few bucks on trafos
I have the schematics of the recent SQN mixers, which use non-input limiters (THAT VCA after the input stage IIRC). They sell it as a CD so I don't want to post it in a public thread, but I could send it to you by mail. PM...

 

[carlmart]

The last version I posted in my thread is final as far as the input stage is concerned :
http://img59.imageshack.us/img59/3407/lcod.png
I don't plan to sell that design, that's why I can spend a few bucks on trafos

Have you already built it? It's a good thing you can use Vactrols (why two?), even if I would have put them to control the gain of IC7A.

In my case I had to practically develop a limiter, using THAT's 4315 and with help from their support. If it works fine, it might end up as a limiter AN at THAT.

I'm also gonna try a limiter chip, NJM2762, and see how it works, if I get to debug it. JRC support, different from THAT, is completely nonexistent.

I have the schematics of the recent SQN mixers, which use non-input limiters (THAT VCA after the input stage IIRC). They sell it as a CD so I don't want to post it in a public thread, but I could send it to you by mail. PM...

Please do. I'm very curious. Send me a PM with your e-mail or if you prefer I PM you.
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