Quote from: Loudthud on March 12, 2017, 03:18:02 AM

Quote from: tarahall on March 11, 2017, 09:28:38 PM

Attached below is a layout I did and an updated schematic. I took you suggestion and added resistors in series with the pots to limit their span and make them easier to adjust.

Would there be any difference/advantage in inserting the FETs between the resistor(s) R4 and/or R9 and the caps(s) C1 and C3 instead of between the caps and ground ?

Quote from: Ironsinthefire on July 21, 2017, 05:01:55 PM
Loudthud,

Given the difficulty of finding appropriate P-channel FETs nowadays

I got some 2n5462's from here:

www.utsource.net

HTH
Gavan

I tried this with a James (passive Baxandall) tone stack circuit derived from a Goldentone/Fender Blonde hybrid combination. It sounded promising too.

Quote from: Loudthud on March 12, 2017, 03:18:02 AM

IIRC Rod's preamp used a Baxandall tone control network. The Fender type network will certainly sound different.

Actually the tone stack in Rod's Project 27 (see attached) is FMV. The Baxandall stack is used in his Project 94.

Project 27 is the one that I used as a test setup - I bought 2 of the boards for experimentation purposes. In the circuit schematic, I replaced resistors R6 & R11 with a 1.5k and 820r respectively and mounted C3 & C8 externally with the JFET network on a daughter prototype board. I set the gate volts at +3.3 and -2.8 as an initial test (aurally) although I realize they would need to be adjusted to suit the specific FETs.

I didn't worry about the diodes at this stage as I don't use OD pedals or active circuit guitars. The results using a standard Strat with Seymour Duncan vintage PUs were extremely encouraging.

Quote from: Loudthud on February 18, 2017, 04:37:26 PM
As a general guideline I would say you want a JFET with an Idss roughly between 2mA and 10mA. The 2N3819 should work fine. If Idss is higher, the pot becomes very touchy, you might need a 10 turn trim pot. This all depends on the resistances in the feedback network. Lower impedances work with higher Idss parts. Just keep the ratio of the resistors the same.

Are the gate voltages for the 2n5462 (+3.3) & mpf102 (-2. the optimum for your circuit or are they just indicative of the voltages you needed to set for the specific devices you used?
I'm curious as we all know how varied the tolerances and ratings of JFETs of the same type can be.

I have tried adding your JFET idea to Rod Elliots's Project 27 preamp (mentioned elsewhere on these forums) and the results are certainly encouraging although I had to adjust the gain of the first 2 OPAmp stages upward. It made the preamp brighter with less mid-range wool and a crispness and bite one expects from a typical Fender front-end.

I breadboarded this up using a 3819 and a 5462. It certainly had the crsipness and brightness I have come to expect from the last blackface deluxe I played thru recently. The 200k pot adjust seems to be really touchy though. I tried using an 18k and 12k in series across the Vcc to ground (and Vee to ground in the MPF102 side) and then bridged the 12k with a 100k linear pot (which I had on hand) and fed the wiper to the gate of the FET. It definitely helped with the ease of adjustment

Quote from: Loudthud on February 18, 2017, 04:37:26 PM
As a general guideline I would say you want a JFET with an Idss roughly between 2mA and 10mA. The 2N3819 should work fine. If Idss is higher, the pot becomes very touchy, you might need a 10 turn trim pot. This all depends on the resistances in the feedback network. Lower impedances work with higher Idss parts. Just keep the ratio of the resistors the same.

Thanks for the heads up.

I have a selection of N-FETs on hand (2sk170, 117, 30, 2n5457, 3819, J201, MPF106) so I'll check their Idss and find some within that spec. The P channel 2n5462s I have some on order.
I also have some LND150 depletion mode mosfets but I'm not sure they would be suitable.

Cheers

I also had the idea of an aux send & return to run a delay/reverb unit. Rod's advice was to take a feed from the output of U1b (pin 7) via a cap, trimpot and 100 ohm resistor, the latter being to stop oscillation induced by shielded cables. The return feed could be re-inserted via a buffer with the output decoupled via a cap and 2.2k summing resistor with another 2.2k summing resistor in the output of U2a before the master volume.

There is also a method of patching in the Tremolo circuit - Project 29.

Cheers

how critical are the FETs? Is it the gate voltage that is the important factor?
If I dont have MPF102's will other FETs suffice - eg:2sk170, 2n3819, 2sk117 etc?

TIA.