Another Cornet 2 completed (longish)

tomw · « **on:** 25 Apr 2006, 12:39 pm »

Folks/Jim

Just completed my Cornet 2 after nearly 5 months. I have a slight hum which is made worse by using Lundahl step ups.

I am now running it without the step ups (MC cartridge) and turning my preamp gain to max.

FYI the mains transformer runs warm (wired for 240V) and my internal voltages are high. Here are my measured voltages (in brackets)
B+ 365 V (382 V)

Left 155 V (168 V) Right 155 V (172 V)
330 V (347 V) 330 V (347 V)
150 V (160 V) 150 V (164 V)
300 V (315 V) 300 V (316 V)
150 V (158 V) 150 V (162 V)

Heater 6.2 V (6.6 V)

I may try reducing the heater voltages and maybe reground my earth lead to minimise the hum. Any suggestions?

Thanks and it sounds dynamic and quiet (hum excepted)

tom w

hagtech · « **Reply #1 on:** 25 Apr 2006, 08:53 pm »

What is the ac line voltage? We can always increase the voltage dropping resistors in the RC supply filters.

What did you use for chassis? Can you post a pic? Hum can come from many places. I need to see what you are doing.

jh

tomw · « **Reply #2 on:** 27 Apr 2006, 01:28 am »

Jim

My AC line voltage is 246 V AC and the B+ today is 375 V DC. The chassis is a Hammond Aluminium 12 x 8 x 2. Am trying to upload pictures but unsuccessful. I think the hum could be my earthing or the earth ground from RCA sockets. I may run the unit without the input from the TT and see if the hum persists.

I will try to upload the images again.

tom w

tomw · « **Reply #3 on:** 27 Apr 2006, 01:34 am »

Please find images at:
http://pg.photos.yahoo.com/ph/twywong_au/album?.dir=/7ca4scd&.src=ph

tom w

troporobo · « **Reply #4 on:** 27 Apr 2006, 05:16 am »

Maybe try twisting opposite pairs of the AC leads, if you haven't already? It's not difficult. There's a thread on this a while back:

http://audiocircle.com/circles/viewtopic.php?=222254&highlight=#222254

hagtech · « **Reply #5 on:** 27 Apr 2006, 06:11 am »

The input wiring should look like the output wiring. Do NOT connect the gnd lug to them. The gnd lug should connect to chassis, the earth on input ac connector, and the earth terminal on the board. That input return (gnd) is for signal only.

jh

tomw · « **Reply #6 on:** 27 Apr 2006, 06:41 am »

Jim and others

Many thanks for your replies. My input and output connections are the same, the ground lug is used only for the chassis ground, the RCA grounds are connected to the pads on the pcb. I know its hard to see from the photos.

I have isolated the hum to the R channel and disconnecting my tonearm leads has stopped the humming altogether - just tube noise.

So I reckon the Cornet 2 is fine but my tonearm lead may be the source of the hum.

More work there then!

Should I replace R220 and R221 to 2K0 or 2K2 to reduce the B+ and replace R223 with 2R2 to reduce the heater voltages?

I may also twist and shorten the transformer leads to make it look neater. All in all an excellent product and a pleasure to build (except for that chassis!)

tom w

Gooch · « **Reply #7 on:** 27 Apr 2006, 01:10 pm »

Nice work Tom I see that you used several different types of resistors very cool. Let us know how it sounds once you get the hum out.

Regards

Dave

hagtech · « **Reply #8 on:** 27 Apr 2006, 10:07 pm »

I'm a little worried about your excessively high and fluctuating power line voltage. Not sure which value to pick to work with.

The obvious way to drop voltage is with the resistors on the secondary. But that doesn't stop the tranny from getting hot. Oh well, it's a good place to start. Best not to run the heaters hot.

jh

mgalusha · « **Reply #9 on:** 27 Apr 2006, 10:37 pm »

Some cartridges are more sensitive to hum than others. I had a Grado for a while and in combination with the Cornet it always had more hum that I liked.

It's since been replaced with a Scheu/Benz and it's very quiet. Both are low output cartridges. The Scheu/Benz is a MC and while the Grado isn't MC it has a low output similar to MC. I installed some Jensen step up's inside the chassis and they've proven trouble free. I also had some Lundahl's and find them very nice as well. Good luck on getting it sorted out, along with your mains voltage.

tomw · « **Reply #10 on:** 28 Apr 2006, 07:17 am »

Hmm

I may have to look into power line conditioners/stabilizers ala PS Audio power plants or even isolation trannys (balanced power?) to stabilize my AC line voltage

FYI my MC cartridge is AT OC9 ML2 and tonearm RB300 rewired internally/externally professionally many years ago with vdh cable. I think this may be causing the hum. I will swap my tonearm with another one to try and see if the hum subsides. Many thanks to all for suggestions and I keep you all informed!

tom w

tomw · « **Reply #11 on:** 7 Jun 2006, 11:12 pm »

Jim and others

I'm still chasing that hum with my Cornet 2:? Recently I purchased another phono stage (Roksan Reference Phono Stage SS from eBay) and installed it in my system and it was dead quiet. Mind you it didn't sound as good as the Cornet 2 plus my 4 tubed 12AX7 based head amp (have abandoned the Lundahls as the hum is even worse). BTW The soundstage was flat with the Roksan

I'm wondering whether installing tube shields over all the 9 pin tubes of the Cornet 2 and my head amp may reduce the hum. Any comments?

Thanks

tom w

hagtech · « **Reply #12 on:** 8 Jun 2006, 04:27 am »

You can simulate tube shields using aluminum foil and tape as an experiment. Easy to check out theory without cutting chassis or buying parts.

jh