All my ASM-1 questions at once!

Magnus Danielso magnus at
Thu Dec 4 11:24:35 CET 1997

>>>>> "KDB" == Kurt Dwight Bleach <gghegel at> writes:

 KDB> So, I'm plugging away at my ASM-1 and have, fortunately, found the
 KDB> answers to almost all of my questions in the archives. However, there's
 KDB> a couple of things I can't figure out, so I thought I'd compile them all
 KDB> into one e-mail, so as not to flood the list with a bunch of stupid
 KDB> newbie questions. Any input would be greatly appreciated!

 KDB> 1) In regards to the NPN pairs in the exponential converter, do 2N3904’s
 KDB> work okay or is it much better to go with the MAT-02’s? If the 2N3904’s
 KDB> are okay (I have a bunch lying around) how do they go into the board? I
 KDB> ordered some LM394’s from Digikey, but stupidly got the 8-dip package.
 KDB> What’s the correct package? TO-5? (Anybody need a couple 8-dip LM394's?)

The MAT-02's is much better paired and is also well thermally
connected, something that is worth noting since thermal differance
between those transistors transforms into detuning...

Personally I went for the MAT-02's and I feel happy about that decission.

Orienting the MAT-02 in metalcap is a little tricky, but not as your
second question below...

 KDB> 2) I’m using 2N4391 JFET’s. In trying to figure out their orientation on
 KDB> the board, I went through the archive and found what looks to me to be
 KDB> conflicting information. Can anybody help me out with this?
 KDB> First I saw this: “The 2N4391 to 2N4393 does not have the right leg
 KDB> pinout, you will have to bend the middle leg to end up on the other side
 KDB> or put the transistor on the soldering side.”
 KDB> But then I noticed: “Primitive ASCII pin diagram for 2N4391/2N4392 JFET:

 KDB> TO-18 Metal Can Package
 KDB>                    D
 KDB>                   ---
 KDB>                  /     \
 KDB>             S |   o   | G
 KDB>                | o   o |
 KDB> tab  --->  /\     /
 KDB>                   \/ ---
 KDB> **** Bottom view - pins pointed towards your eyeballs ****
 KDB> S = source, D = drain, G = gate
 KDB> In the ASM-1 VCO:  Source = goes towards CA3140 (-) input, Drain  = goes
 KDB> towards CA3140 output, Gate   = comes from LM311 output”

 KDB> Comparing this information with the schematic, I ended up putting mine
 KDB> on the board with the tab pointing straight up (away from the CA3140, no
 KDB> crossing pins). Is this correct, or did I screw up?

I found an error that Gene did in the schematic/board transition... I
guess many people know of this by now:

He wrote 2N4391 but he designed the board for the PN4391!

They are the same transistor really, but the capsule and pinning is
diffrent, so you end up with a very quite VCO or you found out somehow
and bended the Gate pin so that you can put it in the PN4391 position.
I had to do this to my 2N4391's...

Here is some horrible ASCII graphic of it (bottom view again):

 2N4391 PN4391

 G    D      D
 o    o      o

 o      o    o
 S      S    G

So you have to bend the Gate (pin2) along the diagonal in the square
for which the pins touches the corners...

Very confusing...

Oh, you migth not get the VCO running cause a little other trouble...
If the output locks to +15V then you should increase the feedback
capacitor over the LM311. This capacitor controls the reset-time and
when it is too small will it not empty the capacitor long enougth and
therefore it locks to +15V.

 KDB> 3) Regarding power supplies, I found a Harrison 802B twin low voltage
 KDB> power supply at a prop house for $3.00. (They thought it was a power
 KDB> amp.) It has knobs on the front to adjust the voltage. I tested it out
 KDB> and it seems quite stable, outputting +15V, -15V and ground. Anybody
 KDB> know if this will work or are there some important criteria of which I’m
 KDB> embarrassingly ignorant? (E.g. noise problems, hum, grounding problems,
 KDB> etc.)

 KDB> 4) This is more of an aesthetic thing, but I want to add a couple LED's
 KDB> to the Envelope generators. Can I just attach them to the ADSR out in
 KDB> the same way that the LED is incorporated in the LFO? I'd like them to
 KDB> flash when the envelope is triggered. Anybody tried this? (I know, it's
 KDB> silly, but I have a couple bright blue LED's collecting dust...)

Put them to work then... it should be OK, but if you run into trouble
use a unity gain buffer with a spare op-amp to reduce the load at the
ADSR output.

 KDB> Well, any help would be grand. Thanks!!!

PS. In case people forgot or didn't know it there is a ASM-1 homepage at
    I admitt that it is not refreshed for a long time... motivate me
    to change that ;)


More information about the Synth-diy mailing list