Op-amp Overdrive

Tom Gorr · From: Three Hills, Alberta

The holy grail of op-amp overdrive/dist/fuzz circuits used to be the JRC4558, made popular by Ibanez in most of their OD circuits. Those chips are probably many decades 'out of date'.

What are the op-amps of choice for these types of circuits using CURRENT technology ?

[This message was edited by Tom Gorr on 07 May 2005 at 06:31 PM.]

Paul Arntson · From: Washington, USA

.

[This message was edited by Paul Arntson on 24 May 2005 at 08:03 PM.]

Jim Peters · From: St. Louis, Missouri, USA, R.I.P.

I modded my new TS9 this last year, new JRC4558,resistors, and whatever else you're supposed to do. It sounds great, but I couldn't pick it apart from my friends bone stock TS9. My older TS5 with 4558( I installed) sounded better A-B'd, than any of the other TS's we had lying around, including some very old and original TS9's and TS8's! Go figure.
A lot of this holy grail stuff is BS. JP

Mike Wheeler · From: Delaware, Ohio, USA

There are differences in internal structures from chip to chip, but my opinion is that the surrounding design is the key. Engineers work with what they are given, ie. budget constraints, pre-selected components, time constraints, etc., etc. Just changing a chip will in most cases not produce much change. Filter circuits, feedback loops and the like can cripple the highest quality chip into mediocrity. A good analysis of the existing design, and a plan of attack would, IMHO, be needed to really get the changes one wants.

I recently ordered a dozen of the Burr Brown OPA2604s to experiment with. I'm impressed with their specs and price point...so we'll see what comes of it all. I've always liked FET based components because of their inherent sound qualities, but the items that were available left a lot to be desired. I'll post the results later, once I run some tests.

ajm · From: Los Angeles

I'd be interested in your results with the Burr Browns. There was a thread on them here a couple of months ago.

And one minor point: Correct me if I'm wrong, but a 4558 is NOT a replacement for a 741. A 4558 is a dual, a 741 is a single.

Paul Arntson · From: Washington, USA

.

[This message was edited by Paul Arntson on 24 May 2005 at 08:03 PM.]

David Deratany · From: Cape Cod Massachusetts

I've been using a Craig Anderton (PAIA Electronics) designed tube sounding fuzz built into my electric for 20+ years. It has a nice warm, smooth sound. It uses a 741 and some hex inverter, if memory serves me well. I tried the 4558, looking for more/better, but while it dropped right in, I didn't like the sound I got.

oops, I may be wrong about it being a 4558. The one I used was said to be a higher fidelity version. Or maybe I used 1/2 of the 4558. Any notes I had are lost in my head.

[This message was edited by David Deratany on 24 May 2005 at 09:12 AM.]

Vern Wall · From: Arizona, USA

If you want to fiddle with op amps you'll appreciate Radio Shack part # 276-175. It allows you to build a circuit by just pushing parts in, and you can pull them out to make another circuit. They also have an identical PC board so you can make a permanent version exactly like your breadboard version.

Bob Lawrence · From: Beaver Bank, Nova Scotia, Canada

From the Ibanez website:

So what's the difference between the TS9 and the TS808? For one, the TS9 reissue is less expensive than the TS808 and for two, they sound somewhat different. Why?

On paper, the original TS9 (circa 1981-84) was supposed to feature a JRC4558 chip. But in practice, that's not what happened. Most TS9's ended up being loaded with the more "high-fidelity" TA75558 chip. Which is why, for historical and tonal accuracy, the TS9 reissue features the TA75558 chip.

The TS808 (circa 1979-80), on the other hand, was spec'd with the JRC4558 chip, and that's what most of the original TS808s did feature and that's what the reissue features.

Players who prefer and cleaner, clearer sound tend to prefer the TS9 with its "high fidelity" TA75558 chip. Players who prefer a warmer sound tend to prefer the TS808.

=========================================
However, to get the warm sound of the original TS808s, Ibanez can't use just any currently manufactured JRC4558. In over two decades since the original TS808's were made, the manufacturer has made improvements in consistency, electrical characteristics and fidelity of the JRC4558. Unfortunately, those aren't necessarily "improvements" when your goal is vintage distortion. (And don't let anyone tell you that a "chip is a chip is a chip." Chips vary from country to country, factory to factory and even foundry to foundry within a factory.)

The result is that present-day chips are different than those of the "good old days." However, we discovered that we can pick out a small quantity from current lots of JRC4558 chips that match our vintage TS808 spec, if we measure them one-by-one to find those with the desired characteristics. This careful and labor intensive selection process limits the production of the TS808 and increases its cost (as does its famous old square footswitch, which had to be completely recreated in order to make for an accurate reissue).

=========================================

Complete doc: http://www.ibanez.com/accessories/spec.asp?m=94

Paul Arntson · From: Washington, USA

.

[This message was edited by Paul Arntson on 24 May 2005 at 08:03 PM.]

db · From: Bethlehem, PA 18015 USA

Musitronics used to use the 4558 everywhere.
But, they would do a selection process . . .
chosing ones with lower offset and noise.
The best ones would be used in the signal path
and the others would be used in the control circuitry.
There was no better spec' device available at the time.
So this was the only way to go.
One other note, the 4558 needs a bi-polar supply to be able to use both op-amps.
The Dan Armstrong boxes were powered by a 9 volt battery, an did not incorperate a bi-polar "simulating" circuit. Maxon makes a single DC to DC bi-polar converter IC (it was not available back then.) This would allow you to use both of the op-amps in the package.
------------------
Dan Balde
U-12/8&5, S-7/D 3&1, S-6/E,A & G3

[This message was edited by db on 25 May 2005 at 07:58 PM.]

Mike Wheeler · From: Delaware, Ohio, USA

I don't know about the Musitronics devices in particular, but the 4558 is a bipolar 8 pin design. "Bipolar" simply means that there is an equal but opposite voltage presented to the op amp's + and - legs, with the mid-point voltage being used as the "ground reference".

The pedals that used a single 9 volt battery would use a voltage divider and bias the opamp at 1/2 the supply voltage (or about 4.5 volts). So the "common ground" line was actually 4.5 volts above actual ground. A few judiciously placed caps would return the input and output connections back to true ground reference. Simplistic, but effective.

A few other, more expensive designs "converted" the 9 volts DC into an actual bipolar power supply. Rather inefficient, (and ate batteries for lunch) but it allowed the circuit designers to create standard designs that could simply be popped into the device without any concern for special biasing.

db · From: Bethlehem, PA 18015 USA

Yes, those old simple "bi-polar supply methods" ate up batteries.

------------------
Dan Balde
U-12/8&5, S-7/D 3&1, S-6/E,A & G3

Forum Index > Electronics		Next oldest topic :: Next newest topic

All times are GMT - 8 Hours	Jump to: