LC EQs and Hum Pick Up

[thermionic]

Hi,

I made an LC EQ for a friend a couple of years back (designed by an esteemed designer at a firm I was involved with). He uses it live, in insane SPL situations. All was fine for the first year, but he's started getting hum pick up with it in recent weeks. However, this pick up seems to only occur in particular venues - noticeably ones in the US. I can't see why a venue in the US would be any worst than one here in the UK? Not all US venues are a problem - some are fine. I've had the EQ back here several times and, on each occasion, it's measured approx 110dB S/N ratio. I can't find a fault. It has a gnd lift switch, and bal / unbal toggles (it's transformer I/O) in case it has to interface with anything that isn't balanced (they simply short the cold leg to gnd - I've told him to experiment for least noise).

It's in a steel case and weighs a ton (literally). The trouble I have is not being able to mimick the situation in my workshop... Flying to the US, on the premise that it *might* hum, is an expensive undertaking. He's using it with a 120v-to-240v converter transformer.

He's also claiming that it has some microphonic-type behaviour. During heavy bass transients from other gear (i.e. no signal present) it hums, but is silent during less bass-heavy passages.

The only ideas I have at the moment are, to supplement the shielding with thick alloy, and replace input transformer (it's a 600r-600r which is unshielded - at line level, I can't see the shielding being an issue...?).

???

Many thanks in advance.

Justin

 

[okgb]

different mains frequency 50hz Vs 60hz
but one wouldn't think that the problem

you've tested this at the insane levels he uses it at
[ or nearest facsimily ? ]

 

[abbey road d enfer]

He's also claiming that it has some microphonic-type behaviour. During heavy bass transients from other gear (i.e. no signal present) it hums, but is silent during less bass-heavy passages.

Think of a transformer that draws more power when there's a lot of bass... There must be an amp, or a step-down transformer that's too close to your EQ.

 

[thermionic]

He claims it's happening when the EQ is away from any other gear, in isolation. He also says that, now the EQ's back in the UK, it's perfectly quiet... He managed to run the noise into a spectrum analyser - see attachment. Looks like multiples of 60hz... The PSU has a large 6,800uF cap for the unregulated supply (it's +24v SE supply) - I've had it on my scope, and with it set to 1mV sensitivity, I can't see anything vaguely resembling ripple.

This is really getting me down... I'm drawing a total blank. An alloy case should give better shielding for low frequency hum, and I could shield the I/P transformers - but it's a lot of time + expense to get no result...

:'(

Thanks,
J

 

[abbey road d enfer]

Another related possiblity, which would also explain the occurrence of noise in loud passages and the noise spectrum: if the pre-reg voltage is somewhat low, the reg is out of range; the more power is drawn from the system, the lower the input voltage and then there is hum in the rails.
Can you chech the  pre and post reg voltages? Maybe you could use a Variac to try reproduce the problem...

 

[thermionic]

Abbey - I think you've nailed it... When VRegs have insufficient input voltage, they emit a large triangular waveform at twice the mains frequency. This would certainly explain the frequencies shown on the SA. I always rate my secondary values conservatively as I hate companies that cook the Vregs (I could show many photos of scorched PCBs due to this - it's a fire risk).

When I was in the US a while back, overseeing installation of some gear, I observed a mains voltage of 128v AC... This has meant the heatsinks in the PSUs have gotten pretty hot. You just can't win, can you?

Many thanks!

Justin