Son of Dork: Active Circuitry

[HarryHaller]

All the Borbely designs with negative feedback are "op amps", whats the distinction?

H.H.

[sonnya]

There is no one. Only in the the type of design. Borbely shown in this thread are what i would call a CFB design where the forselltech design is an VFB design.

Sonny

[jam]

Marc,

That is why I suggested hooking one end of the buffer to ground and letting one end of the coupling capacitor sit at about half the poistive rail. Most people miss the fact that coupling capacitors work best with some dc bias on them. Black Gates particularly, will compete with the best films.

Jam

[HarryHaller]

Another nice thing about the differential pair using same type of fets is that closely matched pairs such as the K389 and J109 are readily available. Also the K170 and J74 do not have the same transconductance. Gains never can be precisely matched for each half of the circuit. These is somewhat aleviated by source degeneration for the fets in the complementary diff pair. Adjustment of source resistors value with a pot for DC offset nulling can further complicate things. DC Offset trim can unbalance the gains for each half of the circuit. I also hate putting a pot in a location in the circuit where resistor quality is so important.

[dorkus]

yeah, i'm not so sure about offset trimming either. pots kind of suck. i would only use them to find optimal values in a circuit, then replace it with proper fixed resistors.

i guess you're also pointing out a fundamental problem with all complementary circuits - lack of symmetry w/devices. i'm hoping with the K170/J74 pair they will be "close enough" but i can see why so many prefer the SE route.

anyway, i just ordered a bunch of JFETs from MCM... K170, J74, K246, J103, K389. the J103's are out of stock but hopefully they will be available soon. i got 100 K170's, gonna have to match them all probably, bleh... i will probably build the balanced SE line amp first, maybe the Forssell too if i have time. when the J103's arrive i'll try the complementary super-buffer. does anyone have any suggested modifications to the original borbely SE design?

i was going to ask another question but i can't remember what it was...

[dorkus]

oh yeah i'm going to try to run the SE line amp off +/- 24V instead of 36V, so i don't have to adjust the supplies to try the other circuits. i'm assumming i just need to adjust the top resistors (R3/R6), the diff pair source resistors (R4/R5), and the current source resistor (R9)? not sure how much i'll need to adjust them, guess i'll have to play around... or i can go hit my old textbooks and do the math i suppose.

[HarryHaller]

Voltage equals current times resistance. Go read Nelson Pass's web site and Erno Borbely's for any more detail than that. This stuff is not rocket science but is a little more difficult than balancing one's check book. An evaluation version of Spice of the web might help also. All the jfet models have been posted I think.

J103's? Don't you want J109s?

H.H.

[dorkus]

Voltage equals current times resistance

yes, thank you harry, i was an EE.
i was not sure off-hand how the current would change as i changed voltage/resistance. i'm assumming a linear approximation is not good enough because of the characteristics of the active devices. i may try to get PSPICE or something set up so i can do some simple DC modelling.

J103's? Don't you want J109s?

i dunno, but borbely shows J103 being the complement to K246... did i make a mistake?

[mlloyd1]

dorkus:
Have you thought about experimenting with different cascoding devices:
1. JFETs as you are currently planning
2. MOSFETs as Borbely (and HH ?) advocates
3. Some high performance (e.g. low collector-base capacitance, high Ft, high beta, etc. like the Sanyo 2SA1210/2SC2912) bipolars

I think some listening tests of the same circuit while changing the cascoding device may be interesting.

Too bad nobody has an Audio Precision handy - it would be nice to see how the numbers correlate with listening observations and preferences.

Ooopps; I said the bad word, "numbers" I'll go hide now..... heh, heh, heh

mlloyd1

[dorkus]

mlloyd1,

nope, haven't thought about tweaking the cascoding yet, but i'm welcome to suggestions. i need to read up first on my cascodes (my analog teacher would start the lecture about how great they are but by the time he got to the details i was usually asleep ) , so i don't feel confident coming up with my own schemes yet but after i get the basic circuit up and running i'd definitely like to try modifications. i'll probably wait until i get a handle on the "sound" of the base circuit first though.

cheers,
marc