Microphone preamp with compressor

Geek · 2004-09-11 5:04 am

I'm staring a new thread here to minimize the off-topic chit-chat from the 12AX7 thread. What we were talking about is here:

http://geek.scorpiorising.ca/MicPreamp.html

Now, to answer specific questions about it

JojoD818 said:
Geek,

I also saw that you have a mic preamp with a compressor (separate schematic) and wowow! Have you actually tried using it? Can you describe how it sounds?

Yes. Go to http://www.scorpiorising.ca/music.shtml and all the ones I did with lyrics, used this mic preamp.

Seems that all the parts are locally available here except for the LED/LDR part. Maybe I can just build a discrete LDR/LED for such use?

JojoD

Newark Electronics was the only place I found the devices known as "vactrol's". They cost me $8 CAD each about two years back.

You can, with equal performance, use a bright (I use 3,000 mcd) LED and a LDR (I used a 3.5k to 50M) face-to-face inside a short piece of drinking straw, seal the ends with auto silicone sealer, then cover the whole gadget in heat shrink tubing. The key to success with this method is use of a fast LDR - 50mS decay max.

Optical · 2004-09-11 5:19 am

hmm, so is this solid state or tube stage compression? or a combo? is any audio signal going through the solid state section is what i mean..

also is the LM3900 choosen for a particular reason? it seems to be discontinued.. i notice its specifically for running from one rail, so would an LM324 work? (lm324, have i got that right??)

oh, and my last question, can the 12AX7 be substituted straight in? i have a bunch lying around and dont like my chnaces of finding a 6N1P

TobWen · 2004-09-11 5:24 am

@Optical: it is a tube stage ... or can you see any solid-state-chips in the preamp-schematics?

@Geek:
So you mean, I can simply connect the microphone with +48V (via http://www.diyaudio.com/forums/attachment.php?s=&postid=472164) to your preamp without damaging anything?

Geek · 2004-09-11 5:25 am

Optical said:
hmm, so is this solid state or tube stage compression? or a combo? is any audio signal going through the solid state section is what i mean..

As with all my audio projects, the signal line never sees silicon

also is the LM3900 choosen for a particular reason? it seems to be discontinued.. i notice its specifically for running from one rail, so would an LM324 work? (lm324, have i got that right??)

The 3900 is still in full production by National.

I chose the 3900 over the 324 for the reason it's a current (Norton) amp and therefore inherently a high impedance device. Perfect for interfacing with tubes! A 324 in this circuit does strange things. I am going to R&D a 324 version because while the 3900 is still in production, wholesale lots of less than 1,000 chips are getting harder to find.

Geek · 2004-09-11 5:28 am

TobWen said:
@Geek:
So you mean, I can simply connect the microphone with +48V (via http://www.diyaudio.com/forums/attachment.php?s=&postid=472164) to your preamp without damaging anything?

Yes, that is correct. I forgot to show in the diagram, the capacitor (DC blocked side) goes to the mic preamp. Putting DC on any preamp input (SS or tube) can damage it.

The 10K resistors limit the current to a level that will not damage the microphone if it is wired incorrectly.

JojoD818 · 2004-09-11 5:35 am

Geek said:
Newark Electronics was the only place I found the devices known as "vactrol's". They cost me $8 CAD each about two years back.

You can, with equal performance, use a bright (I use 3,000 mcd) LED and a LDR (I used a 3.5k to 50M) face-to-face inside a short piece of drinking straw, seal the ends with auto silicone sealer, then cover the whole gadget in heat shrink tubing. The key to success with this method is use of a fast LDR - 50mS decay max.

My thoughts exactly! I already used one before on a sub compressor/limiter circuit and seems the discrete LDR/LED combo is functioning well.

Thanks Geek for a wonderful tube circuit collection!

JojoD

TobWen · 2004-09-11 5:37 am

Would it be bad to put it on a PCB ?

Geek · 2004-09-11 5:45 am

TobWen said:
Would it be bad to put it on a PCB ?

Nope, not at all! It just doesn't look as sexy then

TobWen · 2004-09-11 5:55 am

at the moment sound is more important for me than design.

What do you think about this beast:
http://www.siliconchip.com.au/cms/A_30804/article.html

If link does not work (redirected to paysite), try to visit the site via Google

But now tell me ... where in Europe can I buy two 6N2P?
I think, two 6N2Ps are needed, correct?

Geek · 2004-09-11 6:31 am

I have not heard that amp, nor heard of someone who did, so I can offer no opinion as to it's performance

Sure is cute though

The 6N1P and 6N2P I get straight from a broker/friend in Lithuania, so I can't say of shops. I do know www.evita.lt in Vilnius Lithuania as well as KW Tubes in Russia sell them. Evita is priced well though (prices are in Lita's, not USD like KW)

Colt45 · 2004-09-11 9:04 am

There are a few dealers in germany that sell 6N1P and 6N2P, although they wll be cheaper if you buy from further east.. baltics, russia, ukraina..

still is rather inexpensive, even in germany.

TobWen · 2004-09-11 9:16 am

yeah, I think the same.
But the problem is the customs and shipping to Germany.

customs were about 18-24% for every non-EU-deal, if I'd in store :-((

Prune · 2004-09-11 9:31 am

Why does it say here (http://geek.scorpiorising.ca/schematics/TubeMicPreampPowerFINAL.png that the magnetic shielding should be ungrounded?

Colt45 · 2004-09-11 9:58 am

if all you need is 6N2P, ECC83 is close enough (heater is different though).. 6N1P, there isn't a very close ECCxx tube, so you would have to buy one..

I found one site in germany, http://www.btb-elektronik.de/de/preise_lager_international.htm

6N1P is 6,5 €
Not exactly cheap! look around.. something will come up. check ebay and such..

Geek · 2004-09-11 10:30 am

Prune said:
Why does it say here (http://geek.scorpiorising.ca/schematics/TubeMicPreampPowerFINAL.png that the magnetic shielding should be ungrounded?

Old notes on final schema

Colt45 said:
I found one site in germany, http://www.btb-elektronik.de/de/preise_lager_international.htm

6N1P is 6,5 €
Not exactly cheap! look around.. something will come up. check ebay and such..

6N1P is $0.45 lt. in some eastern stores. Even with duties and taxes, you're well under 6,5 €

Optical · 2004-09-11 12:30 pm

Geek said:
As with all my audio projects, the signal line never sees silicon

The 3900 is still in full production by National.

I chose the 3900 over the 324 for the reason it's a current (Norton) amp and therefore inherently a high impedance device. Perfect for interfacing with tubes! A 324 in this circuit does strange things. I am going to R&D a 324 version because while the 3900 is still in production, wholesale lots of less than 1,000 chips are getting harder to find.

http://www.national.com/pf/LM/LM3900.html

ok, its all looking very good so far.. so how about that 12AX7? if it can take one i will build one this week!

Colt45 · 2004-09-11 10:56 pm

Geek said:
6N1P is $0.45 in some eastern stores. Even with duties and taxes, you're well under 6,5 €

im not totally stupid gregg

They're usually 15 - 30 rub. ($0,50 - $1) at most stores in russia.. of course they will be cheaper at a market if you can deal..

But then again.. we don't want everyone getting good deals on our sweet russkij lampy do we? :$:

Geek · 2004-09-12 12:08 am

Colt45 said:
im not totally stupid gregg

Wasn't talking to you

Colt45 · 2004-09-12 12:57 am

fair enough.

looks like the transmission and/or differential on my car is leaking. that might be 0/2 audi's running.

and it's wet again. starting to look like BC here. but flat and cold.

fdegrove · 2004-09-12 2:03 am

Hi,

http://www.triodes.nl/

2 Euro for a 6N2-P seems very fair to me....Especially when you live close by... as I do.

6N1-P is often mislabeled or disguised as ECC88/6DJ8 which it obviously isn't.
For non critical applications it often "works" as a replacement but then to some so would a dozen other ECCs anyway...
Oh well, it's only audio...

Cheers,

Microphone preamp with compressor

Member

Member

Member

Member

Member

Staff Emeritus

Member

Member

Member

Member

Member

Member

Disabled Account

Member

Member

Member

Member

Member

Member

diyAudio Senior Member