Help with CMC5 DC-DC converter

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camarada78

camarada78

Well-known member
Joined
Apr 4, 2014
Messages
157
Location
Rio de Janeiro - Brasil
Hello folks,

I'm building my second Schoeps clone, but this time with only spares i have in the workshop. So i'm a LDC k12 capsule and 557s for the PNPs and a 547 for the NPN, the FET is a BF245 .  they should be suitable.

For now i'm getting good measurements on the microphone circuit (very close to the schematic indications), the problem appears to be on the DC-DC circuit. I get the 6.1v on the c11 (22pF)/2k pot) before the npn, but after the 547 i get next to nothing on the emitter or the base. The measurement point after D1 (before R4) showing nothing too.

My inductors are 470uH on DR1 and 82uH on DR2 and my C8 is a 10pF (not 6.8pf as the schematic).

some additional info:
- I tested different 547s and a 548 with same results.
- my protoboard apparently is working fine, no missing connections.
- my phantom power is feeding 46v
- i'm using a spare chinese k12 capsule (rayking) that worked fine on another microphone.

Thanks in advance.

Have a good day
 

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camarada78 said:
.... I get the 6.1v on the c11 (22pF)/2k pot) before the npn, but after the 547 i get next to nothing on the emitter or the base. The measurement point after D1 (before R4) showing nothing too....
Click to expand...
You should get some current thru 547 transistor, ad the base should be at 0.6V or so. Check out the transistor, surrounding parts and wiring.
 
There's also the possibility, slight as it may be, that you've toasted / zapped the BC547, seeing as it's rated for "only" 50V (not to mention the 30V-rated BC548).

With inductor ratios closer to 2:1, you'd be expecting something along the lines of 60V on the output; i would imagine the 5-6:1 ratio you're using would result in an even higher voltage.
 
moamps said:
You should get some current thru 547 transistor, ad the base should be at 0.6V or so. Check out the transistor, surrounding parts and wiring.
Click to expand...

After changing some inductor values (470 for a 120uH, and the 82uH for a 50uH) i got 0.7 on the base of 547 and 5.5 at the colector of the DC circuit. Still nothing where it should be around 60v.

These are the measures i got now:

at C12 and C13 i'm reading 31.5v (should be 40v according to schem)

R2 i'm getting 6.3v (should be 6.1v)

At the 557s i'm getting
30.5  at the base
30.3 at the colector (should be 33.4v)
31.3 at the emitter (should be 34.4v)

At the FET i'm getting

3.1v at the drain (should be around 4.5v).
nothing at the gate.
2.9v at the source

The FET was biased using Matador methods and the trimpot is 300k/700k.

****
i'm lost.

I think i'll dismantle and reasamble the circuit on the protoboard. maybe i'm missing something.
 
Khron said:
There's also the possibility, slight as it may be, that you've toasted / zapped the BC547, seeing as it's rated for "only" 50V (not to mention the 30V-rated BC548).

With inductor ratios closer to 2:1, you'd be expecting something along the lines of 60V on the output; i would imagine the 5-6:1 ratio you're using would result in an even higher voltage.
Click to expand...

The BC547 still test good on my component tester. HFE 311
 
A voltage of 0.2 V over the FET (drain-source) seems very low to me...
By the way: I don't see 40 V over C12 and C13 in the schematic. (That is the rated voltage for the capacitor).
If you look closer, you will see: ca. 34.4V. (31.3V is close enough. It depends on the Hfe of the PNP transistors.)
 
RuudNL said:
A voltage of 0.2 V over the FET (drain-source) seems very low to me...
By the way: I don't see 40 V over C12 and C13 in the schematic. (That is the rated voltage for the capacitor).
If you look closer, you will see: ca. 34.4V. (31.3V is close enough. It depends on the Hfe of the PNP transistors.)
Click to expand...

You are right. Its the voltage for the caps not "expected".

I'll try another BF245 to see if i get similar results. But still, the DC area seems problematic also.

i think i will restart from scratch.
 
Just an update:

Restarted from scratch and now it works like a charm. The BC547 indeed has lower output indeed as Khron mentioned. i'm getting 28v where i should get 60v.
I will find a bc546 and see how it goes.

Thanks a lot for the help. Love this community!

i'll post pictures of the build soon.
 
The schematic works perfectly as it is, with a BC547.
I have built many microphones with the same DC/DC converter.
I could even get voltages over 70 Volts!

(In most cases I used 68 uH emitter-to-ground and 150 uH emitter-to-1nF)
 

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