Substitute OPA2134A for 4558P?

[psytechguy]

Any problems with doing this?

Playing around with modifying some old gear.

thanks!


Mike

[dementedchord]

my burr/brown data book is old....doesnt list the a ... but the 134 familily is great opamp...and should work well in most situations....good band width (8 mhz) low quescient current (pico amps lol) slew rate in the 20 v/usec range will operate at2.5 v all the way to 18 v and go pretty much rail to rail..... yeah it will work....

[tINY]

You may need to modify the feedback loop and the bypass capacitors a bit.

The 4558 is limited in bandwidth a bit and not prone to oscilations.



-tINY

[Jim Williams]

Make sure there is a small capacitor across the feedback resistor, a 10~22 pf is good for a 10~22k feedback resistor. Scale them larger for smaller feedback resistors. If it's under 3k ohms, you can usually skip it. Add a pair of .1 uf mono ceramic caps from pins 4 and 8 to a close by ground pad or trace. Now you can play with lots of different parts.

Remember, they don't work if you let the smoke out!

Jim Williams
Audio Upgrades

[psytechguy]

So this is an Orban 622B parametric EQ I'm playing with. Is it even worthwhile to try and upgrade or am I just spinning my wheels?

Any known circuit upgrades for these?

thanks!

Mike

PS which pin would the feedback resistor be associated with?

[tINY]

What don't you like about it now?



-tINY

[dementedchord]

the caps that jim suggested adding to the powersupply pins (4-8) is a good idea in particular... they are refered to as decoupleingcaps incedently and tend to help cleanup the hash in the supply.... but i would watch out for the feedback type as this introduces another "filter" to the circuit... (although i admit i didnt do the math to find the freq and if its well out of hearing no foul )

[Jim Williams]

Originally Posted by psytechguy

So this is an Orban 622B parametric EQ I'm playing with. Is it even worthwhile to try and upgrade or am I just spinning my wheels?

Any known circuit upgrades for these?

thanks!

Mike

PS which pin would the feedback resistor be associated with?

On a dual opamp, it's pins 1+2, 6+7. on a single package opamp it's pins 2 and 6. Those small caps are there to cancel input capacitance and prevent ringing, overshoot and oscillations. They operate around 200k hz to allow minimum phase shift at 20k hz.

In that Orban change opamps IC1,3,5 and 7 to a National LM6171. Pop in those 10 pf caps across the feedback resistors from pins 2 and 6. I use a Linear Technology LT1351 for IC9 due to it's input pins hanging out on a trace to a switch, mucho capacitance on those so only opamps with 2~3 mhz gain bandwidth will work.
OPA2134's will work in the filters where the 4558's reside. Changing the EQ caps to polyprop films help lots on the 2 upper bands.
Add those .1 uf caps on the power rails near each wideband opamp.
Replace C10 and C13 tantalum coupling caps to Panasonic FM series 220 uf 25v. Bypass with a polyprop film cap for air.

Now that wasn't too hard, was it?

Jim Williams
Audio Upgrades

[psytechguy]

Originally Posted by tINY

What don't you like about it now?



-tINY

Little noisy. Probably could use some fresh P/S electrolytics, too.

But seriously, I JUST CAN'T GET DOWN TO "BROWN NOTE" TERRITORY WITH IT!!!!!!!

ahem,

[psytechguy]

Originally Posted by Jim Williams

On a dual opamp, it's pins 1+2, 6+7. on a single package opamp it's pins 2 and 6. Those small caps are there to cancel input capacitance and prevent ringing, overshoot and oscillations. They operate around 200k hz to allow minimum phase shift at 20k hz.

OK. Roger that.

In that Orban change opamps IC1,3,5 and 7 to a National LM6171. Pop in those 10 pf caps across the feedback resistors from pins 2 and 6. I use a Linear Technology LT1351 for IC9 due to it's input pins hanging out on a trace to a switch, mucho capacitance on those so only opamps with 2~3 mhz gain bandwidth will work.
OPA2134's will work in the filters where the 4558's reside. Changing the EQ caps to polyprop films help lots on the 2 upper bands.
----------------------
This would be the existing silver mica caps? At least that's what I think they are. Brown an shiny!
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Add those .1 uf caps on the power rails near each wideband opamp.
------------------------
From the power rail to the v+ of the OPA2134?
-----------------------
Replace C10 and C13 tantalum coupling caps to Panasonic FM series 220 uf 25v. Bypass with a polyprop film cap for air.
-------------------------
What value poly prop for bypass?



Now that wasn't too hard, was it?

Jim Williams
Audio Upgrades

Nah!!! Just need some clarification! It's my first time. Please be gentle. ;0)

[Jim Williams]

Yep, replace the 47 pf silver mica C1 to 10 pf . C12 should be left in, it's a global feedback cap around the output stage, IC9 and it's transistors. Add another 10 pf from pin's 2 and 6 to stabilize with the LT1351.

The .1 uf mono ceramics are placed from pin 4 to ground and from pin 7 to ground on the single opamps, the 5534/LT1357 stages I mentioned.

There's .015 and a .0015 caps on the top band EQ, use quality polyprop film caps there of the highest voltage that fits. Same for the high mids, a .033 and a .0047 uf.

Bypass air caps can be Wima MKP-2 .01 uf 250v from Mouser.

Jim Williams
Audio Upgrades

[sonichermit]

A somewhat related question for Mr. Williams.
Actually two questions.
I also have an Orban 622B that I did some work on last winter. I removed the TL071's in four places that I assume are tied to the frequency bands. I installed sockets after taking out the 071's and substituted LT1351's in all four of those sockets. The EQ has shown some performance improvements related to smoothness. I did replace the power caps as well. I did not go any further with it although I do have a few more caps in the signal path I have yet to replace but did not go as far as some of your mods recommended. I might yet do so.
A separate question is in regards to an old Delta lab digital delay I have. Model ADM 1024. Yes indeed it really is an antique. Some years ago I had some problems with it and replaced the 4558 chip after installing a socket. I recently plugged it in again after it sat for years. It behaves really weird. It really doesn't work and just makes a sort of clicking noise with the clicks being evenly spaced and it also makes some other weird noises. I am wondering if this is likely a chip problem still or something else? I just ordered some RC 4558's from Mouser along with some 082's which this unit also has and was thinking of starting to replace chips until I get lucky and get it working again. Have no idea of what is going wrong inside. There are lots of different chips in this unit and frankly I am just messing about with it for for fun. Any suggestions?

[slor]

Jumping in halfway here, but.... I have one of the earlier 622Bs with the sealed EQ modules. Is it worth doing the upgrades / mods to the rest of the circuit if I can't access the modules themselves?

(By the way, I see that they can be desoldered from the motherboard, but even then I don't know how much of a pain it would be to gain access to the insides.)

Thanks much!
-Seth

[sonichermit]

I recently purchased two Edcor WMA600 transformers for my 622B. Is there anyone on this thread that knows how these are supposed to be connected? I was informed the 622B's also had an option for transformer balancing, I assume we are talking about balancing the output?
Any info is of course greatly appreciated.

[Jim Williams]

A couple of Jensen's nickle core JT-11's would be nice, but those run about $140 each. I have those in my Audio Precision analyzer.

Anything else will be a filter and aural exciter. Moonwalking to me...

Jim Williams
Audio Upgrades