Running JFETS on Bipolar Power?

[RDV]

I have a query.

How would I go about converting a ROG emulator to run on bipolar power like say 15v+/- ?

I'm ready for another electronics lesson.

TIA

RDV

[RDV]

Okay, I guess you can't because they're uni-junction devices.

RDV

[RDV]

Okay then. Does anybody know if you can still bias them with those little trimpots we use at 9v if I jump the voltage up to say 20v or so at a pretty high current level?

RDV

[Ge_Whiz]

Run it between the 0 and +15V lines?

[RDV]

Run it between the 0 and +15V lines?

Thanks for replying Phil. That's the only way.

When I ask questions here they're usually crazy. It just shows my basic lack of knowledge of basic electronics .

RDV

[Ge_Whiz]

Swap ya half my electronics knowledge for half your guitar playing skill? 

[Ardric]

Can't you use the -15V as as if it was the ground?  The gate bias 1M resistor and source resistor would be referenced to the -15V.  As long as you AC couple in and out of the emulator with a series bipolar capacitor, it shouldn't matter.  Maybe 22n on the input side and 1u or bigger on the output.

Are the FET circuits and 100k trim ok at 30V?  I bet ROG's brand new Fetzer calculator would be a great help here.  J201's are rated for 40V maximum.

You should have very well filtered power supply for this sort of thing.  I don't know how to calculate the PSRR of the pre, but I bet it's terrible.

[cd]

I can't find the thread anymore, but someone asked a similar question a few weeks ago, something along the lines of would it be better to run a ROG sim at 18V.  It may have been within the context of another preamp?  Anyway there was a real eloquent explanation of how it breaks down, the bias requirements, and why it wouldn't make any difference.  Someone refresh my memory for me!

[RDV]

I can't find the thread anymore, but someone asked a similar question a few weeks ago, something along the lines of would it be better to run a ROG sim at 18V.  It may have been within the context of another preamp?  Anyway there was a real eloquent explanation of how it breaks down, the bias requirements, and why it wouldn't make any difference.  Someone refresh my memory for me!

I found this old thread that has some info like that. The reason I'm asking is I've tried some of the ROG sims as preamps with my chipamps and they sound okay but don't have enough output for the job. There's a couple of ways to get enough output, which is increase the gain of the chipamp itself or add another gainstage at the end of the ROG circuit with an opamp or transistor gainstage. The downside of another gainstage is more noise. I'm having a hell of a time trying to come up with a direction to go in to even get started. I don't want to do the usual opamp gainstage with diodes to ground like everyone and their grandmother has done before.

*Sigh*

RDV

[Ardric]

The reason I'm asking is I've tried some of the ROG sims as preamps with my chipamps and they sound okay but don't have enough output for the job. There's a couple of ways to get enough output, which is increase the gain of the chipamp itself or add another gainstage at the end of the ROG circuit with an opamp or transistor gainstage.

How much output do you need?  A pair of clipping LED's will limit you to a couple of volts pp.  I'm already getting more output voltage swing than that from my 9V stompboxes with the volume on full.  Do you need more voltage to drive something like that Thomas Vox soft clipper/limiter?  And if so, isn't running the emulator at higher voltage going to help?  I thought LM3886's are usually set up with quite a bit of gain themselves, so you don't need very much voltage in your signal to get full output.

Or is it more a question of current, ie you need low output impedence?  If it's current drive, then another FET as a source follower would be better than a gain stage.  Like the input buffer on the Ruby.  It'll nicely drive tone stacks, volume controls, fx loops and etc without adding any further gain.  I think the chipamp.com kits have a 23k input impedence as supplied, so a buffer after the volume control would certainly help.

So that's my suggestion... emulator at 30V (or 15V) to volume pot to Great Big Ruby Amp.  Set the volume so the emulator does all the clipping.  Maybe a further set of voltage divider resistors to give a reasonable range on the volume pot.  I'd still be worried about TOO MUCH signal.

I'd also be worried about getting DC into my LM3886 input, so I'd put a series capacitor in there.  IMHO those hifi guys are a little strange sometimes.