C414 EB P48 build

[Spencerleehorton]

Not that pretty but should work fine until I try and figure out how to put some switches in there!!

 

[Spencerleehorton]

Getting very low output and sounds muffled with this edge terminated capsule?
Will try with other capsule and try other pcb to see if it components or capsule at fault.

update

circuit is at fault as getting half the voltages i should be getting, will change transistors and check caps.

 

[Spencerleehorton]

Trying to get hold of some 2sk30 -O

Or equivalent, anyone know of any sources to get these please??

 

[Khron]

I wonder how horrible some J113's might be, in that position. They're readily available from Mouser (through-hole version).

 

[Spencerleehorton]

Just tried a 2n5457 and seems to sound ok, the output seems lower but I've also changed the 10uf tants to 15uf as that's as close as I have without making up 10uf from two caps.
The other fet is the pf5102 and using bc550c.
Bit of wind noise which must be this cheap capsule.

 

[RuudNL]

I also have issues with wind noise from time to time, but I know the source!  ;D

 

[Spencerleehorton]

Hi ruud,

What's your source of the noise?

Baked potato and baked bean!!!

 

[Spencerleehorton]

Could anyone point me to the correct 2sk30 datasheet -0 type which is used in the schematic as there seems to be so many different types?
I've ordered some -y types and I've ordered some other j201 to try as these 2n5457 seem to only be 25v drain to gate and gate to source and only 10ma F G C compared to 50ma for the j201 and pf5102.
I'd like to see the specs of the specific 2sk30 -0 to compare as these fets seem to make a world of difference in the overall output and performance.

 

[Spencerleehorton]

Well it seems the 2sk30A is -50v g to d and g to s. With 10ma gate current.
Also the 2sk117 seems pretty similar.
Will try the 2sk30A with the pf5102 and see if it improves the overall sound rather than use the 2n5457.

 

[Khron]

Well, considering you can only ever get what, 14mA or so? with the phantom-power shorted to ground, are the current ratings really relevant, at all?

 

[Gus]

You should test the jfets to make sure they are what you bought

T3 is set-up as a constant current device so R8 and or the voltage divider might need to be adjusted IDSS and Vgs off testing should help

For the input jfet. The gate cap, IDSS range, Vgs off range and Yfs Gfs are what you should look at.

Look back at reply 4

A 2N5457 should work at the input might need a resistor  adjustment

The 2SK30 0 rating shows an IDSS of .6 to 1.4mA
https://datasheet.octopart.com/2SK30A-Toshiba-datasheet-101654.pdf

J202
https://www.mouser.com/datasheet/2/676/jfet-j201-j202-interfet.r00-1649170.pdf

2N5457
https://www.onsemi.com/pub/Collateral/2N5457-D.PDF

 

[Spencerleehorton]

Hi gus, yeah trimmers on r5 and r8 and I will do some tweaking and see if I can get some better voltages and sound.
Will report back, gonna try a 10k for R8 and 20k for r5.

 

[Spencerleehorton]

Not much to report when changing R5 and R8, didnt sound any different really when I tried to set the voltages as per earlier post which was taken from another thread.
On T1 I couldn't seem to get drain high enough,  I was only getting 5v and it should be 7.45v, so will try tweaking something else.
I will also try testing 2sk30a and j201 as I have 2n5457 at the moment in there and i think that's the problem..

 

[Spencerleehorton]

These are the voltages, can anyone verify they are correct or in the ball park please?

Capsule bias voltage is 42v

T1  D  7,64V  S  4,69V  G  0,04V
T2  C  23,2V  B  8,07V  E  7,64V
T3  D  22,8V  S  3,11V  G  2,69V
T4  C  43,2V  B  23,2V  E  22,8V

 

[Spencerleehorton]

So far cant seem to find out why I'm only getting very low output,  I suspect it's probably the bc550c which I need to change as I've changed both fets and now have pf5102 and 2sk30a in.
R5 and R8 are trimmers and I'm thinking of putting in trimmer for r9 to tweak once I get full output, I've also changed capsule so I know it's not that.
Both 10ufs I had 15ufs tants in there but have put 10uf 50v electrolytics now.
Must be the bc550c as they generate the gain and it gain I'm missing!!!

 

[volker]

The BE voltage of T2 looks a bit too low. Gate voltage on T1 is either a problem or more likely measurement error. You need to measure at the junction of R2/R3. It's probably around 4V.
The current in your output transistor is a bit low, at least for my taste. I don't know the ratio of the transformer though.

Other than that, test the amplifier without a capsule. Put in a signal and trace it with the oscilloscope. What you are doing is guessing.

 

[Spencerleehorton]

That's exactly why I asked for some proper working voltages dude!!!
Just going with what I've got at the moment, I know I'm working blind but cant do anything else.
Changed bc550c and still the same low output,  must be caps ?

T1 gate voltage is just mv and jumps up and down? No real reading there?

It's a 1:2 txf step up as far as I know?

I'll do what you suggest with signal and no capsule.

Thank you dude

 

[Gus]

The output  JFET is set up as a constant current device
The output stage is an emitter follower with a CC in the emitter a nice circuit
The voltage at the source is 3.11VDC
3.11VDC / 6.8K is about .46mA

for example  if you had 6.8VDC, 6.8VDC / 6.8K is 1mA so you need  a lower value R8 to turn the JFET on more to increase the current. I would guess it should be < = 1mA because the IDSS of the 2SK30 0 is .6 to 1.4mA

If you look at page 2 bottom right graph of the 2SK30 0 link you should have about .65VDC difference between gate and source at about  .46mA

I have not worked on a 414 eb p48 but I would guess the transformer is a least a 2:1 not 1:2
Why? 1K load with 1V is 1mA
.46mA /2 is .23mA into a 1K load or .23V
2 x.46mA = .92mA so you get .92VDC into a 1k load
so into a 2K load is x2 that

I would try reversing the transformer and then maybe reducing the 6.8K source resistor.
I would guess AKG designed this for under .6mA(the min IDSS with gate connected to source)

 

[Spencerleehorton]

Ok gus that sounds great advice, I'll report back.

 

[Spencerleehorton]

so far reversing the txf has reduced the hiss a little but still very low level in comparison to my other C414 eb p48?
Have 3.11v on source of T3 and getting 22.8v on drain of T3.
but T1 drain is 5v rather than 7.64v, T1 source is 4.6v and on T2 emitter im getting 23v, so it just seems T2 emitter/T1 drain which is the problem?

will check all connections and resistors and caps one more time!!