High-pitched buzz when using generator power

[Mottlefeeder]

I tried searching under generators but found nothing. Please point me in the right direction if this has been covered before.

I just spent some time playing in a band in a field. We started playing before the generator arrived, so I used a 150W invertor with a caravan battery to power my Hartke HA3000 (200W into 8 ohms - 10 Amps peak drawn from the battery).

I noticed that I had a high-pitched buzz, which disappeared when I touched the strings, so I tried connecting the signal-cable-ground directly to the invertor-case ground-lug. It made no difference. Turning down the tone control on my passive bass got rid of the noise.

When the generator arrived (8kVA on a handcart), I thought that my problems would go, but I still had the buzz, and I could still get rid of it by touching the strings, or turning down the bass treble cut.

As far as I can tell, the generator output was floating, and the generator chassis was earthed, since my portable RCD/GFCI refused to allow power up, because it could not sense an earth connection. Again, as far as I can tell, the Hartke audio circuit is totally separate from the mains and earth circuits - effectively, it floats too.

What do I have to do to get rid of this hum?

 

[Psycho Bass Guy]

The buzz isn't a grounding issue. Generators put out peak to peak voltage of 120 volts, but it isn't a pretty 60Hz sine wave like you're used to from your wall. It looks more like a sawtooth or square wave, with jagged peaks and irregular waveforms. Most amp rectifiers, which are designed for a 120 volts RMS sine wave, end up passing large amounts of the waveform through to the filter caps which also are not designed to handle such line noise. The harmonics of the power noise ends up polluting your audio signal, because it is modulating the DC supply rails of your amp.

The reason the noise 'disappears' when you touch your strings is that YOU act as a filter between the noise and ground, the same as the tone cap in tone control. Get yourself an isolation transformer and your problem should go away. Also be VERY glad your amp has s/s rectifiers and not a tube rectifier, or you'd hear some REALLY nasty stuff.

 

[i_got_a_mohawk]

Would a power conditioner not do the trick Psycho ?

 

[Psycho Bass Guy]

If it is a TRUE voltage regulator, which most conditioners are NOT, it COULD work, but even some true voltage regulators use a rectification/ modulation system which could actually make things worse.

 

[steveksux]

I doubt the power conditioners would help. They may fix any voltage irregularities, but the problem is the waveform, not the voltage.

Randy

 

[Psycho Bass Guy]

Agreed, but there are some voltage regulators that use transformers for regulation and they'd be OK. Problem is, they're not going to be your typical rack mount power strip type, and will weigh more than the amp they're feeding, not what most gigging players want to hear. They'll be pretty expensive, too.

I think the best bet is to watch eBay for a single A/C line isolation tranny. It'll weigh about 10-15 lbs and should cost about $10-15 if you're patient.

 

[i_got_a_mohawk]

If it is a TRUE voltage regulator, which most conditioners are NOT, it COULD work, but even some true voltage regulators use a rectification/ modulation system which could actually make things worse.

I wasnt too sure, im not too "big" on what they are anyway, i just know they help get out excess noise, but i spose that will be with noise coming from the right waveform etc

 

[Mottlefeeder]

The buzz isn't a grounding issue. Generators put out peak to peak voltage of 120 volts, but it isn't a pretty 60Hz sine wave like you're used to from your wall. It looks more like a sawtooth or square wave, with jagged peaks and irregular waveforms. Most amp rectifiers, which are designed for a 120 volts RMS sine wave, end up passing large amounts of the waveform through to the filter caps which also are not designed to handle such line noise. The harmonics of the power noise ends up polluting your audio signal, because it is modulating the DC supply rails of your amp.

The reason the noise 'disappears' when you touch your strings is that YOU act as a filter between the noise and ground, the same as the tone cap in tone control. Get yourself an isolation transformer and your problem should go away. Also be VERY glad your amp has s/s rectifiers and not a tube rectifier, or you'd hear some REALLY nasty stuff.

I can see that an invertor will put out a stair-case waveform that approximates to a sine wave, but I would expect a generator to do better, since it is basically coils on a shaft being rotated past each other. However, it is not my field
Either way, if the problem is noise on the supply, would a domestic-supply cleaner (e.g. for a pc) have any effect? I think that they use inductors to kill the radio frequencies, and capacitors to shunt out everything else.

 

[nashvillebill]

(professor voice on) There are two versions of AC generators.

The newest (and least-costly) actually generates DC then uses an inverter to produce AC. This waveform is not going to look very pretty, it's a simulated sine-wave. This could easily account for noise in equipment.

The traditional method uses a speed-regulated gas or diesel engine that turns an alternator. This method does indeed produce a true sine wave; it's just a smaller version of the same technology used in hydroelectric or fossil fuel power plants. Generally this is a pretty clean signal, since there isn't miles of wiring and other equipment on the same power grid.

 

[Mudbass]

I think the problem lies with the generator not producing clean power. I played a number of outdoor gigs years ago and we always rented a big diesel powered generator, the kind that come on a trailer. We had a Hammond B3 in the band and the tone wheels on a B3 are motor driven so any fluctuation in voltage or frequency was totally unacceptable because it would throw the whole organ out of key. We never had any problems of any kind using the larger generators.

 

[Jerrold Tiers]

Inverters do produce dirty power unless filtered more than usually done. They make a buzz in most audio stuff. My 2.5 kVA inverter does for sure.

Most generators will be plenty clean enough, although there can be an issue with "brush noise". That tends to be a sort of ragged "whine" noise.

Newer ones can have an issue with the circuits used to regulate the output voltage. That is an internal function which may be done with something like an inverter, causing noise.

The inverter output type is a new one on me, but obviously would have the problems of any inverter.

The bigger the generator, the more likely it is an "old style" brute force clean one.

The small consumer ones (10kVA and below) are made whatever way is cheapest. These days, electronics is cheaper than "big iron", so....

 

[Mottlefeeder]

Inverters do produce dirty power unless filtered more than usually done. They make a buzz in most audio stuff. My 2.5 kVA inverter does for sure.

Most generators will be plenty clean enough, although there can be an issue with "brush noise". That tends to be a sort of ragged "whine" noise.

That pretty much described what I heard, both in my invertor set-up and with the 8kVA generator.

So, would an interference suppressing delta capacitor network between the generator and the amp get rid of it?

 

[Jerrold Tiers]

It might.... what it probably really needs is some form of "L-C" filter network, involving both an inductor and a capacitor per line.

An inverter can even knock out AM radio reception nearby, not that that is much of a loss in general, unless there's a baseball game on.

 

[Eric Moesle]

Those small "cart" type generators are made more for running lights and power tools, NOT to drive sensitive audio equipment.

Any time my band needs a generator, we rent the truck-mounted or vehicle tow-behind type that run on diesel fuel, its enough to run the entire band plus PA, and we've never had a single problem.