Matt Nolan
Well-known member
It is to calibrate the pad / attenuator switch.
Avantone Pro CV-28 Modification?
- Thread starter elskardio
- Start date
Help Support GroupDIY Audio Forum:
elskardio
Well-known member
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
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
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
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
Well-known member
Thanks Gus for the notes!
elskardio
Well-known member
ln76d said:
Hi In76d, why do you suggest to lower C10 (1000pf) ?
Thanks
elskardio
Well-known member
Gus said:
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
elskardio said:
Try 1kHz to R5/C1 junction through 33-68pF capacitor, oscilloscope probe to drain.
For tube move oscilloscope probe to C6/R11 junction.
elskardio said:
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
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.
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.
