Farfisa - Sferasound

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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lolbou
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Post by lolbou »

A recent discussion brought this unit on top of the pile... This pedal was supposed to be used as a "plug-in" in old Farfisa organs, and can be used for guitar too.

It's been later included as a built-in version in more recent Farfisa organs.

Guts and pics here : http://www.effectsdatabase.com/model/farfisa/sferasound

Demos here:
-
- http://www.gearwire.com/farfisa-sferasound-demo.html

Schematic of the built-in sferasound in a Farfisa 250 RSE:
Sfera Sound - schematic.gif
Parts list for the above schem:
Sfera Sound - parts list.jpg
Note: on this version, the two controls are the two switches: "ON/OFF" and "FAST/SLOW".

Now some comments by AG:
analogguru wrote:Now look at the tricky modulation oscillator how the diodes are arranged. I have never seen this before. With changes it can be used in an overdrive circuit too. If you make Rm25 and Rm26 a potentiometer....

And now compare it to the average bootweak-circuit and you will know, why they are boring.
There's probably a lot to do from this, so let's see what it may lead too... Of course if you have the schematic of the pedal version, feel free to share it here. :wink: It is very different, BUT is supposed to achieve identical purposes and sound. BTW the parts in the schem above are fairly available and still quite cheap!

Any input welcome!
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Post by theehman »

The stand-alone unit is a bit different. I have one at home that's not functioning at the moment that I can provide gut shots of.

Sfrerasound pedal schematic is attached.
FarfisaSfera.pdf
(59.52 KiB) Downloaded 427 times
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Post by lolbou »

Cool Ron! Thanks for sharing! Very different indeed.

I guess there are now plenty of circuitry to analyse... :horsey:
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Post by theehman »

lolbou wrote:Cool Ron! Thanks for sharing! Very different indeed.

I guess there are now plenty of circuitry to analyse... :horsey:

I looked everywhere online for that schematic. I found it it finally in a pile of schematics I bought from a local music store that was closing after about 50 years in the business.
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Post by RnFR »

theehman wrote:The stand-alone unit is a bit different. I have one at home that's not functioning at the moment that I can provide gut shots of.

Sfrerasound pedal schematic is attached.
FarfisaSfera.pdf
what's the point of the FET in that PA-208 module?

BTW- this thing sounds very cool! i think it would be a great FSB project if enough interest was generated.
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Post by analogguru »

RnFR wrote: what's the point of the FET in that P/A208 module?
Compare it with the univibe, replace the LDR with a FET, and you will see that it is a phase stage. So it is basically an 8-stage )univibish) phase-shifter.

This one looks older (PA-180, PA-280) compared to the "organ version" (PA-753, PA-760). If I could read the date in discofreq´s schematic..... The "organ version" was not built earlier than 1973 I would suggest from the chips used.

What they have in common is the intention to reproduce the leslie-sound electronically.
RnFR wrote:I think it would be a great FSB project....
And who will be able to do the adjustment of the eight phase stages ?
theehman wrote:I have one at home that's not functioning at the moment.....
Can´t be so hard to fix....
theehman wrote: ..... that I can provide gut shots of.
Guts are always welcome, especially from units which are much more interesting than dirt bootweak-junk.

analogguru
There´s a sucker born every minute - and too many of them end up in the bootweak pedal biz.

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Post by RnFR »

analogguru wrote: And who will be able to do the adjustment of the eight phase stages ?


analogguru
why, i would send mine to you of course! :wink:

would it involve matching FETs? my knowledge of phasers is admittedly sub par.
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Post by lolbou »

RnFR wrote:would it involve matching FETs?
Not necessarily I believe. The trimpot is sort of biasing the FET to adjust resistor value when the FET is used as a VCR, right? So I don't think any special matching is needed... Can't a procedure be written for any of us?
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Post by borislavgajic »

Compare it with the univibe, replace the LDR with a FET, and you will see that it is a phase stage. So it is basically an 8-stage )univibish) phase-shifter.
it is good idea behind this........replace fets with LDR-s...... :D .......aren't opto phasers better then FET based? :thumbsup

I am very glad to see something like this in Vintage Stompbox Corner-my favourite section
Guts are always welcome, especially from units which are much more interesting than dirt bootweak-junk.
:thumbsup :thumbsup :thumbsup

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Post by Dirk_Hendrik »

theehman wrote:
lolbou wrote:Cool Ron! Thanks for sharing! Very different indeed.
I guess there are now plenty of circuitry to analyse... :horsey:
I looked everywhere online for that schematic. I found it it finally in a pile of schematics I bought from a local music store that was closing after about 50 years in the business.

Bump of a worthy corpse.
Breadboarded about 3 quarters of the circuit today. Sound is very promising!
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Post by RnFR »

right on! please continue, and let us know how it goes. this thing always interested me, and I still think it would be a good forum project. we do have to raise money from now on, might as well have fun doing it. :thumbsup is it really that hard to cal? is a scope needed? that may put off some people from building.


edit: listening to that demo again- this thing is so f'n cool!! looks like some of those trimmers could definitely go external, as well.

one more thing- what voltage are you running it at?
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Post by Dirk_Hendrik »

Ah, Those demo's. That's the pedal. What I breadboarded is the Organ module for which Ron posted the schem above. That's the TCA740 and TDA1022 based circuit. (answering the option of a possible forum project as well)

Still gotta tame it. Promising sounds but the BBD will clip easily. The input preamp has a load of gain so it will.... make the BBD clip extra easy.
All at 15 volts. Easy?... actually, yes. Especially for a breadboard build all went quite fast. Scope required all the time. Ofcourse one can do without but when it doesn't work you're lost for hours.
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Post by lolbou »

Dirk_Hendrik wrote:the Organ module for which Ron posted the schem above
no, that's the one I've posted. Ron's refers to the standalone plug-in pedal. :wink:
Dirk_Hendrik wrote:Still gotta tame it.
Just let me know if you ever happen to need references/scopes/voltages from an original unit : I have one at my parent's that I still have to bring back here (in which all F# are dead...).
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Post by Dirk_Hendrik »

lolbou wrote:
Dirk_Hendrik wrote:the Organ module for which Ron posted the schem above
no, that's the one I've posted. Ron's refers to the standalone plug-in pedal. :wink:
OK.. sorry ;) Thanks nevertheless :thumbsup

Dirk_Hendrik wrote:Still gotta tame it.
Just let me know if you ever happen to need references/scopes/voltages from an original unit : I have one at my parent's that I still have to bring back here (in which all F# are dead...).[/quote]

A measurement of the rms value of the input signal (for say an 440hz 'A') would be nice.
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Post by Dirk_Hendrik »

Meanwhile,
Increased the value of that 33 ohm resistor in the input stage to 2k2 and the value of the output voltage divider 100k resistor to 10k. That stopped overloading the BBD line while maintaining level. (and making the modulation noise more audible as a drawback). Makes it very clear why companding was invented.
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Post by RnFR »

sounds like a redesign might be in order? how does it compare to the sound of the pedal? yes, those chips seem rather obscure, a project for that one would not be The best idea.

I'm interested in putting the pedal version on the BB, but that 33v supply is going to be a pain. it looks like a 24v supply maybe be ok, as it gets knocked down to 23v by the time it hits the goods of the first module. maybe they just had a number of those transformers ready.

I'm guessing you could do as many of the 208 modules as you like in order to lessen or deepen the effect. there is no value for Z100, is this crucial? also for C/R 320, which could have possibly been set at the factory?
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Post by Dirk_Hendrik »

Soundwise, a different beast.
The concept is a LFO driven treble control followed by a delay line. The main difference between a chorus and this design is therefore that treble control (simulating the rotating speaker facing towards you and away again). Different than the pedal.

About that pedal,
I may be wrong about that pedal design but a quick scan suggests the first voltage required is 11.4 volts. A simple Traco DCDC converter will give you 12 from an input voltage of 5. Five is easily obtained from 9 with a 7805. Efficient? Not really but a rather cheap solution. Otherwise make 6 volts out of that 9 volts supply and use a 1044 in a voltage doubler config.

As for that 208 module,
Make it SMD and order a strip of SMD FET's. The chances that you'll have to match those are really small since they're all from the same batch. With SMD you could make a shitload of modules that connect in a 5 pin SIL socket. Give it a "motherboard" that can be jumpered for the amount of modules you want. Componentwise it should be possible to get the costs per module under a dollar or 2.
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Post by Dirk_Hendrik »

Update. I was working on this thing last august and part of september till other stuff came in between and I had to catch some nasty behaviour. A design may sound excellen but will still be as weak as it's weakest link. In this case I introduced a problem I had not ran into in the breadboard variant. The breadboard version was powered by 15 volts from my lab power supply. Because I wanted to be able to power the thing from 9 volts externally I used a Traco 5 to 15 volts DCDC concerter. Going from 9 to 5 volts (7805) and then back to 15 is not very energy efficient but the TCA740 in the circuit draws way too much current for a MAX1044 voltage doubler like solution. Furthermore the Traco's are excellent in their performance vs the size. They do have one drawback and that is that there's a strict limit to the amount of (supply filtering) capacitance that can be added to the output.

So I had way too much LFO noise. Tickticktick that is. And the origin...transfer through the supply lines.
So yesterday I modified the PCB I Made to add a couple of 100 ohms resistors and add filtering capacitancd after these. Brings down the voltage a little but keeps the supply lines going to the various parts of the circuit a lot cleaner. Now this thing is so dead quiet I don't even hear from noise it's on till I touch strings.

As well,
Naturally I could adjust the design to implement rate controls, depth controls, LFO asymmetry, dry-wet mix stuff. Name em and spend days twaking the circuit for optimal performance. Till I realized countless organ players have been playing their organs with Leslies for yearrs having only a fast and a slow mode. Apparently, when the controls are not there, one can do without and spend more time and focus on the playing. Therefore I made the PCB design incorporating a bypass and a fast/slow switch only, basically holding on to the original Sferasound circuit. This way the pedal can be pot in the bottom of a pedalboard suggesting to be more of a control (like an amp footswitch) than an effect pedal.

Andd furthermore,
There's some with the opinion that SMD has no place in pedals. Resorting to (full) through hole would make this thing all but a pedal the size of a footwitch. Waste of space on a pedalboard so to say. So, SMD wherever I could, excluding electrolits beacuse of their height in SMD versions as well (or very low voltage ratings). Because of the space consumed I replaced all but the BBD bias trimmer for fixed resistors as well. Makes initial calibration a little harder though.

Pica's?
top1.jpg
All the "upside down" caps are the later modification for the addditional supply power filtering. The little black block on the left is the Traco DCDC converter.
top2.jpg
Copper side wilt most of the components happening. Top left some trace repairs as I has some argument with my etching tank...
bot1.jpg
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Post by andersom »

Looking good dirk, any chance on soundsamples of the slow fast mod?

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Post by mouse »

Great job you did!
I recently serviced a vintage Farfisa OS 42 SR preamp/effect unit which was in a very bad shape. This unit had the same Sferasound circuit found on the pedal - it's a 8 stages fet-based phaser + a tremolo which acted on the high frequency only - and its schematic was very similar to the one I saw posted previously on this topic.
The main difference was the presence of two LFOs, which differ only for caps values. The player can choose two speed of phasing: in the "fast" mode the player can control the LFO speed, and in the "slow" mode, the frequency is fixed (but you can set it by altering a trimmer inside the machine).
Unfortunately, I forgot to take a picture of the unit after the restoring process, so now I can only post a picture of how the unit looked like the first I opened it...
I can ask the owner to take more accurate shots of the unit, in case someone needs it.

The big circuit boards is the preamp and reverb driver, and the vertical modules are dual transistor amplifiers. I traced some of the modules, while reapiring it, since it seems there are no informations anywhere on the net.
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