BeePre 2 Hum [resolved]

[Rosinante]

I just completed a BeePre 2 build.  All the tests which are part of the assembly procedure were performed with proper results.  When I connected the BeePre to my amplifier, there was a loud low hum.  60Hz is my guess. 

I could hook this up to low-powered gear that I do not care about, in my shop/garage, and conduct some testing.  To see if it is volume-dependent or channel-dependent, etc.  To see if music passes through it.  That sort of thing.  But I also want to avoid damage to the BeePre.

Should I put it on my bench and connect it to a low-powered amp and speakers, and do some testing of audio symptoms?

 

[Rosinante]

I failed to mention:  As the tubes warmed up, and perhaps ten or fifteen seconds after powering the BeePre on, I heard the loud hum and immediately turned it off without touching anything.  I think the volume knob was rotate fully counterclockwise.  I am itching to hook it up again and experiment/gather info about symptoms.  Is this the appropriate next step?

 

[Paul Birkeland]

How do the DC voltages look? Can you post some build photos?  Can you tell us about the power amp and speakers you tried?

If the volume knob is rotated fully CCW, that completely shorts the output of the preamp, and if there's hum under these conditions, that is a problem that will require attention!

 

[Rosinante]

It's Jim, Paul.  I mis-spoke:  I now think the volume knob was rotated fully clockwise.  Apparently, the guy who cannot distinguish a BeePre plate from a Kaiju plate also struggles with clock motions.

Here are a couple of pictures.  I am hoping for permission to hook this thing up and gather more information but will absolutely follow your instructions.  And I think you work too hard.  My problem is not urgent.

 

[Rosinante]

BTW, I checked resistance between the + and - connections at one of the output RCA jacks.  With the volume knob rotated fully clockwise, I saw 9.56 kOhms.  Rotated fully counterclockwise, I saw 1.5 Ohms.

 

[bernieclub]

Maybe a stupid question, but do you have a source connected? I've had some nasty hum from a disconnected cable.    Merry Christmas!

Bernie

 

[Rosinante]

Yes, there were three sources connected.  If I test it again, I will see if the selector switch changes anything.

 

[Rosinante]

How do the DC voltages look? Can you post some build photos?  Can you tell us about the power amp and speakers you tried?

Oops.  I skipped some questions.  The voltages measured during assembly were within spec.  If we need a voltage check somewhere, then tell me where.  As you know, Paul, I have little understanding of electronics but I can generally follow instructions.  The power amp was a pair of Leak monoblocks using EL84s in PP configuration.  Speakers were Klipsch Cornwall.

 

[Paul Birkeland]

In general I would recommend more solder.

With the volume control turned all the way down and almost a dead short across the output jacks, the presence of hum may indicate issues that do not exist in the BeePre.

 

[Rosinante]

I may have found the cause.  At least one mistake was made at the selector switch.  Stay tuned.

 

[Rosinante]

In general I would recommend more solder.

My soldering skills are poor but improving.  Some connections have more solder than others.  All of them are actually connected however, and ought to be functional.  If you feel I should reflow connections and add solder then I will do that.  I am hoping to fix the hum problem first.

With the volume control turned all the way down and almost a dead short across the output jacks, the presence of hum may indicate issues that do not exist in the BeePre.

To be clear, the resistance measurements I posted were not between two jacks.  They are resistance measurements made between the + and - terminals of a single (right side) output jack.

I found a mistake, and have corrected that.  I had put the wrong wires on terminals 2 and 3 of the selector switch.  I am now tempted to connect this BeePre to a test amp and speakers to see if the hum problem is addressed.  Any concerns?

Your help is much appreciated, Paul.  As always.

 

[Rosinante]

How do the DC voltages look?

Between terminals 31 and 33, there is 256 VDC.

On the DC Filament Supply board:

Right side Reg + and Reg -    6.28 VDC
Left side Reg + and Reg -        6.22 VDC

 

[Rosinante]

Terminal 25 - 0 VDC
Terminal 30 - 0 VDC

Terminal 6 - 253.9 VDC
Terminal 16 - 253.8 VDC

All these voltages are with tubes removed.

 

[Paul Birkeland]

Why don't you bring it by when the snow melts a little.

 

[Rosinante]

Alright.

 

[Rosinante]

This problem was resolved by the patient coaching of Paul Birkeland.  It was found to be a combination of builder error (me) and also user error (also me).  This unit is now dead quiet and wonderful-sounding.  Very transparent.  Excellent midrange detail.  I can't wait to finish the Kaiju build.