Forsell's JFET-992 Op-Amps anyone?

[niccolo]

Hi there,
Did anyone try those on their J99?
With what sonic results?

thanks

niccolo

[tpryan]

JFET op-amps will work in the J99, but the input bias current compensation won't have any effect and the compensation caps will likely need to be changed. Consult the op-amp data sheets for details.

[atiller]

Oh dear...

I wish I had of checked with you first, Tim, before going ahead and ordering a quad of Fred's JFET-993 op-amps for my upcoming J99 builds. I read some reports about people using them with great success, and assumed (keyword) that they were plug and play.

They are meant to be drop in replacements for the Hardy op-amps (for the Twin Servo circuits), does this make any difference?

If not, can you please direct me (us) where and what to look for so that I can use these 993s in my J99s? Here is the spec sheet:

http://www.forsselltech.com/products/jf ... stData.pdf

Taking a look at the J99 schematic, am I correct in seeing a 10uF cap on the input of the op-amps (U1, U2)? The spec sheet for the 993 says that the input capacitance is < 30pF. Is this relevant, or is this stating the capacitance of the op-amp itself? I have a feeling I have the wrong idea though...

Thanks very much for your help!

-Adam

P.S. and if you have time to entertain yet another question, how will this affect the tone of the J99, if at all? If it is merely a matter of supply current (making sure the op-amp gets the correct bias current) and presumably that is done correctly, changing these caps shouldn't really affect anything audiowise, correct?

[tpryan]

Taking a look at the J99 schematic, am I correct in seeing a 10uF cap on the input of the op-amps (U1, U2)?

No, those are power supply bypass caps. The compensation caps are C6 and C12. According to the 993 data sheet, the compensation caps should be in the range of 10pF to 33pF, not 680pF as shipped with the kit. You really should check square wave response with a scope to get the optimum value.

if you have time to entertain yet another question, how will this affect the tone of the J99, if at all?

Distortion is dominated by the transformers, so the difference between one very good op-amp and another very good op-amp will be very subtle.

[atiller]

Thanks Tim, as usual!

I was referring to the tonal differences of the capacitors, not the op-amps. But I'm pretty sure that provided you use the right values, to allow the op-amp to operate in an optimal fashion, they (the caps) won't affect the tone. But that is besides the point...

Are there any other components which will need changing, or any other considerations that need to be made when using these JFET opamps? Or will swapping out C6/12 be it?

Thanks for the continuing help, Tim.

Adam

P.S. been enjoying my C84s now for a good while. They are great pres!

[atiller]

Hi again,

Sorry to be a bother Tim, but I just wanted to confirm that those compensation caps are the only things in need of change to retrofit the J99s with Forsell's JFET opamps.

I have received them in the mail and will be placing my order with you soon, so I want to make sure I'm well prepared for the build!

Woo, exciting!

Adam

[tpryan]

They ought to work with the compensation caps recommended in the data sheet, but I can't guarantee it. You should be prepared to deal with any unforeseen complications.

[atiller]

Thanks again Tim!

Hopefully all goes well.

Adam

[infested3d]

I was having troubles, and thought the 990c's I got were blown out since they were REALLY hot and I was getting excess noise- so I ordered the MM99 (which, if I'm correct, was what Mr. Forssell designed before going on his own and making the 992/3)- but I was having problems with the input biasing on the J99...it just wouldn't work. As the Millennia MM99 is also a JFET opamp- I think the compensation cap values definitely were to blame- as the MM99's worked fine in my A12's.

After giving up I plugged in the JH 990c's (after reading the data-sheet, I realized that they're suppose to get that hot).

Actually- if I remember correctly, the culprit was the jumpers for the grounding and I had to jumper to power ground instead of chassis gnd.

[atiller]

Sorry, I meant to reply to this thread much earlier. My J99s have been going strong with the Forsell JFET-993 op amps for some time now.

The compensation caps need to be changed, first of all. In my unit, C6 = 27pF and C12 = 220pF. These values seem to work fine.

Secondly, as the previous poster mentioned, the grounding also needed changing. Instead of the default CGND, use the jumper to change the ground to PGND (power ground).

Hope this saves someone a couple hundred bucks in bench time!

Cheers,
Adam

[Soundman1402]

Do the caps also need to be changed for a Hardy 990c, or is that one a drop-in replacement?