C414 EB P48 build

[Spencerleehorton]

I'm gonna wind the txf next and then set the capsule just to cardioid,  the LC and pad I'll just have to off first of all.
Will then put capsule in mesh and power up and see what we have!!

A few of the caps I've just used electrolyics as the moment as that's what I have on hand, the C7 6.8uf/50v I've used a 10uf/63v, the C11 3.3uf/50v I've used a 4.7uf/50v.
The other 3.3uf are tants, and the other  C10 andC5 10uf/16v I've put in electrolyics.
The 1nf (1000pf) are ceramics at the moment but I'll change for polystyrene when they arrive, the 22nf are film, I do have some 22000pf polystyrene but they are rather large so might just put in some wima 22nf.
The 10nf are film at the moment as well.
The 15pf is 18pf polystyrene along with the 39pf which measures 41pf polystyrene and 150pf polystyrene which measures 144pf.
I've used bc550c for T2 and T4, T1 has a PF5102 in there and the T3 is a 2n5457 until the 2sk30s arrive.
Resistor wise before I've changed any for trimmers I've got R6 is 56k, R5 is 12k, R2 and R3 are 120K, R11, R10 are 300k, R12 is 150k, r13, r14 is 68k, R8 is 6.8k, R7 is 2.1k and R9 is 39k.
All other resistors are as per schematic.

 

[Gus]

R5 and R8 are the ones I would adjust from reply 4.  Built it with the schematic values first and adjust if needed.

I would only use trimmers to ADJUST the circuit  and replace with fixed resistors.

You can parallel resistors like you might find in a 87 for the correct final values.

From Rossis numbers some quick calculations from a piece of paper
48VDC - 43.2VDC =4.8VDC
4.8VDC / 2.15K(R7) =2.23mA lets add 50 ohms(made up) for the transformer for 2.2K and 2.18mA total
Look for about 4.8VDC drop across R7 and the transformer

First stage mA
43.2VDC at the "top" the 56K and 23.2VDC at T2 collector
20VDC /56k = .356mA

Lets check the source of T1 4.69VDC / 13.3K = .352mA good enough check

measure across R7 and the center tap for about 4.8VDC
Measure the source to ground for about 6.7VDC
Adjust R5 and R8 if needed

Remember when using trims add a fixed resistor and try to have the center of the trim and fixed resistor at the value you calculate you will end up at. This give the best adjustably if you need + and - movement

2.18 total mA - .356mA about 1.8mA for the CC, a compromise for transformer drive and a high enough capsule voltage

other current paths are less so disregarded for this first quick check

HOWEVER this does not work out with the phantom resistors, it is not off a lot.
6.8K x (2.18mA /2) = 7.4VDC
or 3.4K x 2.18mA
48VDC - 7.4VDC = about 40.6VDC at the center tap

 

[Spencerleehorton]

Yep I was thinking the same

 

[Spencerleehorton]

Need to check all my wiring as some voltages are pretty close and some are not!!
Getting 40v on txf centre tap?
Phantom only seems to be supplying 44v but also need to check that?
Does it need to deliver exactly 48v?

Spotted acouple of mistakes will give it another look tomorrow

 

[Khron]

Well, as you may or may not recall, the phantom power is supplied through a pair of 6.8k resistors (that reside inside the preamp), so... Unless there's zero load on the circuit (or the initial voltage is higher than 48v), you'll never see 48v there "anyway".

Need to check all my wiring as some voltages are pretty close and some are not!!
Getting 40v on txf centre tap?
Phantom only seems to be supplying 44v but also need to check that?
Does it need to deliver exactly 48v?

Spotted acouple of mistakes will give it another look tomorrow

 

[Gus]

Need to check all my wiring as some voltages are pretty close and some are not!!
Getting 40v on txf centre tap?
Phantom only seems to be supplying 44v but also need to check that?
Does it need to deliver exactly 48v?

Spotted acouple of mistakes will give it another look tomorrow

Did you look at the ohms law math?
You are not far off

What winding resistances do you measure at the transformer?

 

[Spencerleehorton]

I'm not sure if it's a step up or a step down txf, I'm assuming it's a step down?
On the primaries I have about 40ohm per winding, 80ohm in series and secondaries are about 130ohm in series.

Like I said I've found some of my mistakes but I'm on a 12 hour night shift now so wont be able to look at it until the morning!!

I hope I'm not too far off, on the emitter of T2 I had 7.4v but on the collector only 11v!! But then I realised I hadn't made the connection from c2 to c14/c15, the source of T1 was about 4v ish but the gate had way too much voltage there.
T4 and T3 i cant remember but I'll make the changes and  see how it looks.

 

[Spencerleehorton]

Right, well it's sort of working!!
Will need to start tweaking now, I'll write down all the voltages first.

Had to change all the transistors,  ended up putting in some j201 and bc550c.

Hopefully the proper parts will arrive soon!!?

 

[Gus]

OHMs law will help
Lets guess the capsule voltage 40VDC and T1 drain current is .357mA
The gate of the JFET does not draw current
There is a voltage divider R4, R3 and R2 1.242meg total
40VDC/1.242meg = divider current (ignoring t2 base current)

We want about .357mA drain current(From Rossi measurements 20VDC /56K)
so what does T1 Vgs vs drain current current look like
Or DIY style
is the IDSS of the JFET higher than the total current(IIRC J201s are .1mA to about 1mA and a good guess is .6mA)
What voltage different do you need between the gate and source for .357mA?
I would leave R2,R3 and R4 values alone.
R5 with the drain current "lifts" the source voltage above ground compare that to the voltage at the R2 , R3 node. This voltage difference sets the Vgs and the drain current.

SO as long as the JFET IDSS is above the needed  first stage drain current at a reasonable Vgs you should be able to adjust for different JFETS. Look at the 2SK30 and PF5102  specs and compare to the JFETs you have

https://www.mouser.com/datasheet/2/308/PF5102-D-1814630.pdf

 

[Spencerleehorton]

The 2n5457 was a bad choice as its max drain gate voltage was 25v rather than 40v as per pf5102 and 2sk30 is 50v.
J201 seems a good replacement for pf5102 and seems to work as 2sk30 but will replace with 2sk30 when they arrive.
I will get voltages now before I go back into another night shift!!

 

[Spencerleehorton]

Here is a quick test of voltages

 

[Spencerleehorton]

Could anyone also confirm whether the txf is step down or step up please?
I've got the low ohm side as the secondary to output so it's a step down.

 

[Spencerleehorton]

At a quick glance Looks T4 needs about 4v more and this should then improve the voltages?

 

[Spencerleehorton]

My first question is as I've asked about the txf, is it step up or step down, not clear yet in my mind?
As it stands it's a very low output, so this is why I'm thinking it's a step up?

Also I have only connected one capsule and have no capsule or 70pf cap in where the other capsule should be?
Should I put in a 70pf? Or can I leave it disconnected?

Should I change R8 to 4.7k with 5k trim in series and R9 for 20k with 15k trim in series and start to twiddle?

Voltages are all sort of there, it's just T4 that to my mind is the most out?

 

[Spencerleehorton]

ok, after changing the txf to step up and putting on a new capsule this thing sounds amazing!!
Im recording drums at the minute and its easily keeping up with the C12a i have on the overheads, lovely and clear and loads of output, quiet as a mouse as well, quieter than the C12a to be honest, and ive just repaired another Fet47 i had and this c414 with a cheap chinese capsule and not even all the correct components sounds better, must be a really bad capsule in this othe fet47 as i need to change the capsule and give it a spruce up!!!

Im quite shocked it sounds so good!!!

I'll put up a link to some audio once im done having fun!!

 

[Spencerleehorton]

The circuit is inside a fet47 body I've made at the moment, if I get a chance to make a c414 style body I will.
Going to make another version of this now to compare and use correct values as per schematic rather than just some bits I had lying around!!
Also I have a ck12 capsule in my other c12a which I might borrow to see how it sounds with this circuit, along with the body and txf, then compare this c414 to the other I've built.
Ideally I think a good matched pair of these will end up as overheads as noise wise they seem better than the c12a which sound great but not quite as good as these c414s.
Also I might make a 3rd one for my guitar amp to go along side the sm57.

 

[doqmemory]

Do you think the build would fit into a MXL Guitar Cube Pro body?  They are on sale at Musician's Friend right now for $70 US.

 

[Spencerleehorton]

i reckon you could squeeze it all in there yeah.

 

[Spencerleehorton]

Building second version with all tants in rather than electrolyics like last one, will keep 22nf and 10nf as film.
Will compare with first version and then make the 3rd version  based on which I feel is better.

 

[Spencerleehorton]

Two more mics in the making