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Cheaper SSM2212 Replacements (Dual Matched NPN SOIC)
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Cheaper SSM2212 Replacements (Dual Matched NPN SOIC)
thor.lucas
So I'm designing a synth. It's designed to be relatively cheap but good sounding so that it can be accessible to amateurs. So far, the cost is rather affordable. However, then we get to the V/Oct converter… by far the most expensive section. At an incredible $7 per SSM2212. The cost of this chip is so much that it nearly doubles the estimated cost per oscillator in terms of components.

Are there any replacements for this? I'm having a frustrating time with DigiKey… you go from 10% matched components at 30¢ each and then they suddenly jump to the 0.5% matched SSM2212 at $7 each.

I'm trying to keep the design potentially "mass" producible, so that means no manual matching of the run-of-the-mill NPNs.
BugBrand
The best alternatives are those from Linear Systems - eg. LS318/358 (one is NPN, one is PNP)
Cheaper 'matched' but without niceties like log-conformance are things like BC847BS/BC857BS or DMMT5551/5401
thor.lucas
BugBrand wrote:
The best alternatives are those from Linear Systems - eg. LS318/358 (one is NPN, one is PNP)
Cheaper 'matched' but without niceties like log-conformance are things like BC847BS/BC857BS or DMMT5551/5401


I'm annoyed that I can't find those on any major parts distributor (DigiKey, Mouser, AliExpress…) where do we find the LS318?

What does log conformance refer to? I've worked out the math behind matched pairs using the Ebers-Moll model… are you saying the BC* and DMMT* don't obey that model?

Thanks!
thor.lucas
Jeez and I've just taken a look at the LS318 datasheet… 5% typ. match compared to the SSM's 0.5% typ. At $4.50 it represents quite a bit of savings though, and since they're both available as SOIC 8 with the same pinout I can interchange them in case the 5% becomes an issue. Thanks!
UltraViolet
The AS394 is matched to 2% according to Alpha. They are $3.99 from Modular Addict.
BugBrand
UltraViolet wrote:
The AS394 is matched to 2% according to Alpha. They are $3.99 from Modular Addict.


Alfa, not alpha! http://www.alfarzpp.lv/eng/sc/application.php
(Good company - bringing out various other interesting chips at the moment)

I'd not noticed that spec on the LS pairs before - wow, that is a surprise.
ixtern
UltraViolet wrote:
The AS394 is matched to 2% according to Alpha. They are $3.99 from Modular Addict.

AS194 is matched to max 2%"(typ 0.5%). AS394 has max 5%. AS194 costs more than AS394.
www.alfarzpp.lv/eng/sc/AS394CH.pdf
https://www.ericasynths.lv/shop/diy/diy-accessories/as194h/
https://www.ericasynths.lv/shop/diy/diy-accessories/as394-matched-tran sistors/
cygmu
The 5% figure on the LS318 datasheet is the hFE matching. For an exponential converter you care about the Vbe matching (as a proxy for Is matching) which is 0.4mV typical, 1mV max. Still not in the ballpark of the SSM2212 (200uV max) but likely good enough for the purpose.

Don't forget that you also need to compensate the second order temperature effects (effect of temperature on exponential response) and will most likely do so with a PTC resistor with much worse tolerance specs than this anyway.
J3RK
Another vote for Linear Systems. I've been using these for years now. Actually, I've switched to VCA based exp conversion, but when I need transistor pairs, these are what I get.

You can order directly from them, without having to get insane quantities. You can also get them from Trendsetter Electronics, and a couple of other places (one of which mentioned above).

They perform quite well. You can get metal cans or SOIC. The pinouts match the SSM2220 and SSM2212 parts.

The LS318 is the NPN part.
The LS358 is the PNP part if you need those.
guest
ive tested a bunch of different transistors for log conformance (emitter bulk resistance), and the alfa AS134 is really good. id reccomend the SMT package over the through hole if thats an option for you. otherwise ive used the PMP4201 (identical to the BCM847) and they work fine, but just go a bit flat on the top end with no HF compensation. the best bang for the buck is probably the DMMT3904. the thing you get for the expensive transistors (MAT14, LS134, etc) are all the transistors on the same die, which helps them stay at the same temperature. with the cheap ones (BCM, PMP, DMMT, etc) they are two seperate dies very close to one another in an SMT package. so the smaller the package the better here, and then insulate the whole thing with the thermistor for best performance.
thor.lucas
cygmu wrote:
The 5% figure on the LS318 datasheet is the hFE matching. For an exponential converter you care about the Vbe matching (as a proxy for Is matching) which is 0.4mV typical, 1mV max. Still not in the ballpark of the SSM2212 (200uV max) but likely good enough for the purpose.

Don't forget that you also need to compensate the second order temperature effects (effect of temperature on exponential response) and will most likely do so with a PTC resistor with much worse tolerance specs than this anyway.


Good catch, thank you
thor.lucas
J3RK wrote:
Another vote for Linear Systems. I've been using these for years now. Actually, I've switched to VCA based exp conversion, but when I need transistor pairs, these are what I get.

You can order directly from them, without having to get insane quantities. You can also get them from Trendsetter Electronics, and a couple of other places (one of which mentioned above).

They perform quite well. You can get metal cans or SOIC. The pinouts match the SSM2220 and SSM2212 parts.

The LS318 is the NPN part.
The LS358 is the PNP part if you need those.


VCA based exp converter… I took a look around on that. This seems like a very, very interesting option. Unfortunately now we've replaced the difficult to acquire 3300ppm tempcos with the elusive 2164 chip.

Do you have any sources for this chip? Preferably in the U.S.…
guest
there are 3 options for the 2164 (V2164, AS2164, SSI2164). you can buy direct from the manufacturers, or pick up small quantities at smallbear or synthcube, etc.

regular thermistors can still be had, but it takes a lot of searching, as they often arent listed by the temperature coefficient. the prices have gone up a bit on them, and they were never that good to begin with. you can also get platinum thermistors (1.4$@qty100), which are double the price, but very, very good.
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