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Old 3rd March 2011, 08:37 PM   #131
Elbert is offline Elbert  Norway
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Now, after the rebuild, it became easier to move the transformer, so I managed to lift it out of the chassis.

As you can se from the immage, I couldn't move it that much but now it was at least not sitting virtually next to the x.over board.

This didn't really make much difference. When grounding the x-over card from the starpoint, there was slightly less noise on the scope, but not a change I would call significant as such.
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Old 3rd March 2011, 09:23 PM   #132
Elbert is offline Elbert  Norway
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Hmmm..

This is difficult to make any sense of...

PSU has been improved with greatly reduced ripple.

Correct starpoint topologuy has been implemented.

Could it be difference between the wires connecting the amplifier cards to the PSU?

As can be seen from the scope noise measurements, it is allways the third amp-card which has the noisiest output. This card allso has the longest wires from the PSU, especially the 0V wire which is over twice as long as the others.

Any comments??
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Old 4th March 2011, 01:16 AM   #133
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Hi Elbert, only a quick one, have you tried twisting the + 0V - wires together for each amp? It doesn't look like they are on the photos, but I'm making some assumptions on what are power connections.

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Old 4th March 2011, 01:52 AM   #134
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Elbert; Sorry it is still give you a headache after all that work, but now you know what its not. It looks like you are passing power thrue the bulkhead from the cap board to the other side for the amps. That may not be the best thing. I see some jumpers on the cap board side under the stand offs? It may be posible you are getting some voltage differentials through all those different metals. Brass stand offs, aluminum bulkhead, and copper wires all have differant resistances. I think it more likely to cause problem than the lenghts of the wires going to the amps. Can you try to wire the outputs from the cap board strait to the amps for testing? Andy
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Old 4th March 2011, 06:31 AM   #135
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OK Something from left field (and I may have already asked this before). Have you tried completely disconecting the PS from the crossover card, but leaving everything else connected? and doing measurements.

I'm just wondering whether you are getting some amp ps current flowing back through your xover psu rectifier and transformer (since the zero volts of each PS is effectively coupled via the signal cables)....

Tony.
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Old 4th March 2011, 07:52 AM   #136
Elbert is offline Elbert  Norway
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Quote:
Originally Posted by 4pyros View Post
Elbert; Sorry it is still give you a headache after all that work, but now you know what its not. It looks like you are passing power thrue the bulkhead from the cap board to the other side for the amps. That may not be the best thing. I see some jumpers on the cap board side under the stand offs? It may be posible you are getting some voltage differentials through all those different metals. Brass stand offs, aluminum bulkhead, and copper wires all have differant resistances. I think it more likely to cause problem than the lenghts of the wires going to the amps. Can you try to wire the outputs from the cap board strait to the amps for testing? Andy
Well, everything has a resistance, so wether its copper or aluminium shouldn't make anny difference.. all the connections from the rectifier to the bulkhead connection points are now of equal length and far less complicated than before, so this is not my first suspect..

I'm leaning more towards the wiring to the amplifier cards, they are different lenghts and not twisted properly due to the space available and the stiffness/ thickness of the wires..

I'll try do do some measurements focused on this in the afternoon..

Tony, have tried leaving the X-over card dead, i.e disconnected from it's PSU, no luck there either..
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Old 4th March 2011, 09:52 AM   #137
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OK that rules that out then Another thought I had on the way home from the station, you mentioned that amp # 3 was the worst, how did it compare if it was the only one running? That should tell you whether the PS wiring is making a difference for that amp compared to the one with the best performance.

I assume you have checked all three of them in isolation.

I remembered my first amp that I ever built this evening, around 27 years ago. It had a most objectionable hum when I first got it working. The back panel was Masonite but it had a foil coating on it. The input RCA jacks were not insulated and the foil connecting the earth's of the two input shields caused massive hum. As soon as I isolated the the input jacks the hum went away. There is a pic of that amp (in the ugly looking amp thread) Actually looking at that picture the problem could have been an earth loop between input and speaker returns (the speaker connectors were also rca jacks).

I think one of the things you tried before was just connecting the shields of the amp inputs at the XO end without connecting to the x-over board right? and it had the same effect as if they were connected to the crossover, if I remember correctly?

Have you tried not connecting the shields at the amp end of the signal cable and instead running a wire from the shield at the amp end (for each amp) direct to the common 0V point on the PSU board where the amp 0V connections terminate?

Tony.
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Old 4th March 2011, 10:48 AM   #138
Elbert is offline Elbert  Norway
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Awesome Amp there Tony, a case study in utilitarian budget design!

Yes, In isolation, all the amps are fine, it's when I connect them that things start getting problematic...

When re-doing the PSU, I connected the chassis to the star-point, simply by not insulating the 0-V bulkhead penetration from the caps.

Following that, I insulated the RCA input from the chassis. Allso, the speaker - terminals are insulated from the chassis.

regarding the alternating shield connection, you are right Tony.

I have not tried your suggestion for connecting the screens to the starpoint with a dedicated wire as such..

When I had the screens connected at the x-over card and the card connected to the star point with a dedicated wire, that would effectively be the same thing, and that gave a lousy result..

To get to the bottom of the PSU wiring, I can try the following this evening..

1. Measure with the scope between the 0V connections on the amp cards to see if any voltage potentials develops over the wires from the star-point.

2. Use the PSU wires from amp3 to power amp 1 or two and see if the problem with amp 3 follows the wiring or not.

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Old 4th March 2011, 11:11 AM   #139
AndrewT is offline AndrewT  Scotland
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measuring hum or buzz voltage across a wire may give a (near) zero reading.
The amplifier increases the signal by it's gain.
If the hum and/or buzz is 2.1mVac at the output and you have a gain of 30times (+29.5dB) then the input referred interference is ~70uVac. That could be the total over a group of wires. Finding the wire with the worst signal that is very likely to be <100uVpp is not going to be easy.

I know you don't like it, but you are getting closer to stripping the amp down.

Connect ONE channel Power Amp and measure the output noise with the input to the Power Amp shorted and with the input open.

If that is satisfactory, then add on ONE more piece and measure again.
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Old 4th March 2011, 11:35 AM   #140
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Hi Elbert, maybe I'm not understanding what you did but maybe a picture will help with what I am talking about.... did you leave one end of the sheild completely floating and ran a wire to the star point from the crossover?

I've drawn once again how I think you now have things connected. Only showing 0V wires between the amps, PSU and XO card. I believe you have a loop between the 0V on the PSU and through the XO card between the amps. Whether this is significant or not I'm uncertain, but if you break the shield at the points where I put an x, and extend it to the PSU star point (represented by the dotted lines) then It should break the loop between the amp cards, as the only common connection they will have will be the connection to the star point. The signal ground will be via the potentially "dirty" PSU star point though...

I've drawn an extra feint line with arrows on it to indicate where there may be some loops between amps, not sure if this is realistic but seemed a possibility.

Although this suggestion seems very similar to what you posted that you have tried and was worse, the difference (I think) is that the shield is maintained end to end.

Tony.
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