Soundcraft 800B grounding

[dualflip]

You can never have enough power supply decoupling, either way it will benefit your audio, the cleaner and better power you provide, the cleaner and better audio you will have, simple as that, which needs not only having a good amount of decoupling, but a great power supply in general.

Noise on the power supply rails can be very subtle, or can be more drastic as to make your opamps oscillate or create false digital states, etc... A scope or an RTA/FFT can be used to determine the amount of noise present on the voltage rails.

The best thing you can probably do, is get a better power supply.

[Ike Zimbel]

Hi James,
These are not filter caps, but de-coupling caps as Dualflip pointed out. By putting them at the power supply input in the console end, you effectively take the PS cable out of the equation and provide a nice low-impedance source for all of the modules. All high end consoles, Neve, SSL etc are fitted with these caps. A Neve V-3 for example has 2 x 10,000uf per bucket of 12 modules across the +/-16v rails.
Every smaller console that I have added these caps to have benefited from the mod. A while back I was called into a club in an old movie theatre here to sort out some "dimmer noise" in the monitor desk. After checking all of the usual suspects, bad power, bad grounds, multiple neutral bonds, all of which needed some work, the Aux sends on the desk, an A&H ML-4000-48, were still noisy. The noise floor was around -71dbu on Aux-1 and -66dbu on Aux 10. After adding 2 x 2200uf LESR caps across the +/-17 volt rails, 1000uf across the 12v rail and 100uf across the 48v rail, the noise floor dropped to -81 to -82 across the 10 Aux sends. I did the same to several other ML Series consoles with similar results. There was also someone on this forum, a while ago that had similar results with a Soundcraft 600 after I PM'd him this fix. I would suggest no less than 4700uf per rail if starting from scratch.
Best,
Ike

[jnashguitar]

Wow--that's exactly the kind of information I was looking for. HUGE thanks, Ike!

OK, I'm convinced--I won't bother with alligator clip experiments, I'll just add the caps :-)

Is the physical location important ? For instance, better to connect the hot side of these caps near the PSU jack or near the ribbon that feeds the modules ? And is it important to ground the caps directly to the star point, or OK to connect to the four ribbon grounds (which now all run separately to the star point) ?

I ask because the most convenient place to mount large decoupling caps would be near the ribbon transition board, which is near the modules but roughly 1m away from the PSU jack and star point. Does that sound reasonable, or is it worth trying to cram the caps closer to the PSU or star point ?

Can't thank you enough for all your advice, Ike!

-James

[Ike Zimbel]

By the transition board is fine. Closer to the electronics is a good thing.
Best
Ike

[jnashguitar]

Great, thanks, Ike!

I've got a bunch of 4700uF/50V caps lying around, so I tried putting one on each of the four power rails. The effect on the aux sends was outstanding: dropped their noise floors by 10dB!

I was disappointed to see no effect on the master, however. That might imply there's more decoupling already on the master ? But looking at the schematic I don't see that's necessarily true, and at this point I'm getting 8-10dB better signal to noise on the auxes than the master. Is this an indication something's wrong ?

-James

[jnashguitar]

Thanks for the advice, Jim! Makes great sense: the intake caps are a quick way to improve the whole board, but since I'm recapping the modules anyway, I'll upgrade the decoupling on each module.

Looks like the fusing resistors are 47 ohm on my modules--presumably that means more filtering from any given cap ? Any reason to change that resistor, or is 47 ohms fine with a 1000 uF decoupling cap ?

Thanks!

-James