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PostPosted: Mon Jun 11, 2018 4:42 am 
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Is it a mechanical hum or through your speakers? I find the MP301 uses less than stellar iron for the power transformer and needs isolation from the chassis and your shelf. If it comes through the speakers, the 0.1uF and 10 Ohm resistor on the 3rd (grounding) pin of the incoming IEC power connector works 99% of the time while keeping the safety of the circuit intact. Did you upgrade the filters to the larger value or is it still stock? There are other possibilities for grounding but I haven't had to resort to anything else other than what's in this thread for getting rid of hum on even the most sensitive headphones or speakers.

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PostPosted: Mon Jun 11, 2018 10:17 am 
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petersch wrote:
Is it a mechanical hum or through your speakers? I find the MP301 uses less than stellar iron for the power transformer and needs isolation from the chassis and your shelf. If it comes through the speakers, the 0.1uF and 10 Ohm resistor on the 3rd (grounding) pin of the incoming IEC power connector works 99% of the time while keeping the safety of the circuit intact. Did you upgrade the filters to the larger value or is it still stock? There are other possibilities for grounding but I haven't had to resort to anything else other than what's in this thread for getting rid of hum on even the most sensitive headphones or speakers.

It's definitely a hum through the speakers, immediately on power-up and seems to increase a bit as the tubes stabilize.
The amp is stock excepting the removal of C6 and C6A capacitors.
It is on a different power circuit than the rest of my gear, where a tuner used to be. (tuner now removed from the system)


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PostPosted: Tue Jun 12, 2018 2:48 pm 
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devon58 wrote:
petersch wrote:
Is it a mechanical hum or through your speakers? I find the MP301 uses less than stellar iron for the power transformer and needs isolation from the chassis and your shelf. If it comes through the speakers, the 0.1uF and 10 Ohm resistor on the 3rd (grounding) pin of the incoming IEC power connector works 99% of the time while keeping the safety of the circuit intact. Did you upgrade the filters to the larger value or is it still stock? There are other possibilities for grounding but I haven't had to resort to anything else other than what's in this thread for getting rid of hum on even the most sensitive headphones or speakers.

It's definitely a hum through the speakers, immediately on power-up and seems to increase a bit as the tubes stabilize.
The amp is stock excepting the removal of C6 and C6A capacitors.
It is on a different power circuit than the rest of my gear, where a tuner used to be. (tuner now removed from the system)


Petes power supply mods are going to help a lot. One thing you can try if you havnt already is to get all your amps running on the same circuit. The possibility of running at different ground potentials is the genesis for ground loops.


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PostPosted: Fri Jun 15, 2018 8:01 pm 
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So I have confirmed the hum disappears if using a cheater plug on the AC cord.
Listening to it tonight for a bit powering the tweeters and it sounds quite good.
Certainly not a long term solution but I have been curious about active tri-amplification.
I wonder if the grounding mods suggested by petersch will remove the loud hum when using the proper IEC cord...


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PostPosted: Sat Jun 16, 2018 5:49 am 
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Yes!

The ground mod lifts the ground in an electrically safe way that will not compromise protection. It's very east to do, find a 10 Ohm 2 watt (or more) resistor and put a 0.1uF at 250V capacitor across the resistor. Next unsolder the green (or black green) ground wire inside the amp and place your new "circuit" in series with it and most (if not all) of the hum will disappear. If you choose to upgrade the power supply filter caps on top of the grounding mod this will ensure a 100% black background and it's especially effective when using headphones.

Pete

PS, get the amp and other gear on the same outlet and this will also ensure you have less noise in the system.

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PostPosted: Sat Jun 16, 2018 10:54 am 
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petersch wrote:
Yes!

The ground mod lifts the ground in an electrically safe way that will not compromise protection. It's very east to do, find a 10 Ohm 2 watt (or more) resistor and put a 0.1uF at 250V capacitor across the resistor. Next unsolder the green (or black green) ground wire inside the amp and place your new "circuit" in series with it and most (if not all) of the hum will disappear. If you choose to upgrade the power supply filter caps on top of the grounding mod this will ensure a 100% black background and it's especially effective when using headphones.

Pete

PS, get the amp and other gear on the same outlet and this will also ensure you have less noise in the system.

OK thanks Pete. I thought maybe your grounding mod was more of a subtle improver, and with the ridiculous amount of hum my amp has I was sceptical.
I will order that orange drop cap and the resistor from Digikey, and since those are such low cost items I may as well get the larger filter caps to make the shipping cost worthwhile.
Will think about coupling caps, but having looked at the existing caps, the amount of glue used looks like they will be a bear to get out...
My dedicated audio circuit is maxed out, so the amp is on another shared circuit unfortunately.


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PostPosted: Tue Jun 19, 2018 6:39 pm 
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ferdzb wrote:
The Nostalgia Co wrote:
Where is that cap and resistor connected to?


It's at the backend lead of the wire terminal where originally the ground wire was at.

Image

In this photo I am unclear what occurs where the “here” arrow is.
Is that leg of the combined resistor and orange drop cap soldered to the back of the terminal strip in lieu of being plugged in to the front of the strip?
In my amp the short green-yellow wire (screwed to chassis) plugs in to the fourth from right hole.
The longer green-yellow wire that emanates from under the power transformer plugs in to the fifth hole from right. It appears from the photo it is this longer green-yellow wire that the cap and resistor is soldered to, but if so i’m not sure what to do with the short chassis ground wire, if anything...


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PostPosted: Tue Jun 19, 2018 7:18 pm 
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I have been reading this thread for the past few days and am planning to start modding my 301 in the next month or so. I have to order all the parts first. Anyway, I was wondering if anyone using Mundorf silver oil caps has encountered degradation? These are not supposed to be used in high heat locations, such as in tube amps.
Just curious as there is all kinds of conflicting info out there. Thank you for the awesome thread. I am really excited about this! Also the MP-D1 mods.


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PostPosted: Tue Jun 19, 2018 7:48 pm 
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Just put the cap and resistor in series with the yellow/green wire that's bolted to the chassis. In the picture he just soldered the resistor to the back of the terminal block as it's easier than trying to poke it through the front.

devon58 wrote:
ferdzb wrote:
The Nostalgia Co wrote:
Where is that cap and resistor connected to?


It's at the backend lead of the wire terminal where originally the ground wire was at.

Image

In this photo I am unclear what occurs where the “here” arrow is.
Is that leg of the combined resistor and orange drop cap soldered to the back of the terminal strip in lieu of being plugged in to the front of the strip?
In my amp the short green-yellow wire (screwed to chassis) plugs in to the fourth from right hole.
The longer green-yellow wire that emanates from under the power transformer plugs in to the fifth hole from right. It appears from the photo it is this longer green-yellow wire that the cap and resistor is soldered to, but if so i’m not sure what to do with the short chassis ground wire, if anything...

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PostPosted: Tue Jun 19, 2018 10:23 pm 
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Hi Guys,
Newby here. I have been watching this thread for a good bit now and I have to say I'm really impressed with the way you folks help each other and the overall talent here. You guys are incredible and you Peter are amazing with your vast knowledge. :D
Cheers, Ron.


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PostPosted: Wed Jun 20, 2018 11:03 am 
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petersch wrote:
Just put the cap and resistor in series with the yellow/green wire that's bolted to the chassis. In the picture he just soldered the resistor to the back of the terminal block as it's easier than trying to poke it through the front.

Thanks. By removing the hex screws from the IEC socket and voltage selector, is there much gained in terms of room to work?
Once again I found those little hex suckers have been tightened to the nines...


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PostPosted: Wed Jun 20, 2018 11:29 am 
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I’ve done it that way a few times, now I just use the method above. So much easier than dismantling the whole IEC thing.

-- 20 Jun 2018 13:29 --

I’ve don’t it that way a few times, now I just use the method above. So much easier than dismantling the whole IEC thing.

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