Small Stone Noise Issue (with Audio Sample)

[Mr.Kite]

Hi,


I have this '79 Small Stone that is quite noisy: it works and sounds great, but there's a "swoosh" (pretty normal IMO) and a slightly louder and alarming "shhh", like sandpaper noise.

It looks like a V3 but has 5 EH1048 chips, older board, pot etc...like a V2 board inside a V3 case, if that helps...

I suspect a bad\noisy transistor\IC, but I'd like to hear your opinion.

Compared to my Big Box RI it's much noisier, maybe it's slightly noisier than my '79 DEM (after the PS mod)!

Here's a clip: Reissue first, than Vintage. You can clearly hear the noise I'm talking about...recorded with an SM57 (close mic), an AT3035 (room mic), JCM600 head with 2x12 G12T75 closed back cab and a Strat with standard single coils, for that matter...

https://voca.ro/nWxvvVLwfPG

One last thing: the pot code is P0200 (generally a 1M pot?) but measures 4M in the fully CCW position (in circuit). I know that the LM13600 version used a 2M pot but...

How would you proceed? Unfortunately I don't have another vintage one with a similar board to compare: for what I know it could be a design\layout flaw...

Thanks for your help!


Mr.Kite

 

[Mr.Kite]

BUMP

[Mr.Kite]

BUMP again...

[11-90-an]

Ummm hello..

Voltages, i guess, i'm not an expert at all, but i think the other forumites here are experts and can probably see a problem...

[Mr.Kite]

...Voltages, i guess...

I remember that when I checked the voltages were totally comparable with my noise-less Reissue...

To me it sounds like a noisy\bad component or a design\layout flaw, but I don't know where to start...

[11-90-an]

If you have extra ICs that are pin-to-pin compatible with your pedal, maybe you can try to drop them in and see what happens..?

Please take my advice with a pinch of salt, though...

[Mr.Kite]

If you have extra ICs...

Thanks for the suggestion!

Unfortunately I don't have spares, but I'm looking for a "procedure" to investigate the source of the noise before any invasive "surgery", I don't want to proceed for trial and errors and\or ruin this nice vintage pedal...

[ElectricDruid]

Honestly, listening to that sound sample, I'm not really sure what you think the problem is. It's old - over forty years old. It hisses a bit. So far, so normal.

If it's got any trimmers in it, perhaps it could do with some readjustment. Values have undoubtedly shifted with the years. Just ask my optician.

Beyond that, I don't really hear anything that I'd class as a fault. Sorry.

[StephenGiles]

They just make that noise I'm afraid, mine is the same. I tried inserting a compander years ago but phasing lost something.

[Mr.Kite]

Interesting...Seems like a design flaw after all...

Yeah, I know that vintage pedals (especially EHX) can be noisy, but I had a doubt on this unit since my Small CLONE Reissue (the chorus pedal) is noisier than my vintage one, but there's a reason for that: both units use an MN3007 delay chip, but the preset delay time on the reissue is quite longer than on the vintage pedal, and we know that increasing delay time on a BBD without lowering the threshold of the filtering causes more hiss, so I thought that the vintage Small STONE shouldn't necessarily be noisier than the modern one...I was wrong evidently!

I will check the trimpots (thanks!), but other than that, it would be interesting to see what "mods\updates" in the reissue version make it quiter...

...I tried inserting a compander years ago but phasing lost something.

Maybe the reissue has a compander? The vintage pedal, as you can hear in the clip, is louder (no volume drop) and brighter than the reissue...more harmonic content and deeper phasing...I guess that I should just live with some noise to get that authentic 70's phasing tones!

[11-90-an]

Will a simple lowpass/highpass filter fix the hiss? Or maybe a noise gate?

[ElectricDruid]

it would be interesting to see what "mods\updates" in the reissue version make it quiter...

There don't need to be any mods, updates, or design flaws in the original pedal. If you build something with cheap parts in the late 1970's, then it gets used for 40 years while the parts age and their values drift, and then you compare it with a new-built unit to the exact-same design but put together with modern parts that may well be cheaper but almost certainly have a better spec, it should be no surprise that the new unit is quieter than the old one.

[StephenGiles]

But surely it's known that the CA3094 was never designed to be a low noise audio chip any more than the LM13700??

[Mr.Kite]

Will a simple lowpass/highpass filter fix the hiss? Or maybe a noise gate?

Probably, but I don't want to mess with this vintage pedal if it's not broken: as you can hear in the clip, the Reissue sounds close enough if I need less noise for recordings etc...

There don't need to be any mods, updates, or design flaws in the original pedal. If you build something with cheap parts in the late 1970's, then it gets used for 40 years while the parts age and their values drift, and then you compare it with a new-built unit to the exact-same design but put together with modern parts that may well be cheaper but almost certainly have a better spec, it should be no surprise that the new unit is quieter than the old one.

Generally yes (*just a note: the "new" unit is 20yo), but I have several vintage pedals that are less noisy than modern (perfectly working) clones: my '81 Small Clone is less noisy than my reissue (due to the delay time), vintage SD-1 vs modern one, I have even a late 60's rare Italian fuzz with CC resistors, ancient el. caps and transistors that hisses less than my DIY clone etc...

This Small Stone has CF resistors, polyester caps and some electrolytic, some transistor and the OTAs: if (and I say IF) everything is still in spec (as it "should" be) the pedal is as noisy as day 1. IMO, if the pedal is not faulty and there's no particular reason for being noisier (dirty\corroded connections etc), the noise will be the same even 50 years later, at least that's my experience...

But, by your own reasoning, a used\aged\drifted pedal that get noisier because is not in spec anymore IS faulty: if your car has worn tyres, misses cooling liquid and the fuel pipes are clogged, then your car isn't just "old and used", it's broken\faulty and needs repair!

So, getting back to the Small Stone, in your opinion, which components are more likely to develop noise problems? I was thinking the input transistors: a noisy transistor at the input of the circuit can certainly cause much noise, maybe I should try to replace those with modern\known good ones? I'm not very experienced with OTAs: can they develop noise over time and still work perfectly otherwise? Should I investigate in them also? El. caps? Other caps? "Critical" resistors? Or maybe a worn pot could introduce noise in this circuit? The pot works fine but who knows...

Still, it could be simply "as is"...

Let me know how you would proceed, and many thanks for your help!

But surely it's known that the CA3094 was never designed to be a low noise audio chip any more than the LM13700??

Sorry, I'm not sure to understand: both the Reissue and the Vintage pedal use CA3094s, please elaborate your argument...

[11-90-an]

I meant add a noise gate AFTER the small stone in the signal chain, not that you have to make a daughter board or something...

I recommend using a audio probe so you can single out which portion of your pedal is hissy...

[StephenGiles]

In a band situation you probably wouldn't hear the hiss anyway. However, if you are trying to impress your girlfriend with your bedroom setup - she's going to say "you really need to do something about that hiss" !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[ElectricDruid]

But, by your own reasoning, a used\aged\drifted pedal that get noisier because is not in spec anymore IS faulty: if your car has worn tyres, misses cooling liquid and the fuel pipes are clogged, then your car isn't just "old and used", it's broken\faulty and needs repair!

Ok, but I think we're starting to get lost in semantics here.

So, getting back to the Small Stone, in your opinion, which components are more likely to develop noise problems? I was thinking the input transistors: a noisy transistor at the input of the circuit can certainly cause much noise, maybe I should try to replace those with modern\known good ones? I'm not very experienced with OTAs: can they develop noise over time and still work perfectly otherwise? Should I investigate in them also? El. caps? Other caps? "Critical" resistors? Or maybe a worn pot could introduce noise in this circuit? The pot works fine but who knows...

For the electrolytics, I'd only look at the power supply ones - the main smoothing cap and whatever is on the bias/Vref supply. I wouldn't bother with the others unless there was some specific reason to believe they were faulty.
A transistor is a possibility. I had a 1970's Yamaha organ which developed an unpleasant cracking sound. Eventually I traced that to a failed transistor in one of the main oscillators. So they do fail, and they can produce noise when they do.
Bad pots tend to be pretty obvious in my experience. You move them and they crackle and pop, or the wiper loses contact altogether and the circuit only works in certain places in the travel.
I've never seen a noisy-but-working OTA, but that doesn't mean it might not happen, just that it's not that common. Maybe someone else here has. For me, they've usually been dead or alive, and nothing much in between.

But surely it's known that the CA3094 was never designed to be a low noise audio chip any more than the LM13700??

Sorry, I'm not sure to understand: both the Reissue and the Vintage pedal use CA3094s, please elaborate your argument...

He just means the 13700 and the 3094 are both famously noisy chips. They can only cope with about 50mV signal level^* before the distortion goes up and up, so the signal to noise ratio through several of them in a row is basically *atrocious*.

*Without the linearisation diodes, anyway

[StephenGiles]

So my recommendation is regrade the circuit for SSM2164 - 4 vc amps in one chip and a touch quieter!

[Mr.Kite]

For the electrolytics, I'd only look at the power supply ones - the main smoothing cap and whatever is on the bias/Vref supply. I wouldn't bother with the others unless there was some specific reason to believe they were faulty.
A transistor is a possibility. I had a 1970's Yamaha organ which developed an unpleasant cracking sound. Eventually I traced that to a failed transistor in one of the main oscillators. So they do fail, and they can produce noise when they do.
Bad pots tend to be pretty obvious in my experience. You move them and they crackle and pop, or the wiper loses contact altogether and the circuit only works in certain places in the travel.
I've never seen a noisy-but-working OTA, but that doesn't mean it might not happen, just that it's not that common. Maybe someone else here has. For me, they've usually been dead or alive, and nothing much in between.

Thanks, this is useful: there are just three el. caps (2x33uF + 1x10uF), it looks like one of the 33uF is the smoothing cap. They "look" good, but I can always try to replace that one, maybe with a 47uF, although it dosn't sound like "power noise" to me, it's the same with a battery too...

Transistors remain the prime suspect IMO: in the audio clip isn't clear enough, but the annoying noise on part of the sweep sounds very similar to faulty transistors that I've heard, like a crackling sandpaper noise...

Good to know that OTAs usually are dead or alive!

I'm in the doubt with the pot for a single reason: it works perfectly across all the sweep (although it gets VERY slow below 10 o' clock or so, much slower than my reissue) but it measures 4.2Meg (fully CCW, in circuit)! I know that different versions of the SS used 1Meg or 2Meg pots, but 4Meg? My reissue measures 1Meg in circuit...
Unfortunately I can't find the exact schematic for my version, but it looks like a mix of a V1 and a V3...a V2?  Yes, but which variant? It's not an issue J, I know that there's a issue H but I don't have the schem...anyway it looks very similar to the second one in this article: http://falseelectronics.blogspot.com/2019/02/2-70s-electro-harmonic-small-stone.html

The date code on the ICs in the second pedal of this article is 7927, mine is 7941...my pedal shares similarities (but not exactly the same) with these two versions:

https://www.ehx.com/?ACT=25&fid=8&aid=718_SugkEFQXP6fu53LcaymQ&board_id=1

https://www.....org/viewtopic.php?f=1&t=3567&sid=bec61e1245bdcbe6b52c8ec59d4c2b12

What do you think of the pot value? It's curious that the measured value is twice a 2Meg pot (as used in the LM13600 version), but the code on the case is "P0200", generally a 1Meg pot with EHX...? Do you think that something in this particular version prevents accurate measuring of the pot in circuit, but not in other versions?

He just means the 13700 and the 3094 are both famously noisy chips. They can only cope with about 50mV signal leve^l* before the distortion goes up and up, so the signal to noise ratio through several of them in a row is basically *atrocious*.

*Without the linearisation diodes, anyway

Good to know, but the reissue uses the same chips and is basically silent...

In a band situation you probably wouldn't hear the hiss anyway. However, if you are trying to impress your girlfriend with your bedroom setup - she's going to say "you really need to do something about that hiss" !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

It depends: if a song has a part with just a guitar (an intro for example), yes, you can hear it pretty well!

It is not THAT annoying, my point is: should it be there? If yes, I can live with it, but if there's something wrong I'd prefer a perfectly working unit, that's all...



EDIT: the second schematic link doesn't work, probably it doesn't want links from that site. It was the LM13600 version schem anyway...

[mozz]

Tried freeze spray?