Blackface preamp emulation

mybud

Well-known member
Health warning, long and wordy post ahead:

Ok, so I've been revisiting/fixing/housing half-completed builds as 2023 unfolds. I had Veroboarded this singing and dancing all-FET version of the Zender Blackface preamp awhile back (http://redcircuits.com/Page120.htm). But it was decidedly underwhelming with my original substitute 5458s for the 3819s as specified. I revisited it to check whether there was some error in my wiring and heard faint signal if I turned my trusty audio interface up to stun. I surmised that the 5458s weren't getting the right voltage, so tried a quartet of 5457s on a hunch. Gosh darn, it started singing a tune reminiscent of the old Zender Spaceman.

IMG_0913.jpg

Since I have a small handful of 5457s and more J201s, I tried them (J201s) instead and couldn't hear too much difference. The original circuit calls for 18V, so I used two 9V batteries in series in the testing phase. Then I remembered that I had ordered some of Brian's nifty Road Rage boards...

IMG_0915.jpg

These marvellous little chaps do two versions of 9V (regulated and not), 15 and 18V, and even -9V for the Fuzz Face aficionados out there. In testing with the superbright green LED I had in mind, it was blindingly bright. I thought of running the LED off the regulated 9V option and the main board off 18V to mitigate the brightness a bit.

IMG_0916.jpg

Here you see this morning's status quo (in progress). After some tweaking, splicing wires, removing a solder bridge (who me?) and other corrective measures, I finally boxed the whole mess into a 1590BB whose drilling I had earlier borked good and proper. I'll spare you the final gut shot, which is not pretty.

IMG_0919.jpg

In short, I love this thing beyond words. Four JFETs, a couple of resistors and caps, four pots, simple but effective in capturing the Zender Spaceman sound. I especially like the low end and glassy brite switch (you can almost smell the antique Tolex), but there are wealth of toans in there for the patient experimenter and plenty of nice fuzzy goodness for the distortion lovers. I find the whole counter-intuitive (at least to me) aspect of JFETs endlessly fascinating and would love to try getting this to work with 5458s or 5459s. I'll breadboard another in due course and see if my calculations hunches for modding the input stage for these alternatives are in or near the ballpark.

Thanks, as always, for reading. Now go Veroboard something...
 
Your build report reminded me of this schematic. I was going to test it once but apparently forgot. I put it on breadboard yesterday. I don't have 2N3819 so I tested with 2N5458, 2N5457, J201, J113. First of all - damn high level of the output signal. At 18V, gain and vol at max it can be over 15V p-p. There is also a lot at 9V, over 6V p-p. With my standard usage, on headphones, with cabsim, vol does not go past 9 o'clock.

I hope you wont mind if I hijack your thread a little bit and post some testing info:)
2N5458 @ 18V and 50mV input signal - 13.3V p-p output, 400mV - 15.1V p-p
2N5457@ 18V, 50mV - 14.9V p-p, 400mV - 15.1V p-p.
J201 - similar to 5457.

And some oscilloscope views. Yellow trace - output, blue trace -output of the first stage
2N5458 - 50mV and 400mV input. 18V supply. Vp - around -2.3V
DS1Z_QuickPrint1.png DS1Z_QuickPrint2.png

2N5457 - 50mV and 400mV input. 18V supply. Vp - around -1.5V
DS1Z_QuickPrint3.png DS1Z_QuickPrint4.png

9V supply. Some assymetry appears.
DS1Z_QuickPrint5.png DS1Z_QuickPrint6.png

J201 - 50mV and 400mV input. 18V supply. Vp - around -0.7V
DS1Z_QuickPrint7.png DS1Z_QuickPrint8.png

J113 - 50mV and 400mV input. 18V supply. Vp - around -2.4V
DS1Z_QuickPrint10.png DS1Z_QuickPrint11.png

If you're interested I could record something for a comparison between mentioned transistors.
I also have screenshots from the ltspice simulation but I don't know if it's appropriate to publish them in this thread (which is a build report after all)
 
Far from hijacking the thread, @temol, I think you’re adding something really useful. I’ve also found it very loud and similarly don’t go past 10 o’clock for useable volume.

Your build report reminded me of this schematic. I was going to test it once but apparently forgot. I put it on breadboard yesterday. I don't have 2N3819 so I tested with 2N5458, 2N5457, J201, J113. First of all - damn high level of the output signal. At 18V, gain and vol at max it can be over 15V p-p. There is also a lot at 9V, over 6V p-p. With my standard usage, on headphones, with cabsim, vol does not go past 9 o'clock.

I hope you wont mind if I hijack your thread a little bit and post some testing info:)
2N5458 @ 18V and 50mV input signal - 13.3V p-p output, 400mV - 15.1V p-p
2N5457@ 18V, 50mV - 14.9V p-p, 400mV - 15.1V p-p.
J201 - similar to 5457.

And some oscilloscope views. Yellow trace - output, blue trace -output of the first stage
2N5458 - 50mV and 400mV input. 18V supply. Vp - around -2.3V
View attachment 39740View attachment 39741

2N5457 - 50mV and 400mV input. 18V supply. Vp - around -1.5V
View attachment 39742View attachment 39743

9V supply. Some assymetry appears.
View attachment 39744View attachment 39745

J201 - 50mV and 400mV input. 18V supply. Vp - around -0.7V
View attachment 39746View attachment 39747

J113 - 50mV and 400mV input. 18V supply. Vp - around -2.4V
View attachment 39748View attachment 39749

If you're interested I could record something for a comparison between mentioned transistors.
I also have screenshots from the ltspice simulation but I don't know if it's appropriate to publish them in this thread (which is a build report after all)
 
Your build report reminded me of this schematic. I was going to test it once but apparently forgot. I put it on breadboard yesterday. I don't have 2N3819 so I tested with 2N5458, 2N5457, J201, J113. First of all - damn high level of the output signal. At 18V, gain and vol at max it can be over 15V p-p. There is also a lot at 9V, over 6V p-p. With my standard usage, on headphones, with cabsim, vol does not go past 9 o'clock.

I hope you wont mind if I hijack your thread a little bit and post some testing info:)
2N5458 @ 18V and 50mV input signal - 13.3V p-p output, 400mV - 15.1V p-p
2N5457@ 18V, 50mV - 14.9V p-p, 400mV - 15.1V p-p.
J201 - similar to 5457.

And some oscilloscope views. Yellow trace - output, blue trace -output of the first stage
2N5458 - 50mV and 400mV input. 18V supply. Vp - around -2.3V
View attachment 39740View attachment 39741

2N5457 - 50mV and 400mV input. 18V supply. Vp - around -1.5V
View attachment 39742View attachment 39743

9V supply. Some assymetry appears.
View attachment 39744View attachment 39745

J201 - 50mV and 400mV input. 18V supply. Vp - around -0.7V
View attachment 39746View attachment 39747

J113 - 50mV and 400mV input. 18V supply. Vp - around -2.4V
View attachment 39748View attachment 39749

If you're interested I could record something for a comparison between mentioned transistors.
I also have screenshots from the ltspice simulation but I don't know if it's appropriate to publish them in this thread (which is a build report after all)
I love oscilloscope traces! What oscilloscope do you use?
 
Nice build report, @mybud !!

I have built this one with 2n3819 and have found it a bit underwhelming (granted my 2n3819 vary wildly in specs - as do 2n3819 per the datasheet)
I subbed in some 2sk304 (somewhat similar in specs to j201) and like it much better. It's still a bit too bright for my taste, though
 
Nice build report, @mybud !!

I have built this one with 2n3819 and have found it a bit underwhelming (granted my 2n3819 vary wildly in specs - as do 2n3819 per the datasheet)
I subbed in some 2sk304 (somewhat similar in specs to j201) and like it much better. It's still a bit too bright for my taste, though
Thanks, @lrgaraujo. The brightness seems to vary depending on where it is placed in the chain, suggesting there are some impedance mismatches going on, and (as you mentioned) the variance in JFETs, not to mention choice of guitar and other FX in the chain, also surely plays a part.

That said, it seems a good candidate for tweaking (add some trimmers to fine tune the JFETs, moderate the excessive gain, and so on). With the right PCB-mount pots and a bit of forethought to the Vero layout, it should fit in a four-hole pre-drilled 125B enclosure for greater pedalboard-friendliness. I have some 3819s on order and will keep working on it meanwhile.
 
Back
Top