Replacement part for CA3094- RE: MicroSynth

[Shorty]

I've started gathering parts for my next build, a Bass Micro-synth (schematic here: http://files.muziq.be/schematics/eh_bassmicrosynth.pdf)

I cannot find a CA3094 (operational transconductance amplifier) through my regular suppliers, is the LM13700 a suitable replacement?
The datasheets seem to suggest that they vary slightly, so I'm not sure if I can use it.

Also, certain parts of the datasheet show no connection to certain components, is this correct? (e.g. the pot R101, controlling attack delay as well as some of the pins on some CA3094's) 

All input is appreciated!

[R.G.]

The CA3094 was not intended for audio use when designed back in the early 1970s. There is no direct replacement. The CA3080 was the OTA part all by itself. If you could get CA3080s or LM3080s, you could use an MPSA13 after it and get the same function.

The NE5570, the CA3280, the LM13700 and the LM13600 all do almost the same thing as two CA3904s in that they have an OTA followed by a darlington transistor for a buffer. They are all lower noise than the CA3094. I believe the NE5570, CA3280, and LM13600 are now also out of manufacture. The LM13700 is still being made.

The LM13700 has superior performance to the CA3094 in all ways except it's a 16 pin dual chip if size is a factor, and the Iabc pin sits at two diode drops above V-, not one diode drop as in the CA3094, as befits the improved current mirrors inside the 13700.

[Shorty]

The LM13700 has superior performance to the CA3094 in all ways except it's a 16 pin dual chip if size is a factor, and the Iabc pin sits at two diode drops above V-, not one diode drop as in the CA3094, as befits the improved current mirrors inside the 13700.

OK, that reassures me a little. I think the best thing to do is go ahead and build it using LM13700s, as I don't have the option of CA3094s, CA3080s or LM3080s. Then if it doesn't work...ummmmm...at least I had fun. Size doesn't really matter, this thing is gonna be huuuuuge regardless of chip size. That reminds me, I have to go look for large enclosures.
Thanks R.G.

[Mike Burgundy]

The "open ended" pots are used as variable resistors. You can leave that end open, or wire it to the wiper.

[Mike Burgundy]

...and FWIW, even though there are better alternatives, Steve has the 3094E at Smallbear (smallbearelec.com)

That's a BIG project! I'm sure lots of people would be very happy to see a pcb for that one
I know I would.

[StephenGiles]

Built it on veroboard in 1980

I would buy CA3094 chips whatever the price if still available as they make life so much easier in checking the circuit afterwards.

[Mark Hammer]

Mike Matthews has a reputation for buying up the world's stock of critical chips so that he can keep certain product lines alive.  While I realize the HOG is going to eventually eclipse the Microsynth, the latter is still in production.  So, um, what is EHX using these days?  Do they have enough 3094s to take them through a few more years or have they switched to 13600/13700s?

[StephenGiles]

The latest circuits "available" still show CA3094 of course. It would take a brave man to revamp the Microsynth for LM13600!

[StephenGiles]

Perhaps the SSM 2164, a quad VCA could be tricked into doing the same job as 4 x CA3094

[tplaysbass]

I know I'm new here, and I hate to bump as my opening move, but the EHX Bass Micro Synth that I'm trying to fix has LM13700's in it.

I wanted to let everyone know since the question was asked by Mark Hammer:

...So, um, what is EHX using these days?  Do they have enough 3094s to take them through a few more years or have they switched to 13600/13700s?

In the one I'm working on, they are using three LM13700's on the board. It's frustrating since all the schematics only show CA3094's.

Also their PCB is at least 4 layer (top, bottom and two middles), so it's extra hard to figure out where everything goes and comes from.

[StephenGiles]

With the non availability of chips, it's probably a lot easier now to buy a Microsynth , rather than have all the hassle of reconfiguring the circuit for LM13700s, get it to work, and then redesign a pcb. That said, I dare say someone has enough time on their hands to take up the challenge!

Perhaps such a person can also work out how the start frequency CV can be made adaptive, according to how hard a note is played, rather than always starting from the point set by the Start Frequency Pot - never too late to come up with a solution

I'm sure much of the circuitry is already there!

[Mark Hammer]

Given how many Microsynths of older vintage are out there, and given the wide availability of LM13700 chips, somebody at EHX must have pondered an information sheet that might provide direction on how to retrofit a substitute for one or more CA3094s.  Or do they have some sort of secret stash of them to sell for servicing purposes to authorised repair benches?

[StephenGiles]

.....somebody at EHX must have pondered an information sheet that might provide direction on how to retrofit a substitute for one or more CA3094s.  

Quite likely, I was shown an EH information sheet on how the CA3094 was used in their products  - in particular, tricks using pins 1,6 and 8, on one of my visits to EH UK. Unfortunately I never got a copy of it.  I wouldn't be at all surprised if other available chips have been explored for use in the Microsynth - SSM2164 for instance.

[StephenGiles]

From Mike irwin in 2004:

Subbing LM13600 for CA3094 - this can require some care - since the Iabc pin on CA3094/CA3080 sits at one Vbe above V(-), whereas the LM13600/13700 Iabc pin sits at two Vbe above V(-). Can make a difference depending on how current is being supplied to the pin.

Right now I can't figure out which needs a higher current

[StephenGiles]

I've had no success yet on the internet with writings about replacing CA3094 with LM13600/13700. I can't believe there is nothing out there!

[oldschoolanalog]

Small Bear does have CA3094, but @$6.75 each they fall into the "no free lunch" category.
But not too expensive in comparison to some BBD's and such.

[StephenGiles]

Small Bear does have CA3094, but @$6.75 each they fall into the "no free lunch" category.
But not too expensive in comparison to some BBD's and such.

At least they are available. I would say that the cost of not having them is greater than the cost of buying them. Everything is relative really.