driving transistors with op amp and an active eq

[acehobojoe]

so, as I have been very impressed with the peppermill, I'd like to try to create an op amp driving transistor setup.

I know that you can really make caps "shine" when pairing them with an op amp.

A good place to start is some active eq circuits..

http://www.runoffgroove.com/mreq.html

maybe I'll start there and test.

Any ideas are much appreciated. Do you think that 2n5457s sound good, or maybe j201's?

and don't op amps usually have more head room?

[Mark Hammer]

Headroom is largely dependent on supply voltage.  A 2n5457 with an 18V supply (or higher) is going to have more headroom than a TL072 running off 6V.

[anchovie]

I know that you can really make caps "shine" when pairing them with an op amp.

Er...what does this mean and what is the foundation?

[acehobojoe]

Correct, but then why would anyone use an op amp?

[samhay]

After a couple of beers, I'm wondering if this might just about have the makings of another burst box-like thread.

[R.G.]

I know that you can really make caps "shine" when pairing them with an op amp.

I'm with anchovie - I have no idea what you mean by that.

Do you think that 2n5457s sound good, or maybe j201's?

Again, I'm somewhat baffled. I like 2N5088s - in some circuits, and 2SK30As in others, while in other circuits I like 2N3055s. And my preferred JFET is 2N5485, largely because I bought 100 of them.
I think that low-hopped ales are good, better than IPAs.

and don't op amps usually have more head room?

I think you're feeding back things you may have read but without a context behind it to make the questions make sense. Can you explain more?

[acehobojoe]

Well, Mr. RG, what I said was probably just a misunderstanding of the purpose of op amps. I've experimented with both transistors and op amps, and I like them both. Transistors however seem to sound better for me usually, if both are being used for amplification.

I guess I've just wondered what makes an op amp better for some circuits, ( ts-9? or bluesdriver?) when they could also be given the same amount of gain from a transistor. I think I heard from my friend that op amps can help create better EQ circuits, while a single capacitor might be limited, this could all be wrong. .. so yea..

[PRR]

> but then why would anyone use an op amp?

Read Mark's remark. He compared an apple to a grape-- 18V on one, 6V on the other.

> but then why would anyone use an op amp?

Because it is a damsite easier to bias-up and gain-set an opamp than a transistor.

Anything more than a trivial gain/impedance requirement, the opamp is often cheaper and smaller.

> Do you think that 2n5457s sound good, or maybe j201's?

Beer or wine or scotch? Moxie or Dr Pepper? Blondes or red-heads?

Often you take whatever is handy. Rarely is this a joy/pain decision. I do prefer Moxie for taking the rust off the plow, but only her hairdresser knows for sure. (And that may be true in this day of re-labeled JFETs also.)

I will say that all JFETs are the same, except voltage and resistance. So by changing the circuit parameters you can get the same "good". Hot Dr Pepper takes rust off as good as cold Moxie.

[R.G.]

Well, Mr. RG, what I said was probably just a misunderstanding of the purpose of op amps. I've experimented with both transistors and op amps, and I like them both. Transistors however seem to sound better for me usually, if both are being used for amplification.

I really was confused by your question. This helps.

Transistors are almost by definition low gain devices compared to opamps. The term "opamp" comes from "operational amplifier" and that from the concept that it's an amplifier that is defined by its operation - conceptually, the parts surrounding it - almost exclusively. The way this works is to have really, really high voltage gain inside, and allow the entire operation of the amplifier be determined by the feedback and bias components. Opamps usually have internal voltage gains of upwards of 100,000. This is so high that the "imperfections" of the amplifier are almost completely hidden by the linearization of feedback. Transistors can be linearized by feedback, but not nearly to this degree. The fundamental concept of feedback is to use excess gain to "hide" the internal imperfections. Single transistor gain stages can't get there. Two and three-transistor gain stages were the standard for a long time, and many older circuits use a multiplicity of two- and three-transistor sections for this.

I guess I've just wondered what makes an op amp better for some circuits, ( ts-9? or bluesdriver?) when they could also be given the same amount of gain from a transistor. I think I heard from my friend that op amps can help create better EQ circuits, while a single capacitor might be limited, this could all be wrong. .. so yea..

There are a few big items separating whether one sounds better than the other one.  Many of the tricks used in clipping in, say, a TS-9 do not work well for transistors, or do not work so predictably. The two single transistor stages of a Big Muff are diode-feedback clippers. So is the opamp stage with clipping diodes in a TS-9 The muff works out to have the diodes clipping the transistor sound, and the TS-9 has the output being the sum of the original signal and the clipped feedback signal. The input differential stage is a big part of that sound. A single transistor stage CAN be driven at both base and emitter as a differential input, but it's difficult and tricky, and you wind up doing a lot more work to get there than you do with an opamp.

Opamps are vastly more linear than transistors, even with local feedback, right up until they run out of output voltage swing. Then they clip razor sharply, as the feedback "hid" the onset of clipping as long as it could, then gave up. Transistors have idiosyncratic clipping depending on what's put around them. Transistors cannot come near the linearity of opamps, but musicians tend to like the residual distortions they make - until the clip.

Even with the same nominal gain after whatever feedback they use, opamps will be more linear and also have a lower output impedance, often down into one- or two-digits of ohms. Transistors can't do that, except perhaps in the emitter follower setup, and then they have less than unity gain. Most filter stages ("EQ"s) presume a very low impedance drive, so yeah, opamps might be able to do that more simply than even a two-transistor cascade.

But you're flirting with needing to really understand circuits, not just whether an opamp is better than a transistor. This is another of those apparently simple questions that has a very complex set of stuff hiding under it.

[acehobojoe]

Complex set of stuff hiding under it. - That seems to be the basis of everything that a guitar pedal is.

Thank you for that information. It's amazing to think about how an overdrive clipping is the summing of the two together. The two signals are completely different but join together to make the most awesome sound.

On transistors, you described exactly what I have noticed, I never knew why I loved transistors sounds so much, it has to do with that residual distortions! It's not about what is better, but it's just a different "flavor". If we can get all of the necessary nutrition from ramen noodles and tuna, why would we eat anything else? It's about the flavor.. However, you have also described a very complex IC, (The op amp) it seems that the op amp is not necessarily better than a transistor, but it is "more" transistors and characteristically different.

on a side note.. does geofex have anything on pt2399s?

[antonis]

you have also described a very complex IC, (The op amp)

It's not so "complex" even if you want to go "deep" inside...

If you don't, just consider it as a box with two inputs and one output (OK... and power supply pins..) which follows some - not realy - arbitrary rules, like:

There is no current flowing into inputs... and
An opamp will attempt to make both inputs exactly the same voltage (via the feedback path)... and
If it cannot do so, the output will assume the polarity of the most positive input....

It helps a lot to understand HOW (but not WHY) a circuit works, without getting meshed with terms like Open & Close Loop Gains, Bandwith & Gain Products
, Offset Voltages, CMRR e.t.c. - like you do with an "elementary" analysis of transistor amplifier..

[samhay]

^ http://www.runoffgroove.com/mreq.html

You realise this doesn't use any op-amps right?

^It's amazing to think about how an overdrive clipping is the summing of the two together.

This is not a generic statement of how overdrives work, but rather relates to the specific tubescreamer-type clipping arrangement. Also, it is perhaps better thought as an analogy rather than strictly how the circuit works. Nevertheless, as RG mentioned, it's a good example of something easy to do with an op-amp, and much harder to do with a single transistor stage.

As far as the PT2399 goes, these are good:
http://www.valvewizard.co.uk/PT2399_Data_Notes.pdf
http://sound.westhost.com/project26a.htm

[R.G.]

Complex set of stuff hiding under it. - That seems to be the basis of everything that a guitar pedal is.

That is very accurate, especially for many of the earlier designs from the 60s and early 70s. Most of what the effect does is what a mainstream designer would call a side effect of the actual operation.

However, you have also described a very complex IC, (The op amp) it seems that the op amp is not necessarily better than a transistor, but it is "more" transistors and characteristically different.

Yes. It's a different beast. Transistors are what they are. Opamps are what the feedback and other components say they are.

Historically, EE design professionals were trying really hard to iron out the oddnesses and limitations of transistors. For instance, it was a long, hard technical fight to get hifi and other audio gear down to distortion levels low enough to be mostly neglegible. The solutions that were worked out eventually amounted to building opamps out of discrete transistors. That was the general answer for all power amplifiers before PWM digital amps were hacked into practicality, and still is the practice for truly high quality audio. The distortion of single transistor stages sound good to musicians who are looking for it, but they're unacceptable for trying to accurately reproduce sound.

A particularly ugly example of this is that you, as a musician, get a truly lovely sound with your guitar and some inaccurate, but pleasant sounding pedals. This sound is recorded through equipment which does not use opamps, but single transistor gain stages, and they add their own distortions on top of the sound you got. Then it's played back through preamps and power amps with their own distortions, and the sound you get is very different from what you started with - and slaved over to get.

on a side note.. does geofex have anything on pt2399s?

No.

[acehobojoe]

Sweet, thanks to everyone. This all helped.