Modifying Soundcraft 500 Series

Hello all!

I recently picked up a soundcraft 500 series 24 channel board to do mixing OTB. I have a bunch of spring reverbs, tape echoes, compressors etc I like using in my mixes but frustrated by the latency involved with Digital Mixing. This new board has 6 Aux sends so pleanty of routing options.

However I heard a rumor that one downfall of this board is the line inputs are actully not 'traditional' line inputs but rather -30 db padded inputs sent into the mic-pre. Im not sure why they choose to do this, (save $ on 24 additonal 10k Pots?), but I checked the schematics from Soundcraft website and it looks correct. I'd rather not mix through the mic preamps if possible.

I plan to use this board only as a line mixer, (I have other additional preamps), so I am looking to modify all channels for line level and I dont care about disableing the micpreamps. I'd like to accomplish this with as little jumpers and additional components as possible. The faders must be configured to control the line level gain, rather than micpre gain.

Schematic for the input module is shown below:


My initial thoughts were to bypass the transistors and bring the line level input signal directly to IC1A, which happens to be a TL072 Opamp. Then I would use the Fader in the negative feedback of the opamp to control the gain. I put the circuit into SPICE to simulate this.



The overall output gain is a little different when compared to the original configuration, (it clips at about 85% fader level) but it actually looked good otherwise in the simulation and the frequency response was improved!

I am somewhat new to this board and circuit mods, so feedback would be appreciated! Im sure im overlooking many things.

If I read correctly your schemo, I just see an unbalanced input to a voltage follower and a lot of things around it that don't do much. Is it really what you want? You want it unbalanced?
Using the fader as gain control will make the pre/post selection useless; is it what you want also?

Ok so being new to the forum, I took a shot at it....but looks like I have some learning to do...

abbey road d enfer said:

If I read correctly your schemo, I just see an unbalanced input to a voltage follower .... Is it really what you want? You want it unbalanced?

Click to expand...

Abbey Road, Thanks for the response! Would this make it very noisy? Would it be a better idea to connect to IC1A in a differential type config to reduce common-mode noise? Any suggestions? How expensive/hard/necessary would it be to make it transformer balanced?

abbey road d enfer said:

Using the fader as gain control will make the pre/post selection useless; is it what you want also?

Click to expand...

Looking again at the console, I think i may have confused the mic-pre gain knob with the fader. I thought on the schematic the 10k pot was the fader but it is not. Therefore, I probably do not need to modify this at all but rather bypass it or ignore it.

abbey road d enfer said:

and a lot of things around it that don't do much

Click to expand...

Sorry i started by putting the whole circuit into Spice to get an understanding of it, then modifiyed it.... which is why there is a bunch of unused stuff.

If you need no additional gain from the console you could try bringing the signal in via the insert returns for a more direct path.

If you're looking in the direction of recapping or more you may want to consider contacting Jim Williams at Audio Upgrades.  He knows the SC as well as anyone and I can say from firsthand that his mods are excellent.

I didn't think about that! however, i believe the inserts come after the eq which I would like to use.

As far as caps and overall condition, i opened it up and the unit looks like it was kept in a time capsule and should mostly be ok. however i may goto Jim for a master buss Mod? Thanks for the reference.

Additionally,,,,I need a PSU for this thing. (15-17V @ 3A) I pulled a huge old transformer (didnt check the specs on it yet) from my dads old Kenwood amplifier from the 70's I may go DIY. Any advice for this?

Or was looking at this part http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=179-2328-ND

If you want a Balanced line input, it is hard to beat the way they did it.

I did a lot of small work with mostly UN-balanced line inputs. In an electrically quiet room it works fine. But any little ground discrepancy between sources throws hum/buzz into the mix. I got pretty good at finding and solving those issues.

If un-balanced is fine with you, just stick it in some insert points (not shown in your snippet). From what I can see, you'd lose the IC1B rumble-cut. If that is useful, yank one leg of C11 and shove unbalanced line in at SW4's center legs (easy to find!).

But a balanced input is more flexible. Signal (both sides) is separate from shield. Even when taking an unbalanced source, you can use the balanced input to extract the signal without the ground-crap.

The pad is not inherently offensive. The resistors are near-perfect components (and you have a bazillion more resistors in the whole system). Yes, throwing-away level puts you closer to hiss, but the mike-amp is very low hiss. And practical source will have more hiss already in it than this stage with its 20dB-32dB line-pad. (For example, your external mike preamps have similar self-hiss and will usually run more than 20dB gain.) The distortion of the transistor-pair plus the several opamps is quite low.

You also get the dandy VR1 gain control. You can accept inputs which are well off of nominal levels and trim them so the mix faders can all ride near the same level.

The one-opamp differential/balanced inputs work, pretty OK. But when the going gets tough, they don't do very well. For balanced line inputs I've always used transformers for the real tough cases, and 2-opamp plans for compact transparency.

You "could" design-out the transistors. Two ~~50K resistors into the IC1A inputs. You need IC1A to take the difference signal. You need IC6A to supply a balancing feedback signal (the two 36K resistors). And you want IC6B to force the DC difference to zero so you don't get scrathy and thump as you work the controls. You would lose the VR1 gain trimmer, because it needs the extra nodes the transistors bring. And the summing noise of the TL072, of many chips, is higher than the noise of these transistors, maybe high enough to matter.

I think you should use the Inserts; or when ground-buzz is a problem, use the existing balanced line input, and Make Music.

> 15-17V @ 3A

That's +/- 15V.

> I pulled a huge old transformer (didnt check the specs on it yet) from my dads old Kenwood amplifier

That's +/- 40V or so. Mondo hassle getting it down to size.

3 Amps? Really?

I have an old ('81) SC 800 with 1/4 balanced patch bay that I've used for years and it's been a champion in regards to rejecting ground junk/buzz issues.  It seems to interface well with everything I use.  For the money they are really good sounding consoles.

The little blue or gold electrolytics will likely give you some problems as they drift.  Each channel will sound a little different tonally and in noise level.

The master buss mod is probably the best place to start.  It will sound like you just pulled cotton out of your ears and the noise floor will drop considerably.

The stock EQs are . . . eh - ok I suppose.  They've always been a bit too phasey sounding to me.  The one modded channel on my 800 has a much improved and pleasant to use EQ.

OK it seems that maybe just using the inserts may be the best afterall? I suppose I can do eq in my DAW  before I send out to the board.

@Lassoharp:

-Do you use your 800 with the factory power supply or built your own? Happen to know how much current it draws?

-What all does the master buss mod include? Upgrading the caps and such to be back like new, or changing the design?

templemark said:

OK it seems that maybe just using the inserts may be the best afterall? I suppose I can do eq in my DAW  before I send out to the board.

@Lassoharp:

-Do you use your 800 with the factory power supply or built your own? Happen to know how much current it draws?

-What all does the master buss mod include? Upgrading the caps and such to be back like new, or changing the design?

Click to expand...

I use the factory PS which is the rack mount version.  I don't know current draw figure for the entire supply - Just bits and pieces. The phantom supply is rated for 100ma @ 48V.  There's 3 rails @ +-17V audio, + 24 for logic and the phantom. 

The recommended load for fault checking is a 20ohm 20W for the audio rail, 47ohm 5 W for the Logic, and 2.2K 1 W for the phantom.  That would be for 18 input modules plus 8 buss and 4 auxes.  This should give you a safe margin to reverse eng from.

Jim's master buss mod includes all new caps, op amps, transistors and a redesign to extend bandwidth and lower noise.  The last time I spoke with him he did mention the option of just ordering the parts bundle and mod schemos from him which would save some labor cost.

He does build custom supplies for these as well.  I don't know if he offers a 'kit' deal on those but it may be worth asking. 

templemark said:

I didn't think about that! however, i believe the inserts come after the eq which I would like to use.

Click to expand...

Look more closelier at the schematic. The insert is right after the low-pass filter, before the EQ. This is pretty standard.

-a

Andy Peters said:

templemark said:

I didn't think about that! however, i believe the inserts come after the eq which I would like to use.

Click to expand...

Look more closelier at the schematic. The insert is right after the low-pass filter, before the EQ. This is pretty standard.

-a

Click to expand...

? well maybe you should have another look. i didnt include the full schematic, but the insert is actually after both the eq and low-pass filter...,,i was hoping to use the eq ,thus my original post..? .. lassoharp made the point its prob easier just to sacrifice the eq for the insert input and i am thinking i may do this.

but would anyone have a suggestion in the case i wanted to mod it for balanced line input? would the voltage follower idea i had above for un-balanced even have the correct input impedance?