ไอ.ซี. Opamp

The Opamp

The TS808, the TS9, most TS9 Reissues, and some of the 10's and perhaps a few 5's came with the "JRC4558" dual opamp. Most 10's, almost all 5's and a few 9 RI's came with other opamps. The JRC4558 is a dual opamp with "industry standard" pinouts which means that there are perhaps fifty different dual opamps from different manufacturers that will fit in the same holes and work.

There seems to be a difference in sound depending on what opamp is actually in the circuit. In fact, this may be the single biggest effect on the sound of the TS circuit. The JRC parts were undoubtedly chosen by the design engineer primarily because they were cheap. During the time the 808 was in production, the JRC was oneof the cheapest dual opamps on the market with acceptable audio performance. As such, it was used in huge volumes of Japanese audio equipment. The opamp is described in some Japan Radio Corporation (JRC...) literature as "an improved dual 741 type opamp with better noise, drive capability, and slew rate than the original 741 type opamps". Note that this is not saying much, as the 741 is a fairly abysmal audio part.

For some reason, the JRC4558's seem to be well suited to duty in the TS type circuit. JRC later changed its name to New Japan Radio Corporation and made parts titled "NJM4558". When you order these, you get parts that are labeled "JRC4558". By all reports, these sound as good as the original. The tube screamers made with 72558A are reported to sound abysmal. From the brave souls who took the suggestions in the tube screamer clone section over on GEO and put in a socket to swap parts, the following types of opamps DO seem to sound good.

• new manufacture JRC4558D
• LM833
• RC4558, made by a couple of US firms
• TLC2202
• TLC2272
• OP275
• LT1214

What makes for a good opamp versus a bad one? I dug through the data books on the ones that people told me sound good, and came up with a common factor. The ones that sound good seem to all make a point that they recover from overdrive gracefully. I can hear you asking "what the devil does that mean for opamps?" It's like this - in a feedback configuration, an opamp's enormous gain keeps both inputs within a few millivolts of each other all the time, every nanosecond. When the opamp is driven to the point where the output is near a power supply voltage and cannot swing any further, the feedback is no longer keeping the inputs together, and one input is driven further toward a power supply than the other. This can cause some ugly things to happen. In some opamps, the output inverts polarity for a few microseconds. I'll leave it to you to imagine how ugly those little "blatts" sound. In other opamps, the recovery time from overload is slew rate limited; the output does not follow a multiple of the input, but "slews" at a fixed rate until it catches up to the amplified input. This also sounds bad.

While the clipping opamp is not driven into overload, I theorize that the sudden change of gain every time the diodes go into and out of conduction can cause a similar although briefer recovery period at each "corner" of the clipped waveform. In this period, you'd get non-harmonically related artifacts that would neve be very big (all that lowpass filtering, remember) but which would be audible as a harshness or edginess. While this is just a theory, it does seem to meet the only observed facts, and has predicted a couple of opamps that did sound good. You're free to come up with your own theory, of course, but I like this one. It has worked so far.

== This section has been updated based on newer information ==

There is some confusion at present over what is a true or "vintage" JRC4558. Several electronics suppliers list the JRC4558. These are sometimes listed in their catalogs as "NJM4558", but what you get are marked "JRC4558D". By all reports, the current manufacture JRC4558D's sound just as good as the ones current with the manufacture of the original tube screamers.

There were zillions of the real "JRC4558" made when it was current. They are still available in isolated caches of parts, although they can sometimes be hard to find. Does true "New Old Stock" matter? probably not. In addition, the JRC4558D was used in tons of Japanese audio equipment at about the same time, so a junky cheap Japanese stereo or clock radio could have several hidden inside it.

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The best thing to do, I believe, if you're hunting the Tube Screamer Holy Grail is to stick in an 8 pin socket on the board, noting the orientation of the original chip carefully, and then just plug in various dual opamps until you find one that conjures up images of your own personal guitar deity.