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Old 11th September 2004, 04:04 AM   #1
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Default Microphone preamp with compressor

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


Quote:
Originally posted by JojoD818
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.


Quote:
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.
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Old 11th September 2004, 04:19 AM   #2
Optical is offline Optical  New Zealand
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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
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Old 11th September 2004, 04:24 AM   #3
TobWen is offline TobWen  Germany
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@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/attac...&postid=472164) to your preamp without damaging anything?
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Old 11th September 2004, 04:25 AM   #4
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Quote:
Originally posted by Optical
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


Quote:
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.
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Old 11th September 2004, 04:28 AM   #5
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Quote:
Originally posted by TobWen
@Geek:
So you mean, I can simply connect the microphone with +48V (via http://www.diyaudio.com/forums/attac...&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.
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Old 11th September 2004, 04:35 AM   #6
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Default Re: Microphone preamp with compressor

Quote:
Originally posted by Geek



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
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Old 11th September 2004, 04:37 AM   #7
TobWen is offline TobWen  Germany
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Would it be bad to put it on a PCB ?
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Old 11th September 2004, 04:45 AM   #8
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Quote:
Originally posted by TobWen
Would it be bad to put it on a PCB ?
Nope, not at all! It just doesn't look as sexy then
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Old 11th September 2004, 04:55 AM   #9
TobWen is offline TobWen  Germany
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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?
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Old 11th September 2004, 05:31 AM   #10
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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)
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