Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Yamaha CP70 JFET preamp help
Yamaha CP70 JFET preamp help
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th November 2019, 09:06 PM   #21
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by cancon View Post
...Rs changes made to the buffer FETs down the line...27K changed to 1-1.5K each. Any danger of pulling too much current through these parts?
Source followers are pretty insensitive to changes in Rs (though Vgs will increase a wee bit as Rs goes up.) There should be no need to change Rs for these stages.

If you do want to tweak for maximum output headroom, the DC bias voltage at the gate is where the tweaking should be done. Not by changing Rs.

I typically keep power dissipation below 250 mW for anything in a TO-92 package. That's a conservative level that keeps them cool and happy. With a 12V supply, that suggests keeping Id below some 20 - 30 mA (allowing for the fact that the full 12V doesn't appear across the JFET.)

I don't think 2N3819s will ever flow that much current, even at zero Vgs, but some other types of JFETs with lower channel on-resistances and higher Idss specs will.


-Gnobuddy
  Reply With Quote
Old 3rd December 2019, 12:17 AM   #22
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by Gnobuddy View Post
If you do want to tweak for maximum output headroom, the DC bias voltage at the gate is where the tweaking should be done. Not by changing Rs.
To clarify, the above statement is specifically about source followers. (As is my entire previous post.)


-Gnobuddy
  Reply With Quote
Old 3rd December 2019, 01:30 PM   #23
cancon is offline cancon  Canada
diyAudio Member
 
Join Date: Mar 2011
Quote:
Originally Posted by Gnobuddy View Post

So if you have the time and energy to spend, consider tweaking the bias networks to shift the quiescent source voltages so that they have symmetrical headroom for positive and negative half-cycles of the audio signal.

-Gnobuddy

I didn't understand that this advice wasn't meant for the source-follower stages (doh!). I was attempting to put the gate voltage between the drain and source like in FET 1,2,4.

I'll switch those Rs back to 27K.
The output of the pre isn't super low, its better than it was, and I don't have the piano to test it out. I can tweak later if need be.

However, I still have the lone original K30A which came with the preamp. Maybe I'll put it back in to improve the gain since its still here - any suggestion of which position will yield the best results, 1, 2 or 4?

Last edited by cancon; 3rd December 2019 at 01:40 PM.
  Reply With Quote
Old 4th December 2019, 12:36 AM   #24
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by cancon View Post
I didn't understand that this advice wasn't meant for the source-follower stages (doh!).
Well, you can tweak source-follower stages this way too, but you can't tweak away JFETs that want 6 volts of Vgs, instead of 1 or 2 volts! Only a JFET swap will fix that.

Quote:
Originally Posted by cancon View Post
I was attempting to put the gate voltage between the drain and source like in FET 1,2,4.
These are depletion-mode, N-channel JFETs; they bias up like a vacuum triode does, with the cathode (source) more *positive* than the grid (gate.)


So you can't put the gate voltage between drain and source; the source has to be more positive than the gate!

Quote:
Originally Posted by cancon View Post
I'll switch those Rs back to 27K.
Now I'm confused which stages we're talking about. But to clarify: for the source-follower stages, I suggest going back to the original factory value of Rs (2.7k or whatever), and then tweaking the DC voltage at the gate, so that the source ends up roughly at half Vcc, or as near as is reasonably practical.

Quote:
Originally Posted by cancon View Post
...the lone original K30A...which position will yield the best results, 1, 2 or 4?
FET2. That's the only stage where the source resistor is fully bypassed with a capacitor. That means it was originally the stage with the highest voltage gain.


As mentioned previously, using a high Vgs JFET lowers the voltage gain even if the source resistor is fully bypassed - this happens because you end up with less DC voltage across the drain resistor, and that reduces the voltage gain.


So if you currently have a JFET with much greater Vgs_off in the FET2 position, it's producing less voltage gain than that stage was originally supposed to.



-Gnobuddy
  Reply With Quote
Old 4th December 2019, 10:00 PM   #25
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Arrrgh! Looking back at post #23, there is still ongoing confusion, and miscommunication on my part, because my mental cogs slipped for a moment, and I interpreted "source-follower" as "common source", which is totally incorrect.

To clarify and correct:

1) For a source-follower, leave Rs alone, tweak the DC bias voltage on the gate.

2) For a common-source gain stage, start by tweaking Rs. If Vgs_off isn't too far off spec, this will get the stage working. But if Vgs_off is way out of line, you might need to tweak the DC bias voltage at the gate so that Rs doesn't become ridiculously big in the effort to bias the JFET.

I hope that clears it all up!


-Gnobuddy
  Reply With Quote
Old 4th December 2019, 11:38 PM   #26
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
Quote:
Originally Posted by cancon View Post
...original parts list shows all FETs in this circuit (9 total) to be SK30A-Y. However, all but FET 1 are actually p/n 2N3819....
Small Bear will sell you SK30 for a buck; however postal rules to Canada will make the shipping more than the parts. But maybe less than the value of torn-out hair?
Transistor FET 2SK30A-GR - Small Bear Electronics

Yes, it is not the right suffix. I still think it is more-right than '3819. You want a hot motorcycle engine, the '3819 is a medium sedan engine. Yes, I remember a Harley with a 283 Chevy in it, but it was not a good fit or ride, it was better with the original 45.
  Reply With Quote
Old 6th December 2019, 05:37 PM   #27
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by PRR View Post
...You want a hot motorcycle engine...
The J113 is still available at both Digikey and Mouser. It might make a good motorcycle engine. Vgs_off ranges from (-0.5V) to (-3V), and minimum Idss is only 2 mA (max Idss is not specified.) Those samples with Vgs_off and Idss near minimum spec are definitely "hot motorcycle engines".

Too bad the PF5102 has disappeared. It was solidly in the hot motorcycle engine category (small Vgs_off, small Idss.)


-Gnobuddy
  Reply With Quote

Reply


Yamaha CP70 JFET preamp helpHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Yamaha A17 has got Jfet input? As A500 rhythmsandy Solid State 3 23rd October 2015 11:17 PM
JFET BOZ Preamp john65b Swap Meet 6 9th June 2012 02:15 PM
jfet preamp - does a jfet exist that'll work at just 5V supply? peskywinnets Solid State 50 1st May 2012 01:02 PM
FS: Pre 4 jfet preamp hags Swap Meet 0 25th April 2012 03:36 PM
FS: PreAmp with Complimentary JFET Diff input & JFET ouput USAcircuits Vendor's Bazaar 12 21st June 2011 12:02 PM


New To Site? Need Help?

All times are GMT. The time now is 08:00 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki