[andrew]

Cornet sounds great but.....

My Cornet hums as well, especially when the pre-amp is cranked up! I changed the grounding as per Jim's instructions and implemented the C13/C14 fix.

However, what's really worrying is that I can audibly hear a buzz if I put my ear to the rear of the board. Another Cornet owner I know says he can't hear this but he's going to check for me.

Could it be the heater diodes? Are they supposed to buzz like that or have I got a problem with one of them?

   thanks,

[hagtech]

Any audible mechanical buzz probably comes from the power transformer.  It usually indicates a dc offset on the ac mains or too much 2nd order harmonic distortion (caused by some other items on the same line).

Can you try a different outlet (another room) to see if the Cornet still buzzes?

Line conditioning is always a plus.  Another possibility is that there is some magnetic interference from a nearby transformer.  You can quickly tell this by moving the orientation and proximity of the Cornet in every direction (and spin).  All it might take is a few inches.

One way to tell if the noise is from internal heater circuit is to turn off the Cornet while listening.  It continues to play for a few seconds via thermal inertia and capacitancein the B+.  If the buzz goes away, then we can start looking inside.

jh

[andrew]

Hi Jim,

  Thanks for the prompt reply. The problem is definitely inside the box, or rather only there when the Cornet is on. I switched the pre to full without the cornet on, no hum, so it's not external pickup. At switch on I hear the valves warming up and then some hiss, then the hum, the hiss dies down and the hum comes to dominate.

  I mounted the TX behind the PCB. In this layout, the box is squarer the line of valves are therefore in front on the TX, RCA's to the left, IEC input at the back right. Basically, much like your layout but the TX is behind the lines of valves and not at one end.

  The TX is therefore some physical distance away from the diodes, so there's little chance of TX noise being mistaken for hum elsewhere. Something around the C13/C14 - diode area is still humming. The 370 appears a quiet as a mouse. Could it be a bad on connection C13/14 , broken diodes or even C11/12? Any thoughts..

  Last night I re-did the grounding, put a filter on the IEC and re-soldered the diodes from the top - but its still there....

  Does anyone else hear buzz around the heater diode area? Please let me know if you do?

   many thanks,

[hagtech]

Andrew, great if you could post a picture of the guts.

Is PCB grounded to case?  Are you using isolated RCAs?  Not sure what the audible sound could be, maybe a bad electrolytic.  Usually it is a magnetic component that acts like a voice coil.  Or a tube.

jh

[andrew]

-- Jim,

   I scoped this up, hum is 50Hz (UK mains freq) and appears after the anode of V2 all the way through to the output. So it's not the diodes as they would be 100Hz. They do still hum tho'! Can't recall peak-to-peak values, it was late last night, but is was in the order of tens to a hundred of mV.

   I have to confess, fair play to your design, the hum is coming from EMF from the TX and must be getting coupled into the circuit. I should have stuck to the original layout. Hum is greatly reduced by moving the tranny well out of the way. Thankfully this box is only a prototype up to get me going whilst I design the final thing.

   I guess with 43db gain it doesn't take much pickup to cause noticable hum. Also I use a high-output MC cart so the signal to noise is already lower than a typical MM - 2.5mV against 5mV for a nominal MM cart.

   Perhaps you could update your instructions to strongly emphasize TX placement and best grounding tecthniques.

   In the end, I earthed the PCB to the case at the grounding post, and then the earth on the IEC to the case at the grounding post. The RCAs are isolated with the ground on each one taken back to the individual RCA ground points on the PCB.  I wondered if this was optimal as it would give a "bus ground" rather than a "star ground" at the PCB"? Would it be any better to ground each RCA back to the central ground on the PCB where the grounding post comes out?

 What other hum busting techniques do you recommend?

  BTW. I'm going to put the TX inside its own box in the new case and shield it with copper  

 
   thanks for the help,

-- Andrew.