Simplistic NJFET RIAA

[vgeorge]

They are from an old group buy for BOSOZ. Each one is 4x18v. I series paralel the windinds to get 2x36. Unloaded voltage is 21v. I was using them for a year now with no problem.

 

[nicoch58]

humm you can try choke input or semi.. report on tubes forum are that is way better ....

but as you do in separate box and with the great Salas stuff ....not sure

at last a resistor to reduce current charge pulse

 

[Salas]

V check

For the RIAA - it is going to the 57db for my Shelter 901 - I have already matched some 170 BL at about 8.5mA, but I also have a stash of V's. I will measure some and see how many I can match.
Also waiting some teflon capacitors from the states.

Having a stash of V is better to be put in good use. If you can find some circa 15mA to match then you can make your phono in better input noise and interstage drive current. The RIAA values are slightly changed to be adapted and you will not need the 1Meg shunt resistor to R4 also.

 

[vgeorge]

nicoch46,

I tried choke input, but got lower voltage with not enough margin to regulate for the shunt. I may put a small resistor after the diodes though.
Salas, thanks for the schematic! I will measure the V the next days and see what matches I am going to find.

 

[Salas]

Some few V that I got tend around the 15mA mark so I hope yours are alike.

 

[massimo]

nicoch46,
I tried choke input, but got lower voltage with not enough margin to regulate for the shunt. I may put a small resistor after the diodes though.

George, try a small film cap (1-5 uF, what's handy) after the diodes instead.
That should be what nicoch intended when he mentioned you can try choke input or semi...

 

[vgeorge]

George, try a small film cap (1-5 uF, what's handy) after the diodes instead.
That should be what nicoch intended when he mentioned you can try choke input or semi...

I will do when I finish the rest of the RIAA. Now, I put a small resistor in front.
I would be fun to see if I can tell the diffence.

 

[nicoch58]

semi ...under 1µ IMO

 

[massimo]

Nicoch, at first glance, I thought you were referring to semi.... conductor!
Then I realized!
< 1 uF? Really? PIO should be best in this position, possibly better than film.

 

[nicoch58]

this caps is used to adjust the Vdc out or lower the buzz from cheap choke

 

[vgeorge]

Salas, I had so much fun measuring 100 V's tonight!
Anyway, I found 6 around 14.7 ma. Can Q5 be 7 ma?
Also I have some shinkohs 6.830K. Should I parallel them with 2.2M to bring them down?

 

[Salas]

Q5 can surely be 7mA if you will delete the 15 Ohm R3 and you change R12 to 3K3 1W. Yes bring those tantalum to spec by shunting, no problem.

 

[massimo]

this caps is used to adjust the Vdc out or lower the buzz from cheap choke

I know, and it helps small Xfrm too (not George's case anyway). It avoid too much stress to the choke and often helps to rise the voltage a bit.

 

[vgeorge]

Thanks for the advise Salas.
With V's it' s difficult to find matches, especially at 15ma. There are more at 11-13ma.
I have to buy now some resistor values that I' missing to complete the build.
This time I' m not going to use trimmers for the source resistors, and hope everything is going to be fine.

 

[Salas]

Franz was once saying the same and his V were stronger in the end... Double check the conditions. Are you using straight current mode DMM measuring and a 9V battery, or you add a resistor in series also, then you translate voltage drop? It can be dropping enough with V and changing the VDS passing lower IDSS.

 

[vgeorge]

Franz was once saying the same and his V were stronger in the end... Double check the conditions. Are you using straight current mode DMM measuring and a 9V battery, or you add a resistor in series also, then you translate voltage drop? It can be dropping enough with V and changing the VDS passing lower IDSS.

I am measuring with a staight current mode DMM and a 9v battery. I tested few again with a fresh battery and matching is the same, only absolute value is around 0.20mA more. Should I test all of them again? As I understand, this is the recommended method.

 

[Salas]

Its OK since you tested that way, I wanted to be sure. Didn't you get also some between 15-20mA in the batch? Its expected by spec. Unless it wasn't mixed with the very high yield.

 

[vgeorge]

I have 8 fets above 15ma in 100 pcs. Maybe they were from the same batch. They came in 2 strips.

 

[vgeorge]

The best 6 I could get near 15ma are:
14,87
14,91
14,88
14,88
14,91
14,99

Should I put the two exact pairs on the first two stages and the 14,87 and 14,99 at the last one?

 

[Salas]

Yes, that arrangement will do fine.