An EL34 to replace a 12AX7?
That’s interesting considering size and octal pin format.
You’ll need a transformer to power that thing at 325v minimum.
I might need to design a module with a 6550 in. Zoinks!
Thogre, perhaps you meant an EL-84
But the pinout is radically different as are the voltage and current requirements.
You can run a 12ax on low VDC and still get some saturation.
TBH, I don’t know. I just went with the silkscreen.
Whoa… that is so trippy! <3
Oh no, please, no updates on hardware !
Now I have to update my v1 ones… 
I like the Bartok reference
I ran out of 220 but had 1k on hand for my rush build.
pins 13 and 4 on the LM13700’s had optional values on silkscreen and here in the schematic
Looks like the 1k may increase the Gain of the amp, but one of you wizards likely knows for sure
R22 and R24 form a voltage divider to drop the input down to an acceptable range for the LM13700.
This is because the device will start to distort if the differential input goes above about 40mV:
If R22 is 10k and R24 is 220, the input will be divided by about 50.
R23 should match whatever R24 is to balance out any offset currents/voltages.
If you’ve got R24 as 220, but R23 at 1k, I don’t think you’ll notice much of a difference, maybe some slight offset.
But if you’ve got both R23 and R24 at 1k, the input voltage is only going to be divided by about 10, so you’ll probably get some significant distortion if you don’t reduce the input levels.
If R24 is 1k you could try bumping R22 up to around 50k (or 47k if you’ve got one) to keep the input division around 50.
Cheers
Bartok, yes, not BTS. The Music Thing module it was based on is called Mikrophonie (a Stockhausen reference); when I did my Kosmo version I couldn’t resist.
Thanks for this in-depth explanation of the gain structure. Truly appreciated.
Only the resistor on the inverted inputs are 1k. I’ll swap them out to 220’s now that I have ample restock on that resistor value
Nice!
Did you know that Bela’s son was an Audio Recording Engineer?
What a world.
Love the colours on that! So vibrant =D
Started to breadboard along to the @moritzklein x es.EDU VCO. I should state at this point that I didn’t buy a kit, I just have most of the components already, so though why not. Got as far as page 17 of the PDF instructions and have hit an issue.
I don’t have any BC548 transistors, so used a 2N3904, turned around so the pins were still the same. Plugged it in and noticed that the POT wasn’t changing the very faint buzzing, then I noticed a burning smell, so whipped the power off. The CD40106 Schmitt Trigger was rather warm, but I think I got to it before it went pop.
I thought a 2N3904 was a valid replacement for a BC548, or am I mistaken…?
In the last 2 ADSR+Gate-modules I was working on I used the BC547 (very similar to a BC548) in one and the 2N3904 in the other (indeed turning the device around) without noticing a major difference. But of course that may be different if a circuit uses a specific feature of a BC548 which a 2N3904 does not support.
I finished the semi-modular synth build! I’m quite excited about this and going to make a short video on it in the next couple of days.
That’s adorable …
(extra text ignore me)
Instructions say
So we will replace our drain resistor with a BC548 transistor – which is just a standard general purpose NPN variant.
and a 2n3904 also is a standard general purpose NPN variant. I very much doubt the substitution is your problem.
If literally it worked with the 40106 not overheating with the configuration on p. 15 but it did overheat with p. 17 then there’s not much to look at. As far as the 40106 is concerned you’ve only moved the diode and added the connection to the transistor. My bet is that there’s a mistake in your assembly somewhere, either with the diode or with the transistor and its associated components. Check those. Double check you really did get the transistor orientation right. Then you could try something like disconnecting the transistor from the 40106 and instead connecting the collector via a 1k to ground, then power it on and verify the transistor, pot, and associated resistors don’t overheat. Check the voltage on the collector to make sure it’s positive, and make sure the emitter is grounded. If not there almost certainly is some assembly mistake there. Another possibility is that the transistor is bad (or the 40106 is), if you have extras you could try subbing them but see if you find another problem first.
