Does this mean my opamps are already bypassed?

Hi, I'm working on modding a Soundcraft Delta 200 SR.  I have done lots of reading about adding a 22pf cap between pins 1+2 and 6+7, and then adding a .1uf cap from pins 4 and 8 to ground.  I recently noticed that there are two .1uf caps way off to the side and when I looked them up on the schematic I found that they are apparently attached to pins 4 and 8 on all of the opamps on the board.  Is this sufficient?  Can I start swapping out opamps without having to add additional .1uf caps?

https://i.redd.it/2lc47l1peui01.png

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I am not at all familiar with the mod you're talking about but I will assume for a second that you want to swap in high speed op amps which do require ceramic bypass to keep them stable. But they have to be right next to the op amp. Specifically you'll want to get the body of the cap within 5mm or so. The other lead can be long. So if those caps you speak of are 5cm away, that's not good enough because there is a tiny inductance and resistance in the wire / trace to the op amp. Also, they must be ceramic because they have very low ESR.

More generally though, I doubt changing op amps is going to give you a noticeable performance improvement because the differential stage in front is where most of the gain is and is what gives the circuit good noise performance. And believe it or not the lowly TL072 is actually not a horrible part. That circuit with that bipolar differential front end is known to be low noise already. Your time would be better spent replacing scratchy pots or re-capping dried out electrolytics. Although it may not be necessary to recap. I have pulled caps out of really old gear and tested them and they were fine. Sometimes. Sometimes not.

22pf and the pin location described sounds like feedback caps to me....... I don't know the schematic....
But regarding the bypass caps, I usually have added the bypass caps from + and - to ground even if there are some around.... You have to with the newer faster ones for sure...

I have to agree that the results from swapping op amps are not mind blowing if at all better sometimes.... The risk to reward really comes into play if it's not something your used to doing as well...... The Delta pcb  is nice but, not so much by some other manufacturers...

You can damage stuff......... Although, workarounds can be done and, it's an excellent learning experience for finding your way around the schematic to repair stuff that would've been perfect if left alone, the resale goes out the door.......more or less

But it's cool and I do like some of my upgraded stuff more than I did before......

Then you have the current draw from faster chips.....it adds up fast...... I've ended up  checking out different op amps of the same characteristics and found I like the one's closer to the original specs more times than not....

I'd take squarewave's advice and recap...maybe even bypass the coupling caps if you're into that...... I've noticed improvements from swapping out the lentil  type and other ceramics with some higher quality cog types as well.......

I'm not sure why the link isn't showing up for some people.  I'll try again with the little img tags.

Try imgur.com. But make sure you use the direct link to the image file itself.

And when you add images to your posts, do it like this:



So that only a thumbnail is displayed but when someone clicks on it they get the high res version. Otherwise you're sort of torturing people on mobile devices.

That seems to be the general technique used around here anyway.

Although just in case imgur.com does not live forever, I insert my images at the top of a neverending blogspot.com post, hit save, and then right click on the image and select "Open image in new tab" to determine the direct URL link. Even though the post is never published, the images are still available to the WWW. If blogspot does not live forever, maybe I'll have a chance of downloading them first.

On my Soundcraft series6000 I upgraded all the caps and swapped out the tlo72 for Texas Instruments LM6172IN  on the preamp input and eq  - you have to decouple the power rails and I put a 1000 nf wima in series on the back of the input filter caps for some smooth high end also upgraded the ekectrolytics to 47uf -  the difference is night and day.



I've since swapped back and fourth between the TLO72, LM6172IN and some 5532s on the input buffer, I'm favouring the 5532s right now, but I like LM6172IN.  I've come to the conclusion that, though perfectly functional TLO72s kinda suck next to modern op amps and they suck next to 5532/4s because they were a dilibrately budget design, they found their way into proaudio because the 5534 was expensive and they could handle high impedance inputs they his. You will notice a difference between different op amps, if you don't you shouldn't be an Sound Engineer.

Interesting aside my old friend Ade Emsley at Orange has made a discrete dual op amp to replace TLO72 or 5532s

https://m.youtube.com/watch?feature=youtu.be&v=tKanR1aM16I

He's a real cool dude, beautiful soulful guitar player and brilliant engineer. But they're too expensive!

I'll stop waffling now

Tim

Timjag said:

You will notice a difference between different op amps, if you don't you shouldn't be an Sound Engineer.
Tim

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And to add, knowing when an older and slower op amp might be better suited ....

Favoring a 9 v/us slew 5532 with its 10mhz ugb over the 3000v/us slew  6172 with it's ugb that is 10x the 5532 shows that newer and faster doesn't always mean better.  I have a pretty nice stash of faster chips now after realizing that a much more simple recap with some added "paralell" bypass caps made the improvements I was looking for and the newer op amps just didn't sound as good to me or even right sometimes.

I've actually found that I don't like the TL072 as much as the pretty much equivalent LF353 in some circuits..... not too sure why but, it just sounds better to me...

I also  favor the NJM4562 over the LM4562 in some places as well..... Unfortunately harder to find those for some reason...

And the 5532 can't be faulted in many places.....

At the end of the day for me, the differences heard from pushing certain pieces to their optimal operating points has always yielded  more satisfaction  compared to replacing op amps . Don't get me wrong, I like having my stuff working at it's best potential and sometimes a recap may not be all that can do this but, I also know I can put that un modded piece in front of someone who is more skilled at the art of using it to it's maximum potential and they will most certainly reveal what I may have been looking for without having to touch a soldering iron.....

If modding and soldering is something comfortable to you, then it's no big deal to try for yourself. If it's not something you're used to, and there are no sockets there to experiment with, there's a possibility you will notice the new jumper wires that weren't there before more than you will notice any major improvements that could've been had with the same op amp or similar......

But we've all been there and this is DIY so, I guess it kinda has to be done. :-[....Just be careful.

dysenterygary said:

Can I start swapping out opamps without having to add additional .1uf caps?

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It depends on the op amps you intend to use. 

Yeah, I think you'll get into difficulties with noise if you just start swapping op amps. High speed amps are awfully noisy if you don't decouple

On a sonic note we spent last week going through several op amps to find the winner. And it was only really the winner for that project, I've split the desk into sections for different sounds. It's on rich harmonic textures you hear it the most, piano and overdriven guitar being the most obvious examples. Still decoupling the power rails is a good thing to do.

Soundcrafts are excellent desks that are spoiled by cheaper components - back in the day the TLO72 allowed them to crack the market in live sound and higher end project studios. They made a name for themselves offering very well spec'd desks with tons of ins and outs, inserts, separate modules, for a fraction of the cost of high end boards.

The great thing about that 20 years later is that they're a really solid base for smart upgrades they're built like tanks. And they're cheap - I saw a series 5 going for 1,600 quid, 1600!! That's a lot of desk for that money