KB300: 4558 replacement

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dak
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KB300: 4558 replacement

Post by dak » Sun Jan 02, 2022 7:02 pm

Hi, have a KB300 here that had a defect reverb unit: after powering up it was fine for a minute (approximately), then when there was any reverb level above zero on the general reverb pot, there was a tremendous racket (essentially like amplified contact noise). The preamp does not have a lot of active circuitry other than 4558 opamps. The racket still was there when pulling the reverb spring off completely, so it essentially was the second opamp of the 4558 employed for the reverb. I swapped it at first with one of those preamp 4558s which did not use their second opamp.

Fixed the problem, but a disconcerting state to leave the amp in (who knows what kind of a bad influence the faulty opamp might be on its twin eventually). So I ordered a few 4558 chips as replacement and wanted to swap out the older ones: I mean the old ones were 35 years old or so (I think the amp is from 1987 but a 1983 model, no XLR input) so I'd expect current versions of it to fare better, and they are cheap.
Old 4558 to the left, "new" one to the right.
Old 4558 to the left, "new" one to the right.
DSC06577.JPG (365.54 KiB) Viewed 288 times
Problem was that the "modern" ones turned out distinctly noisier and I prefer my amps to be quiet when I don't do anything so I don't have to keep switching them on and off between activities (like an acoustic set in between or discussions or some pause or such). So I moved things around in order to just replace one original 4558, outside of the input stages so that its noise level would not likely matter.

Still, I wouldn't mind less noise (you might ask why I bothered fixing the reverb at all since its use does not quite fit in with the goal of maximum quietness; I don't have a really good answer for that but don't like half-broken things even when I am just using the other half). And given the experience with the modern replacements, the noise level appears to be not a mere consequence of the overall design but relevantly dependent on opamp performance/quality.

Now there's been positively a deluge of opamps in that form factor released since that time and swapping them out is straightforward as they are socketed. But the attractive ones appear to contain some significant differences:
  1. JFET input stages: very high input impedance which is generally nice but not necessarily the lowest-noise fit. TL072 or successors.
  2. diode-clamped inputs which make me a bit worried they will behave strangely when overdriven. NE5532
  3. PNP input stages. MC33078, mentioned as "for improved performance" in the LM833 specifications, where the latter in turn is missing both a circuit diagram (so no idea about the input circuitry) in the ON Semi data sheet as well as some core specs.
And something like LM4562 looks also nice spec-wise (and is advertised as intended for high fidelity audio applications with low noise). With my 4558 noise experience, I'd probably give a bit of preference to TI as a supplier over ON semi or similar though there tends to be a price difference.

I may be overthinking this, but am partially worried about using stuff out of specs. On the other hand, the circuit diagrams of the KB300 show discrete clamping diodes on the opamp inputs to the ±16V rails which would be quite insufficient for keeping the original 4558 out of its absolute maximum ratings. So maybe I should worry less and experiment more. But it's possible that someone else already made some more systematic experiments, and since I have to mail-order for any experiment, minimising the number of experiments reduces cost and waste (some parts are cheap enough that one can order just tentatively, at least for small numbers).

It's also likely that it's nonsensical to replace anything past the instrument input stage chips (and reverb input stage?) with regard to the noise performance, but low distortion should theoretically be useful also in the higher-level stages should one make use of the line-out for recording or something. Though very much pointless otherwise when the speakers are high-efficiency instrument ones (here Black Widow, and piezo tweeters). I certainly won't get a wonderfully uncolored and distortion-free auditory response, but I do have a probably exaggerated appreciation of low-noise quiescence. Not a fan of "comfort noise" for addressing the "is this thing on?" question.

Thanks for any insights!

PostScriptum: I find that the JFET input variants, at least TL072, tend to have distinctly higher noise voltage numbers (15nV/sqrt(Hz) rather than about 5nv/sqrt(Hz)) but negligible noise current. For the 220kOhm termination of the open input, that would deliver by far the quietest results and would make sense for directly connecting a high impedance source like a pedal-less guitar. But this is essentially a keyboard amp so it probably was a mistake to evaluate noise levels without anything connected: the actual use case would be more like a 2k source to which the input circuitry adds 4k7. The LM4562 breaks even with noise voltage/current at about 2k so I'd fare better with something like the MC33078 (or LM833) which have their break-even noise point at about 9k, meaning that the voltage noise figure will actually be relevant in the input circuitry of the KB300.
input.png
input.png (45.22 KiB) Viewed 287 times
That's for connecting something with reasonably low impedance. And to be honest, quieting a guitar input is sort of a fool's errand, and 220k is sort of a lowish input impedance for guitar anyway. So I'll probably not shoot for one of the JFET opamps since my main objective is for quieter operation for my use cases.

And another PostScriptum: it would appear that at a temperature of 300K, a 5k resistance alone is already good for 9nV/sqrt(Hz) of noise. Which kind of explains the low input resistance (before negative feedback) of the "good" opamps and means I should not expect too much of an improvement just from swapping opamps around. If I interpret stuff correctly.

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dak
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Re: KB300: 4558 replacement

Post by dak » Fri Jan 07, 2022 4:21 pm

I ended up getting a number of MC33078 which were reasonably affordable. To my surprise, they are already somewhat of an improvement regarding the noise level for open inputs (and should be definitely more quiet for low impedance sources). The largest obvious improvement was with the reverb unit: the opamps there have considerable amplification factors and (after the reverb unit) weak signal levels at reasonably low impedance. The reverb now is not a lot above a "no show" with regard to noise. There is not much of a point in meddling with the second 4558 in the input stages (tone control) or the final amplification stage: the ones worth swapping regarding noise reduction are really the three input stage opamps (U1, 2U3 and 3U3) and the reverb driver and receiver opamp (U5).

Other than with the reverb, the improvements (namely in the input stages) were modest but definite. But swapping the original 4558 from TI for a new replacement (possibly ON semi but I am not sure about the supplier) was definitely a change for the worse, so if you have to replace one anyway, it makes sense to not get another "4558". Or at least change things around until the better ones (likely the old ones) end up in input stage and reverb.

It's also noticeable that the LM4562 (which I did not end up getting) is pretty expensive unless you buy a bunch of them at Chinese sources on Ebay that have several negative reviews of the "sands down ICs and relabels them" kind. To be fair, there are also sources there which are less of a bargain but have no such reviews. Expensive parts that are functionally and pin compatible with quite cheaper ones (with worse characteristics) are somewhat obvious targets for such shenanigans I guess.

I doubt that I'd see much of a further improvement with one of those, however: their specs look like they work best with rather low input impedances that they don't get in the KB300 circuitry. And if I had a guitar amp (and no pedal or active pickup in front that would have given lower impedance), I'd likely have rooted for one of the opamps with high-impedance JFET inputs.

Another point I see is that "recommended operating conditions" for a 4558 are a maximum of ±15V supply voltage for normal operation conditions while the KB300 gives them ±15.6 (labeled as 16V in the diagrams). The MC33078 are specified for ±18V maximum supply voltage so they are not taxed to their limits to a similar degree and thus are more likely to actually behave according to their specs.

Thanks for bearing with me.

TLDR: four of the eight 4558 opamps in the KB300 may be worth upgrading in order to reduce noise, new 4558 ICs may be worse than the old ones, and using MC33078 for those four gives a noticeable reduction in noise compared to the original ones, particularly so for the reverb.

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Enzo
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Re: KB300: 4558 replacement

Post by Enzo » Fri Jan 07, 2022 6:37 pm

"Temperature of 300K"?????

To be kind, I think you may be overthinking this a bit. Just my opinions , of course, but they came out with 4558s a while back. Modern day 4558s are not improved really. "Better" 4558s are things like the 4560 and 4580. Slap a 4580 in there if you want newer. But 4558s have served well for years. I never found them especially noisy. SOme individual chips more than others maybe, but that is any part.

Also I didn't notice you gave a source for the parts. If you bought from Mouser or Digikey, fine. But if you bought from ebay, the parts you bought may well have been factory seconds.

I guess it also depends on what your idea of too much noise is versus mine for a stage amp. But you can sub other lower noise dual op amps with ease. TL072. You can look at JFETs input being not necessarily the lowest noise. Sure, but they will sit right in place and work fine. Just because a certain technology has POTENTIAL for more noise doesn't mean it will in fact be noisier. Poke one in there and find out. These are not picky circuits.

5532 worried about how they perform in overdrive? Well consider you'd have to overdrive the input differentially, not just drive to clip. Those diodes are there to protect the chip. You can generally drop one in place of a 4558. In fact in many mixers of the era they did just that. A 5532 at the input and 4558s the rest of the way.

Power supply for 4558 from the TI data sheet is up to 18v. +/-15 was the signal limit. The 4558 in this amp are not stressed by the existing power supply Note also that Peavey in their solid state amps is fairly consistent in putting the clamping diodes across the input as seen in your little schematic swatch. That limits extreme signal excursion past the power rails.

And while spinning chips I am kinda surprised you didn't include the OP series of op amps that are very popular.

But the amp will accept pretty much any dual op amp you care to try, so experiment away.

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Re: KB300: 4558 replacement

Post by dak » Fri Jan 07, 2022 9:13 pm

Enzo wrote:
Fri Jan 07, 2022 6:37 pm
"Temperature of 300K"?????

To be kind, I think you may be overthinking this a bit.
Thinking is cheap and fun. If I were paid for my time, of course "get one of everything and try" would be saner.
Also I didn't notice you gave a source for the parts. If you bought from Mouser or Digikey, fine. But if you bought from ebay, the parts you bought may well have been factory seconds.
Kessler electronic, a German mail-order electronics shop. They don't actually specify a certain manufacturer; the data sheets were ON Semi.
I guess it also depends on what your idea of too much noise is versus mine for a stage amp.
Now you are being unfair. I explicitly stated that my noise target was definitely not relevant for a stage amp and explained why.

Another amp of mine I tampered with is used as a bass accordion amp in an accordion ensemble. It's a Leslie clone (motor is off: now that would be noisy indeed) and looks more like odd furniture than an amp and also is acoustically inconspicuous: it's just that the bass accordion sounds louder than it is. Even the player himself has no clue whether the amp is on or off other than the volume. In that "acoustic reinforcement" setting, having the amp silent by default is really a boon.
But you can sub other lower noise dual op amps with ease.
I did.
TL072. You can look at JFETs input being not necessarily the lowest noise. Sure, but they will sit right in place and work fine. Just because a certain technology has POTENTIAL for more noise doesn't mean it will in fact be noisier. Poke one in there and find out. These are not picky circuits.
As I said: I reckoned from their spec sheets that they would likely be quieter on a high-impedance source (or an open input, but why turn that up in the first place?). But neither the reverb coil nor the keyboard arranger I use with the KB300 are high-impedance. And the TL072 are only specced to ±15V, and when I am overthinking things, I prefer heeding the specs.
5532 worried about how they perform in overdrive? Well consider you'd have to overdrive the input differentially, not just drive to clip.
Drive to clip and then 0.6V more on the input. Peavey somewhat optimistically declares that the KB300 inputs take "unlimited" voltage. If you want to take this somewhat seriously, the limiting diodes in the opamp inputs should not get more current than they can withstand. They probably wouldn't but then going through all the cases is a bit of a drag.
Power supply for 4558 from the TI data sheet is up to 18v.
Nope. See here. 18V is the "absolute maximum rating" where sustained operation or behavior according to specs is not guaranteed. The normal operation maximum is ±15V.
That limits extreme signal excursion past the power rails.
Past 0.6V beyond the power rails. Better than nothing and worse than the "absolute maximum ratings".
And while spinning chips I am kinda surprised you didn't include the OP series of op amps that are very popular.
Pricy, partly only available as SMD, and a bit silly regarding the specs. I really was only interested in the noise figures, with the rest being mostly "meh". I do admit, while at it, to have ordered two OP27 in the same order in order to replace two 14=pin 741CP (if I counted the pins right, the 8-pin OP27 can be used when placed correctly) in the reverb unit of the Leslie speaker clone. Which is currently disconnected because it is too noisy for my taste. But if the KB300 reverb can be quiet enough... But the Leslie clone is not a Peavey (a "Solton Turbojet"), and I won't get at it before next weekend. I can report on the results then but I don't even know what dual-opamp version would correspond to the OP27.

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Re: KB300: 4558 replacement

Post by Enzo » Sat Jan 08, 2022 2:26 am

Since I don't know you, couldn't guess as to suppliers. Just too many folks buy fakes from ebay or seconds from ebay. Real suppliers sell legit parts. Fakes can and do have many brands on them.

Wasn't trying to be unfair. The amp is designed as a stage amp, even if you don't use it that way. SO I was just wondering how little noise you might be expecting from such an amp. I am sure one can reduce noise, just wondering if there might be a floor to improvements

TL072 and 4558 have recommended supplies of 15v, but they are rated to 18v or 36v if you prefer. Plenty of other makers who run 16v rails and even 17v rails in mixers. I assume for headroom.

I think running the input of a 5532 a volt or two past rail would be anathema to looking for low noise. And quite frankly, I doubt any of the other candidates would be very happy having their inputs abused like that. Over my career I never had 5532 inputs failing any more often than any other chips.

And a note, the 5532 is still a good chip. And a small number of them subbed into an ap is OK, but they do require about twice the current of a 4558, so when we look at mixers with a large number of op amps, switching them all to 5532 might overtax the power supply.

OP27 appears to have the common pinout, like TL071 or 5534. Oh, and I see it has 22v supply ratings. 44v, wow.

But actually, I made a typo earlier, I meant to type OPA series, the Burr-Brown stuff. They come in DIP still and also are cheap.

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Re: KB300: 4558 replacement

Post by dak » Sat Jan 08, 2022 4:58 am

Well, let's just state that we disagree about what "absolute maximum ratings" in the specs mean. The TI spec sheet states
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress
ratings only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under
Recommended Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device
reliability.
and you may chalk it up to my lack of experience that I would much prefer taking them by their word. It's worth noting that my looking for replacements was predicated on one of the 4558 having permanent damage where one of its opamps became tremendously noisy after about a minute of warmup.

I did look at a few OPA chips, but given that I paid €100 for the amp in the first place and that it has 8 of those ICs (and the ones I looked at were not cheap here: sometimes it's hard to tell why some article may be priced quite differently on the other side of the Atlantic) and I am not sure this will be a keeper (paradoxically partly because it is hard to move), I went with "should be good enough".

The specs for KB4/KB5 mention about -100dB noise floor below capacity while the older KB300 states -90dB so obviously even Peavey was of the opinion that there was a market for a lower noise floor in a stage amp. The KB300 actually manages hum pretty well. I have a 5W or so guitar practice amp (late 90s I guess) that is considerably noisier.

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Re: KB300: 4558 replacement

Post by Enzo » Sat Jan 08, 2022 3:51 pm

I don't know what the OPA stuff sells for where you are, and there are numerous different OPA chips. The ones I saw were selling for $1.35 and up. For noise, I would be thinking just the input ones. That is just me.

I think you will find the KB4 stuff is using 4560, to which I would attribute the better noise figure.

I don't want to get hung up on voltage. All I mean is I don't think chip failure is due to 15.6v rails instead of 15v rails, when the chip is rated for up to 18v. We were not planning to exceed 18v.

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Re: KB300: 4558 replacement

Post by dak » Tue Jan 18, 2022 2:32 pm

dak wrote:
Fri Jan 07, 2022 9:13 pm
Enzo wrote:
Fri Jan 07, 2022 6:37 pm
And while spinning chips I am kinda surprised you didn't include the OP series of op amps that are very popular.
Pricy, partly only available as SMD, and a bit silly regarding the specs. I really was only interested in the noise figures, with the rest being mostly "meh". I do admit, while at it, to have ordered two OP27 in the same order in order to replace two 14=pin 741CP (if I counted the pins right, the 8-pin OP27 can be used when placed correctly) in the reverb unit of the Leslie speaker clone. Which is currently disconnected because it is too noisy for my taste. But if the KB300 reverb can be quiet enough... But the Leslie clone is not a Peavey (a "Solton Turbojet"), and I won't get at it before next weekend. I can report on the results then but I don't even know what dual-opamp version would correspond to the OP27.
With regard to the Solton Turbojet, replacing the original "741CP" (that's in the schematics but the actual devices were not ceramic package but plastic) did work after finding and fixing a hair fault in the circuit board and for good measure, modifying the reverb tank in order to avoid a ground loop.

And now that reverb does not come with the slew of add-on noise it had previously. Though I forgot how staggered the echo of that old long (12.75") 2-spring tank is. The shorter reverb tank in the Peavey KB300 definitely has a smoother sound.

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