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Old 15th March 2013, 01:05 PM   #2291
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wires are tooooo long
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Old 15th March 2013, 02:00 PM   #2292
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Yes, most of my wires to the PBC are a bit long. Without this length, though, I wouldn't have been able to lift the board and desolder my adjustment pot. Perhaps when I am "finished" tweaking I'll try to shorten them a bit.
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Old 15th March 2013, 02:26 PM   #2293
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you can't count on non-oscillating results , with wires so long

look at solution in Alep J X - daughter board is much closer to mainboard
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Old 15th March 2013, 08:29 PM   #2294
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Zen: Looking again at the photo, I'm wondering if I just took the picture from a poor angle. All of my wires to the daughter board are less than 3" in length. The brown and white twisted pair that runs from the bottom foreground of the image to the top is the extension for the VR2 trimpot for offset adjustment. The daughter board is mounted just behind where the original 9610 pair were.

Here are some before and after comparisons with the JFET front end.

Original 9610 differential (technical details of images are in the image headers):
Click the image to open in full size.
8 ohm output = 73w, bandwidth = 210kHz
4 ohm output = 90w, bandwidth = 140kHz.

New JFET differential (technical details of images are in the image headers):
Click the image to open in full size.
8 ohm output = 66w, bandwidth = 95kHz
4 ohm output = 62w, bandwidth = 120kHz

The traces with the JFET front end look more rounded with more overshoot on the leading edge. This doesn't make sense to me. It looks like increasing the value of the feedback caps would "fix" the overshoot on the leading edge of the square wave, but would also decrease bandwidth... Grabbing the feedback caps with my fingers confirms a reduced overshoot on the leading edge.

JFET Sine Wave:
Click the image to open in full size.

This one has me more baffled: the sine wave is not symmetric, it leans to the right. Sine wave from the 9610 looks perfectly symmetric (no image).

The top and bottom of the traces with the JFET differential look thicker than with the 9610. Is this oscillation? I don't understand the "messier" output and reduced power with the JFET differential. I was expecting cleaner output than from the 9610 differential. Any insights?
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Last edited by Eric; 15th March 2013 at 08:37 PM.
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Old 15th March 2013, 09:09 PM   #2295
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Subjectively on my main speakers, the JFET front end seems to be coaxing a bit more sparkle out of the tweeters and perhaps a pinch less weight or "tubby-ness" out of the midrange drivers. I would characterize these as positive improvements.

I'm also getting a bit more hum out of the speaker now, too... Perhaps it is time to work on shortening those wires to the daughter board. This will be a bit of a challenge as it will leave me with little room to work with the wires. I wanted to keep the daughter board from overlapping the main PCB so I could tweak the resistor values.
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Old 15th March 2013, 09:10 PM   #2296
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give me the link to Graeme's circ
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Old 15th March 2013, 09:57 PM   #2297
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Graeme's JFET Schematic

I only changed out the transistors, removed the diodes, and removed the gate stoppers for the 9610. I didn't change any resistors - I figured one step at a time. My resistor values for the input front end are listed a few posts up.

The JFETs are nicely matched (from Spencer), but I did not make any effort to match the ZTX550 transistors.
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Last edited by Eric; 15th March 2013 at 10:11 PM.
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Old 15th March 2013, 10:19 PM   #2298
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looking at pointed schm , I can think of few things :

- for voltage bias of cascodes - put them to 12V , changing D3 to 12V ; nice cap - 2u2 will do , is fine to put across diode

that's where , at least to my experience , Jfets are somewhat happier than on under 9


DZ - you can put plain 1N4148 in series , cathode pointing down of course - it will somewhat null tempco of DZ itself , but dunno for overall CCS tempco ..... I'm used to use two-bjt- CCS ( Widlar ) which is having somewhat better Tempco than sole mosfet CCS

regarding other things - you're pretty much there on your own in your own physical layout , to fiddle with drain resistor (at LTP) values and current through LTP , to achieve best performance , sound wise and CRO wise
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Old 16th March 2013, 01:17 PM   #2299
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Thanks, Zen! I can get some 12v Zeners from a local place today and I have a few 2.2uF 50v electrolytics in my parts box.

You've lost me, thought with some of your abbreviations, what are DZ, LTP, and CRO? I am also unclear about your intentions for the 4148 diode...
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Old 16th March 2013, 01:36 PM   #2300
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DZ is name of that 9V1 zenner in CCS above Long Tailed Pair ( other name for input differential pair )

or is it D2 - not clear enough on schm

CRO is scope ( Cathode Ray one )
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