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.
I am just curious about all of the recommendations I keep seeing about use of the Ragin Cajun in the Super Champ XD.
I have a band of advance musicians and we have two Champs, one with the stock speaker and one with the Ragin Cagun. We ran them side by side with an A/B switch and the entire band said the stock speaker beat the Ragin Cajun hands down. I was stunned since I owned the upgraded Champ. I clearly agreed after hearing it. Whats your take on this? Also do you know where I can get a deal on a use stock speaker?
Thanks,
Tim
Hey Bill,
Have you considered a Presence control mod on the Super Champ XD? I have replaced the stock speaker with an Eminence Legend 1058, tubes with Tung Sols and the speaker cable with Van Den Hul. I’m very happy with it but was curious if the addition of a presence control might add some sparkle and liveliness to some of the modeled voices, in particular voices 1 and 3 which are my favorite. I use Voice 3 primarily with Gain at only 2 for a dirty clean Fender breakup tone with my Strat and it is great but a little dark even with the bass rolled down to 6. Any thoughts or other ingredients I could add to this setup I may have overlooked?
Thanks!!
The SCXD uses a very simple negative feedback loop to improve output stage accuracy. It doesn’t have any of the tone shaping components found in the all-tube amps, with or without an adjustable presence control. Since the PCB is so crowded and hard to work on, I think it would be a bad idea for most owners to attempt to add a presence control.
Voice 3 is a tweed amp and I agree that it is a little dark compared to the blackface voices. Voice 1 (Champ) seems plenty bright to me, but we all have different ideal tones in our heads.
Thanks Bill. Have you tried any OD pedals into this amp? I wonder how an OD pedal would sound into Channel 1 or Voice 1 since those are Blackface cleans.
I appreciate your feedback!
I don’t use OD pedals with mine. Since there are bunches of OD voices on the other channel, I can’t think of a reason why I’d want one.
I use a Digitech Bad Monkey. A very inexpensive OD pedal. Sounds awesome on any channel. The higher gain channels have a bit of a volume boost. On the tweed and blackface settings, it has a noticable breakup making the amp sound so much sweeter.
Bill,
I have a SCXD and I also replaced my speaker with a Eminence Ragin Cajun. Just changing out the speaker is a big improvement. I have not tried changing any tubes. Are there any other tubes you recommend over the stock ones?
Try the JJ 6V6-S output tubes.
Bill,
Is the scxd fixed bias like the blues jr?
The SCXD has adjustable bias and a 1 ohm resistor for measuring idle current. You can download the schematic from Fender.com–the specs are on the schematic.
Is changing the stock 12AX7A worth the mod, in that it’ll significantly improve the sound?
If so, what’s a good model to buy?
Tubes hardly make any difference in the sound. It’s pretty subtle.
Bill, I was wondering if there is a pedal or could a pedal be made that could be switch between the various voicings and FX of the SCXD. My SCXD does a great job creating the sounds our band needs, but we need to be able to access several different voicings on demand during our live performances. If you know of anything we could do please let me know. thanks
That would be a bit of a hack. The knob generates binary codes that are read by the DSP. Each code (1-16) has a voice associated with it. You’d need to preset the codes with dipswitches or additional knobs that generate the binary code. Then you’d need a circuit that switches from preset to preset. Then you’d have to make it accessible from a pedal.
While you’re at it, it would be cool to store away the tone, gain, and volume settings.
Sounds like a Cyber Deluxe. Or a good multi-FX pedal.
I’m interested in running my SCXD through an external 4 ohm cab. I see that the low profile TO20 transformer allows occasional 4 ohm use, but I’m concerned about the “occasional” qualifier. Could the TO20 be special ordered for 4 ohms or could a version of the TO22 be ordered for the SCXD?
I’m running a Ragin Cajun right now but I would consider something with a lower profile if needed to fit a larger transformer. At that point, though, I would also consider swapping speakers or swapping cabs to run 8 ohms.
Your best bet would be to use the TO22 transformer, which has a 4 ohm tap. You’d have to drill an additional mounting hole for it and find a place for either a second output jack or the 4/8 ohm switch that I usually provide with the TO22. I have not done this on a SCXD, so I can’t offer much guidance in where to put the jack or switch. It’s a little crowded back there, and you might have to loosen the circuit board to drill the hole for the larger footprint (1/2 inch) of the TO22.
I replaced the stock speaker with the Celestion Greenback G10 and had a great result. I also have an extra Fender Superchamp Cabinet that had the amp removed. I installed a Ragin Cajun in it. I bought a 1/4 to 1/4 speaker cable at Radio shack and stripped one end and wired it to the Rajun Cajun Speaker, so now I can just swap the speaker cables between the two speakers. I can get a variation in sound without a major hassle. The Ragin Cajun has a little more edge to it, and volume. The Celestion keeps a little of that Fender Mellow vibe and warmth.
Now if someone can tell me how to run both speakers from the same amp or how to make it into a cab extension.
The aux jack kit would let you plug in a second speaker. You’d have to unplug the speaker from the amp in order to use the internal speaker as an extension speaker.
Bill, I’m all new to biasing. What is the Max i can set the bias at on my SCXD? Bias was factory set at 65 (resistor and pin 8 values). Pretty hot from what I’ve been reading. I biased it to 40 (20 per tube/GT’s stock) and I didn’t like that as much. I found a calculator online (http://www.ax84.com/contrib/biascalc.php) and if I am correct the 6v6′s are at 400v plate voltage so @ 70% I could set it to 49? Could I go 80 or 90%? – realising I’d have to replace tubes more often or would this put too much stress on other components that would toast the amp.
Thanks
Many manufacturers consider the 6V6 a 12 watt tube, which is why Fender uses a more conservative 40 millivolts. In a way, it’s kind of a shame that you got used to the 65mV tone–it’s mushy and inaccurate. Maybe it adds something to the distortion tones, but I think it takes away from the clean tones. In any event, you shouldn’t consider the bias a tone control. It’s an operating parameter. If you bias at 80 or 90 percent (that’s the “idle speed,” remember), your tubes will easily go over 100 percent when you play. That seriously shortens the life of the tubes and doesn’t allow them to “hand off” to one another the way push-pull tubes are supposed to. The screen grid is far more likely to fail if you hot-bias, and that’s not pretty. A shorted screen grid usually takes a couple of power supply components with it when it blows.
I bias JJ 6V6s at 50mV. They’re much heavier-duty than typical 6V6s; I can believe that they’re really 14 watt tubes. And they sound great at 50mV.
I’ve both Super & Vibro XD’s. The Super has a Weber 10A150 speaker (a warm Weber with Alnico magnet). Stock bias should be 40ma, came at 36ma, which as is said above, a lower bias, is cleaner with later breakup. Tried higher and will confirm, an earlier breakup and more warmth/mush. I tend to wince at treble bite, so a brite cap C43@470pf on the Super circuit has been disabled, by unsoldering one leg from the pcb.
The Vibro has a 10″ Celestion Greenback, and also the bright cap disabled. C47@470pf on the vibro circuit. Both speakers have warm ribbed cones, with the Celestions ceramic magnet being more precise. The Weber’s alnico magnet is warmer & I prefer its sound on a hollow bodied guitar. Over all though the Greenback is superior, particularily for a solid body guitar.
Reinstalled the bright cap on the Super, but I didn’t like it, & took it out again. 10″ speakers have enough inherent brightness for me. Also the Blackface voice (the real gem in these amps) is plenty clean & bright anyway. Installing higher watt rated speaker in these cabinets produces a richer more substantial voice.
I have been reading some of the comments on here, as well as some of the forums, about speaker upgrades for the SCXD. I have shopped around, read and scratched my head about the right replacement speaker. I went on the web to a place called http://www.mojotone.com and purchased a speaker called the American Vintage AV10C. It is the perfect speaker for the amp. After I purchased the speaker I took the amp back to the store I purchased it from and several of the employees there tried my amp out compared to the stock amp and they all agreed it is a completely different amp with the speaker. One of the guys has a SCXD with a Rajun Cajun and he ordered this speaker to replace it. I know tone is a matter of taste but this speaker makes all of the settings sound much better. Just a option if you are looking for a great replacement speaker.