Discrete Opamp Open Design

[bcarso]

Originally Posted by scott wurcer

My bad, I set up my first experiment with 2SK170's and left out the cascodes for simplicity. The high Cgs caused the oscillations, back on track now. The input stage looks to match simulation for voltage to current transfer very well. Now to put it all together.

Seems to be the season. I strung together a mostly-air-circuit to do a quick test of a vacuum tube stage and used a base-current-recycling thing along the lines of Aldridge, with moderately slow bipolars (2SA1015, 2SA1249, running at about 15mA). I tested it with a resistive load to begin with and found that it was rather intolerant of capacitive/transmission-line loading. After some help from base resistors I wound up with about 820 ohms in series with the output collector to make it quit convincingly. It was taking off at about 65MHz.

With a much tighter layout it probably wouldn't require as much series R.

[linuxguru]

Just completed the layout of a folded Kaneda-style discrete opamp with 7 actives, all currently-available commodity SMDs. Here's the link to the schematic and layout:

kaneda preamp

It's about 0.5" x 0.6". Some more space could be saved by using a dual JFET like the 2sk2145, but the singles (2sk209) are easier to source in BL- grade.

Note that it differs from the stock Kaneda - this one is folded, and has rudimentary output stage bias current stabilisation.

[scott wurcer]

Originally Posted by linuxguru

Just completed the layout of a folded Kaneda-style discrete opamp with 7 actives, all currently-available commodity SMDs. Here's the link to the schematic and layout:

This looks very nice, have you tried it in a few of the standard applications to see where it might shine or not? I/V, line amp, pre-amp, input to PA, etc.

[qusp]

Scott, just wondering how you think the SW-OPA might go as part of a discrete DSD FIR filter?
seems that given the BF862 are RF jfets and DSD128/256 is effectively almost RF range before the filter, it might be a good match?

[linuxguru]

Originally Posted by scott wurcer

...have you tried it in a few of the standard applications to see where it might shine or not? I/V, line amp, pre-amp, input to PA, etc.

I just gave the dual, through-hole version for PCB fabrication today - probably a month for bare PCBs and a week or two after that for assembly and test. I'm hoping it will do OK in line stages in CDPs, DACs, soundcards, etc. - anything with a high-Z load. It could in principle drive low-Z headphones with a heftier output pair and higher output stage bias, though it wasn't designed for that.

Meanwhile, I've upgraded the SMD version to use Rohm IMX2/T2A duals in SC74 packages, which have higher (300 mW) Pd. I'll probably send it out for fabrication only after the through-hole version comes back and passes sanity checks.

[scott wurcer]

Originally Posted by qusp

Scott, just wondering how you think the SW-OPA might go as part of a discrete DSD FIR filter?
seems that given the BF862 are RF jfets and DSD128/256 is effectively almost RF range before the filter, it might be a good match?

In fact this looks to be a little over the top in BW and slew rate. I need to build a tighter prototype since this one seems a little to hot. I just got a little too sloppy on the breadboard. Everything looks great but I can still coax a little fur on the output under some conditions. The basic topology seems to be doing everything intended. I will be piggy-backing some real PC boards over the holiday break on some work projects.

[scott wurcer]

Originally Posted by linuxguru

I just gave the dual, through-hole version for PCB fabrication today - probably a month for bare PCBs and a week or two after that for assembly and test. I'm hoping it will do OK in line stages in CDPs, DACs, soundcards, etc. - anything with a high-Z load. It could in principle drive low-Z headphones with a heftier output pair and higher output stage bias, though it wasn't designed for that.

Meanwhile, I've upgraded the SMD version to use Rohm IMX2/T2A duals in SC74 packages, which have higher (300 mW) Pd. I'll probably send it out for fabrication only after the through-hole version comes back and passes sanity checks.

Look forward to your results, I'm not prejudiced in any direction.

[PHEONIX]

Originally Posted by scott wurcer

In fact this looks to be a little over the top in BW and slew rate. I need to build a tighter prototype since this one seems a little to hot. I just got a little too sloppy on the breadboard. Everything looks great but I can still coax a little fur on the output under some conditions. The basic topology seems to be doing everything intended. I will be piggy-backing some real PC boards over the holiday break on some work projects.

Hello Scott,

Have you measured THD yet at say @6Khz.

Arthur

[paul kemble]

Originally Posted by bear

Afaik, the 990 used this for the front end:

http://www.ti.com/lit/ds/symlink/lm394.pdf

_-_-bear

Also, why is this going to be anything worthwhile (wrt the above specs for example) since there are a slew of ultra super low distortion monolithic IC opamps now available??

Agreed. I think I'd be more inclined to find something old and contemporary that's in need of some tlc unless some custom deck came up.

[scott wurcer]

Originally Posted by PHEONIX

Hello Scott,

Have you measured THD yet at say @6Khz.

Arthur

Briefly (with properly applied fingers ) at 10kHz, G = 20dB, 500 Ohm load, and 20V p-p the distortion went into the noise. There also does not appear to be any pathological behavior, foldover, supply sticking, etc.

Remember this started out for me with the intent of using 140V devices and making a PA front end. The available IC PA front ends are not that great.

And this thread has dragged out a great variety of good discrete device options for all projects.