Avantone BV-12 Modding

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Kroc

Member
Joined
Nov 2, 2013
Messages
19
Hey guys,
I just got my hands on an Avantone BV-12 tube microphone last week. It's really nice, but I want to mod it to make it even better! (I realise 'better' is subjective, but you know what I mean  :) )
It's quite sibilant (on my voice), pops quite easily, and with the 12AX7 I have in it at the moment it starts to saturate very easily (practically the second you hit a chorus with vocals). So I'm going to replace the stock tube with a NOS GE JAN 6072A from 1984, which I'm hoping will clean up the sound a bit. But I'm curious as to whether there is any way I could enhance the sound further by tweaking the circuit board or upgrading to higher quality components.
Ideas? :)
Cheers,
Dan
 
You may want to start by reading Brian Fox's technical evaluation of the BV-12:

http://foxaudioresearch.ca/BV-12Review.htm
 
Interesting. Thanks for the link.
The distortion that Brian describes certainly reflects the trouble I was having, and it's interesting that he says modding the circuit increases the headroom by 20dB or so. In its stock form, the mic will sound great when you're singing quietly, but it quickly gets very fuzzy and messy.
He also says:
"I think I know the answer at this point. In the past with the CV-12 I also saw high distortion in the stock circuit and my solution was always to re-wire the mic to a CCDA circuit or to a replica c12 single triode circuit, depending on the customer's budget. (Replica circuit is a big re-wire so it costs a little more)"
Which makes me wonder whether I'd be better off maybe re-wiring to a replica C12 circuit, and what sort of difference this would make?
 
Ok I'll jump in.

:)

The 2 stage circuit on the CV-12 and BV-12 is in theory a very nice circuit.  It gives you the gain stage of the old mics in the 1st stage and a low impedance output circuit to drive the transformer.

Two issues:

1.  You have to build it right.  These mics have used the easiest form of capacitor coupled cathode follower for the 2nd stage and its not great.  distortion can run over 13% in the high end. However re-wiring that stage makes it transparent. Almost like it's not there  except for... noise

(BTW it is NOT the same as the APEX460, which uses a direct coupled 2 stage circuit, which is much cleaner)

2.  Running 2 triodes means they both have to be quiet.  Not impossible, but you have to do the testing or listening or both.  It's easier to get 1 good triode in a bottle.

After that there other problems with the design, like installing RF bypass caps on the output that are 100 time bigger than they should be and so they get involved with the transformer and screw up the high end.  They are a big contributor to the fuzzy high end on these mics.  It's called capacitive loading and most tube circuits don't like it.

So we should be aware that a 2 stage tube mic can sound real good. It is more to the solid-state sound because of the electrical "stiffness" of the low impedance output stage.
It just has to be built correctly.

But then you may be a purist and want the complex distortion that comes from running a single triode like the old mics.
They do have a sound we seem to enjoy.  There is something about them that I still like.

Hope that helps.

BF
 
Hi Brian! Thanks for your reply, and for your fantastic article. I very much appreciate you taking the time to write back. :)

After much contemplation, I think I want to rewire it the way you have described in your article, to get the best out of the mic without completely changing it. After all, if I wanted to do that I'm guessing I'd probably be better off getting a TNC ACM-1200/APEX 460 (or several) and pimping it out with upgraded high-end components. (Which I might do one day anyway)
But I really like the whole vibe of the BV-12, so I think I'll try to honour the original design as you have. I think it will give a very usable sound once that distortion has been improved.

I used it in a session the other day on a female vocalist friend of mine with a big, powerful voice, and we had to set the gain to next to nothing and compress on the way in and it was still saturating in the last chorus. I can kind of understand it distorting when a singer like that starts belting, but my voice is on the softer Jeff Buckley/John Mayer side of things, and even I have to restrain myself. I imagine it would work like a charm in stock form if you're just using it for really hushed, intimate singing and guitar work without many changes in volume (Nick Drake type stuff). Probably why Taylor Swift loves it.

I have a few potentially stupid questions (I'm quite new to mic modding :) ):
1. Are there particular brands of components that sound better for these mods? What did you use?
2. Would it be worth upgrading the Chinese BV11 tranny, or the capsule? Or again, is this excessive, and something that would be a better thing to try out on a cheap modding mic?
3. There don't seem to be any schematics for the BV-12 anywhere. I'm not quite sure where to start with this whole process?
4. How big of a difference am I likely to see when I stick a 12AY7 in instead of the current AX?

Cheers,
Dan
 
I also just noticed something concerning. Apart from the fact that it came stock with a 12AX7B (which I didn't think was the case for BV-12s), the circuit board says CV-12.
The chassis says BV-12, and it came with all the BV-12 documentation. I'm sure there might be a perfectly reasonable explanation, though. Any thoughts?
 

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I don't think it matters much what it says. It's still a regular chinese circuit board and components, exactly same than in a mic of mine with a different brand name.
 
Kroc said:
I also just noticed something concerning. Apart from the fact that it came stock with a 12AX7B (which I didn't think was the case for BV-12s), the circuit board says CV-12.

...because the BV-12 is simply a dressed-up CV-12 upgraded with an edge-terminated capsule...
 
Kroc said:
Hi Brian! Thanks for your reply, and for your fantastic article. I very much appreciate you taking the time to write back. :)

After much contemplation, I think I want to rewire it the way you have described in your article, to get the best out of the mic without completely changing it. After all, if I wanted to do that I'm guessing I'd probably be better off getting a TNC ACM-1200/APEX 460 (or several) and pimping it out with upgraded high-end components. (Which I might do one day anyway)
But I really like the whole vibe of the BV-12, so I think I'll try to honour the original design as you have. I think it will give a very usable sound once that distortion has been improved.

I used it in a session the other day on a female vocalist friend of mine with a big, powerful voice, and we had to set the gain to next to nothing and compress on the way in and it was still saturating in the last chorus. I can kind of understand it distorting when a singer like that starts belting, but my voice is on the softer Jeff Buckley/John Mayer side of things, and even I have to restrain myself. I imagine it would work like a charm in stock form if you're just using it for really hushed, intimate singing and guitar work without many changes in volume (Nick Drake type stuff). Probably why Taylor Swift loves it.

I have a few potentially stupid questions (I'm quite new to mic modding :) ):
1. Are there particular brands of components that sound better for these mods? What did you use?
2. Would it be worth upgrading the Chinese BV11 tranny, or the capsule? Or again, is this excessive, and something that would be a better thing to try out on a cheap modding mic?
3. There don't seem to be any schematics for the BV-12 anywhere. I'm not quite sure where to start with this whole process?
4. How big of a difference am I likely to see when I stick a 12AY7 in instead of the current AX?

Cheers,
Dan


1. Based on my measurements and listening, I am more a fan of what the parts are made of (the underlying technology) versus brand names.
So metal film resistors are a newish invention but are very stable.
Polypropylene dialectrics in capacitors seem to have a general vote of sounding better than some others.
I Like polystyrene caps for the input but that is just a preference.  YMMV

2. The  BV-12 Transformer is quite good.  Tests like a Peluso BVP11.  Higher output than is common in an AKG style mic. Not needed, but no harm either as long as the circuit can drive it properly.

3. I make/send a schematic for all my mics.  You can get one if I mod it for you. :)

4.  6072/12AX7 :  Gain drops by about 2 to 3 dB.  Output impedance of the cathode follower drops to 1500 ohms or so (a good thing for that transformer)  Noise is less if (and only if) you have a good tube.  Distortion drops which makes the mic sound cleaner. 

But if you want a big difference just cut or de-solder and remove C9 and C10.  Amp goes to flat response.  All the high end comes back. High end distortion drops way down.


BF
 
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