bixonic expandora

[Chuck D. Bones]

I hope I'm not too late jumping on this thread.  I'm fairly new to this forum.  I have questions regarding the Expandora's opto operation.  I'm an EE, so please forgive me if I'm overthinking things here.  My questions pertain to the opto's FET resistance and how it affects circuit operation.  In a nutshell, I think the FET resistance is too high to do much of anything except in Forbidden mode.  Here's why I think that... 
The opto's maximum LED current is determined by the resistor between the opto and the PNP transistor.  In all of the schematics I've seen, that resistor is 4.7K.  Without getting into a detailed circuit analysis, the 4.7K resistor sets the max LED current somewhere between 0.6mA and 0.7mA.  The PC419 and H11F1 datasheets contain curves for FET resistance vs. LED current.  At 0.6mA, the FET resistance is  in the neighborhood of 3.5K (I had to extrapolate since the curves only go down to 1mA).  This is the *minimum* opto FET resistance in the Expandora.  Depending on the switch settings, the second stage feedback resistors present either 371, 560 or 1.1K in parallel with the opto's 3.5K resistance (not including the "forbidden" setting).  My question comes down to this: how can the opto make and difference in the sound when the resistors in parallel with it are so much lower in value?  Wouldn't it make sense to reduce the 4.7K series resistor down to 1K or lower?  Has anyone on this forum looked inside a production Expandora and can they confirm that the series resistor is in fact 4.7K? 
I built an Expandora with a 10K pot in place of the 560 Ohm and 1.1K feedback resistors and a switch to disable the opto drive so I can compare sound with / without the opto in the feedback loop.  I'm still experimenting with the opto drive current and will report my findings.

[rankot]

This one's on my "ToDo" list, so I will prepare some popcorn, since the list is very long Please share your experiences with this with us.

[digi2t]

If memory serves me correctly, I've compared this schematic to my original 2000R unit, and it matches. Note; the nomenclature on the schematic does not match the board. The 4K7 resistor in the schematic (R6) is R14 on the board, and so on.







There was also a change of transistors between the 2000 and 2000R versions. The 2000 version used the RN2206, which was internally biased, while the 2000R used a 2SA684R, which was a more "garden variety" transistor.

Insofar as the gain switches are concerned, I did away with these, favoring a pot instead. I called it "Edge" control. I kept the "Forbidden mode" as a switch, which I wired to a second footswitch. The pot covers the same range as the switches, but with all the in between settings that the switches can't give you.



And if that isn't enough to keep your brain cells busy, here the 2001 version schematic. Different layout and structure, including different opto;



Enjoy!

[Chuck D. Bones]

Thanks for the board pix!  I looked up the datasheet for the PC827 used in the 2001 model.  Very different beast from the PC419 or the H11F1.  The PC419 and the H11F1 are single optos and have FETs as the switch / variable resistor element; the PC827 is a dual opto and has BJTs as the switch / variable resistor.  In the 2001, the two optos in the PC827 have the LEDs connected in series and the BJTs connected back-to-back in parallel, for bidirectional operation.  The LEDs are operated at a much lower current in the 2001.  According to the datasheet, the transfer gain is higher in the PC827 and the operation in the ohmic region is not specified.  I would  expect the 2001 version to sound different from the other revisions based on the different opto.

[digi2t]

I would  expect the 2001 version to sound different from the other revisions based on the different opto.

The 2001 is a different sound to be sure, but the different opto angle, I think, only plays a small role. The overall structure is the same, but the IC's are different. The LM308 has been replaced by a TL072, which lends for a slightly more sterile tone. That's not to say that one is better/worse than the other. Just different. Both are good, but within their own respect. As such, I own both. They're that good, at least, to my ears.

[rankot]

If memory serves me correctly, I've compared this schematic to my original 2000R unit, and it matches. Note; the nomenclature on the schematic does not match the board. The 4K7 resistor in the schematic (R6) is R14 on the board, and so on.

And if that isn't enough to keep your brain cells busy, here the 2001 version schematic. Different layout and structure, including different opto;
Enjoy!

What's the purpose of LM324 part of a circuit, it is missing in later designs?

[Chuck D. Bones]

UPDATE:
I have learned a few things so far from my Expandora / RAT build.  First & foremost, there is an undocumented feature in the H11F1: Pin 5 is internally connected to something, probably the FET substrate.  My circuit is built on a Vero board.  I did not put a cut between pins 2 & 5 because (according to the datasheet) pin 5 is not connected to anything.  That turned out to not be the case.  The problem was particularly vexing because the H11F1 tested fine on a prototype board, but the FET would not turn on when installed on the Vero board.  I bent pin 5 up, I put the H11F1 back into the socket on the Vero, and voila!  Suddenly the opto started working. 

As I mentioned in my previous post, I suspected that the on-resistance of the opto's FET, as well as its saturation current, were not a good match with the Expandora's circuit impedances.  I tried increasing the impedance of all of the components in the 2nd stage's feedback network by 5x.  Using the reference designators from the Expandora 2000R v3 schematic (posted above), R9 was changed to 2.7K, R10 was changed to 240 Ohms, C8 was changed to 1uF and C9 was changed to 470nF.  The DRIVE control (replaces SW1, SW2, R19 & R20) is A10K in series with 1.2K.  I had also reduced R6 from 4.7K to 240 Ohms in an effort to increase the opto's LED current.  Once the opto started working, the effect was extreme gating and a very sputtery sound at lower GAIN settings and/or softer playing.  Clearly, the opto was now doing too much.  I raised R6 to 1K, but the gating and crossover distortion were still too severe.  R6 is now back to 4.7K and this is definitely in the ballpark.  The note attack and decay are more natural sounding.  There is smooth noise gating at the tail end of the note's decay.  When the strings are muted, the output is quiet.  I'll probably try 2.2K to see how that sounds.

Next, I started probing the signals at various points with my trusty USB oscilloscope.  The output of the 1st-stage opamp was close to 4V instead of the expected 4.5V.  I now doubt that the NE5534 is the best opamp for the 1st stage because its bias current is quite high, typically 500nA.  In the Expandora, and in my circuit, the input's DC path is 1Meg (GAIN pot all the way up).  This causes the offset voltage to be -0.5V in my circuit.  This offset consumes headroom and leads to asymmetric clipping when the 1st stage is overdriven.  This by itself may not be a big deal, but the other effect of the -0.5V offset is it interferes with the operation of the peak detector because the peak detector is referenced to 4.5V.  I replaced the NE5534 with a (vintage) LT1001.  Any opamp that has low current noise and very low bias current will work.  The LT1001 also has a fairly slow slew-rate, just like the LM308.  There are many other choices, but the LT1001 was at hand and works well.  Note that the 2000R v3 uses a TL072 for the 1st & 2nd stages.

I'm still experimenting with the 2nd-stage opamp.  For the moment, I have an LT1001 installed there.  I started out with NE5534s because they are low-noise and externally compensated.  That and JHS uses them in The Kilt.  Externally-compensated opamps like the LM308 and NE5534 are desirable if you want to dial in the slew rate.  I'll try an LM308 at some point to see if I can hear the difference.
Cheers!

[digi2t]

If memory serves me correctly, I've compared this schematic to my original 2000R unit, and it matches. Note; the nomenclature on the schematic does not match the board. The 4K7 resistor in the schematic (R6) is R14 on the board, and so on.

And if that isn't enough to keep your brain cells busy, here the 2001 version schematic. Different layout and structure, including different opto;
Enjoy!

What's the purpose of LM324 part of a circuit, it is missing in later designs?

There is a set of 4 small LED's in this version that act as a VU indication. The harder you strum, the more LED's light up. The 324 is acting as a comparator for the LED's.

[Chuck D. Bones]

At this point I'm ready to retract a fair amount of what I said in my previous post.  After more playing & listening, I found that the note decay still had some sputtering / gating noise at the tail end.  My circuit is now more-or-less back to the original Expandora component values.  The things that depart from the Expandora v3 are:
1. First stage is non-inverting.  Input impedance is 500K.
2. First stage gain range is 0db to 21dB.
3. First & second stages are direct coupled via R4 (C3, C4, R2 & R3 are not there).
4. R19 & R20 are replace by a 10KA Drive control.
5. C6 is 1nF.
6. C10 returns to GND instead of Vb.
7. I put a 100 Ohm resistor in series with D1.  Loading opamps with capacitors (C10) is generally bad practice.
8. Last stage is a unity gain buffer with a 10KA Level control in place of R14.  Output is taken from the Level control's terminal 2.
9. I added a "Contour" control consisting of a 10KC pot, 22 Ohm resistor & 47uF cap in series, all that in parallel with C9.  With the Contour control at 0, the freq response is the same as RAT & Expandora.  Rotating the Contour control clockwise increases the low mids & bass.  At 10, the freq response is flat down to 55Hz.
10. There is a 3 position Diode switch that selects combinations of silicon, schottky & LED clipping diodes, similar to the Proco Solo RAT.
11. There is a 4.7K resistor between IC3b and Q1 base and a 22K resistor from Q1 base to +9V.  This ensures that IC3b can turn Q1 completely off.
12. SW3 was deleted because the Contour control takes care of tailoring the low-freq response.  C14 is 1uF.
13. SW4 is 3PDT, the 3rd pole takes the place of Q2.
14. I implemented the power filtering and Vb supply slightly differently.  C15 goes from Va to GND.  C16 is 10uF.  R16 & R17 are 47K.  R15 is 100 Ohms.  D4 is a series schottky instead of a shunt zener.  D5 is not there.
15. The opamps in stages 1 & 2 are LT1001.  The output buffer & peak detector are TL072.

The Expander effect is subtle, but noticeable.  With the Drive at 5 (equivalent to about 2K resistance) there is 5dB of muting at low signal levels.  With the Drive at 10, there is about 13dB of muting.  Because the FET in the opto acts like a resistor when the voltage across it is <100mV and transitions to constant-current mode at higher voltage drops, it introduces crossover distortion.  A tiny bit of crossover distortion adds dimension to the sound.  Too much crossover distortion results in gating and sputtering at low signal levels.  If you like that kind of thing, putting a 5K trimpot with a 1K series resistor in place of R6 will get you there.

Now about the peak detector.  D1 and R11 set the threshold at about 0.3V.  For signal levels below 0.3V at the output of the first stage, the peak detector applies max current, about 650uA to the opto's LED.  Above that level, the peak detector reduces the opto LED current and when the first stage output reaches 1V the LED is fully off.  Because the Gain control is ahead of the peak detector, it also acts as a threshold control.  At lower Gain settings, it takes a bigger guitar signal to turn the opto off.

The last thing I fiddled with was R12.  I've found the hot end of the Tone control (clockwise in the case of v3) to be of little use, so I changed R12 to 6.8K.  With the Tone at 10, the cutoff freq is now 7KHz instead of 32KHz.  There is still plenty of top end and more useful Tone control range.

I'm done messing with the innards, on with the playing!

[Chuck D. Bones]

And here's my schematic, in case anyone is interested.

[rankot]

Great! Why using LT1001?

[Chuck D. Bones]

LT1001 has low noise and low bias current.  There are other good choices, but I had some of those on hand.  Did not like NE5534 there and have not tried LM308 yet.  TL071 should work well also.

[digi2t]

Well... it finally came to pass. I managed to get my hands on the rarity. The very first version of the Expandora.



Time to clear a corner of the bench and dig into the circuit. Plugged it into an amp and gave it a whirl. It's munchy crunchy. 

[ultrasun]

Hi everyone,
I need to change the footswitch on my Expandora v3 (with 3 external dipswitches). Loud static noise when I stomp on it, like it might or might not turn the pedal on everytime.
the footswitch looks smaller than most pedals. Would anyone have a link to the exact same footswitch?
Thanks a million!

[ultrasun]

Bump!
Anyone could indicate the appropriate footswitch that would fit in that small expandora?
Thanks!!!

[zedsnotdead]

Hi guys!
First of all I wish you a merry xmas and a 2021 full of health and wealth (we all hope so).

I was watching some JHS Kilt demos and I was wondering about the "Cut/Flat" switch... what do you guys reckon it is doing? I uderstand it's maybe swapping some caps, but where exactly?

Cheers!

[pinkjimiphoton]

Bump!
Anyone could indicate the appropriate footswitch that would fit in that small expandora?
Thanks!!!

i got one from small bear years ago. wasn't perfect, but it worked with a bit of adaptation