gripes and more on cheap sampling

Entropy Struct entropy at thecia.net
Wed Jan 28 07:56:35 CET 1998


(will this go through? I hope so... It's been a few months since I've been  
on the list. There's a lot of griping/troubleshooting in this mail with a
little bit of intelligent discussion at the bottom..)

I finally got my asm-1 working to some degree. (2 asm boards, 2xVCF,
3xVCA, 3xVCO, 2xLFO, 1xnoise, 4xADSR, 1xglide) I had awful problems with
the LFO apparently bleeding into the outputs (most notably VCF and noise
generator), but after I disconnected the two 10uF tantalum conditioning
caps it went away like a charm, so i guess it was coming from the noise
source, not the LFO. Bad caps? 

Basically the only problem I have left is that the VCO's are still dead. I
checked the placement of the JFET and 3904's.. no go. I have the
FET properly placed and the 3904's right with the flat sides facing the
MAT-2 square.. There's a bit of low current running through the outputs,
but nothing more. Adjusting the trimpots doesn't help. 1 bad VCO I can
understand, but *3*? I have to be doing something fundementally wrong
here. It's the only "mystery" about this synth that's still bugging me...
and also, well, keeping it from being a usable synthesizer. If anybody
has ideas about what could be wrong I would *really* appreciate it. 

I know this has been mentioned, but I would also like to say that the 30pF
cap across the VCA outs really should be required. It sounds so much
better, and doesn't have that "buzz" at a certain point in the envelope. 

I also shorted out something when I accidentally slipped my trimpot
screwdriver down inbetween the 3140 by the Q out and the offset trimpot..
I thought I nailed either the closest 3140 or the 3080 but I replaced
those with no improvement... no mystery here though, I'll probably end up
replacing the chips one by one until something clicks. <sigh> I seem to be
pretty jinxed; my paia midi->cv died on me, too. The only thing I can get
out of the thing in trigger mode is a ground hum when I hook a proper
ground up to it... this never used to happen. It responds to midi data, so
something on the analog end must have gotten jostled in the move.
<grumble> It's too damn small, what could have possibly gotten fried? 

Although it's not strictly synthesizer-related, I remember a while ago
people asking me about the project I was working on involving cheap little
toy samplers... I've moved on a bit and found a little chip at radio shack
called the ISD1000A. Really nifty, and it's triggered by a momentary +5V
connection, which means you can use trigger outs or a gate signal to play
samples. (ah! finally a sign that it might be useful!) The only drawback
is you have to have some sort of battery-backed system lest you loose your
samples... so at best it's a plaything or studio item. It does however
have 20 seconds of sample time, and you can choose the sampling address
pretty easily with an 8 position DIP switch. (I'm just using 8 SPDT
switches probably for easy of twiddling) It also has a looping option...
hmmm... maybe the latest DJ-hyped-wassamajigger toy? 

I plan on putting a filter of some type at the output of mine. There are
no schematics needed for the sampler really, since a full spec packet
comes with the chip. (BTW i think it's regularly 17.99, i think i got it
on sale for 14.99 or somesuch, which means you can have a little sk-1 type
sampler up and running for at most $25 if you toss in the cost of a spiffy
plastic box) And one last thing... you can hook 'em together in parallel
for more sampling time... heh, I kinda hope somebody goes to buy a lot of
a dozen to hook together just to prove it can be done, although I'm sure
the sound quality is nothing to go nuts about. 

Andrew
NP: Autechre, "Chichlisuite"




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