[macm75]

Simple question...
To start my Pimeta without class A biasing of the opamps, can I install the JFET's with R10 open? Or will I damage the transistor with V- connected to the drain (or source?) and an open source (or drain?).
If I run the opamps in class A at a later date, I rather just have to install R10 because if the JFET's sit they will undoubtably get lost

Thank You.

 

[jl123]

I would leave them out...but that's just me. I'm thinking they would be fine but you never know. That's not their intended use.

Tape them to the inside of your case.

 

[tangent]

There's no problem with having the JFETs in and R10 out. It does just what you suspect it does, macm75: they're as good as out of circuit.

 

[luvdunhill]

I had a quick question above the current sources in the PIMETA. The docs say that "you should also check that OPALR's (OPA 2227) output pins are equal to V-". I measured all the pins on OPALR with IG as reference point and I get:

pin 1: -19 mV
pin 2: -0.1 mV
pin 3: -0.3 mV
pin 4: -11.4 V
pin 5: -0.4 mV
pin 6: -0.4 mV
pin 7: -20.5 mV
pin 8: 11.56 V

I purchased the JFETs from Tangent, and carefully installed them in the correct positions. I don't have any other connections to the board other than the power. Any recommendations?! Am I measuring the voltages on pin 1 and pin 7 correctly?

Thank you!

 

[luvdunhill]

well, I'm getting antsy and I'll probably just go ahead and try the PIEMTAs out without understanding if my current sources are working. See above posts for my voltages with respect to ground on my OPALR. My guess is I'm not measure with reference to the correct thing, as further down in Tangent's tutorial it mentions measure the DC offset in a similar fashion.

Thanks!

 

[FallenAngel]

Well, quick question.
Did it work properly before Class-A?
If yes, of course install R10 and try the amp out.

 

[luvdunhill]

Obviously, I just installed the devices I got from Tangent and didn't test it before

 

[FallenAngel]

Well, in that case, don't install the Class-A yet, your voltages look good, put some opamps in, turn it on, make sure nothing gets hot - finger poke the opamps, then measure your DC offset (Output Right - Output Ground and Output Left - Output Ground), if they are small (less than 10mV), you're good to go. Plug some headphones in and listen to some music. If everything sounds good, then install Class-A and do the same thing again.

 

[luvdunhill]

heh, what I meant to say is .. yes, I've already installed the current sources and the resistors. Could you perhaps clearify this statement "If you've installed the current sources already, you should also check that OPALR's output pins are equal to V-." Do I measure the outputs pins from IG?

 

[FallenAngel]

Yup, Output pins (1 & 7) are connected to V- through the CCS, so they should be showing V- when measured to Input Ground.

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif heh, what I meant to say is .. yes, I've already installed the current sources and the resistors. Could you perhaps clearify this statement "If you've installed the current sources already, you should also check that OPALR's output pins are equal to V-." Do I measure the outputs pins from IG?

 

[luvdunhill]

ok, maybe I'm just not understanding the schematic...I don't see how the output of the opamp to IG could possibly be V-...oh well. I guess they're not working then. I'll double check tonite that everything is connected properly I guess, then report back.

 

[FallenAngel]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif ok, maybe I'm just not understanding the schematic...I don't see how the output of the opamp to IG could possibly be V-...oh well. I guess they're not working then. I'll double check tonite that everything is connected properly I guess, then report back.

Simple, do you see the output of the opamp going through R10 and the CCS to V-. When the opamp isn't plugged in, it's basically connected to V- so when you measure from ground to opamp Output, you'll see V-.

 

[luvdunhill]

"When the opamp isn't plugged in"

The light just went on! I've been measuring everything with the opamps plugged in to the sockets!

Thanks!

 

[FallenAngel]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif "When the opamp isn't plugged in" The light just went on! I've been measuring everything with the opamps plugged in to the sockets! Thanks!

Ahahahahaha!

, that's just too funny buddy. Well, I guess we know that you're getting reasonable DC offset off the opamps.

So now measure the DC offset of the amp and you're hopefully done