H11F1M - a Vactrol alternative

[electronicpresskit]

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?

[decaying.sine]

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.
http://electro-music.com/event2008/foru ... highlight=

[electronicpresskit]

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.

[Moog$FooL$]

hhhmmmm.....

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.

[Moog$FooL$]

yeah, i wondering about distortion issues myself.
seems to be the case with these.

[electronicpresskit]

another thing that sets these apart from vactrols is the very low on resistance..it'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.

[felixer]

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!

[daverj]

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.

[wsy]

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

[ActiveFilter]

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

[zaphod betamax]

I have started to use the H11V in all my breadboard experiments.
They now run over $5CAN, but at least they can still be purchased during the chip shortage.

[imrae]

They do look ideal for making a multi-pole filter with simple stages and a common CV that works up to FM rates. I assume the Doepfer A-101-6 is exactly this, or something similar?

Maybe a bit simpler to design than using OTAs, but less space-efficient?

[Kevin Mitchell]

Reminds me of the use of the H11F3 used in the "Neutron" which was a clone of the Mu-tron III envelope filter by R.G. Keen. A pair of those replaced an unusual opto that had dual LEDs.
That project is over a decade old, so this is not a new solution and sadly it doesn't quite fit the bill for many of our projects.

The opto struggle is real. I'm down to my last few new-old-stock VTL5C3s. I actually used a pair in my Neutron which outperformed any opto chip.

[jorg]

CdS photo cells seem to be easily available - why not just make your own vactrols?

[wackelpeter]

Would second that notion... because i never bought a real vactrol, rolled them all at home from LED's and LDR's... Takes some more work, and one has to alter some parts, like resistors etc., especially if you use low current LED's like i do but they work.

Perhaps the only downside is, you can't get those huge bags of LDR's with 50 or 100 pieces from China for just a few bucks with shipping anymore and they probably are more expensive now and you might also have to pay custom fees or such.