New BeePre2 hum troubleshooting

[cacofonix]

I've just completed a new BeePree2 build and hooked it up to discover the dreaded hum.

Currently going through and trying to find bad solder joints and the such and trying to reflow as many as I can. However I haven't been able to eliminate the hum/buzz yet

I've attached some build pictures to see if keener eyes can help me figure out whats going on.

- Hum is from both channels
- Hum increases with volume
- Preamp is hooked up to various solid state amps and speakers - hum exists on all of them
- Passed on voltage / resistance checks in the manual

I've also included a mov file with the audio of the noise if that helps

Thanks for your help

 

[Paul Birkeland]

How are the resistances and voltages?  Did you end up getting this with the two extra upgrades? 

There is a minor mod we can do to knock that down, but the same effect can be obtained by putting in a portion of the second upgrade module.

 

[cacofonix]

Resistances and voltages were all within manual spec. I’m not at my desk right now but will send them along as soon as I get back.

I did get both upgrade kits but per instructions I was waiting to install them till I get the stock kit right.

Paul, am I to consider this amount of noise normal for the Beepre?

 

[Paul Birkeland]

Can you open up the second upgrade, then perform the steps on pages 10-12, then 21-22 (up to installing the pair of 220 ohm carbon film resistors).

This will give you the 6CM7 sockets and heater wiring, and won't interfere with the first upgrade when you get around to installing it, but it provides an extra ground reference for the third heater winding which can be helpful in nudging the noise down a bit. 

 

[cacofonix]

As requested

Voltages and resistances (recorded while building the kit)

Incoming AC = 122 - 122.3 V

PT Secondary
terminals 7 - 8 = 192.4-192.5 V
terminals 9 - 10 = 6.56 - 6.57 V
terminals 11 - 14 = 6.56 - 6.57 V
terminals 12 - 13 = 6.56 - 6.57 V

High Voltage Power Supply Test
31L - 33L = 260.2 V DC

Filament Voltage Supply Test
Left side = 6.3 V DC
Right side = 6.3 V DC

Resistance Checks
4 - A4 = 446.8 Ohm
4 - A1 = 446.1 Ohm
14 - B4 = 447.5 Ohm
14 - B1 = 447.4 Ohm

DC Voltage Tests
14U - 25 = 9.73 V
14U - 30 = 10.22 V
14U - 6 = 102.6 V
14U - 16 = 100.1 V

All of those looked normal per the manual.
Note: I have not measured all of these again after completing the kit, but will do so if you think it useful.

So, continue with installing parts of the second upgrade you think without debugging further?

 

[Paul Birkeland]

Yes, I think it's worthwhile to pop the 9 pin sockets and the minimal amount of wiring in just to see if it helps.

 

[cacofonix]

I just finished the steps you recommended from the output upgrade kit and you are right - it nudges the noise level down significantly.

Just for clarity, this was the last step I performed
"Attach the twisted leads of the 220Ω resistors to 48L. Attach the free lead of one resistor to C4. Attach the other free lead to C5. Solder C4, C5, and 48L."

Thank you so much for your help Paul! I'm going to enjoy some tunes tonight

One follow up question
- There noise still exists - especially at higher volumes (anything past 12 on the volume). While its a whole lot quieter, its not totally quiet.
- Are there further steps I can perform to drop this down further?
- Will the upgrades improve this further?

I've attached a phone recording from right next to my speakers

 

[Paul Birkeland]

The upgrades will further drop the noise floor. You should find that at those high volume where you can hear some noise, that this is not a realistic listening level (if you tried to play music at this listening level, it would be unreasonably loud).