H11F1 Opto-Isolated FET is back!

There was a wonderful component, the H11F1, which was multi-sourced and widely used in a host of appications requiring a remotely adjustable resistance - VCA's VCF's to name a couple.

This part was discontinued by every supplier in the wake of the RoHS madness - and there was no replacement - the closest replacement were the fragile, horribly slow, and extortionately priced Cadmium Sulphide LED/Cell devices.

I gave up looking for and using these (H11F1) parts because they were only available from 'obsolete component' suppliers at high prices.
I have now discovered (by accident) that Fairchild Semiconductors is again producing these parts, in RoHS compliant form.. Price has gone up a bit (£1.70 vs £1.10) but they are worth it for applications that need them.

Applying a current via the LED causes the FET resistance to vary - from ~100 ohms to ~300M ohms, and these isolators have a fast response (25us!) They can be used to make remote attenuators with unmeasurable distortion..

It is really nice to see this part return!
(available from Digi-key.. http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=H11F1M-ND )

Data-Sheet: http://www.fairchildsemi.com/ds/H1%2FH11F1M.pdf

Oh - One other thing.. Because the gate of the FET is not exposed, one can handle this part without taking extreme* care about ESD (*one should always take care with all components - but high-Z inputs, like FET gates, are the most likely to get zapped)

p.s. " attenuators with unmeasurable distortion " only applies if one keeps signals across the FET low - easily done. If one overdrives the FET, distortions occur - and this can be useful, as the distortion resembles 'soft clipping' and can be a useful 'tube' type effect.

I think this part is probably the simplest way to implement volume control for a Theremin.. One could take the audio from the mixer directly to the FET, and the CV via a resistor to the LED.

(Back in 1979 I obtained some of the first pre-release samples of this part from GE, and was the first importer of them to the U.K. .. So I am probably a bit more 'sentimental' about them than I should be! ;-)
Fred.

He Fred.
Jipp! A just beautiful component - in my ears, too. I discovered the features the last month after by chance having found it listed at a german distributor . VCA´s are not too easy to handle (to me). So i replaced a LED/LDR coupler with the H11F1 and: the sound appeared to be heavily distorted. Ok, i thought, it seems to be fast but... Then i lowered the signal! Hui. And just after "no distortion" by pushing the amplitude again it sounded pretty well tubish.
So, nothing new to your statements. But one more sentimentalist.
Dominik

Hi Dominick,

Glad you found the H11F1 - its good to have 'confirmation' that I am not deluding myself! - I have not heard anyone else comment on the 'tube' quality imparted by this device.. and was almost afraid to mention this - I can see (and measure) the deliberate distortion, but there are many other circuits which give similar distortion but do not sound 'tube-like'.. I see no particular reason for this quality - and if I dont see a reason, I tend to doubt my judgement.. There are probably about 100k H11F1's in audio products I have designed (although I expect most are now in land-fill.. they were cassette based AV type products) and no-one ever commented on them sounding 'tube-like'.. but it was a long time ago.. and I left the company soon after the units went into production.. The major feedback I got was "why did you pick such a b***dy expensive and difficut part to source, for the VCA?" - LOL ..

[i]" VCA´s are not too easy to handle (to me) " - Dominick [/i]

VCA's have several issues which can cause difficulty .. particularly if adding these to an existing design (no problem if one designs everything oneself) - You need to get the control voltage range and polarity right, which can involve level shifting and/or inversion of the CV, and one needs to get the signal levels right.. (also supplies for the VCA) all a bit of a pain if retro-fitting to something already built.

Interfacing to an opto-isolated device is usually a whole lot simpler.. Worst case, one needs an op-amp, transistor, and a few resistors to provide the correct LED current for any given input CV range, and there are no critical factors - one can fiddle with a couple of resistor values (or fit presets) and easily adjust to taste.

The isolated resistor or FET can be similarly trimmed with a couple of resistors.. Worst case one may need to add an op-amp to the input circuit .. but a small board containing one dual op-amp, 6 resistors, one transistor and a few C's (and the H11F1) makes a universal linear VCA which can accept any normal CV range or polarity, and any normal signal levels, and can be optionally fitted with a potentiometer to give 'tube' distortion control.

Now that the H11F1 is available again, I am going to produce the above boards - price will depend on quantity I can afford to make, but a fully assembled and tested board will certainly be under £50 but I hope to get price to £25-> £35. Anyone interested, please email me:

fred
[at]
fundes
[dot]
co
[dot]
uk


These would be boards fitted with trimmers, having instructions (one will require a multimeter) and set to give unity gain with a CV of 5V, and -90db at CV=0 .. The trimmers will allow any CV range and polarity to be selected.. There will be connectors for wiring a distortion potentiometer, or fitting a resistor to set distortion..

Please, send no money now (LOL - as if.. ;-) - no committment - I am really interested in knowing what possible level of interest there would be for this board, so that I can decide how many to build.

(I will publish the schematic on E-14 and sell PCB's to those who want to do their own build - details in about 2 weeks..)

Fred.

Vactrols are more linear, but of course more expensive. You can still get them, though.

The SSM2018T is a "cadillac" VCA chip. It works quite well, too, but again it is expensive.

Neither of these are available from Mouser or Digikey, unfortunately.

Don

I imagine interest in this would extend beyond the theremin community. It might be worth mentioning it on electro-music.com's DIY Hardware and Software forum, for instance.

[i]"It might be worth mentioning it on electro-music.com's DIY Hardware and Software forum, for instance." - Gordon [/i]

Thanks Gordon, I will - But only when I have boards done and dusted, so to speak.. ;-)

strange ol life.. Ive been in a real panic about scraping pennies together to survive.. And, as always seems to happen, at the last moment I start getting some cash-earning work to keep me going until the next crisis ;-)

But this job I landed today is the strangest I have had yet - I am designing a controller for an extremely high-end TOASTER! (absolute intrinsic safety, and all sorts of incredible features I cannot disclose).. My client selected me because (wait for it..) he wants this toaster to incorperate an analogue synthesiser where each toaster can be set to produce a specific sound suitable to his customers individual tastes! [i](I was the only person he could find who had both intrinsically safe and analogue synth design expierience)[/i]

I thought this was someone having a joke on me - but no, its real.. Apparently there are a large number of extremely wealthy people (many in oil producing nations) who happily spend >£100k on a custom mobile phone and will happily pay large (undisclosed) sums for a high-end toaster!

In the past, when I have designed products for the kitchen market (food mixers etc) cost cutting has been paramount - one needs seriously good reason to add 5p to the cost of the product..

It is extremely strange to be designing a toaster with safety constraints normally applied to medical equipment or explosive atmospheres, and incorporating a 'synth' into this.. LOL!

As I said .. funny old life.. ;-)

Oh - Reason I mention this.. ;-) ... Well, this work will delay everything else I am doing by about 3 weeks..

Fred.

Hahahahaha!

Tonight I'll be jamming on a toaster!

[i]"Apparently there are a large number of extremely wealthy people (many in oil producing nations) .. will happily pay large (undisclosed) sums for a high-end toaster!"[/i]

Well - the "large number" turned out to be not quite so large.. LOL .. and the project has been dropped [i](well, technically suspended - it 'may' be revived.. reminds me a bit of Hitch-hikers guide.. wait until the galactic economy recovers then start selling again)[/i] - Real sign of the times.. Even the super-wealthy are thinking twice about spending perhaps >£1000 on a farsically stupid idea!

I got 'reasonably' paid for the work I did (nearly enough to perhaps buy one toaster if the price was a bit lower than I guessed!) - and there are some other projects they want me to look at (.. no, Im not even going to try to guess! ;-) but in the meantime I can get back to work on my marginally less stupid endevours!

Fred.

NEW / REPLACEMENT QUAD 4-QUADRANT MULTIPLIER IC:

http://www.ichaus.com/BM_datasheet_en

On the theme of parts - There was a quad 4Q multiplier (MLT-04) that was discontinued some time ago. A new pin-compatible replacement has just appeared.. This 4QM has a better specification, and will be of particular interest to analogue synth developers (each 4QM can be a multiplier [x*y*0.4],squaring [input connects to both x and y so output = input*input*0.4], a VCA [+x*+y or -x*-y.. uses only 2Q - bit of a waste, but as there are 4 on the chip, if one has a spare after doing heterodyning, linearizing, and VCF..), a filter element, a ring modulator, a hetrodyning mixer, etc..)

I have obtained samples (I need lots of multipliers in my scheme - have been using parts which end up taking a lot of board space and using a lot of components) and I suspect I will be using these parts.

I dont know where they can be obtained in small quantities - I have not found any distributor with minimum order quantity less than 40 parts, and at £12 each.. I got a few samples from the manufacturer, but it took a lot of haggling.

I should know in a few weeks whether I will be using these parts, so anyone interested in them, please email me - with VAT etc an order for 40 will cost me about £570 (I only need about 20 for HO 2011) so I could sell individual parts for about £16 (ouch!)

If you find a supplier who will sell smaller quantities and / or have lower prices, PLEASE let me know!

Fred [at] fundes [dot] co [dot] uk [*no spaces*]

This 4 x 4Q multiplier seems interesting but I'll wait until I find something similar in a classical DIL package. I'm too old to handle SMDs