I have an old Peavey XR1200 that i use for summing out of protools and have recently wired it to take advantage of the pre amps (anything is better than the 002's). The chips used in the pre's are the TI RC4558P's. The pre's actually have a decent sound to them although a bit dark for my taste. Is there a better chip that could be popped in there? Also how hard would it be (or worth the effort) to try to integrate some discrete circuitry in there?
Almost anything would be better than 4558s! 
For starters, in order of cost: LM833, OPA2227, LT1124 would be my first choices for most audio use. All are low-noise bipolar chips with substantially better bandwidth and slew than the 4558. They're drop-in replacements in most applications. The 833 is dirt cheap and has good GBW but has poor low-Z drive capability. The 2227 and 1124 are better at low-Z drive and have much lower offset voltage, but draw more power supply current, and aren't as fast as the 833. The 1124 has the best noise specs and better speed than the 2227, but it ain't cheap.
For summing amps try the OPA2228 or LT1126. They're decompensated versions of the 2227 and 1124 respectively.
You might want to try a video op amp like the LM6172 or LT1358 for the tone controls. The 6172 is cheaper and faster but has higher input noise. The noise is unlikely to be an issue at line levels.
You'll probably get lots of suggestions about using FET input op amps. I prefer to save those for high Z input stages. That said, it's hard to go wrong with the OPA2134.
If you see a uA747 in there, don't replace it!! Peavey's DDT circuit depends on behavior peculiar to a 747.
For starters, in order of cost: LM833, OPA2227, LT1124 would be my first choices for most audio use. All are low-noise bipolar chips with substantially better bandwidth and slew than the 4558. They're drop-in replacements in most applications. The 833 is dirt cheap and has good GBW but has poor low-Z drive capability. The 2227 and 1124 are better at low-Z drive and have much lower offset voltage, but draw more power supply current, and aren't as fast as the 833. The 1124 has the best noise specs and better speed than the 2227, but it ain't cheap.
For summing amps try the OPA2228 or LT1126. They're decompensated versions of the 2227 and 1124 respectively.
You might want to try a video op amp like the LM6172 or LT1358 for the tone controls. The 6172 is cheaper and faster but has higher input noise. The noise is unlikely to be an issue at line levels.
You'll probably get lots of suggestions about using FET input op amps. I prefer to save those for high Z input stages. That said, it's hard to go wrong with the OPA2134.
If you see a uA747 in there, don't replace it!! Peavey's DDT circuit depends on behavior peculiar to a 747.
one caveat.... make sure the power supply can handle the new chips.... for instance, 5532 op amps replacing 4558, or worse TL072 op amps in quantity can nuke a power supply designed to handle the 4558 or tl072 load. make sure you check the operating current of the new op amp against that of the old one, and allow for that by beefing up your power supply if neccesary.
Don't even think about video opamps in a Peavey mixer, the layout and power supply bypassing aren't good enough.
The biggest improvement will come when you replace all the electrolytic caps in the signal path and bypass them with a small film type. There are about ten per channel in the MKIII mixers, only a few less in the XR series.
I removed about 200 electrolytics in the signal path of a 16 ch MKIII and replaced with only 3 (per channel) good film types. The 4558s were replaced with TL072s (this was in 1980). The MKIII had a 5534 for the mic input, that stays.
The biggest improvement will come when you replace all the electrolytic caps in the signal path and bypass them with a small film type. There are about ten per channel in the MKIII mixers, only a few less in the XR series.
I removed about 200 electrolytics in the signal path of a 16 ch MKIII and replaced with only 3 (per channel) good film types. The 4558s were replaced with TL072s (this was in 1980). The MKIII had a 5534 for the mic input, that stays.
I don't know much about opamps but I just replaced an LT1012 chip in a DC nulling circuit in my dc coupled gfa-565 and the specs on the OP97 that I used supercede the LT1012 and other OP07 chips. Although the opamp used in my application is purely to null out DC offset, the OP97 was suggested by Walt Jung (a diyaudio.com member) in that it uses significantly less power than an OP07 op-amp.
In addition, I have seen some Burr-Brown units that are quite pricey and look well-built: Burr-Brown opamps
Has anyone had any experience with Burr Brown op amps?
In addition, I have seen some Burr-Brown units that are quite pricey and look well-built: Burr-Brown opamps
Has anyone had any experience with Burr Brown op amps?
The 2132/4's mentioned above are Burr Browns, but since they were developed, BB was bought by Texas Instruments. IIRC, the 213X family was the first; since then there are several BB's/TI that are descendants of the original design. They were designed specifically for high quality audio, and listening was part of the design process. (What a radical notion!)
wow, wasn't expecting so much interest for such and old junker. i actually obtained this for free from an old church system and starting using it since the relatively long throw faders. This seems to be a luxury on mixers nowdays. djk, i actually did consider just recapping the whole bugger since it would do me better since i use the pre's relatively little.
As far as the power supply was concerned, it is going to be rebuilt so i can adjust it for any of the voltage/current requirements of the chips i finally decide on.
The summing opamps were the next to go, so thanks for jumping the gun on that one. As far as the sound of these new chips... i usually have 2 standards for rating the sound of equipment...API for the fast in your face sound and Neve for the smoother laid back sound. Where would you rate those chips you recommended chucko? I will get to the complete tear down of this board this weekend and will include pics of the progress.
Upgrades I will be including are:
new chips for pre's
recap
summing chips
addition of VU style analog meters
balanced outputs (transformer)
Thanks again for everyone's help.
As far as the power supply was concerned, it is going to be rebuilt so i can adjust it for any of the voltage/current requirements of the chips i finally decide on.
The summing opamps were the next to go, so thanks for jumping the gun on that one. As far as the sound of these new chips... i usually have 2 standards for rating the sound of equipment...API for the fast in your face sound and Neve for the smoother laid back sound. Where would you rate those chips you recommended chucko? I will get to the complete tear down of this board this weekend and will include pics of the progress.
Upgrades I will be including are:
new chips for pre's
recap
summing chips
addition of VU style analog meters
balanced outputs (transformer)
Thanks again for everyone's help.
drop in replacement
Hi all,
what timing as I had a question on the subject of replacing one op amp for another. I'm very new at this but understanding more with each project and using the search feature of this site.
Chucko, you said that there was a handfull of op amps that were a 'drop in replacement' for the RC4558. I understand that you would have to make sure the replacement's operating range fell within the supplied voltage/current of the circuit but aren't there other spec that must be looked at and compensated for in the circuit or is that implied here? Or is the 4558 so bad that replacing it with a wad of bubble gum would be an improvement? << that's funny right there, I don't care who ya are ....
I'm curious because I just finished one of the flavors of a CMoy headamp using an OPA2134. To test the difference in sound I want to try some other ops like the opa2227 but I would have thought the original circuit would have to be tweeked to accommodate the replacement. (such as feedback loop, etc.)
Yes, I'm reading data sheets but not always understanding what's relative to the project.
Hi all,
what timing as I had a question on the subject of replacing one op amp for another. I'm very new at this but understanding more with each project and using the search feature of this site.
Chucko, you said that there was a handfull of op amps that were a 'drop in replacement' for the RC4558. I understand that you would have to make sure the replacement's operating range fell within the supplied voltage/current of the circuit but aren't there other spec that must be looked at and compensated for in the circuit or is that implied here? Or is the 4558 so bad that replacing it with a wad of bubble gum would be an improvement? << that's funny right there, I don't care who ya are ....
I'm curious because I just finished one of the flavors of a CMoy headamp using an OPA2134. To test the difference in sound I want to try some other ops like the opa2227 but I would have thought the original circuit would have to be tweeked to accommodate the replacement. (such as feedback loop, etc.)
Yes, I'm reading data sheets but not always understanding what's relative to the project.
Re: drop in replacement
To be honest, I don't have golden ears, nor experience with expensive studio gear. I'll be interested to hear your observations.
The 4558 is a chip whose only saving grace is that it's cheap. It's noisy and slow. It may have been state of the art in 1972. This doesn't prevent the musical equipment industry from using them by the ton in "prosumer" gear to this day!
When I say "drop in replacement" I mean a chip that is similar enough in input characteristics (e.g. bias current, noise voltage/current ratio) and power draw that it is virtually guaranteed to work in most 4558 circuits. The chips I suggested are faster, but not so fast that they would be hard to stabilize. I've used all 3 in various musical equipment POOGE projects with good results.
You can usually substitute a FET input op amp for a bipolar. The reverse is not always true. From what I remember of the cmoy circuit, it should work fine with a variety of chips.
If you can get your hands on a few, they might also be good in the summing stages. The 4562's specs suggest that it likes to be driven from low impedance sources (including feedback networks). It probably won't give best results when the source impedances are much higher than 2K ohms. It also has enough GBW that I would watch for unintended oscillation when replacing slower chips in marginally designed and laid-out circuits.
Binaural said:
To be honest, I don't have golden ears, nor experience with expensive studio gear. I'll be interested to hear your observations.
tamasic1 said:
The 4558 is a chip whose only saving grace is that it's cheap. It's noisy and slow. It may have been state of the art in 1972. This doesn't prevent the musical equipment industry from using them by the ton in "prosumer" gear to this day!
When I say "drop in replacement" I mean a chip that is similar enough in input characteristics (e.g. bias current, noise voltage/current ratio) and power draw that it is virtually guaranteed to work in most 4558 circuits. The chips I suggested are faster, but not so fast that they would be hard to stabilize. I've used all 3 in various musical equipment POOGE projects with good results.
You can usually substitute a FET input op amp for a bipolar. The reverse is not always true. From what I remember of the cmoy circuit, it should work fine with a variety of chips.
danny_66 said:
If you can get your hands on a few, they might also be good in the summing stages. The 4562's specs suggest that it likes to be driven from low impedance sources (including feedback networks). It probably won't give best results when the source impedances are much higher than 2K ohms. It also has enough GBW that I would watch for unintended oscillation when replacing slower chips in marginally designed and laid-out circuits.
ge.... it's getting to where we need to replace our old 30Mhz o-scopes with 100Mhz ones now...... iirc the 4562 has a GBW of 45 or 50 Mhz? my 30 Mhz scope would probably "see" the oscillation, but not be able to sweep fast enough to do much more than that (such as identify what freq it's oscillating at)......
ok so, with this whole recap business, what is the best recommended type of film capacitor. i've done some research and from what i read it seems that the polystyrene or teflon caps are the best for audio. am i heading in the right direction?
also with the opamps, i'm not able to find the LT1124 anywhere. where's the best place to purchase these bad boys?
also with the opamps, i'm not able to find the LT1124 anywhere. where's the best place to purchase these bad boys?
DigiKey usually has the LT1124, but you might prefer to buy from Linear Technology directly:
http://www.linear.com/
Their small quantity prices are less than DigiKey's, and they almost always have stock.
http://www.linear.com/
Their small quantity prices are less than DigiKey's, and they almost always have stock.
Binaural said:
Yes polystyrene and teflon are the "best" choices. Both are expensive. Construction details also matter. Some rules of thumb, in ascending order: metallized mylar, metallized polypropylene (some are pretty good, some pretty bad, lot of variation), polyprop/aluminum foil, polyprop/tin foil, polyprop/copper or silver foil, polystyrene/tin foil, teflon/copper or tin or silver. Again, just rules of thumb. Generally, higher voltage caps sound better, all other things being equal.
Humblehomemadehifi.com Cap info link Some test info, that agrees with what I've observed, but my experience is not nearly as broad.
There is a lot more info around. One major pitfall is that it takes quite a bit of breakin before you can asess the sonic virtues of the components used. I prefer to listen briefly initially, then break in silently to prevent adaptation issues, and then listen again at 50 and 100 hours. It's done when it stops changing. A given cap's characteristics may be a bit different in a tube circuit with a high voltage bias, in an opamp circuit with low voltages, or in a passive crossover network, with high currents.
Ok, so the tax return has come in and this project is underway. I have decided to go with the Ti/Burr Brown OPA2134. It seems to have the best cost/performance ratio. I recently found out that TI has a samples program where they will send you up to 5 of each product. Chucko reccommended the OPA2227. Are there any other TI/BB chips that could be used here? I can receive up to 5 of 8 different products so basically I'm trying to get as much as i can for free (hey why pay if you don't have to?)
Well, I just received the 5 samples of the OPA2134's. The board is layed out in pairs (one board for every 2 channels) and has 4 chips per boars so i swapped out 4 on the first board for the 4 new BB's. On the input board is the same deal, 1 chip for 2 channels so i popped that one out (all chips were 4558's by the way) and fired it up. NIGHT AND DAY DIFFERENCE!!! I can't wait to get the rest of the board done. Thanks for everyone's help!
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