More cornet 2 hum questions

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cyto

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More cornet 2 hum questions
« on: 7 Aug 2013, 05:15 pm »
As posted before I have the hum (60hz) at a reasonable level but want to see how low I can get it. This is what I have got:

With the amp volume at 1/2 and no input signal (or inputs shorted) I get 40-41db of 60hz hum right at the speakers. At that level most music is about 95db. This being the only tube phono pre I have owned I wonder if I am being too picky or unrealistic

I have the transformer and all the other grounds going to a star on the back plate (I am using a wood Hammond case with aluminum top plate, steel bottom plate, steel back plate and aluminum shielding on the wood. When I was using the TT ground lug as a star the hum was louder. After that moving the grounds around and trying a homemade ground loop isolator didn't affect the hum level either way.

If I turn the power switch off the hum fades to nothing rather than abruptly stopping.

I did reflow the solder joints just to be certain.

To me it seems that the power supply is the source, maybe magnetic interference from the tranny?

I haven't done this type of troubleshooting since 1976, so I am rusty and apologize if I am being a pita.  :duh:

Thanks guys

David

hagtech

Re: More cornet 2 hum questions
« Reply #1 on: 7 Aug 2013, 05:37 pm »
It's all about wiring and component placement relative to the power tranny.  And preventing ground loops.  Can you post a photo so that we can see what you did?

Eventually you should have nothing but "hiss". 

jh

Speedskater

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  • Kevin
Re: More cornet 2 hum questions
« Reply #2 on: 7 Aug 2013, 06:13 pm »

cyto

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Re: More cornet 2 hum questions
« Reply #3 on: 7 Aug 2013, 10:48 pm »
Can you post a photo so that we can see what you did?
Here are a few:






« Last Edit: 8 Aug 2013, 02:27 am by cyto »

hagtech

Re: More cornet 2 hum questions
« Reply #4 on: 8 Aug 2013, 06:00 am »
Hmmm.  I don't see anything obvious.  Except maybe one channel doesn't seem to have its shield connected.  But you said you had hum even without anything connected to the inputs?

jh

cyto

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Re: More cornet 2 hum questions
« Reply #5 on: 8 Aug 2013, 11:03 am »
Hmmm.  I don't see anything obvious.  Except maybe one channel doesn't seem to have its shield connected.  But you said you had hum even without anything connected to the inputs?

jh

The shield (ground) is connected, it just looks funny in the photo. Yes it is the same with nothing on the inputs or the inputs shorted.

The unconnected green wire in the last photo goes to the steel bottom plate.

poty

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Re: More cornet 2 hum questions
« Reply #6 on: 8 Aug 2013, 04:58 pm »
If I turn the power switch off the hum fades to nothing rather than abruptly stopping.
...
To me it seems that the power supply is the source, maybe magnetic interference from the tranny?
If you switch off the device there is no magnetic fields from the transformer.
First of all, I'd made the input/output RCA connections tidy. There should not be loose loops and the length should be as short as possible.
I don't see where the E-wire from the IEC connector goes? How the TT-ground jack is grounded? Where is the ground wire from the PCB? IMHO, the E terminal from the IEC should go to the ground jack together with the wire from the PCB. All other (case+transformer) ground wires can be connected to other star-point, but then - to the ground jack also.
Do you use nylon standoffs?
Are you shure your RCA is isolated from the metal part of the back panel?
Try to take out the V200 tube (from powered off device and after the tube becomes cold!!!) and listen to the level of hum. Repeat your listening test after powering off the device, but this time - pull the plug from the mains outlet, not just switch off with help of switch.

cyto

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Re: More cornet 2 hum questions
« Reply #7 on: 8 Aug 2013, 06:42 pm »
If you switch off the device there is no magnetic fields from the transformer.
First of all, I'd made the input/output RCA connections tidy. There should not be loose loops and the length should be as short as possible.

I used a little extra ground wire to have room to work. Originally I had straight wire going to the PCB untwisted. I did this to twist them. Both sounded the same.

I don't see where the E-wire from the IEC connector goes?

To the star ground

How the TT-ground jack is grounded?

To the chassis back plate. Originally I had it as the star, moving the star to where it is lowered the noise

Where is the ground wire from the PCB?

On the star ground

IMHO, the E terminal from the IEC should go to the ground jack together with the wire from the PCB. All other (case+transformer) ground wires can be connected to other star-point, but then - to the ground jack also.

So you are saying 1 star point for the IEC & PCB ground wire, then another star for everything else connected to the first star? And the 2 stars should be electrically separate except for the connecting wire?

Do you use nylon standoffs?

Yes

Are you shure your RCA is isolated from the metal part of the back panel?

100% certain

Try to take out the V200 tube (from powered off device and after the tube becomes cold!!!) and listen to the level of hum. Repeat your listening test after powering off the device, but this time - pull the plug from the mains outlet, not just switch off with help of switch.


OK I will try that

thanks

poty

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Re: More cornet 2 hum questions
« Reply #8 on: 8 Aug 2013, 08:01 pm »
I used a little extra ground wire to have room to work. Originally I had straight wire going to the PCB untwisted. I did this to twist them. Both sounded the same.
Its better to do this right, because other way the main problem could be backed by this. We'll solve one problem, but won't feel improvements because of the other.
So you are saying 1 star point for the IEC & PCB ground wire, then another star for everything else connected to the first star? And the 2 stars should be electrically separate except for the connecting wire?
No, it would be OK if they are connected to each other through the metal part of the case. I'm against only that the signal currents flow through the case in any possibility.

cyto

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Re: More cornet 2 hum questions
« Reply #9 on: 8 Aug 2013, 10:10 pm »
Try to take out the V200 tube (from powered off device and after the tube becomes cold!!!) and listen to the level of hum. Repeat your listening test after powering off the device, but this time - pull the plug from the mains outlet, not just switch off with help of switch.

Did that, the noise was about the same, it measured 1 or 2 db less. Pulling the plug the noise still died slowly as the caps discharged.

Speedskater

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Re: More cornet 2 hum questions
« Reply #10 on: 8 Aug 2013, 10:36 pm »
With an all metal chassis, the ground wire from the IEC connector should go directly to the chassis.  The audio circuit grounds should go to the chassis at the I/O jacks area.  The power line ground is not of the audio circuit ground system although both are connected to the same metal chassis.

Once again Bill Whitlock has detailed troubleshooting procedures.

cyto

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Re: More cornet 2 hum questions
« Reply #11 on: 9 Aug 2013, 02:06 am »
With an all metal chassis, the ground wire from the IEC connector should go directly to the chassis.  The audio circuit grounds should go to the chassis at the I/O jacks area.  The power line ground is not of the audio circuit ground system although both are connected to the same metal chassis.

Once again Bill Whitlock has detailed troubleshooting procedures.

Not an all metal chassis


IMHO, the E terminal from the IEC should go to the ground jack together with the wire from the PCB. All other (case+transformer) ground wires can be connected to other star-point, but then - to the ground jack also.

I put another star ground on the back plate on the other side of the IEC, the IEC earth and PCB earth are connected to it. The tranny and chassis grounds are connected to the original star and the two stars are connected by a wire.

I cleaned up the input wires, didn't have time for the output but they don't really hace extra wire.

No change

cyto

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Re: More cornet 2 hum questions FIXED!!!
« Reply #12 on: 9 Aug 2013, 12:08 pm »
After all that I figured it out. My first hint was that removing V200 didn't change the noise. Putting V200 in the V201 = no noise.

Swapping the 12AX7's resulted in more noise, 

I put the 6N2P-EV I have (with adapters) and no noise, I mean dead quit darned near my SS phono stage I had.

I think the 12AX7 in V201 was bad or noisy which is why swapping and putting this tube in V200 created more noise.

Thanks for all the help guys!

 :green: :green: :green: :green:

poty

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Re: More cornet 2 hum questions
« Reply #13 on: 9 Aug 2013, 12:19 pm »
Congratulations! In what position do you use the 6N2P? Have you checked the resulting RIAA (either by measurements or by listening)?
It is the first time in my practice that a tube is involved in hum, not noise. 6N2P has internal screen, which can be grounded and improves external shielding and triodes inter-talk. Have you grounded the screen in your usage?

cyto

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Re: More cornet 2 hum questions
« Reply #14 on: 10 Aug 2013, 11:22 am »
Congratulations! In what position do you use the 6N2P?

V200, I have the good TungSol 12AX7 in V201, the sound is better than the reverse.

Have you checked the resulting RIAA (either by measurements or by listening)?

I do not have a way to measure the RIAA. At first the sound was bright and harsh (the 6N2P is NOS 1984) but after 2 or 3 hours it settled down. By my ear the RIAA sounds accurate and there is also more gain. I played several LPs that I am very familiar with and the sound is great, my wife thought it sounded better too.

It is the first time in my practice that a tube is involved in hum, not noise.

It may have been "inaccurate reporting" on my part

6N2P has internal screen, which can be grounded and improves external shielding and triodes inter-talk. Have you grounded the screen in your usage?

No I haven't grounded the screen. The adapter I am using has pins 5 and 9 shorted in the adapter, no connection for the tube to pin 9 and pin 5 for adapter to PCB socket removed. So running a ground wire to pin 9 should be easy. If I do that should I just run a wire to the star?

poty

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Re: More cornet 2 hum questions
« Reply #15 on: 10 Aug 2013, 12:18 pm »
V200
Great! In my device it brings more "hiss", but sound was definitely better.
I do not have a way to measure the RIAA.
It could be easily checked by iRIAA, but I think the only testing that matter is our ears! My question is just curiosity as soon as I had to change some parts to achieve comparable results.
No I haven't grounded the screen. The adapter I am using has pins 5 and 9 shorted in the adapter, no connection for the tube to pin 9 and pin 5 for adapter to PCB socket removed. So running a ground wire to pin 9 should be easy. If I do that should I just run a wire to the star?
IMHO its better to connect to signal ground at bottom of R203s, C202s or R207s.

cyto

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Re: More cornet 2 hum questions
« Reply #16 on: 7 Sep 2013, 11:31 am »
IMHO its better to connect to signal ground at bottom of R203s, C202s or R207s.
I finally got around to adding a wire from the ground side of R203 to pin 9 of the 6N2P-EV in V200, it cleaned up the sound a bit more. I am still using the adapter, don't want to rewire the socket.

If I also put a 6N2P-EV in V201 with an adapter would I be able to get away with just running a ground wire from the pin 9 wire of V200 or should I run a new wire from R203 to the screen ground?

poty

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Re: More cornet 2 hum questions
« Reply #17 on: 8 Sep 2013, 05:04 pm »
The screen of the V201 is better be grounded to R210 or C204.