Dificult problem with buzz noise...

[Santa]

I have Krell KSA-150 amplifier which work with Class Audio 6R preamplifier.

It have buzz noise in speakers. If I connect amplifier alone without inputs than I have just a bit of that noise in both speakers. On 1 channel is a bit more but almost the same. I have to put speaker close to my ear to hear it. Is it normal??

If I short inputs of amplifier I have a bit more noise than when its open.

I replaced all electrolytic capacitors in Krell and it fixed about 50% of noise.

Now if I connect and preamp than I have more noise.

If I connect that preamp with another aplifier I do have noise but almost not noticable...

I checked ground lines of both of them.

Because I am not in audio too much it would be nice if somebody can explane me how to localise what unit cause the noise and what to check.

I had similar problems when power cord is bad so im thinking maybe relays in Krell does not make good contact and it cause noise..

If you guys need more informations let me know.


Thanks

 

[gyraf]

Santa,

There's no simple answer to your question. There will always be SOME noise - but wether it's an error or not depends entirely on what the unit is specified for, and how much noise there is.

Consult original technical spect for s/n ratio. Measure s/n ratio. Then you'll know if there's anything wrong.

Power amplifiers are NOT easy to alter/upgrade regarding such problems - you may want to have a specialist look at it.

Jakob E.

 

[Santa]

The noise when input is shorted should be at least better than -95db. Using a reference of full power, 150 watts into an 8 ohm load, a 1khz.

How do I measure noise? What is proper instrument for that??

Its easy to bring stuff to specialist but that way I will learn nothing.... :wink:


Cya

 

[bcarso]

[quote author="Santa"]The noise when input is shorted should be at least better than -95db. Using a reference of full power, 150 watts into an 8 ohm load, a 1khz.

How do I measure noise? What is proper instrument for that??

Cya[/quote]

95 dB below 150W into 8 would be an rms voltage of about 620 uV. This would be audible for most speakers with your ear next to them. There is no telling from that spec what fraction of the noise is hum related and how much is hiss. Some medium-priced DMM's could get a reading of this voltage, although many are notoriously error-prone for low level a.c. voltage measurements, so beware. The frequency response is also often very level-dependent.

-95 dB is not very good btw. A power amp ought to do at least -110 or better. This is not usually a practical concern where I live in the city though :razz:

The standard instruments for noise measurement these days are good audio analyzers but these are generally very pricey.

 

[Santa]

95 dB below 150W into 8 would be an rms voltage of about 620 uV.

How did you calculate that?

 

[bcarso]

[quote author="Santa"]

95 dB below 150W into 8 would be an rms voltage of about 620 uV.

How did you calculate that?[/quote]

Look at what the output is that gives 150W into 8 ohms: (E^2)/R = 150, so E^2 = 1200, and E = 34.64V rms. 95 dB below that means that the noise is divided by the ratio corresponding to 95 dB. This is 10 raised to the power of 95/20, or 10^4.75, which is 56.2k. So 34.64V/56.2k = 616 uV.

 

[Santa]

Can Fluke 187 measure that noise?

Also what should be the maximum alowed noise on DC + - voltage after rectifiers?
It using +-80V.

(I checked all GND paths and tight them up!)

 

[Santa]

www.tangentsoft.com

There is a nice low-noise amplifier that can be used to measure noise. Did anybody use it?

Thanks

:wink: