Sound differences from different dual op amps in SS amps

[Slobrain]

Well, I'll probably get flak for this but I wanted to upgrade the RC4558s in an old Peavey Special 1x12 amp from 1982. Its a pretty ok amp but not all that great. I see on some forums that the guys are swapping out the 4558s with either NE5532 or the famous Burr-Brown OPA2134. I know that there are differences within the RC4558, the NE5532 and the OPA2134 but wanted to ask if anyone here has used these in mods to improve the old Peavey amp sounds?

If you have let me know what your thoughts are.

OK... Yes I have some really good tube amps but I do like the Peavey SS amps...maybe just sentimental... After all sounds are like different flavors of Ice cream... lol...

Cheers

Slo

[Enzo]

It is all anecdotal, and further, a matter of taste. ANy subtle changes might ne liked by one person and disliked by another.

What does upgrade mean? In what way does the amp fall short that you feel different op amps will cure? In other words, what are your goals here? What does improve mean?

Modern 4560 or 4580 op amps will drop in place of 4558s. Whether that will give any audible difference is debatable.

5532 are "low noise" op amps, but that is potentially only. In this guitar amp, they may be no less noisy than the 4558s.

I have never bothered with fancy op amps, so I cannot comment on the BB ones.

I don't know what the BBs are selling for these days, but the others are cheap and can be used most anywhere op amps go in our equipment, so for a few dollars you can have a handful of types and just try them out. Aren't they already in sockets?

[Leo_Gnardo]

Whoaaah, hooold on there. Isn't it the "vintage" op amps that have the sound abound, the secret sauce that golden ears crave? Like those early Ibanez Tube Screamers people pay $450 for? Must be something attractive to part with all that loot for an old stomp box. One potential problem, swapping in "faster" op amps can sometimes result in ultrasonic oscillations, not something you want. In some cases I've found even going from a lowly RC1458 to a slightly better 4558 can bring this on.

In wide-band hi fi gear, where people are looking for enough bandwidth to entertain dogs and bats, yes, newer better op amps can help, but they run into the same oscillation problems. Also - sometimes but not always - upgrading pro mixing consoles. When oscillaton is encountered, you can alter the compensation network. And then you're back to square one, except you're poorer in cash and time.

If the op amps are socketed, you could do as Enzo suggests, get a couple and try 'em out. Tag the old ones so you can put 'em right back if your experiment doesn't work out. If they're soldered-in, it takes skill and luck both to get 'em out without damaging the board or op amp. Whenever I remove one, I put in a socket because I never want to do that again.

I'm not saying you won't hear any differences, maybe you will, and maybe for the better. A more useful and obvious change could be accomplished by swapping in a different speaker you know your ears like.

[J M Fahey]

I see on some forums that the guys are swapping out the 4558s with either NE5532 or the famous Burr-Brown OPA2134. I know that there are differences within the RC4558, the NE5532 and the OPA2134 but wanted to ask if anyone here has used these in mods to improve the old Peavey amp sounds?

All Op Amps work and sound the same, as long as they are used within their limitations.

They are "transparent" and sound comes from whatever "external" networks they have attached.

As long as they have enough gain to follow what NFB "orders" , they'll do exactly that.



Suppose you have an Op Amp, set for 10X inverting gain (the one on the right) , with a 10k input resistor R and a 100k NFB resistor Rf, ... it will have 10X gain (100/10) at all frequencies, from DC to the point where its internal gain reaches 10X or less, in which case it will lack "muscle" to obey orders.

Let's see a couple Op Amps, from Obsolete to cutting edge, to see whether they can sound different (meaning WE can hear differences) ... and remember we will use guitar speakers with sharp cutoff about some 4 kHz .
Usually around 24 dB/Octave.



See that after 3kHz it drops some 25dB by 8kHz, a real "brickwall".

Ok, let's see some Op Amp curves.
These are "open loop gain" curves, meaning the internal gain at different frequencies.

While the internal gain is higher than what NFB asks for, Op Amp will behave as ordered; when internal is same or lower, it will not, so at that frequency and higher different Op Amps may sound different, while below they will all be exactly the same.

So in this example, we'll check at what frequency internal gain meets 10X (20 dB)

1) very old, classic book example but completely obsolete RC741

Both graphs (gain and bandwidth) show basically the same, but we'll use #4 where they kindly indicated horizontal red lines showing different gain values , meeting the dropping-with-frequency open loop gain so we clearly see where Op Amp runs out of steam.

Looking at the 10X gain horizontal line, we see it meets the sloping one at 10^5 Hz frequency, meaning this old despised obsolete Op Amp is ruler flat to 100kHz , if used with 10X gain.

Ruler flat to 10kHz (poor for Hi Fi but fine for Guitar) at 100X (twice as much gain as what a 12AX7 can give).
Not bad, huh?
FWIW MXR Dist+ used a 741 with gain: 1M/4k7 for 200X gain and 5kHz bandwidth.

Just at the edge of performance, and do not even imagine for a second it was by chance .... Mr MXR knew what he was doing.

2) much improved, designed specifically for Audio RC4558:

now 10X gain (20dB) reaches 300kHz .
Even very high 100X gain reaches 30kHz ... well above what ears, soundcards, MP3 , CDs or guitar speakers can reach.

3) Famous OPA2134

now we have a fast one: at 20dB/10X gain it reaches 900kHz ; at 100X still 90kHz ..... impressive.

Will WE hear any difference, specially through guitar speakers?
Very much doubt so ..
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HEYYYYYY!!!!!! , I DO !!!!!!!!



HOW do you think I catch dinner?

[Silvertone Jockey]

I am inclined to think that everything makes a difference, but in the end it really comes from your fingers......finger tip control, vibrato and pick attack, learning the pinch harmonic positions, muting etc. You can try equipment forever but if you aren't creating the tone first, you're never going to find it in any amp. Once you discover the secret that's not a secret you sound good on any amp

[Enzo]

And not to put too fine a point on it, we are talking about this peavey amp, not a tube screamer.

[dai h.]

I don't think it's crazy to hear differences between op amps when swapping them willy nilly, but (from what I gather and from my experience doing this) perceived differences could be due to non-optimization (bypassing or whatever) or instability. Op amps seem to be (correct me if I'm wrong here) more of a passenger vehicle, or a "school bus" (intended for "safe, boring, predictable linearity") compared to a "stunt driver on a motocross bike" (some topology with less feedback with distortion as a pleasing artifact or goal as in fuzz, guitar amp, etc.).

I remember a thread on another forum (which I'm too lazy to look up right now) where there was someone who was adamantly (or even arrogantly) opining that such and such older op amps (the original types in a mixer) were out of date and inferior, but later (when they had taken time and effort to optimize the mixer for different op amps) could not tell the difference between the older ones and supposedly newer superior types.

I suppose you could just swap op amps and go with what sounds best subjectively and ignore the technical stuff (with the risk of something giving out from oscillation or whatever).

re: Juan's point about "us(ing) them within their limitations":

this seems to be a good one. (Again, from what I gather) if used within their limitations (not trying to obtain too much gain, not being forced to drive too low a load or what have you, etc., correctly bypassed, correct circuit layout, grounding, etc.) different ones should be more or less "neutral" (ideally). So I suppose you have to figure out whether you are indeed hearing "a better op amp", or the sound of an incorrectly utilized, non-optimized application of an op amp (which might still sound better subjectively).

[Steve L]

Peavey Modifications

My personal opinion (based on 35 years of experience) is that you will not hear much of a difference in tone. The external op-amp circuitry is really what matters most.
You may hear a little less noise if you only play the amp in a recording situation or your bedroom, but if you play on stage forget hearing any difference in noise.

However, that being said, check out these attachments from Peavey themselves:

Peavey Steel Amp Modifications - Nashville 400.pdf
Peavey Steel Amp Modifications - Session 500.pdf
Peavey Steel Amp Modifications - Session 400 LTD.pdf
Peavey Steel Amp Modifications - Vegas 400.pdf

None of these are your particular amp, however Peavey does include changing out 4558 op-amps with OPA2604 op-amps in these amps for enhanced tone.
Does it matter? I am not sure. As a guitar player I don't personally care for the tone of any of their amps other than their pedal steel ones noted here, however I do suspect Peavey knows something about tone.

If you notice the lists of suggested modifications from Peavey, most of them involve opening up the bandwidth of the preamp circuitry.
High bandwidth is important for pedal steel amps and changing the external circuitry accomplishes this.
I should note it is possible that Peavey's note to replace the 4558 op-amps is because the increased bandwidth might increase the noise. I suppose you could ask them.

As suggested, put in IC sockets and try it for yourself if you want to see the difference other op-amps make.

Steve

[J M Fahey]

LOVED the catastrophe headline sized warning about not messing with the amp for unqualified people

[tedmich]

these are supposed to sound great

More Info Page

now if I can just fit them in...

[Slobrain]

I am inclined to think that everything makes a difference, but in the end it really comes from your fingers......finger tip control, vibrato and pick attack, learning the pinch harmonic positions, muting etc. You can try equipment forever but if you aren't creating the tone first, you're never going to find it in any amp. Once you discover the secret that's not a secret you sound good on any amp

I agree about having the touch to get the sound.

The reason for my question about the dual op-amps are to strictly make an old SS amp sound better if possible without going into too much modding. This PV Special was one of the first to come out in 1981, then in a few years the Peavey engineers did upgrades to make certain amps sound better. Not sure why I like old Peavey amps. I suppose the same reason some guys like old Silvertone amps or any other old amp odd brand... Really You have to hand it to Hartley Peavey that he built some really good roadworthy amps and spawned many musicians to go on to careers from those low cost amps. Without Peavey lots of guys couldn't afford a Marshall or an old Fender amp.

I'm sure someone with good electronics engineering background could probably look at one of those 30 year old schematics and improve on something tone wise. I'm not an electronics engineer but just in repair. I'm just looking for en easy method to make a old SS amp a small bit more sweeter sounding.

After reading on another forum about the steel players modding the old Peavey amps I figured why not give it a try.

[g1]

Just going by anecdotes, the OPA's are a popular mod for pedal steel players with big clean peavey's, like Nashville 400s.
Those guy's IMO have pretty good ears. But then again, people may be throwing those mods at them for free .

[Enzo]

And we still don't know what you mean by improve or better.

[Silvertone Jockey]

After reading on another forum about the steel players modding the old Peavey amps I figured why not give it a try.

I didn't mean to come across like modding a Peavey amp or any amp isn't worth while. I have a Classic VTX that is dead in the water currently because I was doing exactly what this thread is about. I was installing different OP amps and I plugged one in with all the legs on one side out of the socket and it hasn't worked since then. I checked all the obvious stuff, but had to put it on the back burner to work on other projects with higher priority. It's going to take an oscilloscope to repair it now when I get the time

[Silvertone Jockey]

these are supposed to sound great

More Info Page

now if I can just fit them in...

A snip from the info page from the link quoted above

Sonic Quality

Listening to an IC op-amp is like hearing a concert through a keyhole, or admiring the Mona Lisa on a stamp! The resolution and texture of the Supreme Sound Op-amp completely surpasses IC op-amps of any kind. Compared with any IC op-amps, the SSO provides more texture and nuance with more refined detail. The bass frequencies are well controlled and punchy. Double bass can clearly take shape on a very three-dimensional soundstage. Classical music reproduction becomes much more realistic and spacious, string sections become a group of individual musicians rather than just a homogenized clump. The Burson SS Op-amp is a extremely listenable and engaging performer bringing pure music to your soul without listening fatigue.