Op amp to Discrete Conversion - Possible?

[tedmich]

Would it be possible to convert a preamp using op amps to transistors?

Yes! Check this unit:


its a Burson audio all discrete dual op amp ready to snap into your DIP socket and tower over your circuit board,,,for $147.67 at hificollective

[Enzo]

Does it come in surface mount too?

[imaradiostar]

That's probably because of the circuit topology, though, not the components. What do you think op-amps are made of inside?

I disagree with R.G. on the objectionable distortion, though. A discrete JFET preamp with no global feedback will have sweet-sounding distortion that an op-amp (either monolithic or discrete) won't.

I once simulated and designed boards for (but never completed) a neve ba283 module with slight mods so it would replace the mic pre op-amp in my yamaha console. The console already had transformers out front for CMR so all I needed was gain and the Neve circuit would do nicely.

If I had to do it today I'd probably arrive at a J201 or LND150 or similar with a mosfet source follower. I'd use a large cathode bypass cap in line with a trim pot for a "factory set gain trim" and use resistive dividers and negative feedback with multi-pole switches for user gain adjustment a la early neve or trident a-range.

That said- the best thing to do with an ART tube mp is leave it alone. It works so let it be. Build something different and learn from it. Try some of the stuff from Tape Op magazine. check this out: Hamptone JFP DIY

jamie

[Steve Conner]

Yes! Check this unit:
its a Burson audio all discrete dual op amp ready to snap into your DIP socket and tower over your circuit board,,,for $147.67 at hificollective

:O

Do the guys who buy these things ever think about how many op-amps their album went through in the studio? I don't see why they have to hate on our little eight-legged friends. It's a scientific fact that the distortion on something like a NE5532 is inaudible, in fact you can prove it to yourself in a home lab using a null test.

Burson quote THD of 0.0000whatever% for their discrete op-amp, so to me that says their distortion is inaudible too, and you're paying $148 to get no change at all.

Of course the above statements apply at low gains, where negative feedback hides all of the amp's inherent distortion. If you take most of that feedback away, pushing the amp towards the limits of the gain it's capable of, for instance if you're running an op-amp with a gain of 60dB in a mic pre, then it might start to show its character. Since the open-loop gain of an op-amp falls with frequency, it'll show that character in the high end first. If the amp has a Class-AB output stage, it may be an unpleasant character with crossover distortion.

So for something like a mic pre, I'd choose a circuit like Jamie's. A single-ended JFET input to make the distortion sweet, and a Class-A source follower output that can't generate nasty crossover distortion. Some designers like to take ordinary op-amps with Class-AB outputs, and hang a big DC load off the output, which essentially skews them into Class-A.

And remember "All components on the silicone dice are formed by droplet of chemical." So they must sound bad, right?

[dai h.]

That said- the best thing to do with an ART tube mp is leave it alone. It works so let it be. Build something different and learn from it. Try some of the stuff from Tape Op magazine. check this out: Hamptone JFP DIY

jamie

seems it'd make more sense to start with something better if you want a good sounding mic pre rather than going off on a esoteric direction with something that doesn't appear particularly great to begin with. My take on those starved plate "tube pre amps" are that at best they provide a bit of colour to the signal but at worst cheap junk hitching a ride to the "warm tubes" bandwagon.

[jesucio]

Discrete opamps and IC opamps are basically made of the same thing using the same topology. One big issue however is that when you build yerself a dicrete op amp you can pick the parts yourself. You'd be surprised how a set of capacitors could affect your sound. Another issue is that when people make discrete opamps they tend to go for the gold and make them class A for that high definition audiophile quality. IC opamps are made pretty damn well nowadays and their distortion specs can reach almost unreadable levels, but the choice of a discrete opamp is always there. If the IC in your circuit is set to a gain in the range of say 50, or 100, or more...then you may have a hard time getting that much gain out of a single discrete opamp without getting a lot of distortion. You also gotta realize that a good part of the opamps sound is based on the parts around it. Good decoupling of the power rail that supply the IC really help. Especially if your trying to get that all too ellusive low end. Theres a lot of parts that go into a circuit. Not just the opamp. Just some thoughts on it. I personally like discrete op amps, but IC opamps definately have they're benefits. Not the least of which, their reliability and efficiency. Some of them are tough little troopers.

-jesucio

[Robert M. Martinelli]

.....Burson quote THD of 0.0000whatever% for their discrete op-amp, so to me that says their distortion is inaudible too, and you're paying $148 to get no change at all.

And remember "All components on the silicone dice are formed by droplet of chemical." So they must sound bad, right?

Well Steve, that's not entirely true, a change actually takes place, as $ 148 fly from their wallets into Burson's......

The main problem with audiophools is that you can argue with them all the way to h&ll and back, still they DON'T want to be convinced ( or should I better say "educated", or "saved" ? ). As long as they're willing to pay thou$ands of $$$ to hear differences/improvements that exist only within their twisted minds, or to try to impress their friends/ladies ( a slight variation of the old "small pr!ck, big machine" syndrome ) that's OK with me....my only regret at the moment is not running an "audiophools-oriented" business....Maybe I'm too honest to run one, and this explains why I'll probably die poor and forgotten...

Some tube purists aren't immune from this syndrome too....they spend tons of money on tube amps, and half of their time backbiting SS amps, still they stick loads of "silicon based" boxes before their beloved "all-tube-amp" unflinchingly....

To me, if it sounds good, it sounds good, period, I've heard good-sounding SS amps and terrible-sounding tube amps ( and vice-versa ), but I usually don't take my o-scope with me when I'm going to a gig, I tend to use my ears, together with the little knowledge and common sense I still happen to possess.

It's a mad, mad world.....and common sense is becoming less and less common IMHO.

Cheers

Bob

[Steve Conner]

I quite agree, I think there's a lot of Emperor's New Clothes effect there. Most people just hear what the advertising copy primed them to expect, and you never hear from the dissidents because the audiophile community censors them. Or they self-censor through a kind of herd mentality. "Don't you think that sounds so fat, tight and yet warm?" "Hell Yeah! And the soundstage is almost 5-dimensional." Etc.

I'm currently trying to design a good-sounding guitar amp inspired by an old Fender, but made entirely of JFETs and BJTs, a project I've been working on over the last few years. Progress has been easier than I thought so far. JFETs don't sound quite the same as tubes when you overdrive them, but they don't sound "bad", at least not to me, and not in the circuit I developed. There just doesn't seem to be any evil "transistor sound" that needs to be fought every step of the way.

I was previously working on a hybrid with a BJT output stage transformer-driven by an EL84. I built a prototype of that, but the purely solid-state amp kind of fought its way to the top of the pile!

[Jazz P Bass]

Bob Pease

His troubleshooting analog circuits book is a personal favorite - I learn something new every time I read it.

I like when Bob throws the computer off the roof.

[Mark Hammer]

1) Do not mistake how much epoxy surrounds something in a standardized shape/size with the quality of the component inside. The transistors inside a chip are generally like the transistors inside a TO-92 package. The T)-92 has to have a wad of epoxy around it to conform to the spec for mounting/installation purposes.

2) Discrete circuits can be custom-tailored to create different sorts of operating characteristics, but then there are a helluva lot of different op-amps in chip form too, many of which may well replicate the performance of a discrete circuit, except no one knows it yet because that op-amp is off the radar...or perhaps out of production.

3) Discrete circuits are designed from the ground up, not simply as higher-quality substitutes for chips. Conversely, designs predicated on op-amps in chip form are based around the property of the chip (usually). Plywood consists of multiple layers glued together with orthogonal grains in each layer for added strength. You can do that too with popsicle sticks, and the popsicle sticks will likely have no knots in them, unlike plywood. But would it make any sense to do so? On the other hand, if one was sculpting or carving a shape, it would make eminent sense to begin with a solid piece of wood, rather than sandwiched layers of plywood.

[tedmich]

1) Do not mistake how much epoxy surrounds something in a standardized shape/size with the quality of the component inside. The transistors inside a chip are generally like the transistors inside a TO-92 package. The T)-92 has to have a wad of epoxy around it to conform to the spec for mounting/installation purposes.

2) Discrete circuits can be custom-tailored to create different sorts of operating characteristics, but then there are a helluva lot of different op-amps in chip form too, many of which may well replicate the performance of a discrete circuit, except no one knows it yet because that op-amp is off the radar...or perhaps out of production.

3) Discrete circuits are designed from the ground up, not simply as higher-quality substitutes for chips. Conversely, designs predicated on op-amps in chip form are based around the property of the chip (usually). Plywood consists of multiple layers glued together with orthogonal grains in each layer for added strength. You can do that too with popsicle sticks, and the popsicle sticks will likely have no knots in them, unlike plywood. But would it make any sense to do so? On the other hand, if one was sculpting or carving a shape, it would make eminent sense to begin with a solid piece of wood, rather than sandwiched layers of plywood.

I'd argue the converse: an audio integrated op amp designer is free to include almost any values while discrete components are available in a finite variety. Any testing which "found" a discrete op amp circuits superior to an integrated audio op amp is most likely correlated to a lower sitting position with a depleted wallet than any real difference, IMHO.