replacing console opamps - and comparing the results..

[cwatkins]

Ok, after browsing I'm not sure I could find the answers to these questions..

background..
I got a ramsa wrt-820 off ebay for $152. runs great, I don't think I've found any major issues yet. There is some question to whether group 8 works right in the master section so I will tackle that last.

So I've taken one of the "modules" it's 4 channels at a time, and I've socketed the line input opamp, which is both the tape and the line opamp and the fader opamp. ( I left panning alone). It used 4560DD's and I have biased two 4450DD's into class a by shorting with a 10k resistor (I that does it? cross 1/2 and 6/7) and on channels 3 and 4 I have replaced line and the fader opamps with 5532's.
I then compare with two other 4560's channels and run a stereo program through them.

Here's the questions..

I run a 1k test tone through and make everybodies gane stage as close as possible before clipping with the meter on pre fader. (The knob position has changed for the 4560DD's in biased mode headroom loss?)
(Does it break the peak light? accuracy?)
5532 is about the same as the 4560's normal.

then I setup the fader's to match so that everything is equal outbound, everything is different there as well. 5532 being closes I beleive.
(run RR tests and the all the noise stats are similar.)

so now that all the "gain" stages are matched? (is that an acceptable way to do it?)

I listened...

5532 won very close to the 4560DD's with 10k resistors and then the normal channels where third, but not much difference at all, the 5532 was clearer and the bass had more solid punch.. It kind of made me think I was listening to a burbrown in comparison..

So, when I read the 5532 stats on power, Is it 4vs per side = 8vs and isn't the 4460DD the same, 4vs per side?

 

[cwatkins]

hey shameless bump too, but:
but how about the 4570 or the opa275 if the ne5532 turns
out to be too much juice?

I mean I suppose I could leave it alone, it turns out the main's L/R is a ne5532 so it looks like for the "master" opamp they used the ne5532, I don't know if that was for quality or just line driving specs?

 

[rodabod]

You could have a go at some TI samples.

Check out the OPA2228 (IIRC). I think it might be a nice if it is suitable for your circuit in question.

What are the signal path caps like?

 

[cwatkins]

22/33pf in the feedback loop?

 

[smallbutfine]

opa 2228 seems to be a very good try. i used it's unity gain stable counterpart 2227 and the 2228 alot in modding analogue effects as 4558 and TL072 drop-in replacement with good and stable results. As 4558 and 4560/4580 are 100% drop-in compatible this would be a very quiet and good sounding upgrade but could turn out to be a bit pricey.
4580DD is the direct follower of 4560 with better noisespecs, if everything else turns out to be too instable or pricey a drop-in replacement with this one may be worth a try under all circumstances.
Peak accuracy is dependent of the point where the peak is measured.
(channel peak(?) at the channels out normally, so i don't think of an inaccuracy...but don't know your device...)

 

[cwatkins]

the 5532 seems to be working wonderfully, how to I figure out
how many I can put in before I "tax" the power supply..

and then, generally what happens? a pooof?

 

[Svart]

as far as the PSU is concerned, you should be able to find the ratings on most of the parts and figure out what the max output should be. You can figure out the max draw by really just counting the opamps and multiplying the number of opamps( and other components) by their full current draw(look in datasheets) and estimate how close you are to the max output of the PSU.

that should get you fairly close.

usually not a *poof*. more like *melt* pop!

you'll sag the rails if you aren't too close to the edge, but if you are over the edge, you might get thermal runaway or just blow up parts. always good to put fuses/breakers/fuse resistors in the system if they are not there.

 

[smallbutfine]

Compare between the current draw of the old and the new opamp from their datasheets and multiply by the number of opamps changed. Sometimes this is in the tolerances of what the psu can stand... If the psu can't handle this anymore, there can be a few components blown up...the bridge rectifier(s), diodes...or even the power transformer (if you are out of luck and the fuse is too slow).
EDIT: Svart was faster...

Fuse resistors tend to burn a hole into the pcb...it gives quite some smoke...

 

[bcarso]

Check out the loading due to the feedback and following input(s) you are driving, as well as the quiescent current, if you want to avoid overkill but still want to sleep well. Most of the time these parts are working harder when there are signals running around, but not so much harder that the maximum output currents flow very often.

One of those IR thermometers would be handy---run some fairly aggressive material through the system for a while and look at the temp rise of power supply components before and after the mods.

One thing that works in one's favor is that many manufacturers have a conservative spec for max quiescent current. For example the BA4560 says 7mA but is typically 4mA. However, the console maker may have taken this into account and lived with a smaller power supply to begin with.

 

[SonsOfThunder]

[quote author="cwatkins"]I have biased two 4450DD's into class a by shorting with a 10k resistor (I that does it? cross 1/2 and 6/7)...(The knob position has changed for the 4560DD's in biased mode headroom loss?)
[/quote]
I saw this when first posted and thought I'd wait a bit before responding to make sure my head was on straight.

Seems to me you have only lowered the gain on these opamps by putting these resistors in place as evidenced by the change in knob position. To bias into class A, you'd want to put the resistors from pin 1 to 4 and pin 7 to 4 (if biasing to the negative rail...)

The 4560 has decent drive ("DD" denotes guaranteed low noise version, usually) (not as much as 5532 but some mixers use them to drive headphones directly). I'd look at only replacing the input opamps on each module and then modifying the master section. First, I'd check the schematic to make sure that the frequency response is not being choked by a bunch of caps in the signal path (feedback paths). This is all too often the case on mixers with this grade of opamp.

A good low-noise, slightly lower current draw alternate to the 5532 is the LM833 (Kev's favorite!)

HTH!
Charlie