Super Champ XD: Swapping Op Amps
When I got my second Super Champ XD, I decided to explore the effect of the op amps on the tone. I installed sockets for the three op amps that are in the audio signal path and tried some different chips in there.
The incoming guitar signal first sees a TL072, and it goes through both stages. From there, it’s routed to the DSP. When it emerges from the DSP, the signal is balanced, with + and – outputs. Another TL072 integrates the signals and turns them back into a single-ended mono signal. Then the signal goes to the first stage of a 4560. From there it goes to half of the 12AX7, which buffers the signal and drives the phase inverter half of the 12AX7.
All three op amps apply bandpass filtering to keep the signal in the audible range and to prevent oscillation.
The TL072 has good specs and is a reasonable, low-noise op amp, but guys who hack their DVD players consider them “jellybean” parts– $.50 cheapies with barely acceptable tone. I generally don’t think about op amps having tone, but they do, just like tubes.
The question is how much improvement can you get, and is it worth the effort?
“Effort” is a definite factor. The SCXD circuit board is very difficult to remove and it’s hard to desolder the op amps from the board without damaging the board.
After successfully installing sockets, I tried two different kinds of op amps: the 5532 and 2132. The 5532 is well-regarded by most audiophiles, but the highest praise generally goes to the 2132. The 5532 is a bit more expensive than the TL072; the 2132 is a $4-$5 part.
I reloaded the amp with TL072s and a 4560 and fired it up–it sounded normal. I checked for oscillation on the op amps and there was none. I replaced the preamp TL072 with a 5532 and the sound was noticeably brighter and crisper. Not to an earth-shattering degree, but noticeable. Then I replaced the 5532 with a 2132. It was slightly better than the 5532–crisper bass, brighter highs, better note definition. I switched back and forth between the TL072 and the 2132 and the difference was fairly obvious.
Changing the second TL072, the differential integrator, to a 2132 added a slight bit of brightness, but the increase was not as dramatic as it was in the preamp.
Changing the 4560 to a 2132 again increased the brightness and crispness, but again, just a small amount.
I did all the bench testing through a well-broken-in Eminence Swamp Thang, which is a clean, loud, mostly colorless speaker. My son hears high frequencies better than I, and was there for all the testing to double-check my ears.
We then reassembled the amp and brought it upstairs to play against my other SCXD. I connected them with an A/B switch and adjusted them for identical volume levels. The first thing we noticed was that the SCXD speaker was not as crisp or articulate as the Swamp Thang–it added a certain blurry quality to the tone, which tended to make the amps sound more alike.
After switching back and forth a bit, though, we could easily tell which amp was playing. The difference is not as dramatic as upgrading the speaker to a Ragin Cajun, but it’s there. Pick attack is cleaner, note harmonics bloom more fully, and the amp is brighter, with more top end.
Would I recommend that you do this mod? No.
It’s a lot of work for a little bit of progress–half a speaker upgrade’s worth of tone. The chances of ruining your amp are considerable, especially if you don’t have desoldering equipment that heats and applies suction at the same time. I work on PCBs all the time, and I still managed to tear a trace when a pin got stuck.
I will say, however, that combining the upgraded op amps with the Ragin Cajun gives the amp a degree of transparency, the open, airy tone that the audiophiles talk about, that wasn’t there before.
Putting it in perspective, however, as soon as you turn on effects, you blur out most of the gain–reverb, chorus, delay all overlay the sound with more sound, and no more crisp edges, many fewer ringing, airy harmonics. And we are talking electric guitars here, not critical listening to a flute and harpsichord concerto.
There’s still more tone to be had from the SCXD; my exploration isn’t over yet.