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H11F1M - a Vactrol alternative
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author H11F1M - a Vactrol alternative
I haven't see the H11F1M talked about much but I think it deserves some attention! Here's the Datasheet

It's an LED and a FET configured as an optocoupler in a 6 pin DIP package designed to be used as a remote variable resistor. You can get them from mouser for $2 a piece so they're much cheaper than vactrols, but the DIP package means a little more work if you like to solder directly to existing components of the circuit you're modding (like I do).

I just pick up some proto board from radioshack and chop it (dremel) into 3 row pieces suitable for the DIP.

Another nice thing is their LED is robust, or perhaps there is some resistance in the LED path that's not listed on the schematic. Bottom line is you won't burn out the LED if you forget to add a resistor like I've done so many times with vactrols.

I've only just begun using these but so far they seem really good! Anyone else tried these?
Nice find. Post some tunes if you do tests. I was trying to find out if any synths used these and came across this EM post from our own doctorvague...

There are some samples and some more info. &postorder=asc&highlight=
oh yeah that thread and a couple brief emails on the SDIY list are the only discussions I was able to find on it. That thread also reminds me to mention there is distortion in some configurations. So far I've just used these on my MB-808 and I've only noticed distortion when I added CV control of pitch on the Toms.

Here's the Low Tom being modulation by a dual slope generator. You'll hear there are certain frequencies I hit that just peak the hell out of the circuit and I don't know if that's the H11F1M or if it's due to the circuit itself having a tendency to resonate more at certain frequencies.

news to me.... i've never even heard of this chip before.

thanx for the info. i'd like to know more.
6.4 Billion
I tried to use the H11F3 for a project and it didn't work out. There was horrible distortion at lower CV levels.
yeah, i wondering about distortion issues myself.
seems to be the case with these.
another thing that sets these apart from vactrols is the very low on's listed at less than 100 ohms and the one I measured was 47 ohms. For the Tom tuning circuit on the MB-808 this was really important since the tuning pots are only 500 ohm, so being able to get down to 47 ohm gets most of the tuning range. I measured several VTL5C3's and the lowest on resistance was around 600 ohm which doesn't even get into a usable range.
plus it's FAST. prob not too good for audio signals (due to high distortion) but nice to hack cv for 'expression pedal' inputs?
must try this .... thanks for the tip! thumbs up
The slow speed of Vactrols is one of the "features" that a lot of people seem to really like in synths. They are 200 to 2,000 times slower to turn on and off than the H11F1M.
daverj wrote:
The slow speed of Vactrols is one of the "features" that a lot of people seem to really like in synths. They are 200 to 2,000 times slower to turn on and off than the H11F1M.

It's easy to make things go slower - drop a series low-ohm resistor and add a parallel
electrolytic cap before the LED input and that should do it...

- Bill
Is FredMundell lurking on this list??

He showed me the design for a commercial VCF he did with H11F1's in the early 80's...

IIRC the company went bust before thesynth came to market
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