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Old 19th May 2012, 02:56 PM   #1
Atjan is offline Atjan  South Africa
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Location: Johannesburg
Default Hum on Avondale NCC200 derivative amp

Good day,

I've built an Avondale NCC200 derivative and have been using it on and off for the last while.

My issue is that the amp picks up a hum once I connect the RCA cables. It does not matter whether the RCA's are connected to the pre-amp or not. Once connected it gets slightly louder but remains at that level irrespective of the volume I'm playing at.

Its bad enough to hear the hum in the background when listening to piano music where there's lots of quieter passages, hence detracting from the whole experience. When player music louder, there's no problem. On less efficient speakers it's not a problem, but on my ZRTs (which is not exactly efficient) I simply can't live with it.

I've tried the following without success:
> Different RCA cables. I've tried some reasonably good ones and a couple of the ones you get with a cheap DVD player. The problem remains the same.

> Shorten both the RCA and signal lines to the shortest practical length.

> Change the signal line from a twisted pair CAT5 wire to a shielded wire. No change and I reverted to the CAT5 wires as they're lighter.

> Unplug the ground wire from star ground.

> Swop live and neutral on transformer input.

> Set-up the 2 PSU's in a manner where the phases are apposed. Basically have the one +/- and the other -/+.

>Inserted a 8R2 resistor between the PSU ground and star. This made things considerably worse.

I found a thread on this forum from 2005 where there appears to be good advice, but since the pictures are no longer available it is difficult to follow. In that case the problem was sorted out by using a *special* RCA cable. Which in my view is not a good solution as the amp still does not behave as one would expect with regular cables.

A picture of my grounding scheme: Click the image to open in full size.

My apologies if this is old hat - and for the fact that I'm not yet familiar with the forum software.

You help is much appreciated!

Attie
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Old 19th May 2012, 11:26 PM   #2
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Default NCC200 clone

You don't give details of what you have physically built or how you have wired it to the power supplies, mains earth, chassis connection,
star ground etc. These are critical arrangements that must be correctly set out and connected to prevent hum and hum loops in amplifiers.

If you simply copied the NCC200 schematic, it's understandable how you might have missed the necessary details but perhaps there are other
wiring or shielding matters that need attention too. Can you post a pic of your assembly so others can look to spot the problems, if any?
I think you may need to try posting the diagram again too.

Meantime, you could search the subject "Star grounding" on the forum or Wikipaedia etc. and look at the wiring arrangements in the many Pics
of the similar "NAP140 kit on Ebay" thread.
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Last edited by Ian Finch; 19th May 2012 at 11:31 PM.
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Old 20th May 2012, 09:03 PM   #3
Atjan is offline Atjan  South Africa
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Thanks for your reply Ian. I tried attaching a picture of the grounding arrangement in my first post but it appears I need more practice.....

I built 2 amp boards and 2 PSU boards, which includes the speaker protect circuit. Both PSU's are supplied from a single 250VA 33-0-33 transformer.

I did not copy the boards - it was a group buy type thing on a local SA forum, put together by one of the local guys. I've asked for advice there as well - which I listed in my first post.

PSU boards are located directly below the amp boards.

RCA connectors are isolated from the chassis.

>There is a single star ground point for everything on the chassis
> PSU grounds to star (in the attached pic black)
> Amp ground goes to PSU ground
> Transformer center tap to star (black)
> Power outlet ground to star (white)

I try again to post pics of the above:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Last edited by Atjan; 20th May 2012 at 09:23 PM.
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Old 21st May 2012, 01:28 AM   #4
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A couple of other details:
1) You are using 33V AC supplies so I guess you measure about 47V rails. That's no problem for MJ15003/4 outputs,
but I assume all the other components are up-rated to suit the higher than 40V rails. Those semis in Avondale's design
would mostly be on their maximum unloaded limits here.
2) The position you place the amplifiers, directly above the PSUs is not so good from the POV of EMR going straight to
the inputs. It may be a greater distance than it appears to me but I would try to increase the distance between the
rectifiers, PSU wiring and the amplifiers and signal leads as much as possible to minimise 100Hz hum.
3)The ~3mm aluminium heatsink bracket is rather thin for a 200W transistor and they are positioned on the far edge.
You will have to derate the usage or overheating of the case is certain at that supply voltage.
------------------------------------------------
The transformer grounding is not obvious, I think you say you have made chassis earth the star earth and this is
not correct. Star earth is at the PSU common and a strap leads to chassis earth from there, otherwise you introduce
lead resistance into the earth path. Obviously, the resistance between each PSU star ground must be as close to zero
as possible and the benefits of separate supplies vanish without separate windings to permit isolated supplies.

Normally, star ground is above chassis earth but then strapped to it at one point, which may be safety/chassis earth
or another chassis point, according to safety rules which are endlessly changing to satisfy bureaucratic whims.
Safety/chassis earth accepts mains earth, transformer frame earth and a single lead from PSU star ground.
this connection may even be made at another chassis point, depending on power authority rules.

Perhaps this isn't the case but I would hope the transformer secondary common or CT is returned direct to PSU or rather star
ground. If there is a transformer frame or shield strap connection, by all means connect this to chassis/safety earth.
Safety rules may insist on this anyway.

I hope that the problem is just that simple and you get some improvement
if I have understood you correctly. Let's hear what you and others may have to say on it
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Old 21st May 2012, 11:27 AM   #5
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Suggestion ....

Give the safety ground its own chassis connection about an inch away and secure it with a locking nut/washer so you dont keep disturbing it . Then check it has a good electrical connection to the chassis with a meter.

Connect all amp and psu grounds directly to the star ground , including the loudspeaker output returns . Make the psu grounds as short as possible .

Last edited by epicyclic; 21st May 2012 at 11:30 AM.
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Old 22nd May 2012, 08:19 PM   #6
Atjan is offline Atjan  South Africa
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Thanks for your detailed and concise replies!

I'm a novice so it takes a while to take it all in. I'll make some sketches and post here before I go ahead and drill holes/cut wires.
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Old 22nd May 2012, 08:33 PM   #7
Atjan is offline Atjan  South Africa
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Location: Johannesburg
Quote:
Originally Posted by Ian Finch View Post
A couple of other details:
1) You are using 33V AC supplies so I guess you measure about 47V rails. That's no problem for MJ15003/4 outputs,
but I assume all the other components are up-rated to suit the higher than 40V rails. Those semis in Avondale's design
would mostly be on their maximum unloaded limits here.
2) The position you place the amplifiers, directly above the PSUs is not so good from the POV of EMR going straight to
the inputs. It may be a greater distance than it appears to me but I would try to increase the distance between the
rectifiers, PSU wiring and the amplifiers and signal leads as much as possible to minimise 100Hz hum.
3)The ~3mm aluminium heatsink bracket is rather thin for a 200W transistor and they are positioned on the far edge.
You will have to derate the usage or overheating of the case is certain at that supply voltage.
------------------------------------------------
The transformer grounding is not obvious, I think you say you have made chassis earth the star earth and this is
not correct. Star earth is at the PSU common and a strap leads to chassis earth from there, otherwise you introduce
lead resistance into the earth path. Obviously, the resistance between each PSU star ground must be as close to zero
as possible and the benefits of separate supplies vanish without separate windings to permit isolated supplies.

Normally, star ground is above chassis earth but then strapped to it at one point, which may be safety/chassis earth
or another chassis point, according to safety rules which are endlessly changing to satisfy bureaucratic whims.
Safety/chassis earth accepts mains earth, transformer frame earth and a single lead from PSU star ground.
this connection may even be made at another chassis point, depending on power authority rules.

Perhaps this isn't the case but I would hope the transformer secondary common or CT is returned direct to PSU or rather star
ground. If there is a transformer frame or shield strap connection, by all means connect this to chassis/safety earth.
Safety rules may insist on this anyway.

I hope that the problem is just that simple and you get some improvement
if I have understood you correctly. Let's hear what you and others may have to say on it
There's actually a few of these I can address now:
1) The lowest rated component is 63V, so I should be safe on the voltage margin.

2) Which PSU component would emit the most EMR? The amp board is not very far (3cm max I think) from the PSU caps. The rest of the components are about 5cm from the amp board though. I'll make a plan to tidy up my wiring and see if I can keep them further away from the amp board.

3) I had a bit of difficulty sourcing a small piece of alu angle of a reasonable thickness and was also a bit worried about that. However, this weekend I pushed the amp a bit - ran it for an entire CD's duration with the volume on about 2 o'clock. My normal listening volume is 9-10 o'clock. The heat sinks got the warmest yet, but I don't think more than 40 deg C. The back panel and alu angle was just about the same temperature. But you're right - that's another thing I should look at bullet proofing.
-----------
The next bit of of your comment I'll study a bit and see if I can figure out what needs to be done.

BTW, the transformer was meant just to get me going. The 'pie in the sky' plan is to get dual toroidals.

I know this is a bit of a contentious issue, but what VA rating would you consider 'good' for my purpose? The amp is meant to deliver around 90W/c into 8 Ohms. I was thinking 220-300VA / channel, but then an engineer, who builds amps, mentioned he's using 110VA for 80W/c with very little electrical evidence for problems when straining the amp.
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Old 22nd May 2012, 09:01 PM   #8
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I cannot see any kind of heatsink other than the flimsty steel back panel.
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Old 22nd May 2012, 10:55 PM   #9
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+1 to Epicyclic
but i think you will have to move the mains socket and wiring!
Shorten that white earth wire. but first try a ferrite on it!
Shorten all mains wiring too, cant see from the pics where its routed, but keep it at least 2cm away from all other wiring.
Rotate the transformer orientation slowly while listening for hum
also try grounding the RCA sockets, disconnecting the signal ground wires from sockets.

Last edited by whizgeek; 22nd May 2012 at 10:57 PM. Reason: woops!
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Old 22nd May 2012, 11:02 PM   #10
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Are the input RCA sockets insulated from the chassis?
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