Avantone Pro CV-28 Modification?

[elskardio]

Hi guys,

I have a pair of Avantone CV-28. There are pretty decent microphones, especially with the cardiod capsule, but I'm curious if there's modifications I could do to improve them?? Maybe replace the transformer?

Here's a picture from recordinghacks. They look exactly the same.

Any idea would be appreciated 
Thanks

 

[Gus]

Why do think you should replace the transformer?

Why don't you like about the microphone?
That is where you start

 

[elskardio]

Why do think you should replace the transformer?

Why don't you like about the microphone?
That is where you start

Hi Gus,

I would like to open up the top end. Get a bit more definition in the high mid frequencies. I usually use a little shelf EQ while tracking to compensate.

I mentioned the transformer because when I replaced them in my Cascade microphones the result was a more open and detailed sound. That's what I'm aiming for... if possible...

Thanks

 

[Gus]

I did a quick search and did not find a schematic.  A schematic would help a lot.

Can yo trace the transformer primary to the coupling cap?
If so what type of cap and what value do they use?  This is where I would start.

I see a transistor, is this a cascode circuit or a 2 stage in series circuit(jfet to tube)?

I could be the capsule or the circuit.  Capsule looks somewhat like the china build removable jfet microphone capsules. Can you try a different capsule?

 

[elskardio]

Hi Gus,

I think I found more info on the mic. It looks identical to the Advanced Audio CM28. Here's the CM28 description:

Our circuit provides at least 6db more headroom than traditional fet/transformer circuits. We do this with a fet front end designed to capture all the detail of the capsule, which is then coupled into a 6J1b tube driving our BV11 transformer. This approach takes full advantage of a low noise fet circuit. The tube stage then needs to provide very little gain so it is configured as a Low Z driver. The tube provides the fet with a very high impedance load so the fet’s headroom can be maximized.

I'll try to measure the coupling cap tonight.

Thanks

 

[ln76d]

I would make from it  simple plate follower.

 

[Gus]

From the description is sounds like it might be a jfet gain stage to a cathode follower.
This might have at least three important coupling caps, Input, between stages and the tube to transformer.
A hybrid approach can sound good. 
A jfet does not have the grid noise issue
Is the Jfet something like a 2SK30A Y?

 

[elskardio]

The transistor is a 2SK170.

 

[ln76d]

Cathode follower output have sense but it don't add well to the BV11 chinese transformer.
There's possibility that Dave from AAM gave wrong info.
BTW.
If you are unable to draw schematic from PCB, and want to keep original topology i would keep transformer (these are usual very linear) but change the tube and potentially few caps. For 6j1b 5840/EF732 should be really good replacement, same as 6ak5/EF95/5654 for 6j1.
Woth schematic would be much more options to improve it.

 

[Gus]

It might be the capsule and grill interaction
Have you tried the microphone with the grill removed?

 

[elskardio]

I will try to draw a schematic from the PCB. The pcb assembly is not very clean 

I'll probably change some parts anyway so I might as well put it apart and draw the schematic. I'll keep you posted 

 

[elskardio]

It might be the capsule and grill interaction
Have you tried the microphone with the grill removed?

Hey Gus,
I'm in the studio today. I will try to test the mic with the grill removed. Also it's a 2 layer grill and one layer is easily removable from the inside. I'll also test it with one layer only.

 

[elskardio]

Ok... By searching the web i found the Alesis AM40 tube (Groovetubes GT-44) microphone that looked very similar on picture. I contacted their customer service (Fender) and they were kind enough to provide a schematic 

I checked quickly with my Avantone and it seems to be an exact copy. I'll have more time tomorrow and will trace the PCB to check if everything is the same. Any thoughts on the schematic?

Cheers

 

[ln76d]

If the circuit is exactly the same (i would verify that first, in your place), for quick some propositions.

C7 replace with film cap 470nF-1uF
C10 with 470pF-680pF Ceramic NP0/C0G
*in both cases i woul rather use lowest values.
C4 i would replace with with electrolytic elna silmic 10uF.
C2 i would replace with 22uF-47uF eln silmic.
*both electrolytics 10V-16V rated,  should fit without any problem, due to size.
C8, C9 - check are both Cermic C0G/NP0 type, if not replace (black dot, black edge marking or just written).
If you have scope and generator, check is it fet biased.

 

[Matt Nolan]

That's a hybrid cascode. With the right set-up it can be made to perform very well indeed.

 

[ln76d]

That's a hybrid cascode. With the right set-up it can be made to perform very well indeed.

Yeah, i'm surprised, i was expecting much worse circuit.

 

[mics]

Yet another OEM.  Exactly the same as the OEM Peluso P28!

I've found previously that the high z resistors in these Mics are not so good.  Try  some reidons or similar and it will give you an improved starting point for your other mods.

 

[ln76d]

Yet another OEM.  Exactly the same as the OEM Peluso P28!

I've found previously that the high z resistors in these Mics are not so good.  Try  some reidons or similar and it will give you an improved starting point for your other mods.

What are the symptoms of  "not so good". Noisier?

 

[elskardio]

Hi guys,

what's the utility of R7 TRIMPOT ? Bias the FET ?

 

[elskardio]

Hi guys,

what's the utility of R7 TRIMPOT ? Bias the FET ?

I keep looking at various mic schematic and R12 (2k21) looks like the Bias resistor... ? Still curious about that Trimmer R7  ???