GSSL OpAmp and ceramics question: oscillation expected?

I'm nearly there with my "zero cost" GSSL...means everything / every part ripped of from electronics waste, ebay industry spares (pcb material, home etched), reused 19" casing from an ole exciter etc....

1)
Now i wonder if i can use MC33078 as a replacement for the NE5532 (specs do look good, and as i understand it, only in use at a symmetrical input NE5532 are better because of their CMRR specs, right? Both BJT, right?)

2)
...and a FET type single opamp as a replacement on the inputs (maybe a Burr Brown i have for spare? opa227 or opa134 or some similar, having quite some right here?) as FET can mostly replace BJT but not the other way round? Right?

Next question:
3)
I can't find 100pF ceramics in my box so i thought of paralleling some 22PF or 33pF, but there's not enough room for that on the pcb...
As i understand it, the resistors limit the gain and these caps limit the bandwidth according to the gain, am i right?
What happens if i lower these to 33pF?
4)
How critical are the capacitor values anyway to prevent oscillation for these opamps (especially the ne5532, but the TL074 as well, even if not in the audiopath)?
5)
In this circuit, is the opamp behind the vca more prone to oscillation than the others?

I wish to get a more intuitive understanding in analog audio design, so i decided to learn from my first diy units this (maybe hard) way....and my green I was a great experience built from used parts - you measure every part, think about its role in the game, think about tolerances and experiment alot without thinking about lost money or wrong orders or things like that. Improvising as a way to more knowledge, not a fast ripoff.

Any comments highly appreciated.

Kind regards
and happy DIY

Martin

MC33078 :grin:
FET for BJT :grin:
BJT for FET :sad: (check DC offset)
22pf-33pf :grin:


lowering the feedback caps less than 22pf will start to put you into oscillation territory. You tend to want fairly good quality and tolerance to heat.. some debate the need for good caps in the feedback path but personal experience says use good ones.

I used 22pf feedback caps in all my compressors.. I also used opa2604 and opa627s in them too(much faster FET opamps) with no ill effect.

Aah
I am thinking of forking out some $$$$ for OPA627 to replace the 5524's and already have a coupple of opa2604's.
Would I notice the difference in quality in the GSSL or would I just be waisting money I don't have??

i used some samples when you were still able to get them.. they sound great but i wouldn't waste the $$ on them as there isn't that much difference in sound.. I'm going to try some ad825 on a little adapter board i made though..

The feedback cap value is dictated by the feedback resistor value, you probably already knew that...
Go with Polyprop. caps, very good..

HEY MIKE
I can,t find 22pf Polyprop caps in Australia. Do you have any other suggestions.

Digikey is where I get most of my caps,
I have some COG 22pf caps I could send, shipping would be more than the caps...

Digikeys part # P4841-ND 22pf 100V COG, a very high quality cap..

[quote author="Svart"]I used 22pf feedback caps in all my compressors.. I also used opa2604 and opa627s in them too(much faster FET opamps) with no ill effect.[/quote]
I'll guess you mean putting a cap in the feedback. You should use it for certain purpose, not just in case. This cap is sometimes needed to compensate for the input capacitance. If not there = oscillations

... but has also to do with the values of the resistors in the feedback. Lower values = less sensitive.

yes I mean that I traded out the 100pf feedbacks for 22pf feedbacks.

As a side note, I have one time tested a whole bunch of clones without any ceramics but in the sidechain - all behaved nice. This because I attended a DIY-arrangment, where all ceramics had been mysteriously lost. I managed to find enough to get the sidechains going, and the units worked.

But still - to avoid possible future problems - it is recommended not to skip on the hf stability compensation..

Jakob E.

Thanks alot for your helpful comments on this!
:grin: :thumb:
I will try to stuff sidechain only first with the last two C0G 100pf i found and replace with 22pf where 100pf were recommended...maybe i try out 75pf styroflex as well...more on that later... :roll:

Kind regards
and happy diy

Martin

[quote author="rrs"]HEY MIKE
I can,t find 22pf Polyprop caps in Australia. Do you have any other suggestions.[/quote]

I've never been able to locate any low value polyprop's either and the lowest value polystyrene that still seems to be readily available is 100pf.

For low value feedback and miller compensation caps, I've been using silver-mica caps. These are easily found in values down to 1pf.

I can get polypropylene caps down to 47pF easily. But note that they are often wound caps, so they are NOT good for this purpose, because of their bad HF behaviour.

Using the C0G/NP0 ceramic caps or the silver/mica types is a much better idea.

Best regards,

Mikkel C. SImonsen