Understanding suitable transistor and opamp substitutions

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Understanding suitable transistor and opamp substitutions

Post by rumblesan » Tue Oct 27, 2020 5:11 pm

Hey all,

currently in the process of getting back into DIY electronics after a bit of a hiatus and attempting to get back up to speed. I'm trying to understand how I can go about picking suitable transistors and opamps when I'm subbing in and out parts in a circuit, specifically what would I be checking for?

Transistors I get that it's primarily going to be about hfe, and in my current situation building a CGS panel with elby pcbs, pinout is going to be a consideration as well. Beyond that I guess that vbe values, noise and power rating maybe? but what else might differ?
For a concrete question/example, the CGS VCA specifies bc558b transistors which are apparently obsolete, and I'm wondering if I could just use 2n3906 instead or if there's something I've missed?

Op-amps feels a bit tricker, as beyond the obvious it's not actually clear to me what the functional difference is between say an lm358 and a tl072? Is it mainly that the latter has jfet inputs so a higher input impedance?

Sorry if this is a bit obvious, but searching back through these forums I found a fair bit of info on substitutions, but not much explaining what the logic/reasoning is behind them.

cheers for any help

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Re: Understanding suitable transistor and opamp substitutions

Post by MikeDB » Tue Oct 27, 2020 5:57 pm

Firstly BC558 isn't obsolete. Lots of them at many places such as Farnell.

To be honest though in 95% of places the 2N3904/3906 pair are usually okay if the collector current is around 5-10mA. They can handle the full voltage a synth PSU delivers and have reasonable gain, though a bit less than the BC55x. If they turn out to be too noisy then 2N4401/4403 is one go-to pair, whilst if you need higher voltage then MPSA42/92 are good. Or if you need higher gain then the BC is a good choice. If you need a little more current handling then 2N2222 has been the go-to for decades. Where you might need something better is for exponential convertors where you are after a wide range of current without the gain changing too much, and mic preamps where you are after a very low base resistance to give low noise.

Similarly with op-amps. Probably 95% of audio gear uses TL072s for general amplification and 5532s as line drivers. But if noise is critical there are lots of better ones all the way up to the OPA2134. Most op-amps are pretty stable in all circumstances, the 5532 being the exception. If you are running off batteries then there are better low voltage op-amps to consider so choose one that operates well down to whatever voltage you want the battery to go before the circuit may stop working. To be honest I can't think of a use for the LM358 nowadays but someone may be able to point to where it is actually needed.

Diodes you need 1N4148 for normal and BAT85 as a Schottky. For 1 Amp rectifiers I standardised on FR107 years ago but others prefer the 1N400x series. Higher current then choose something that can handle double the current.

So my suggestion is for any circuit try the 2N3904/6 and TL072 first and see how well or badly they perform. Doing that will teach you when you need something better far quicker than any advice on a forum.
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Re: Understanding suitable transistor and opamp substitutions

Post by guest » Tue Oct 27, 2020 6:05 pm

unfortunately there isnt a single answer here, as it really depends upon what the part is being used for. lets take your BC558B as an example. first off, they do have them at digikey, so you can get the part if you want it, but maybe its not strictly necessary? in the CGS64 that transistor is used as a voltage to current converter. the way its being used makes it a fairly innacurate voltage to current converter, so any PNP transistor would probably work as well. some things i might look for, though, in this case are:

1. can it handle the rail voltages? Vce > 16V? Vcb > 15V?
2. will it shut off fully? Icbo < 15nA (the BC558 spec)?
3. will the base current contribute little to the error? hfe > 200 (min BC558B spec)?

but, because its not such a great V to I converter, 3 isnt so critical, so a 3906 would probably be fine.

with transistors, there are probably only a dozen parameters or so that might matter, but with opamps there are many more, which makes it more complicated. but, the first place to start is by looking at the datasheet and getting to know the device. once youve done that, you can make some guesses at to which ones are the important ones, and then go to a distrubutor or manufacturer website that allows searching by parameters. you enter your values and see what returns. its a lot of practice, reading schematics, understanding whats going on, whats important, and then remember which opamps you looked up last time. you can also ask here, and folks are pretty happy to chip in. its a decent way to get started figuring out whats important. there are also cross-reference sheets that list substitutes. i use to find the NTE equivalent, and then see what else cross referenced that same NTE part.
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Re: Understanding suitable transistor and opamp substitutions

Post by rumblesan » Wed Oct 28, 2020 1:52 pm

thankyou both for your input. I get that it's going to be hard to have cut and dry rules for this, but the info you've given has answered a lot of questions.

I think that I'm just going to have to treat the CGS panels as a bit of a laboratory as well and see how some of these subs affect things.

that and a lot of bread boarding and reading data sheets. kind of like being a student again :D

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Re: Understanding suitable transistor and opamp substitutions

Post by devinw1 » Wed Oct 28, 2020 3:36 pm

One thing that you want to look at with op amps is the slew rate (and open loop gain/frequency). The reason I say this is that some old circuits might use something like a 741 op amp which has a very low slew rate (like .5 uV/s) and the bandwidth is therefor pretty low and something like this or even a 4558 might cause an older audio circuit to color the sound in a certain way and this would change a lot if you swapped in something better like the venerable TL07x. Now, if you are building new circuits, then yeah you can pretty much use a TL07x for almost everything.

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Re: Understanding suitable transistor and opamp substitutions

Post by loki » Wed Oct 28, 2020 5:18 pm

I recommend that everyone interested in classic synthesizer circuits download: the National Semiconductor Discrete Databook 1978, available from the internet archive. It catalogs the different processes and their associated part numbers. It will help you understand that the transistors of the seventies were not magic.

In my opinion, the best cheap general purpose op-amp is the OPA1678. Low input noise of 4.5 nV root Hz, low bias current, good bandwidth and slew rate, low distortion. In 250 quantity from DigiKey at 44 cents each compared to the TL072 at 22 each cents in the same quantity.

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Re: Understanding suitable transistor and opamp substitutions

Post by devinw1 » Wed Oct 28, 2020 5:42 pm

loki wrote:
Wed Oct 28, 2020 5:18 pm
In my opinion, the best cheap general purpose op-amp is the OPA1678.
*best SMT general purpose op-amp ;)

Don't forget there's still a ton of people building thru hole stuff. Hence the popularity of the TL07x

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Re: Understanding suitable transistor and opamp substitutions

Post by KSS » Wed Oct 28, 2020 6:03 pm

I completely agree with loki on the '78 databook. These older databooks often have so much more useful info than the later ones.

I use alltransistors.com to get comparables. I also have an NTE book behind me and do what guest mentioned. Though I almost never actually buy NTE parts.

As others have said, it's really up to digging in and gaining experience by doing.
rumblesan wrote:I think that I'm just going to have to treat the CGS panels as a bit of a laboratory as well and see how some of these subs affect things.
that and a lot of bread boarding and reading data sheets. kind of like being a student again
Toward that goal, I'll share a favored method when TH parts are used. I will make up or buy a PCB and then instead of loading it with components I load it with connectors. Different types depending on the expected lead size of the part. So breakaway HDR sockets for most diodes, resistors and caps, and then for the larger leads I've got quite a few millmax or harwin type sockets. two sizes covers nearly all the parts we'd use Sometimes even using pins cut from dual-wipe IC sockets. Basically every part plugs in like a breadboard, but with all the right connections already in place due to it being the actual PCB. And by now I'm not really doing full boards this way; only the expected places where I want to experiment. This also means the part locations and trace lengths are essentially what they will be in a finished build.

By plugging in different components to the actual circuit you can learn pretty quickly what and where the differences lie. I find this method especially effective for updating old synth circuits for new parts and A/B'ing new layouts.
For your VCA maybe you only put machined pin breakaways HDRs for the transistors you're wondering about.

Now the down side is that not everty circuit can handle the added resistances and other variables of all the connectors. But many can. And as you gain experience you'll start having fewer 'question' parts to set up this way.

But I MUCH prefer this connector-studded PCB-based prototyping on an actual laid out PCB to faffing about with a whiteboard and all its foibles and frustrations!

And while you can hardly beat buying PCBs from the various inexpensive fabs now online, I'm still glad I have a muriatic-hydrogen peroxide and aquarium pump in cereal keeper etching tank and a slightly modified laminator for consistent toner transfers, and a small CNC to drill holes. In other words, a system that lets me go from layout to board in hand in an hour or two at most.

But I always point to CraigyB's digisound build thread in the 5U section here for an example of how you can get it done with a sharpie and drill bit. Be sure to have a look at that thread.

The point is, we all have developed methods of USING the information available in consistent and repeatable ways. That goes a very long way in helping learn which transistor or opamp makes sense for a given build.

It's not all about the databooks and specs. <-- But it kinda is, too. ;)
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Re: Understanding suitable transistor and opamp substitutions

Post by MikeDB » Wed Oct 28, 2020 7:21 pm

loki wrote:
Wed Oct 28, 2020 5:18 pm
I recommend that everyone interested in classic synthesizer circuits download: the National Semiconductor Discrete Databook 1978, available from the internet archive. It catalogs the different processes and their associated part numbers. It will help you understand that the transistors of the seventies were not magic.

In my opinion, the best cheap general purpose op-amp is the OPA1678. Low input noise of 4.5 nV root Hz, low bias current, good bandwidth and slew rate, low distortion. In 250 quantity from DigiKey at 44 cents each compared to the TL072 at 22 each cents in the same quantity.
If you're going to buy that sort of quantity you may as well register at TI direct and buy 1000 offs. TL072 is 7 cents each while OPA1678, which I agree is a great op-amp, is 22 cents each.
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Re: Understanding suitable transistor and opamp substitutions

Post by The Real MC » Thu Oct 29, 2020 2:29 am

There is no single perfect opamp, they all have tradeoffs.

TL08x has lower offset errors than TL07x. I use those in CV circuits to minimize drift. TL07x have lower noise so they are optimal for audio circuits.

LF35x has even lower offset errors and has a better output stage for driving a capacitive load.

Some low power opamps sacrifice lower GBW product which can cause a filter circuit to malfunction. While 553x are the standard for output stages and low noise, they draw high current and their lower Avd spec may not substitute in active filter circuits. Some opamps have output stages that can drive very close to the power rails, make sure your substitution can do the same thing.

I have run into a circuit in the Minimoog where ONLY the 741 opamp would work. Sometimes you can't use modern substitutions.

Beware - power pins on older opamps may not be in same locations as substitutes (esp dual and quad packages! You need to watch the current draw on the rails for any substitution - the big unknown is the current capacity of the power supply.

Yes Virginia there are still places where the awful ten cent 1458 can still be used. LFOs...

Can you substitute a 2N2222 for a 2N2222A? Nope! Those suffixes are there for a REASON!

Transistor substitutions is not just hfe; you need to look at what Ic/VCE the hfe is rated at, VCE(sat), IcMax, BVCEO. Some OEMs years ago did not use JEDEC packages, which can throw you for a loop.

That's just the BJT transistors. We haven't even talked about FETs or dual discrete devices.

While guitar players get all hyped up over germanium transistors, they are not worth the trouble. Germanium substrates have too many problems and the NOS parts are substandard quality. Heck, even the Fuzzface pedals varied from unit to unit because the quality of Ge transistors was not consistent.

Yes the old 1970s databooks and application notes are a treasure trove of technical info.
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Re: Understanding suitable transistor and opamp substitutions

Post by franziskano » Thu Oct 29, 2020 4:08 am

Most Dual and Quad Opams are interchangeable considering their pinouts. But some are better for certain applications than others. I mainly use 2 types TL072 and NE4558. Mainly NE4558 for breadboarding, because they are cheap, reliable and suitable for most purposes and if I destroy one it isn't that bad and I can just pop another one. If there is noise coming from my breadboarded circuit, I doubt that it comes from the Op Amp.

Nevertheless, using less noisy and precise Op Amps can be mandatory in certain applications, where having a very stable voltage is important (CV output voltages for example, 1V/oct tracking).

In some digital applications, you want your voltage to reach rail voltages: TL072 (or noisier TL062 and TL082) or "equivalent" opamps are not suited. You might prefer to use MCP 6002 or MCP 6004.

Just for curiosity: Here is a Video from Brian Wampler I found very interesting on the use of opamps and diodes in distortion circuits:

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Re: Understanding suitable transistor and opamp substitutions

Post by Radiance2021 » Thu Oct 29, 2020 4:43 am


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Re: Understanding suitable transistor and opamp substitutions

Post by MikeDB » Thu Oct 29, 2020 7:06 am

Radiance2021 wrote:
Thu Oct 29, 2020 4:43 am
Here some audio opamps compared...

https://www.nanovolt.ch/resources/ic_op ... ortion.pdf
Wow - great resource ! Thanks.

Interesting to see most of the discrete designs are pretty poor, but one or two do perform well showing it can be done.
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Re: Understanding suitable transistor and opamp substitutions

Post by KSS » Thu Oct 29, 2020 1:15 pm

The Real MC wrote:
Thu Oct 29, 2020 2:29 am
Can you substitute a 2N2222 for a 2N2222A? Nope! Those suffixes are there for a REASON!
Please share a circuit where the 2222 cannot replace 2222A?

While there is truth to what you wrote, it's more often a matter of degree than a stict yep or nope as you imply.

The "reasons" for the suffixes may have absolutley NO meaning to any given circuit.

I DO agree it's often about more than gain.
But let's not over-state for beginners to worry about things that won't functionally matter in typical synth circuits.

As an example a suffix might mean pinout difference, or gain range. The first is no problem as long as you know it, and the second may or may not be a problem.

To simply remove a great many of the possible substitutes that WILL work because they have a slightly different P/N is to miss the point in an opposite direction.

PN2222's an PN2222As are functionally identical to their 2N cousins. <--Sisters or brothers, more-like.

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Re: Understanding suitable transistor and opamp substitutions

Post by MikeDB » Thu Oct 29, 2020 2:56 pm

KSS wrote:
Thu Oct 29, 2020 1:15 pm
The Real MC wrote:
Thu Oct 29, 2020 2:29 am
Can you substitute a 2N2222 for a 2N2222A? Nope! Those suffixes are there for a REASON!
Please share a circuit where the 2222 cannot replace 2222A?
I thought the same :-) And even harder show me a circuit where a 2222A can't replace a 2222 !

As you said earlier it's all about gaining experience on what works and what doesn't. And to be honest if a circuit is so sensitive to the gain of the transistor perhaps it isn't a very good circuit in the first place. I remember when I joined HP we all had to use a generic NPN called the 1854-0071 unless we could justify needing something better, which meant even in the 70s HP got them for a few cents because of the immense volume. Then everyone gradually migrated to the 2N3904 because it had a better collector-base leakage and that became just as cheap.

I'd also love to know what this Moog circuit is that needs a 741. Behringer managed to clone the whole thing using modern devices with no problems.
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Re: Understanding suitable transistor and opamp substitutions

Post by emmaker » Thu Oct 29, 2020 3:09 pm

MikeDB wrote:
Thu Oct 29, 2020 2:56 pm
I thought the same :-) And even harder show me a circuit where a 2222A can't replace a 2222 !
At least for me the CGS Stiener filter (forget which version) 2N2222 and 2N2907 transistors worked way better than the A parts.

Jay S

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Re: Understanding suitable transistor and opamp substitutions

Post by MikeDB » Thu Oct 29, 2020 5:52 pm

emmaker wrote:
Thu Oct 29, 2020 3:09 pm
MikeDB wrote:
Thu Oct 29, 2020 2:56 pm
I thought the same :-) And even harder show me a circuit where a 2222A can't replace a 2222 !
At least for me the CGS Stiener filter (forget which version) 2N2222 and 2N2907 transistors worked way better than the A parts.

Jay S
Probably because the ones you had happened to be better matched than the A ones. Almost any pair of matched NPN transistors will work well in that circuit, ideally an integrated matched pair which later versions of the schematics show.

This guy tried lots of combinations and as he keeps saying, they all sounded the same.
http://yusynth.net/Modular/EN/STEINERVCF/
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Re: Understanding suitable transistor and opamp substitutions

Post by emmaker » Thu Oct 29, 2020 6:06 pm

MikeDB wrote:
Thu Oct 29, 2020 5:52 pm
emmaker wrote:
Thu Oct 29, 2020 3:09 pm
MikeDB wrote:
Thu Oct 29, 2020 2:56 pm
I thought the same :-) And even harder show me a circuit where a 2222A can't replace a 2222 !
At least for me the CGS Stiener filter (forget which version) 2N2222 and 2N2907 transistors worked way better than the A parts.

Jay S
Probably because the ones you had happened to be better matched than the A ones. Almost any pair of matched NPN transistors will work well in that circuit, ideally an integrated matched pair which later versions of the schematics show.

This guy tried lots of combinations and as he keeps saying, they all sounded the same.
http://yusynth.net/Modular/EN/STEINERVCF/
The ones I was talking about are the output/feedback amp, not the diff pair controlling the frequency.

Jay S.

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Re: Understanding suitable transistor and opamp substitutions

Post by MikeDB » Thu Oct 29, 2020 7:08 pm

emmaker wrote:
Thu Oct 29, 2020 6:06 pm
The ones I was talking about are the output/feedback amp, not the diff pair controlling the frequency.

Jay S.
As the link shows, the best solution is a TL072 so that you aren't relying on obscure parameters of a transistor but have a repeatable design.
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Re: Understanding suitable transistor and opamp substitutions

Post by rumblesan » Fri Oct 30, 2020 5:14 am

Lots here for me to dig into, so thanks again all.

when I started doing DIY electronics it was all guitar stomp boxes, and I spent a bunch of time reading up on germanium vs silicon and the supposed benefits of certain op amps in a tube screamer circuit, so I'm well aware of that side of this.

at the moment I'm really just interested in getting circuits that work and understanding why they work.

my thinking is mostly, will a component substitution change the sound of a circuit? probably
will I be able to hear the change? probably not
will it sound "better" with one and not the other? highly unlikely

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Re: Understanding suitable transistor and opamp substitutions

Post by MikeDB » Fri Oct 30, 2020 5:51 am

rumblesan wrote:
Fri Oct 30, 2020 5:14 am
Lots here for me to dig into, so thanks again all.

when I started doing DIY electronics it was all guitar stomp boxes, and I spent a bunch of time reading up on germanium vs silicon and the supposed benefits of certain op amps in a tube screamer circuit, so I'm well aware of that side of this.

at the moment I'm really just interested in getting circuits that work and understanding why they work.

my thinking is mostly, will a component substitution change the sound of a circuit? probably
will I be able to hear the change? probably not
will it sound "better" with one and not the other? highly unlikely
The main thing is that electronics has moved on since the times when we were component limited. With early stomp boxes, even mass manufacturers were depending on select on test. There were even the odd one or two of those in some items of HP test equipment although there was both disdain and derision for anybody who did implement such a thing, and the permanent hatred from manufacturing.

But nowadays with modern components the goal should be to analyse what is the sound you actually want, and then design a circuit that delivers that mathematical function whether it is built once or one million times. Remember the best Mini-Moog ever built wasn't one of those you see perched on top of Rick Wakeman's setup, it's the software emulator Moog sell as they have modelled the sound Robert Moog intended to be produced exactly so it does this repeatedly. That isn't to say there isn't a place for analogue synths - of course there is. It's often far easier to achieve a sound in the analogue rather than the digital domain - modelling the Mini-Moog probably took longer and cost more than designing the real thing in the first place. But I still think synths, indeed any electronic equipment, should be designed to do the same thing repeatedly, not depend on a particular low-gain/high-gain/noisy/whatever variable and often undefined characteristic of a particular component. Without such attention to good design, synth owners and designers risk being treated with the same derision currently reserved for the 'gold plated jack' brigade who pretend to be audio experts.

Anyway good luck with your future designs, which I hope go well and sound exactly as you want.
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Re: Understanding suitable transistor and opamp substitutions

Post by The Real MC » Fri Oct 30, 2020 11:42 am

KSS wrote:
Thu Oct 29, 2020 1:15 pm
The Real MC wrote:
Thu Oct 29, 2020 2:29 am
Can you substitute a 2N2222 for a 2N2222A? Nope! Those suffixes are there for a REASON!
Please share a circuit where the 2222 cannot replace 2222A?
I built a string of LED drivers on a 15volt rail using 2N2222s. All the 2222s were full on and would not switch off. Changing the circuit from 15 to 5 volt design fixed the problem. Later I experimented on breadboard and found that the original circuit using 2N2222A on 15 volt rail worked fine.

This design is as conservative as they get (simple switch) and I'm not slamming the base with excess current. Even with no base current these 2N2222s leak current as the C-E voltage differential (not reverse) exceeds 5 volts. That was not in my databook.
While there is truth to what you wrote, it's more often a matter of degree than a stict yep or nope as you imply.
(sigh) Just where in the forum rules am I required to give a complete dissertation on electronics on every post? My time is limited, so I give enough information for interested parties to research more on their own. I'm not going to hold their hands.

The morals of this example are 1) to pay attention to part numbers, as something as subtle as a missing suffix can render a circuit inoperative and 2) databooks don't tell you everything. Here endeth the lesson.
But let's not over-state for beginners to worry about things that won't functionally matter in typical synth circuits.
Famous last words. A substituted Q with worse VCE(sat) can render a circuit inoperative. You better believe these things matter.
MikeDB wrote:
Thu Oct 29, 2020 2:56 pm
I'd also love to know what this Moog circuit is that needs a 741.
IC6 and IC9 of the 3046 version of the Minimoog oscillator card. I can understand IC9 requiring a 741, but IC6 should had worked with modern FET input parts.

While experimenting with substitutes on the other opamps on the 3046 board, I found that LF351s would work but not TL071s.
Behringer managed to clone the whole thing using modern devices with no problems.
The B****** is not a 100% clone. If you compare the (-)5v source circuit that uses IC9, B******* changed the circuit to work with a modern part.
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Re: Understanding suitable transistor and opamp substitutions

Post by MikeDB » Fri Oct 30, 2020 12:07 pm

The Real MC wrote:
Fri Oct 30, 2020 11:42 am
Behringer managed to clone the whole thing using modern devices with no problems.
The B****** is not a 100% clone. If you compare the (-)5v source circuit that uses IC9, B******* changed the circuit to work with a modern part.
Of course it's not a 100% clone component wise - as I clearly stated it uses all modern parts. But it sounds exactly the same which is what counts - you can't tell the difference when using it, apart from the fact it doesn't go out of tune so quickly.
Last edited by MikeDB on Fri Oct 30, 2020 2:39 pm, edited 1 time in total.
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Re: Understanding suitable transistor and opamp substitutions

Post by KSS » Fri Oct 30, 2020 12:33 pm

@MC
What part of please share a circuit did you misunderstand?
Or is that something I'm supposed to "research more on my own", now that you've "given enough information" in your "limited time"?
(sigh) Just where in the forum rules am I required to give a complete dissertation on electronics on every post?
I dunno. But I've never found the rule that says YOU have to mention in EVERY post how educated you are eeither.. <--typo on purpose

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Re: Understanding suitable transistor and opamp substitutions

Post by MikeDB » Fri Oct 30, 2020 2:35 pm

The Real MC wrote:
Fri Oct 30, 2020 11:42 am
MikeDB wrote:
Thu Oct 29, 2020 2:56 pm
I'd also love to know what this Moog circuit is that needs a 741.
IC6 and IC9 of the 3046 version of the Minimoog oscillator card. I can understand IC9 requiring a 741, but IC6 should had worked with modern FET input parts.

While experimenting with substitutes on the other opamps on the 3046 board, I found that LF351s would work but not TL071s.
Well it took a while to find it (hint: maybe link to which one of about ten different Mini-Moog schematics you meant) but the 3046 version I found already has IC6 as a TL071. IC9 is a 1458 because someone chose to drive a 1.5uF tantalum directly from the output and no way a TL07x would take that. But I'd regard doing this as a bad design no matter which decade it was designed in.
Personally I hate cats ! Charlie the jackdaw on my shoulder also hates cats as his mother was killed by one so I had to raise him until he set off to procreate his lineage.

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