C414 EB P48 build

[RuudNL]

Wouldn't it be an idea to inject a signal at the gate of the first FET and compare the AC and DC values with a 'good' working C414EB?

 

[Spencerleehorton]

your full of good ideas Ruud!!!

gonna take apart my first one and look at voltages

 

[Gus]

T2 emitter voltage should be the same a T1 drain.

A picture of the PCB top and bottom will help.

 

[Spencerleehorton]

What I have noticed on both of these other two versions I've made is the handling noise of the mics is much worse than my first version also the wind noise?
I used j201 in my first version for both fets and bc550c, pretty sure from memory used electrolyics rather than tants as well!!

 

[Spencerleehorton]

Well it seems i was mistaken, the first version i built i thought worked the best, but now checking it, it is very low volume and has wind noise!
I only have one version sort of working nearly right but it is loud and is a little noisey, could just be capsule but its a brand new chinese edge terminated capsule, other than sawping the capsule over into a working circuit of something thats the only way of testing the capsule i suppose!

So i have two versions which dont work very well, ive taken out trimmers and gone back to stock values, i am using PF5102 and 2SK30A along with BC550C's.
I have setup transformer to be step down at 2:1, my input voltage is 48v but goes down to 45v at pin 2, I seem to be at least a 1v down on T1 Drain, gate looks fine, T2 emitter obviously is 1v down as well along with the base and collector.
T3 is 1v down on drain.

My working version has the transformer as step up, so something is very weird here?

 

[Spencerleehorton]

The only thing thats different is in the working one i've used a J201 rather than 2SK30A which is in the low volume versions?
I dont have anymore J201 here yet but i do have some 2N5457 which i could try rather than the 2sk30?

this is a real headscratcher as Gus has been saying it should be a step down transformer and im using step up, might try and swap round and see if output level stays about the same but the S/N gets better?

starting to clutch for straws!!!!

 

[Spencerleehorton]

I'm going to wind another transformer, seems one of the secondaries is faulty on one of these with low output and also the other was using a oep txf which I think was loading it too much and that's why the output was down.
I think it's all working just the transformer is faulty!!!

 

[Spencerleehorton]

Seen in another thread to put a 100uf in parallel with R8 6.8k to make it sound a bit thicker, will try this little mod, anyone else done this mod?

 

[Spencerleehorton]

Getting correct voltages on one version now but still low output!! Must be either bad capsule  or transformer?
Transformer checks out fine and I get slightly more output obviously when stepping up 1:2.

 

[Spencerleehorton]

Well the problem I'm having is correct working  info, it looks like I have very close voltages and all my mics work to a point  but none seem to work without a little bit of wind noise?
Have ordered another few capsules and will wind another transformer or two and see if I can get them a bit better!!

 

[RuudNL]

Put a fixed capacitor in place of the capsule. (~68 pF for each side.)
If the wind noise is now gone, you know it is the capsule.
On the other hand: if the noise is still there, it is the circuit!

 

[Spencerleehorton]

Hi ruud,  thanks again, which is the front capsule and which is the rear?
I've taken it as the front capsule is the left hand drawing on the schematic and the rear the right?
I'm only wanting to use this in cardioid so I have nothing connected for the rear, should it be connected to work?

 

[RuudNL]

In that case you can substitute only the front membrane with a capacitor.
(The capacitor should be rated for the polarisation voltage used of course.)

 

[Spencerleehorton]

Also where should I test for the polarisation voltage? Think I read it should be 42v?

 

[Gus]

The polarisation voltage is the same as the circuit voltage
R15, R6, R4, R11, R7, C7 node

For cardiod only and to help figure out what is wrong. The capsule half to the left should be the front because the schematic shows 180 at the right side half. So wire the left side of the capsule to start

You might want to remove the switches and pattern control  wire one side of the capsule to the input set the output to flat so the filter network is out of circuit. Note the output EF is biased off the collector of T2 in this setting

Keep R16, R16 and C1 for the capsule voltage supply

Once you get one working in cardiod you can add in the other sections

Do you have a scope and signal generator?

You can build a test circuit without the transformer if you have a scope
You will need two resistors for the circuit voltage from phantom like a Neumann 8x circuit(2.2ks etc.)that connect to R7 you can start with 100 ohms for each I would think
A 100pf cap to the gate other side connected to the generator under a 1V P to P signal to start.
no pattern control, no pad and no filter control
Get this working
then add a transformer and test again
then add pad caps and test
then add the filter section.
then add the pattern control section

The first stage with out the C to G caps  is at the maximum  gain.
The gain of the first stage is reduced to LESS than, the CAPSULE pf / C1 or (C1 +C2) or (C1 + C3) then is buffered or buffered and HP filtered by the next stage.  The more open loop gain the closer to the equation also don't forget the JFET capacitance this matters more with voltage gain stages then in followeres

I t looks to me that the current in the output is selected so there is enough voltage for the capsule without using a DC to DC converter and can drive the output  with a step down transformer.

Close up pictures of the PCB might help troubleshoot.

 

[Spencerleehorton]

Yeah I got a digital scope on a laptop and a signal gen.
Other capsule just arrived along with some j201s.

 

[Gus]

something that might help

The 80pf is a simple capsule sim
circuit in flat mode adjusted for 2N5484s
Build this fragment on the PCB and test it out of the microphone
Change C8 and note how the gain changes
Then add the other sections and test each time you change something

 

[Spencerleehorton]

Ok, thanks gus, that's actually really helpful, I think I'm getting to hung up on getting correct original values when using alt fets, like what you have done I see I need to change the source resistor to get the 22v and the correct gate value.
Obviously the source voltage will be different but I need to concentrate on the other voltages.
I think my problem might also be a dodgy batch of pf5102, as my working version has 2 x j201 in it and does seem to work and be fairly quiet.
All my versions have 15pf in along with all caps which are 50v.
I'll put some trimmers back in and dial in the 22v and then take out and then make up resistors for the source resistor.
Hopefully then I'll have some better results.
Also I'll do what you suggest about 68pf cap as capsule so I know this isn't a problem.
Then if I get correct voltages put a capsule on and see if I get good signal!!!

 

[camarada78]

Well the problem I'm having is correct working  info, it looks like I have very close voltages and all my mics work to a point  but none seem to work without a little bit of wind noise?
Have ordered another few capsules and will wind another transformer or two and see if I can get them a bit better!!

Why dont you try using Cinemag CM-11021 with center tap?

 

[Spencerleehorton]

I can wind my own transformers with same spec, some of the problems I'm having are not to do with transformer.
I'm fairly confident it's a combination of adjusting resistors, capsule and dodgy fets that is the problem.