BeePre Hiss/White Noise

[datsun510]

Hi All,

I just completed building my BeePre and after checking voltages and resistances, everything looks and sounds fine; however, I have a fairly loud (ie. audible from about 12ft away) hiss that sounds like white noise coming from both speakers. The hiss is constant and steady and is not affected by volume. With music playing it sounds great. I've isolated the noise to the BeePre by swapping components with another system. There is a bit of 60hz buzz too, but I was able to mostly get rid of that by using a cheater plug.

Any ideas where to start looking?

Thanks

[Doc B.]

What amp are you using?

[datsun510]

I'm using Bryston 3BSST amps, around 29db gain I believe

[datsun510]

After doing some research I think I need to attenuate via an L pad at the amplifier input. I'm a noob at this but want to try and make my own rather than purchase the inline ones from Parts Express, the Harrison brand. I found on the forum that the impedance on these is around 10k and this may be unsatisfactory.

In looking at L pad calculators to determine resistor values, should the desired impedance be that of the amplifier's input (50k in my case)? Or does it even matter so long as it isn't too low? ie < 10k?

[Paul Birkeland]

If the L-pad is right at the input of the amplifier, then the impedance isn't such a huge problem.

Since the amplifier has a 50K input impedance, you can pop the hot wire off each of the RCA jacks on the inside, then solder in a pair of 150K resistors between each wire and the center pin of the RCA jack. (They don't have to be exactly 150K, 125K-175K will be close enough)

This will give you 12dB of attenuation, which seems like a reasonable place to start.

-PB

[datsun510]

Just adding an update and another question here after using the preamp for about 20 hours.

Attenuating the amplifier gain worked well to bring the amp down to usable levels with the BeePre... no more noticeable noise floor.
Wound up at about 18db gain. Currently paired with a Hypex Ncore amp and the sound is phenomenal.

I have run into one issue however with one of the kit supplied 300B tubes. After the amp is warmed up, the tube begins to glow blue on one side and flicker intermittently, say once every couple seconds. When it flickers, the corresponding channel makes a click noise. The problem follows the tube. Is this a dead tube?

Thanks

[Paul Birkeland]

Is this flickering constant, or just during warmup?

If you pop the tube out and jiggle it in your hand, do you see anything moving that's attached to the top mica wafer inside the tube?

[datsun510]

The flickering doesn't show up until it's warm, after say ~30 minutes. When it does start glowing and flickering, it does it pretty constant every 2 or so seconds.

I don't notice anything moving inside the tube.

[Paul Birkeland]

I might consider leaving the BeePre on overnight, just to see if there's some goop on the filament that needs to cook off.

Are there unusual voltages that follow the tube also?  (across the paralleled cathode resistors perhaps)

[datsun510]

I've tried leaving it on for an extended time before, but the same thing seems to happen. There are ~20 hours of on-time on the build. Is this enough to let the tubes settle in? Next time I will check if there are funny voltage readings.

With my other tube gear I wouldn't hesitate and just replace the tube, but the 300Bs are pricy and this one is new. Doc, is there a warranty on the EH tubes?

Thanks for your help PB, it is greatly appreciated.

[Doc B.]

You can return the tube to us for a replacement. Do let us know that it is coming so we can watch for it.