[SHIF]

Using a scope I measure tens of millivolts of 120Hz noise at various points in my Cornet, from the outputs up stream to V2 but not beyond this point.  My system is extremely revealing and this hum is definitely audible, even at reasonable listening levels, so I wish to fix it.

There is no noise in the circuit leading up to V2 so I think the problem is in my power supply.  Doubling C4 and C10 helps a little but this is not the solution.  I've tested with several rectifier tubes and not found a cure.

Anyony try the Weber Copper Cap rectifiers yet?
These look interesting:

http://www.webervst.com/ccap.html

Now I'm thinking it may be a bad tranny.  Is the 370 less prone to humming that this BX270?  Looking for advice from you "hum busters" out there...

Through all this effort I'm still loving the great music that this thing produces.  I have my iRIAA filter hooked into my tape loop function so I listen to the Cornet often.

-SHIF

[hagtech]

Sure, you can change C2 and C4, but I don't think it will make a difference.  Most hum is caused by the relatively high currents in the heater circuit.  Or by input wiring and RCA jacks.

Measuring low signals with a scope is not easy.  What you see may not really be there.  Probes often pickup or inject more noise than you realize.  Differential probes help.  Watch where you put the ground.  DVMs are even worse.

jh

[eweitzman]

I have extremely sensitive speakers and low power DHT SET tube amps. It took about 50,000uF of filtering -- 2nd cap in pi filter on the 5V heater supply -- to reduce the hum on the speaker outputs to less than 1mV. This is just barely audible a few feet from the speakers.

A local friend brought his Cornet to my house on Saturday. With my passive linestage's pot set up high, the hum from the Cornet was very loud, louder than my solid state Yamaha C-2 (which I use for MC phono duty). It seems that the Cornet's filaments can be too noisy.

If you add more filter capacitance to reduce the hum, make sure to put it after R21. With excess capacitance before R21, the inrush current drawn by the first cap bank on turn-on may exceed the current ratings of the transformer.

- Eric

[SHIF]

I’ve increased caps in the B+ and heater circuits in another effort to eliminate the 120Hz ripple that comes through as audible hum:

C1 & 3 = 1uF (metallized PP)
C2 = 90uF
C4 = 200uF
C15 = 40,000uF
C16 = 40,000uF

C10-C14 = original values

Still humming, actually slightly worse with C15 and C16 bumped up to 4X stock value.  OK now I’m starting to think a new tranny is in order and also returning all the electrolytic caps to original values.  Or perhaps maybe I’ve got some bum diodes.  More scope measurements required.

Still seeking ideas from others who have chased down hum.

A side benefit of all this, I’m getting quite handy with a soldering iron.

-SHIF

[hagtech]

Can you post a photo of your Cornet chassis?  Often the wiring is a source of hum.  The circuit itself can be built extremely quiet.  The first batch of Cornet boards required a "hum fix" that involved C13 and C14 sharing the same ground hole.  That has been fixed on second batch.

Changing circuit values will probably get you nowhere.  In terms of hum, anyway.  Also, keep in mind that first 12AX7 needs to be a quiet one.

jh

[SHIF]

Here is a shot showing my stack of caps.  Hot melt glue holds them in place.  
I was wrong thinking these did nothing, my scope shows the ripple smoothed considerably. Listening to vinyl confirms positive, albeit limited improvement.  These caps can stay for a while:

http://www.fototime.com/A29464699920F2C/orig.jpg" border=0>

As another experiment, I did a radical star grounding mod on the heater circuit.  Cutting many traces, I lifted all of the 6.3v ground points and tied them to a single bus bar that can be either isolated or tied to the ground plane at a single point on the board.  This was an interesting exercise that yielded little sonic variation.

I’m willing to try almost anything, that’s what this project is all about.  Meanwhile the darn thing continues to make wonderful music.  I replaced my Mullard 7025s with Raytheon 1959 blackplate AX7s.  These seem a little quieter.  Overall I’m still a happy enthusiast.

-SHIF

[hagtech]

The pic looks fine.  Nice clean work.  Everything seems to be in place.  Did you use insulated RCA jacks on the input?  Is that panel black anodized?   And are the standoffs metal?

Star grounding the heater circuit is a good thing.  Hard to do on the board.  Cornet2 improved in this area.

jh

[SHIF]

Thanks.  The rear panel is milled from black Delrin plastic so all the connectors are insulated.  The inputs leads are made using cheap coax video cable, the shield being used as the ground lead and soldered at both ends.  The PCB is sitting on quarter inch metal stand-offs.  The ground lug has a wire running to a corner mounting screw since I also cut the ground traces from the mounting holes in one of my experiments, oops.  I’ve checked the ground continuity at many locations and it is fine.

The underside of the board really looks like a science experiment with bus wire running between parallel caps and also the isolated heater ground and the LED leads.  Teflon tubing insulates all discrete wiring.

-SHIF

[hagtech]

SHIF,

Maybe I missed something, where does the circuit board (and RCA shields) connect to chassis ground?

jh

[SHIF]

Jim,
The circuit board ground connects to the chassis through the mounting screw seen in the lower right corner.  I cut the ground plane traces around the mounting holes during a past experiment so I use a wire from under the screw head to the ground lug (between the input jacks).  It is visible in the photo.  I verified ground continuity with my DMM.

-SHIF

[SHIF]

Hum be gone!
After playing around with caps and ground paths and a few other modifications, I finally rewired my I/O leads as per Jim’s original reply to my initial post.  I simply swapped the locations of the input and output jacks which keeps the input wiring well away from V1 in my particular configuration.  The net effect, the hum is reduced to miniscule levels.  I’ve not yet scoped the unit to quantify this change, I’m too busy enjoying the music now to even bother.  I’m going to keep the additional caps for the filaments and B+ as these help smooth out the power supply ripples, whether audible or not, I like the idea.

-SHIF