To me the obvious question is whether diode compression works on this deal?   :twisted:

U know JD,
that's sure doesn't look like an "opamp" 2 me.
i have several "transistorized OA" schemos.
Remember the one from SWTPC?  (Meyrs?)
All use about 10 trannys.
But, these R *true* operational amplifiers.
Where U just use resistors 2 program gain.
I tried simulating your circuit and all i got was spikes.
Not linear...mayB a "fuzz" of some sort(?)
Saying this is an OA is *really* gonna confuse noobies.
stayclear
tone

Quote from: Paul MarossySo, this differential preamp would be something of a virtual XLR jack?!

Actually, I think if you wanted to, you could just install an XLR output jack in your guitar. But, then you'd probably need a converter box anyway, to be able to plug into amps and pedals and such. So, maybe it's more like an onboard balanced/unbalanced converter.

~ Charlie

Well, moosapotamus, keep us posted if you come up with something!  :wink:

Quote from: tonemanU know JD,
that's sure doesn't look like an "opamp" 2 me.
i have several "transistorized OA" schemos.
Remember the one from SWTPC?  (Meyrs?)
All use about 10 trannys.
But, these R *true* operational amplifiers.
Where U just use resistors 2 program gain.
I tried simulating your circuit and all i got was spikes.
Not linear...mayB a "fuzz" of some sort(?)
Saying this is an OA is *really* gonna confuse noobies.
stayclear
tone

A majority of transistors in something like an opamp are really to control current - in other words - they act like the resistors in the simple opamp schematic.

One thing about opamps is that the current in the device is very controlled, and when you demand the limit from the device a lot of the dynamics are lost. This is why 2 circuits like the Guv'nor and BSIAB sound somewhat similar is some ways and totally different when it comes to sensitivity and dynamics.

What would be pretty cool would be to combine FETs, and Ge transistors to produce a circuit equivlent of an opamp - leveraging the best parts of the sound form each device. I think that might make more of a triode sound - sensitive yet warm sound (instead of the pentode like sound of a FET - sensitive and edggy)... just a thought..

A balanced active pickup and cable is only going to help with hum pickup, if the hum isn't being picked up by magnetic induction to the coil.
In practice, I doubt very much whether there would be any improvement over the usual buffer in a guitar to present a low-impedance drive to the usual cable.

The Boss "turbo overdrive" pedal uses discrete opamps, and I think it sounds really good.

The turbo "off" path is a discrete "super overdriver".

The turbo "on" is two discrete opamp based stages, cascaded, AND starved (powered by 4.5V instead of 9V).

The discrete opamps in the turbo od are more "normal", it's two NJFETS, a 4.7K "long-tail" to gnd, a 2.2K in the collector of the (+) input fet, this makes a diff input and the output is a common emitter PNP with 2.2k on the collector.

The parts count is similar (there's one extra resitor compared to Joe's opamp).

Joe, are there any advantages in using the complementary input instead of the "normal" diff pair?

PS. I still have no idea on how the "charge pump" transistor in the shoctave works

Quote from: mojotronWhat would be pretty cool would be to combine FETs, and Ge transistors to produce a circuit equivlent of an opamp - leveraging the best parts of the sound form each device. I think that might make more of a triode sound - sensitive yet warm sound (instead of the pentode like sound of a FET - sensitive and edggy)... just a thought..

I hope someone picks this up & runs with it, there must be tons of possibilities of 'mixed' circuitry, I saw a mid 60s electronics book (before transistor dcircuit design had 'settled down') and peoplewere making all kinds of tube/transistor hybrids.
That is the beauty of DIY, we can make stuff that has no commercial sense, but SOUNDS good

Meanwhile, in a parallel Universe, there are a bunch of guys thinking, "Hey! Maybe we could combine FETs and Ge transistors in a sort of hybrid op-amp to give us both clean amplification AND uncontrollable radio pickup, in a highly temperature-sensitive manner..."   :lol:

Cool, as you say it may be, but I just can't help thinking that it would be very easy to get in one hell of a muddle with transistors all over the place. With an opamp you know where you are, you know what each pin is but .........................I'm not saying it's not a good idea, but it's not going to compicate things for me.
Stephen

QuoteJoe, are there any advantages in using the complementary input instead of the "normal" diff pair?

The advantage is keeping it simple, but probably not much else. The other possible advantage is being able to "flip" the entire circuit, depending on which transistors you have/want to use. I need to put up an example of that.

QuotePS. I still have no idea on how the "charge pump" transistor in the shoctave works

The input stage distorts slightly, sending pulses to the charge pump. Each pulse turns on the diode, which in turn biases the transistor. At the moment the transistor is on, power goes from collector to emitter, through the diode, and then to the oscillator. When the diode turns off again, the circuit shuts down.

There are other applications of that idea that I never got around to, including having something "sense" an input signal, various voltage/polarity conversion, etc. You can also sense a frequency with it, which has some interesting possibilities (simple tuners?).

Quote from: Joe Davisson

QuoteJoe, are there any advantages in using the complementary input instead of the "normal" diff pair?

The advantage is keeping it simple, but probably not much else. The other possible advantage is being able to "flip" the entire circuit, depending on which transistors you have/want to use. I need to put up an example of that.

Well that's 2 P devices, 1 N device, 2 resistors against 2 N devices, 1 P device, 3 resistors in the "normal" case. And you can flip the normal circuit too.

I don't know, maybe the fact that the current in both transistors in the input is always the same can give a different overload characteristic (more assymetric?) than in the normal case where the current in one device goes up when the other goes down. :?:

Quote

QuotePS. I still have no idea on how the "charge pump" transistor in the shoctave works

The input stage distorts slightly, sending pulses to the charge pump.

This I understand

Quote
Each pulse turns on the diode, which in turn biases the transistor.

When the diode is on the be junction of the transistor is off because the anode "points" to the base.

Quote
At the moment the transistor is on, power goes from collector to emitter, through the diode, and then to the oscillator.

Again I can't see how the diode and transistor can be on at the same time.

Quote
When the diode turns off again, the circuit shuts down.

When the diode is on, the transistor is off, the 0.47u cap is charged, then the transistor is off, the diode is low and the cap is discharged, alternating  as the output of the preamp goes high/low. There's how I see it.

I'm really intrigued because the shoctave does work nicely, so I must be mistaken in some point.

Quote from: moosapotamusCool! Looks like this would also work as an onboard differential preamp, yes/no? One PU wire to (+), the other to (-). Make your axe immune to humm/buzz, maybe?

~ Charlie

Well, you could do that with an opamp chip, too, right?

Ben

Sure. This just looks a lot more compact.

~ Charlie

QuoteWell that's 2 P devices, 1 N device, 2 resistors against 2 N devices, 1 P device, 3 resistors in the "normal" case. And you can flip the normal circuit too.

Well, that's true. So there's only a resistor to gain.

QuoteI don't know, maybe the fact that the current in both transistors in the input is always the same can give a different overload characteristic (more assymetric?) than in the normal case where the current in one device goes up when the other goes down.

Without some clamping in the feedback loop the distortion characteristic with bipolars is kinda crappy (and more unstable) but in the "screamer" example it sounds pretty good, much like a regular opamp. I should add that other opamp situations work, such as inverting amplifiers with programmable gain (r2 / r1). It really is an opamp, just not an outstanding one

Ok, now for the shocktave again:
1. Transistor and diode are initially off.
2. Input pulse is high, forward-biases diode, raising the NPN's base voltage. This turns on the transistor.
3. Power cannot go between base and collector. But it can go through the collector, emitter, diode, then oscillator.
4. Input pulse is low, diode & transistor turns off. Now no power can flow until the next high pulse.

The transistor can only conduct when the diode is forward-biased.  The diode is very useful here because it has a sharp off/on characteristic, meaning with no signal the oscillator is dead. Hope that makes sense.

Hi Joe,

i have a question about the very nice Circuit..

at the Top the CollectorPin of the top/right Transistor (2n5087)
to what is it connected? 9Volt + ??

sorry if my question is a bit dump, but i want to build that TS
and dont want to kill the parts by soldering to wrong connection  

thanxx !!!

bye
Oliver

The 2n5087's emitters go up, not the collectors. The upward arrows all go to to +9v. So the 2N5089 will go in the normal way, while the 2N5087's will look like they are put in backwards.

Quote from: Joe DavissonThe 2n5087's emitters go up, not the collectors. The upward arrows all go to to +9v. So the 2N5089 will go in the normal way, while the 2N5087's will look like they are put in backwards.

Hi Joe,

Thanks very much for the fast Answere !!!!!

bye

Oliver

Some observations:
- this opamp is useful only for AC signal circuits, as the two inputs inherently can't be at the same DC voltage. Not a big deal for AC-signal-only circuits, but a huge problem for any DC applications. The inputs have to be about 1.2V apart.
- A simpler version can be made by replacing the input PNP with a resistor. If you look at it that way, The input PNP is only acting like an emitter follower to drive the first transistor's emitter with a low impedance. If you accept the low input impedance on the (-) input, the emitter works just fine. The feedback network then goes between the collector of the second PNP and the emitter of the first transistor. If you are using it as simply a follower, connecting the collector of the second PNP to the emitter resistor of the first, you can replace the first emitter resistor and second collector resistor with a single resistor. There are a number of these two-transistor feedback pairs that exist.
- a single opamp needs five pins, four resistors, and two capacitors to operate as a non-inverting gain stage. This one has nine pins to connect and six resistors plus the same number of caps. When you need to evaluate circuit complexity, count all the device pins to be soldered. The crossover to an opamp being simpler is at about two transistors.
- two-transistor feedback circuits have an inherent frequency advantage over opamps. They're a two-stage feedback circuit, and thus inherently stable in feedback (unless you do something silly) and so they can have a much better high frequency performance than all except the last decade of opamps. They don't need frequency compensation. Three (and more) stage amplifiers have to be compensated to be stable in feedback. One- and two-stages *can* be stable without compensation.

I don't pretend to understand all of this, but part of the point of doing your own op amp is to be able to use different transistors to tweak the distortions???  

Mosfet here, Jfet there, BJT in between...  All Mosfet, all Jfet, another mixed blessing.

I guess it is similar to trying the OP275, TL072, 4558, OPA2262, 5532, 353, 833, etc.