Op amp replacement in Peavey Bandit 65

[Christopher Kux]

I have a Peavey Solo Series Bandit 65 and have heard tell of replacing the RC4558P op amps with newer, higher tech chips that replaced the RC4558P. It’s a good amp but seems unbalanced when it’s loud, even with a Texas Heat in there, and gets icy especially with the reverb up.

Some forum posters have recommended using a Burr Brown OPA2134PA to quell some noise inherent to the 4558P (I understand changing the electrolytics will also help as they’re quite old at this point). Is the FET input stage of the OPA2134PA going to be a problem? Are there other direct replacements for the -4558P that will yield different results? Interested in your experiences. And would you change the reverb driver 4558 or leave it be?

[Dave Mudgett]

The only way you'll know if you like something besides a 4558 in there is to try it. I personally like the 4558 in these 80s Peaveys like the Bandit/Bandit-65, Studio Pro 40/50, and so on. But I tend to use them more for guitar than pedal steel. I guess if I was using a lot of gain, I might go for something more low-noise.

The good news is that changing these chips is a breeze because they're socketed. Just unbolt and pull out the chassis, you'll see them sitting there. There are lots of subs for 4558 op-amps.

I put a TL082 in the reverb driver of my Studio Pro 50. But only because the 4558 was shot when I got it and that's what I happened to have around at the time. Works fine.

One thing that can be an issue with extended frequency active elements like op-amps is that in some circuit layouts, you can wind up with parasitic oscillations - that is, oscillations above the intended frequency range of the circuit, usually higher than the range of human hearing. This can cause all kinds of crazy problems.

Some recommendations from diy stop box people, who use a lot of these things - https://www.diystompboxes.com/smfforum/ ... c=114168.0 - I kinda' like the TL072 in guitar pedal/amp circuits. But the older 4558, especially the JRC4558D used in early Tube Screamers, are highly coveted. Honestly, though, I think there's a certain amount of 'cork-sniffer cachet' involved with a lot of this stuff. I understand wanting to lower the noise floor in some situations, but I think at audio frequencies, worrying about things like slew rate and frequency response into the 100 MHz plus range is a bit much. So I kinda' agree with this guy - https://www.youtube.com/watch?v=bpTv2jAree8

[Dave Grafe]

What Dave M. said, all of it.

The levels of noise involved are negligible, especially in an instrument amp, with little to no difference in actual tone. On the other hand the current requirements can vary widely, which can cause power distribution problems if not taken into consideration, and stuffing a high performance chip in there can cause ultrasonic oscillations capable of taking the amp down entirely.

Bear in mind that many of the most revered analog audio mixers of the past 50 years used 4558's throughout. My Midas Pro02 and Pro05 had some but not all of them replaced over the years but you couldn't tell which were "upgraded" or not by listening, even in a quiet room.

That being said I have lots of LF352 and "Biamp N" - untested 5532 chips that Biamp purchased in batches in the 1980's, testing them in-house to keep costs down, also a few LF351, LF357, TL072 and other audio opamps here at the day camp that I would be happy to move on.

[Bill Terry]

Bear in mind that many of the most revered analog audio mixers of the past 50 years used 4558's throughout. My Midas Pro02 and Pro05 had some but not all of them replaced over the years but you couldn't tell which were "upgraded" or not by listening, even in a quiet room.

Back in the early days of my recording 'career' I owned a Tascam Model 5 Mixer with the 12 channel expander. This made for a pretty nice 20x4 bus console actually, and it was FULL of 4558 opamps. I read somwehere that replacing those 4558s with TL072s was the hot thing to do, providing more 'air' etc. So I bought a boatload of those TL072s (I think there were 3 or 4 per channel, not counting the busses). They were all soldered down, not socketed, so I bought sockets as well and that little home project ended up costing me a LOT of hours of work. End result? I couldn't hear a bit of difference.. YMMV.

I also own a first year Bandit (pre Bandit 65) and those are really fine little guitar amps, especially in the bang for the buck category. I'm not touching it..

[Christopher Kux]

I agree, a newer faster op amp isn’t necessarily desirable if the circuit was designed around a specific original chip that was available at the time of manufacture. I’ve encountered this with Tube Screamer modifications in the past — sometimes it’s a win, sometimes it’s not. Diodes aside, there are several replacements for the 4558 in a TS, some of which were even supplied by Ibanez in reissues, that leave you scratching your head as to why Ibanez thought those new chips were acceptable in the reissues.

I should probably be looking at the electrolytics to fix noise first, before changing op amps, but (honesty) had a project lined up for the 4558s if they were to come out of the Bandito.

By the way, the Eminence Texas Heat turned a perfectly good Bandit into a really great perfectly good Bandit… better for warming up a six string than my steels, but very nice nonetheless. It came with someone’s ancient Celestion G12M-70 in there… since I got no guitars shaped like hockey sticks, that guy got nailed on the wall where it belongs

I’ll report back!

[Craig A Davidson]

I have a couple Bandits and I'm thinking of trying this. How many chips get replaced?

[Bill Terry]

How many chips get replaced?

My Solo series has four 4558s. I found a list of schematics with board layouts here, find your model and it should be easy to count:
https://www.audioservicemanuals.com/p/p ... ey-bandit/

Just checked, Bandit 65 also has four.

[Jack Stoner]

The OPA2134PA is what Ken Fox sells for Peavey steel amps. I like them as it cleans up the "Peavey Honk" in those amps.
If I were to try other op amps I would go with those. The 2134 was designed for audio, not a general purpose op amp like most others.

[Pat Chong]

Hello,

I've heard the term peavey "honk", a few times. What sound or characteristic is this referring too?

Thanks.....Pat.

[Christopher Kux]

Hello,

I've heard the term peavey "honk", a few times. What sound or characteristic is this referring too?

Thanks.....Pat.

The honk is a pronounced mid range response… and either mitigated, complemented, or exaggerated by different speakers, effects, pickups (channeling Hawkeye Pierce in that sentence)… Depending on all those factors, a distinctive midrange can be a blessing or an obstacle. One benefit it that the Bandit can give you a clear, warm, thuggy tone that can be heard above the band.

I’ve got a handful of Burr Brown chips on the way and a few TL072s in my parts stash. Should be a fun weekend. I’ll report back.

I’m curious about mixing and matching these at different stages. Sometimes it’s as much fun to tinker as it is to play music.

[Pat Chong]

Hi Christopher,

Thanks for the answer.

I just purchased a Nashville 400, and will replace the 4558's, hopefully to de-honk my amp. Just having gotten it, I have not used it at 'band' volume yet, just at practice volume. However, sometimes the high notes are shrill sounding.

Peavey sent me TL072's to replace 5 of the 6 op amps. With jfet inputs, I suppose the input impedance is higher than the 4558 and might, just in itself, alter the sound quality. Anyway, hope it goes well on your project.

...........Pat

[Ken Fox]

Yes the OPA2134PA works well in the Bandit amp. But beware of cheap counterfeit chips on eBay

I buy directly from Texas Instruments. I buy 250 at a time so I can offer them at a great price including shipping and a board layout with directions

[Christopher Kux]

I installed the four OPA2134PA chips today in the Bandit 65, testing continuity and function as I went. Dripped contact cleaner in the sockets. Thn I played through the Bandit’s Texas Heat 12/8 speaker and did a 15 minute test drive on my Stratocaster. Haven’t plugged in a steel yet, no time, unfortunately.

My report:
1) Much improved clarity and separation at all volumes.
2) Middle range control is more discernible now, and when used in conjunction with the treble and bass controls, I hear a lot more interaction where those ranges overlap. So basically, there are more useable tones on the amplifier than with the stock TI RC4558P chips.
3) Reverb is markedly different… clearer, more usable and toneful around the 12 o’clock mark but splashy when cranked.
4) The screechy high freqs and annoying, persistent resonant freq in the middle range is gone! And the new op amps do not sound noisy at any volume on an isolated power supply.
5) Tested the electrolytics and several resistors. Resistors seem drifted or too far on the left side of the tolerance curve, and so those parts will come out next.
6) The Texas Heat sounds a lot more like the model I demoed in a shop last year. There’s more “top end bite” as Eminence called it, while the bass of still quite full it is no longer what I would call tubby or unmanageable.

Overall I really like the progress this has made for the amplifier.

Thanks everyone who is replying… I know this is a tired old topic, but your input and experiences helped me make a few decisions.