Please help me understand the Soundcraft Delta 200 SR EQ

[dysenterygary]

I have spent hours trying to figure this out and I'm left feeling foolish and stumped.  If anyone would hold my hand and explain some of this I would be extremely grateful! 

1.  On the HF which of those 10n caps are the "eq" caps, and how the heck does that circuit equal 12khz?
2.  What resistor/caps would I change to make the frequency 10k or 8k?
3.  In the LF and MF what does the lower value capacitor do(C18 and C20)?
4.  How is the Q set?  Is there a way to make the Q adjustable in this circuit without adding opamps?

 

[SnizzleD]

I have spent hours trying to figure this out and I'm left feeling foolish and stumped. If anyone would hold my hand and explain some of this I would be extremely grateful!

1. On the HF which of those 10n caps are the "eq" caps, and how the heck does that circuit equal 12khz?
2. What resistor/caps would I change to make the frequency 10k or 8k?
3. In the LF and MF what does the lower value capacitor do(C18 and C20)?
4. How is the Q set? Is there a way to make the Q adjustable in this circuit without adding opamps?

Did you ever get a handle on modding this eq?

 

[gyraf]

Sorry, never saw that

A very approximated answer:

1) All the three 10nF caps are frequency determining. the 10KLin in parallel with the 4K7 mixer gives some 3K2'ish total load to the HF filter, the two capacitors at the pot ends are series'ish, so I'm seeing 5nF - and 5nF/3K2 gives you some 9952.2Hz (my guess is that it's set for 10K, not 12K, but it's a very soft curve, so it probably is an undetectable difference)

2) Whatever it's frequency is as-is, just scale these three capacitors with the factor you wish. Doubling or halving gives you octaves, you do the math. All non-inductor EQ's react like this to capacitor change. Just put a bit in parallel with the existing ones to lower frequency.

3) The two sweep bands consist of kinda-synchronized combinations of first-order passive high- and lowpass filters - forming an approximate bell shape that can be moved. C18 and C20 are the high-cut/lowpass part of this equation.

4) Can't Q here, it's a first-order at unity gain - it is what it is. Unless you're going inductor. (Edit - well, you could make Q lower, (bandwidth even wider) by increasing distance between hpf and lpf filters by changing capacitors. But you can't get Q higher than 0.6'ish because it's first-order-passive)

/Jakob E.

 

[SnizzleD]

Sorry, never saw that

A very approximated answer:

1) All the three 10nF caps are frequency determining. the 10KLin in parallel with the 4K7 mixer gives some 3K2'ish total load to the HF filter, the two capacitors at the pot ends are series'ish, so I'm seeing 5nF - and 5nF/3K2 gives you some 9952.2Hz (my guess is that it's set for 10K, not 12K, but it's a very soft curve, so it probably is an undetectable difference)

2) Whatever it's frequency is as-is, just scale these three capacitors with the factor you wish. Doubling or halving gives you octaves, you do the math. All non-inductor EQ's react like this to capacitor change. Just put a bit in parallel with the existing ones to lower frequency.

3) The two sweep bands consist of kinda-synchronized combinations of first-order passive high- and lowpass filters - forming an approximate bell shape that can be moved. C18 and C20 are the high-cut/lowpass part of this equation.

4) Can't Q here, it's a first-order at unity gain - it is what it is. Unless you're going inductor.

/Jakob E.

Thank you for the info...more specifically i was hoping to switch out the eq caps for wima and I'm very green when it comes to electronics ibwas maybe looking for some help on identifying which caps are the eq and the values of them heres a pic...I have no idea what the coding on them means just sorta looking for some guidance

 

[gyraf]

I don't see any benefit to be had from replacing the capacitors - they're film capacitors already, probably some standard Panasonic type if I read the markings right..

In stuff like this, best route is to replace the electrolytics every 10 years or so - and learn to get the best out of it as it is..

/Jakob E.

 

[SnizzleD]

Sure makes sense...there are just rumours floating around that the wimas smooth the top end a lil and make the bass defined...now I know there are other mods that do this on a much greater levels but i was just looking for a way to get my feet wet with modding my board...i plan to get the power supply and master done by a professional but was just looking for ways to improve it myself that are not too involved considering I dont understand electronics but I'm very mechanically inclined and can follow directions but that actual physical understanding of why or how the mods would work I am cluesless... I also see a place on the pcb where there are jumpers which looks like to wire in transformers...do you know anything about that?

 

[gyraf]

the idea of wima sounding significantly different than panasonics is just wishful thinking or uninformed internet BS, best to ignore that altogether

IMO the wima has an advantage in better quality control resulting in predictable tolerances and less mechanical variation, and are more robust to violent soldering practices - and yes, I use wima everywhere as I'm a violent solderer (also because my customers expect me to)

You can probably put a transformer in there, but unless you are haunted by RF interference on your inputs, I don't think you'll gain much (or anything) from that

/Jakob E.

 

[Newmarket]

Sure makes sense...there are just rumours floating around that the wimas smooth the top end a lil and make the bass defined..

Please don't waste your time chasing internet 'rumours'. Those terms have no real meaning.
If you don't like the nature of this EQ section then you need a different one ie different frequency response / control characteristics.

For a real and measurable improvement you could look at changing the 072 op amps for something quieter eg OPA2134.

 

[JohnRoberts]

+1 I am not a fan of second guessing console designers... I do advocate benchmark measurements. Measure the frequency response of every channel and identify outliers. Weak LF response can indicate bad (dried out) electrolytic capacitors.

In general if you find one bad electrolytic, replace all of the same value because just like cockroaches there isn't just the one you found.

JR

 

[living sounds]

For a real and measurable improvement you could look at changing the 072 op amps for something quieter eg OPA2134.

And get the whole console to sing by itself. Been there, done that. The 072 is among the least demanding as far as power decoupling, placement etc. are concerned. That's what the circuits and PCBs were designed for. And old 80s-90s op amps of the same type often sound better than modern ones for some reason.

Put in OPA2134 and you likely need additional decoupling. Adding lot's of different decoupling caps in parallel can (and likely will) create it's own problems. Plus, they dump more current into the audio ground, and you may end up with another kind of audible problem.

This comes from someone who wasted way too much time modding a Soundcraft console of that era. I would maybe replace the signal path electrolytics and maybe add bypass caps (polyester, small size - be careful, like with PSU decoupling the potential problem is anti-resonance) and power filter electrolytics (but not the ultra-low-ESR types). And change EQ curves if you like, but there are reasons why they were selected as they are.

 

[SnizzleD]

Thank you for your help everyone...is there any guide to identifying the values on top of those yellowish caps that I have in the picture and you dont think adding Jensen's transformers will improve the sound? I'm new to this whole thing and I'm quite mechanically inclined but know not a lick of anything to do with electronics...and where there are a few sources to have professional mods done their waiting list is long or they are soon retiring...I honestly haven't tried to sum a mix thru this thing yet I am still in the tracking phase but I was trying to get any idea of there was anything I could do myself to improve the sweetness of the highs and make the lows more define but this is soley coming from what ive heard thru forums...you may be right and I won't see any improvements...but I'm just trying to get into replacing some caps while I'm tracking so it's done by the time I go to mix...i have a really nice pultec clone but I'm trying to avoid having to print everything one at a time to mix into the delta afterwards i was hoping to use the boards eq for most of the moves and was wondering if new caps in the eq section would make it sound a little better

 

[Newmarket]

Put in OPA2134 and you likely need additional decoupling. Adding lot's of different decoupling caps in parallel can (and likely will) create it's own problems. Plus, they dump more current into the audio ground, and you may end up with another kind of audible problem.

Disagree wrt the OPA2134. I've used them loads in both commercial applications and various DIY bits on stripboard etc. Just standard decoupling - no issues.
Note that I deliberately stayed away from suggesting 'faster' higher BW 'uber' opamps for exactly the reasons you highlight but not a problem with the 'workhorse' 134 series.
Adding different types of decoupling caps in parallel can indeed be counterproductive. But an increase in current isn't one of the problems. They are essentially dealing with the current that the circuit is drawing and that basically stays the same.

 

[gyraf]

..just put in IC sockets, then it's easy enough to go back..

But if it was me, I wouldn't change anything until I'd worked with the console long enough to know its quirks and habits really well - only that way you'd know for sure if a change is for the better or not. This keeps out the internet rumours..

/Jakob E.

 

[Newmarket]

and you dont think adding Jensen's transformers will improve the sound?

Depends on what you mean by improve. The real 'utility' aspect of transformers is high CMRR.
As for 'sound quality' - some people really like putting stuff through them. Meanwhile others (including some participants on here) have spent considerable time and effort designing low noise high gain mic amps to be able to avoid them (not needing the 'free' voltage gain).
Really the technically 'better' transformer you use the less it alters the signal. So for 'colour' you might want to drive it hard and/or look at a 'lower quality' transformer but be careful about loss of bandwidth esp bottom end. There's always the question of whether you want that 'baked in' to your desk or available as desired eg a separate unit.

I'm new to this whole thing and I'm quite mechanically inclined but know not a lick of anything to do with electronics...and where there are a few sources to have professional mods done their waiting list is long or they are soon retiring...I honestly haven't tried to sum a mix thru this thing yet I am still in the tracking phase but I was trying to get any idea of there was anything I could do myself to improve the sweetness of the highs and make the lows more define but this is soley coming from what ive heard thru forums...you may be right and I won't see any improvements...but I'm just trying to get into replacing some caps while I'm tracking so it's done by the time I go to mix...i have a really nice pultec clone but I'm trying to avoid having to print everything one at a time to mix into the delta afterwards i was hoping to use the boards eq for most of the moves and was wondering if new caps in the eq section would make it sound a little better

 

[Newmarket]

..just put in IC sockets, then it's easy enough to go back..

But you're ignoring the extra inductance from the socket legs !
No, really - good suggestion.

 

[living sounds]

There's always the question of whether you want that 'baked in' to your desk or available as desired eg a separate unit.

I put lot's of transformers in the console. Turns out I prefer a console without trafos (or a single mostly transparent one on the mix output as in my current console). And Jensen are as clean as they come anyway (if implemented correctly). Unless you need the best CMRR (unlikely in today's typical studio mixing console applications) there really is no reason to put in trafos at the console level. IMO, of course.

 

[JohnRoberts]

But you're ignoring the extra inductance from the socket legs !
No, really - good suggestion.

Just to be clear... he was kidding about sockets adding inductance, not remotely an issue for audio. Sockets are OK.

JR

 

[Newmarket]

Just to be clear... he was kidding about sockets adding inductance, not remotely an issue for audio. Sockets are OK.

JR

Just to confirm Yes - socket thru hole ICs - esp for DIY where minor component cost is not a real consideration. Not for SMT of course !
No one ever said "That would have been a great record if they hadn't socketed the op amps"...

 

[SnizzleD]

Does anyone on here have the schematic for the delta Sr mono input

 

[moamps]