Avantone Pro CV-28 Modification?

[Matt Nolan]

It is to calibrate the pad / attenuator switch.

 

[mics]

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

Yes introduced noise into the circuit is the problem. 

 

[ln76d]

Yes introduced noise into the circuit is the problem.

I would give you a like, but the button is currently missing ;D

 

[ricardo]

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.

Can you measure the response of the shelf EQ at the settings you use on this mike and post it here?

Presumably it makes the mike sound like what you want.

 

[elskardio]

Hy Guys,

I got the Avantone schematic (SYT-28). It's the same design as the Alesis AM40... only a few component values are different. I already ordered some replacement tube and a few capacitors!

Any thoughts?  ;D

 

[Gus]

Found my GT33 I believe it has the same  capsule as the 44
The sound changes with the grill unscrewed from the body I do get hum with it removed
It would be interesting to try different capsules on the same stock electronics
You MIGHT make it worse changing parts

First measure and record the JFET S and D and tube pin voltages
First gain stage device I would change(if I was to change anything) is the JFET, However you need the drain voltage for selecting a replacement(drain to gate max voltage etc.) Maybe something like a 2SK30A  x select the IDSS range I would not change the tube it might be fine.
You might need to change R6 however it is not as simple as a KM84 bias

First thing for EQ
Before you change any parts think about the EQ you like, you could try changing(reducing in value)C8 for a high end lift

C6 is kind of a large value however it looks like an electrolytic so maybe that is why someone picked a 4.7uf

Circuit has mixed bias
R6 source bias
R7 and R8 Grid 1 bias (look up cascode)
Voltage bias for the JFET with RP(look up the Vishay app note AN104?)I am thinking about why the voltage comes from the plate(more feedback gain reduction)
C2 sets some of the gain(charge amp) C3 is the pad gain reduction (change amp)
R9 looks to be to reduce pops when switching the pad

 

[elskardio]

Thanks Gus for the notes!

 

[elskardio]

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

C10 with 470pF-680pF Ceramic NP0/C0G

Hi In76d, why do you suggest to lower C10 (1000pf) ?
Thanks

 

[Khron]

The Vishay appnote is actually the AN102 http://www.datasheetcatalog.com/datasheets_pdf/A/N/1/0/AN102.shtml

 

[elskardio]

Circuit has mixed bias
R6 source bias
R7 and R8 Grid 1 bias (look up cascode)

Hi Gus,
Could this method work for biasing the circuit? (this is a wild guess)

-Apply 1kHz sinewave to R1/C1 junction
-Connect oscilloscope to Q1 drain and select R6 for symmetrical clipping
-Then connect Oscilloscope to Tube Plate (Pin 5) and select R8 for symmetrical clipping

thanks

 

[ln76d]

Hi Gus,
Could this method work for biasing the circuit? (this is a wild guess)

-Apply 1kHz sinewave to R1/C1 junction
-Connect oscilloscope to Q1 drain and select R6 for symmetrical clipping
-Then connect Oscilloscope to Tube Plate (Pin 5) and select R8 for symmetrical clipping

thanks

Try 1kHz to R5/C1 junction through 33-68pF capacitor, oscilloscope probe to drain.
For tube move oscilloscope probe to C6/R11 junction.

Hi In76d, why do you suggest to lower C10 (1000pf) ?
Thanks

You shouldn't feel any difference in low end response the more that it's SDC microphone.
Good quality C0G/NP0 ceramic cap. Sometimes lower capacitance at the input (also output as many other places) can change a little microphone clarity (not exactly in the meaning of different low end cut) - explanation in simplified way
In opposite to Gus opinion:
- if the FET is 2SK170BL (possibly GR) i would keep it.
- definately i would change the tube, chinese tubes usual sucks, you should notice here difference between tubes.

My proposition is - set bias for the tube and FET which already have inside microphone and change caps which i listed (you can ommit for now pad caps for simplifiction). Then you will have some reference point. You can make it step by step, checking is there any improvement or it's worse.
Back to original values take less time than writing multiple posts and getting opposite opinions from different members

 

[Gus]

ln76d

Why would you keep a 170 in a microphone even if it is a cascode.

 

[ln76d]

Gus

Why not?

 

[freeccm]

I want to change the tube of microphone from 6j1b to 5840, however, I failed to find the information of exact PIN number that I've changed. 6j1b is 7pin and 5840 is 8pin. Would you guys can give some help, please?

 

[Khron]

Well, you see, you don't necessarily need to KNOW anything by heart - looking at pictures and seeing connections / pinouts should be plenty.



In the case of the 5840 it's even easier, since you have the pinout list in english. Pins 2, 4 and 8 are all connected together (the cathode and grid 3). The 6J1B has all the connections independent.

Whether that still works with your intended application, depends on whatever stock(?) circuit is currently in your mic.

 

[Nen'O]

Hey, I’m about to get 2 syt 28 aka advanced audio cm 28.
Does anyone know what mods Dave from Advanced Audio do to this mics ?


Thanks in advance

 

[bounce]

Here are both a Sterling ST44 (left) and Advanced Audio CM28 (right) for comparison if useful to anyone…

 

[Nen'O]

Here are both a Sterling ST44 (left) and Advanced Audio CM28 (right) for comparison if useful to anyone…

Hey, Thanks for sharing photos.
To me it looks identical circuit with different tube and output capacitor. Is there any noticeable difference in how they capture sound?
The one with wima cap and GT tube is Advanced audio cm28?