dBu Madness or Help my Soundcraft Auto32!

Help Support GroupDIY:


Well-known member
Sep 21, 2004
Hello Every-body - I'd like to ask for comments or suggestions on my
'real world' noise measuring attempts !

Thanx in advance for ploughing thru my long post :grin:


I've been working on 'noise floor measurements' of my Soundcraft Spirit Auto32 and connected gear for a couple of weeks now.

Using a software RTA with freq spectrum and dBu display and a PC with a Motu 2408MKII balanced analog inputs and outputs. (Should be high accuracy)

SO - I calibrate my RTA with 0dBu (300Hz sine) = 0.776VAC rms (voltmeter checked) at the Motu and start measuring various things : first loop back on Motu then plugging the Motu to the desk intputs and outputs.

My Motu shows -84dBu on loopback with mains harmonics (50Hz) primarily the noise, as well as some hi-freq build up. All below -100dBu and with a cumulative reading of -85dBu.

Disappointing but acceptable. :roll:

THEN continue to my desk. Calibrate the balanced input and (ground compensated outputs - btw wtf is that?) outputs of desk with Motu RTA to 0dBu = 0.776VAC such that I'm confident my levels are consistent. (input trims at 0dB gain and outputs generally at min or unity).

It all lined up really well - everything consistent according to my 'real world' test (I'm no expert in the very technical world of standardised noise measurement!). My RTA and desk tone generator and desk goup and mix LEDs were all reading well at '0' (LEDS go up to '+18')

My direct channel outs did well at -81dBu and , noise components
were mains harmonics mostly each below -90dBu.

OK, all basically good.


NOW the problems!

I looked at the group busses : -74dBu with pretty big mains harmonics each below -80dBu. This with no channels assigned and bus faders at unity. Not sooo bad but not real good either! :cry:

Then the mix bus : -77dBu again similar shape as buses - mains noise.
No channels assigned, mix faders at unity. Slightly better than group bus but again, no great shakes.

AND then the real horror! :twisted:

I started methodically assigning channels to the group bus (channel faders at unity, connected to Motu but with no signal)

Each one added pretty horrendous noise : 2 chns assigned, group = -69dBu, 4chans assigned = -66dBu and so on to a worse case of -60dBu

Ouch! What no usable busses??? :evil: :evil:

Same on mix bus when assigning channels. Drops to -65dBu worst case.

WTF?? :shock:

So if you will - help by perhaps relating some of your experiences. My mates tell me 'you can't hear it, so what the hell does it matter? Stop measuring stuff!'

:?: : what would the main contributor of mains noise and bus-assignment noise in a reasonable quality built analog desk in good but well used condition ???


what is wrong with me??? Can I be rehabilitated? Don't laugh! Once I was a musician.


I also tested in another room with nothing except RTA connected and no possibility of power cable/signal cable interactions. Was slightly better but still same trends.

Phew! I;ve said my peice and I thank you.

PS - I could provide RTA shots!! :twisted: