Small Stone Inquiry

[Baran Ismen]

Good morning beautiful people of the forum.

For my next project, I want to start an EHX Small Stone. There are a couple of them around the internet, and I think the one in the tonepad seems the one mostly used so far.

I just wanted to be sure which layout I should follow. I like the original Small Stone sound. I don't know which mods are there, but I don't think I'd ask for extras.

As you may follow from another topic, I had some issues with Bassballs for a non-proper layout I've found -even though it's said to be confirmed-, and I just don't want to experience the same issues again. I'm open to any suggestions as well, like there might be another project that sounds exactly like SS and has some other functionalities in its nature etc..

Another side question is, the projects I've seen so far are 2 types, some are based on LM13600/700, and some are OTA3080/94, I wonder what the difference is in-between.

[antonis]

There are, as far as I know, 4 Small stone versions..
Which one is of your preference..??

[ElectricDruid]

Another side question is, the projects I've seen so far are 2 types, some are based on LM13600/700, and some are OTA3080/94, I wonder what the difference is in-between.

I don't know much about the Small Stone, but I can answer this a bit since I've been playing with OTAs for about 30 years.

The differences between the 13700 and 3080 are minimal, unless you start using the linearisation diodes and drving the chip a bit harder. At that point, the 13700 is able to produce a better S/N ratio than the 3080. This comes at the cost of a harder distortion charateristic, which will alter the sound subtly.
Whether the 13700 versions of the Small Stone use the diodes, I couldn't say without seeing a schematic.

3094 was basically a 3080 with a darlington transistor output added, so extremely close to the 13700 also.

[Baran Ismen]

There are, as far as I know, 4 Small stone versions..
Which one is of your preference..??

I'm not sure about the spatial differences inbetween the varieties, but the one with red background, I believe its the v3

[Chillums]

I've built a couple of the tonepad small stones.   Well worth the effort in my opinion.   I also made a layout based on the tonepad schematic and if you'd like I can post the gerbers later tonight when I get outta work.

[Mark Hammer]

The first issue, using 6 chips, instead of 5, was the cleverest, AFAIC. One of the things it included was a simple lowpass filter on the output of the LFO, that essentially reduced sweep width at faster speeds >1.6hz).  In a full-fledged 2 or 3-knob phaser, there would be a control for adjusting sweep width to be narrower at faster speeds, but the SS did it for you automatically. 

The Color switch changes amount of feedback, speed range, and sweep shape, all at the same time.  The sweep and feedback functions can be disaggregated, though, by using two separate toggles instead of a DPDT slide switch, and is well worth doing.

The black Ross Phaser, which Francisco Pena lists as the "Ropez" project on the Tonepad site, is a sort of LM13600 adaptation of the Small Stone.  It uses the 2 stages of an LM13600 dual OTA to replace the 2xCA3094 LFO on the first issue SS.  While it uses the 10k series resistor on the LFO output, it omits the 10uf cap to ground that the original used for automatic sweep-width adjustment.  But it allows for continuously variable feedback, instead of the less/more that the original Color switch provided.

On one I built for myself, I reduced that 10k LFO output resistor to 9k1 or so, and was able to achieved a jarringly-wide sweep.  The result was a sweep not unlike the Electric Light Orchestra song "Strange Magic".  Of course, because extra-wide sweeps are annoying at fast speeds, I placed a 10k variable resistance in series with the 9K1, for limiting width when needed.  Note that OTAs can fry with too much Iabc current, from what I understand, so any reduction of the current-limiting resistance on the LFO output has to be done judiciously.

As mentioned, the Color switch on the SS changes the shape of the sweep, from triangular (and faster speed range) to "hypertriangular", where the sweep becomes sinusoidal as it sweeps "lower", and then triangular as it moves to the "top" of the sweep.  Hypertriangular is optimal for slow dramatic sweeps with lots of feedback, since it emphasizes the movement of the notches through the range of note fundamentals and lower-order harmonics by slowing down the perceived movement.  It's a little annoying for fast speeds, though, and sounds a little too "boing-ey".  It doesn't sound all that musical if one lifts the dry signal to produce vibrato, either.  The Ropez only implements the Hypertriangular sweep, and not the two sweep shapes of the Color switch.

There is a temptation to want to add phase-shift stages, with a more-is-better strategy.  Francisco drafted and posted a layout for bumping 4 stages up to 8, via an add-on board.  My own view is that, when it comes to phasing for guitars, 6 stages is the sweet spot, and adding more stages may not improve the sound, while simultaneously increasing the risk of unwanted oscillation.  Remember that each of those phase-shift stages, whether using OTAs or FETs, is supposed to be unity-gain.  Even if one uses 1% resistors, one or more stages could provide just a smidgen above unity-gain.  And when you feed the output back to the input, oscillation can be the result.  Rolling the bass off the feedback signal, by reducing the series cap value, can help avoid that.

[Phend]

Here's one I have,

[CheapPedalCollector]

I'm not sure about the spatial differences inbetween the varieties, but the one with red background, I believe its the v3

Can confirm, mine is V3 and from 81, non LED, Red and Black.

Uses CA3094.

I also did true bypass and analog man volume mods to it, since it was rather beat and I had to repair it anyways.

LFO mod like the V1 would be sweet, I need to look into doing that, thanks Mark.

[Baran Ismen]

There are, as far as I know, 4 Small stone versions..
Which one is of your preference..??

Actually it's the latest version according to my research, the USA reissue. I'm not sure sound-wise there's a drastic difference in between.

[StephenGiles]

I managed to cobble up a Small Stone using SSM2164 around 1988, I can report that it sounded exactly the same as the CA3094 version, can't find the circuit unfortunately.

[Baran Ismen]

I think I'll give this one a try. Both CA3080 & 13600 ICs are locally available at the same price around 3$ each. This has 2 ICs, tonepad one needs 5 of 3080. Confused in between. I don't want to spend my time & parts for nothing, though.

http://dirtboxlayouts.blogspot.com/2020/11/electro-harmonix-small-stone-v3.html

[ElectricDruid]

Remember that the CA3080 is a single OTA (with no darlington buffer) and the LM13600/13700 is a dual OTA (with darlington buffers). So $3 each makes the 13600 twice as cheap. Plus the CA3080 version probably needs extra parts for the buffers too. It also explains some of the difference in the number of ICs - each 13600 can do *two* stages, whereas each 3080 can only do one.

If you want to build a really authentic V.1 though, the CA3080's would be the way to go. For V.3, use the LM13600.

[Baran Ismen]

Remember that the CA3080 is a single OTA (with no darlington buffer) and the LM13600/13700 is a dual OTA (with darlington buffers). So $3 each makes the 13600 twice as cheap. Plus the CA3080 version probably needs extra parts for the buffers too. It also explains some of the difference in the number of ICs - each 13600 can do *two* stages, whereas each 3080 can only do one.

If you want to build a really authentic V.1 though, the CA3080's would be the way to go. For V.3, use the LM13600.

I don't particularly ask for the V1 tho, all I want is the authentic Small Stone sound, version is regardless, but ofc reachability and costfulness are a plus. A working & confirmed layout too, also   

[ElectricDruid]

I don't particularly ask for the V1 tho, all I want is the authentic Small Stone sound, version is regardless, but ofc reachability and costfulness are a plus. A working & confirmed layout too, also   

Well, that Dirtbox layout looks pretty good then! Pity no-one has verified it.

[Baran Ismen]

I don't particularly ask for the V1 tho, all I want is the authentic Small Stone sound, version is regardless, but ofc reachability and costfulness are a plus. A working & confirmed layout too, also   

Well, that Dirtbox layout looks pretty good then! Pity no-one has verified it.

Hope ill be the first then.

[Baran Ismen]

Got a quick question for Rate Led.

Is it only active when the pedal is on, or its always blinking?

[Mark Hammer]

Likely always blinking.  I gather the assumption is that  when you see blinking, the effect is engaged, and when the blinking stops, the circuit is bypasses.  Since the drawing does not indicate any explicit switch for turning it off, it may also be the case that it blinks constantly.

One thing I have done in past is to use the 3PDT stompswitch to alter the current limiting resistance to a blinking LED although it is hard to discern what the resistor is in this case (yet one more reason why I hate stripboard layouts).  In any event, if for instance there was a 6k8 resistor going to the LED, increasing that to 12 or 15k would allow it to flash but much more dimly.  That allows the player to use the flashing LED to set their desired modulation rate, even while in bypass, but still tell the difference between effect and bypass by the brightness of the flash.

[Baran Ismen]

I gather the assumption is that  when you see blinking, the effect is engaged, and when the blinking stops, the circuit is bypasses.

That's what my intention is actually Especially depending on the below design I created (on the left side where the swollen part is).

[Baran Ismen]

And 3 more questions that;

1- The highest log type of pot I could find is A1m, and I found b2m as a linear type. I'll give them both a go, but may I ask, how the sweep rate will be affected according to the pot's resistance value?

2- Is it unorthodox to use a Linear pot with this circuit, if 2m is better overall?

3- To which lug shall I connect an extra 1m resistance in order to increase the log A1m pot's value to A2m if a log-type pot is a must?

[ElectricDruid]

1- The highest log type of pot I could find is A1m, and I found b2m as a linear type. I'll give them both a go, but may I ask, how the sweep rate will be affected according to the pot's resistance value?

If the pot is the LFO Rate pot, changing the pot value will affect the slowest speed that's possible. Smaller pot = faster "slowest" speed, bigger pot = slower "slowest" speed. If the pot halves in value, the speed doubles in value.

2- Is it unorthodox to use a Linear pot with this circuit, if 2m is better overall?

If the original schematic you're working from (which is it?  Can we see it please?) uses a log pot, you'd be well advised to do the same. A linear pot will likely bunch most of the action up at one end of the knob's travel.

3- To which lug shall I connect an extra 1m resistance in order to increase the log A1m pot's value to A2m if a log-type pot is a must?

While this would work in as far as it would keep the slowest speed the same, it will massively affect the fastest speed possible and effectively cut away most of the control's range. If you need a 2M Log pot and can't find one, you'd be *much* better off using a 1M Log pot and then doubling the LFO's timing capacitor to compensate for the smaller pot - so a 1u cap would become a 2u2 or whatever. Need to see the schematic to give you any guidance here.

HTH