The jfets were chosen to work in the middle range of the 50 ohm pot in
the circuit, this to achieve the specific Drain and Current values for the
character wanted - 1% 2nd harmonic at 0.5V out.
I tested J113's for Vp, the pinch off voltage where the Drain is at +12V,
the Gate is grounded, and the Vp is measured on the Source attached to
ground through 1 megohm. This at 22 degrees C.
In this batch, I chose Vp because the Idss test heats the jfet up quickly,
skewing the result which is temperature dependent.
I chose Vp at 2.50 to 2.59 volts, which Idss corresponds to about 28 mA.
In the adjusted circuit the jfet settles into a Drain voltage between
4.40V and 4.60V at about 27 mA.
Other values of Vp will want different voltages at T2 - with R3 at 221
ohms, that voltage is about 4.1 times Vp. You can adjust R6 to get in
range for the pot.
This is an approximation. If you want precision you will need to measure
the distortion when making the adjustment.
Thank you Nelson, great explanation, even I understand how to measure Vp now. Lab supply, jumpers and and a 1M ohm resistor.
I think I'll order some J113's to try it, not going to risk the FET's you sent us.
The voltmeter is connected from source to ground to measure Vp, across the resistor, if I read it correctly. How long before the measurement is taken?
The hard part will be getting to/maintaining 22'C in my work room.
I think I'll order some J113's to try it, not going to risk the FET's you sent us.
The voltmeter is connected from source to ground to measure Vp, across the resistor, if I read it correctly. How long before the measurement is taken?
The hard part will be getting to/maintaining 22'C in my work room.
Thanks Papa!
Until my Hammond case gets here I’ll test it as is. R1&2 set for Unity gain. EPCOS 4.7uF film caps instead of electrolytics. Panasonic FCs…..Dales are a bit big…
Initially testing it added onto the outputs of my B1. Can it match the loveliness of my B1K?….we’ll see…
Happy building everyone.
Cheers, Pete
Until my Hammond case gets here I’ll test it as is. R1&2 set for Unity gain. EPCOS 4.7uF film caps instead of electrolytics. Panasonic FCs…..Dales are a bit big…
Initially testing it added onto the outputs of my B1. Can it match the loveliness of my B1K?….we’ll see…
Happy building everyone.
Cheers, Pete
Attachments
Until my Hammond case gets here I’ll test it as is. R1&2 set for Unity gain. EPCOS 4.7uF film caps instead of electrolytics. Panasonic FCs…..Dales are a bit big…
Initially testing it added onto the outputs of my B1. Can it match the loveliness of my B1K?….we’ll see…
Happy building everyone.
Cheers, Pete
Nice, Pete
What is the Epcos series? Never saw a so small 4u7 with 5mm LS!
Nice, Pete
What is the Epcos series? Never saw a so small 4u7 with 5mm LS!
EPCOS/TDK Polyethylene Terephthalate (PET) film caps &
some creative bending of the leads
WIMA MKS 4.7uF would have been my other choice, with the same lead bending.
PPs would have (obviously) been outrageously big…
Pete
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blush
ashamed
Yes, the speaker leads still have to be switched, but I'm just wondering if there would be a difference sonically. Where would be the best place to "inject" the H2 signature? Before or after the preamp? I haven't compared it, though.
Maybe it doesn't matter with the B1 buffer, since it has no gain, but maybe there is a difference with other preamps.
.
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ThanksEPCOS/TDK Polyethylene Terephthalate (PET) film caps &
some creative bending of the leads
WIMA MKS 4.7uF would have been my other choice, with the same lead bending.
PPs would have (obviously) been outrageously big…
Pete
I’ll admit I’m a paranoid audiofool and really don’t like to put electrolytics in signal paths unless there is no other choice. I bought the C1 caps Papa had in his BOM and will try them just to see if there is a difference.
Pete
Pete
Never did a distribution before..was kinda nervous some demons would arise somewhere. Just want everyone to get their boards & get them out of my house. . Also hope the boards sound magical after all this…
I didn’t think my initial request to Nelson for the purchase of some matched jfets & permission to copy the older version of the board would have snowballed into this …
LOL
Being proud of my build (the first build pictured)? …. everyone musta known that was gonna happen….hahahaha
Pete
. Also hope the boards sound magical after all this…I didn’t think my initial request to Nelson for the purchase of some matched jfets & permission to copy the older version of the board would have snowballed into this …
LOLBeing proud of my build (the first build pictured)? …. everyone musta known that was gonna happen….hahahaha
Pete
C‘mon, admit it: you‘re proud
I did some tests this morning and compared the sound with the H2 after the B1 - as opposed to before the B1, and it sounds the same to my ears.
I also removed the B1 altogether and it still sounds good. I'm now considering to not use the B1. Less components, you know... Papa's H2 article actually mentions that the H2 can be used as a preamp and also how to change the gain which is 9dB in stock form.
.
I got my board and am still waiting on parts from Digi-Key. I'm sure managing the distribution was a ton of work and I want to let you know how grateful I am for your effort here. And of course, thanks to Papa for his generosity in providing the boards and parts which is really quite extraordinary.Never did a distribution before..was kinda nervous some demons would arise somewhere. Just want everyone to get their boards & get them out of my house.