Need help re-designing power supply for high-end amplifier

Lestraveled

Joined May 19, 2014
1,946
...................Since transformers were changed that is the area I would suspect..........
I am in this camp.

.............I emailed the company and was told that they don't have the schematic available for this amp, but they did send me some schematics of their equivalent connections, basically a schematic showing the topology that applies to all of their early amps..............
- Do the schematics show an electrostatic shield in the power transformer??
- Do the power transformers in your amps have an electrostatic shield??
 

SLK001

Joined Nov 29, 2011
1,548
I am in this camp.
I see now that the transformers are NOT original, so I, too am in this camp with you. The original transformer may have also had a flux band around it (a brass or copper band surrounding the transformer).

This is why a picture would be helpful.
 

Thread Starter

earth1

Joined Mar 25, 2016
9
His was a solid state amp on a PCB. The ground was sort of "U" shaped with the low level inputs on one leg of the "U" and the power output on the other leg of the "U". The power supply filter caps connected to the bottom of the "U".

The flaw in the PCB layout was that the charging current for the caps flowed through part of the bottom of the "U". The effect was that this current was added to the ground reference between the input circuit and the output circuit. The difference voltage caused by the current flowing in the ground trace was amplified by the low level input circuit that spanned this section of the ground trace.
It's been quite a while, so I wanted to post a follow-up to this thread.
YES INDEED, I have found the causes of this amplifier's hum problem and corrected them.

The issue that RichardO cited with his friend's solid state amplifier, is EXACTLY the same issue here on this exotic expensive pair of VTL Deluxe 120 mono-blocks.
It appears that whomever designed the PCB layout, thought that they could just indiscriminately tied ground at any reference point along an existing ground bus. Just like the "U" shaped ground buss in RichardO's example, this too is a U, but then at the far end of the U, it dead ends into a T. So let's trace this out.
First, there is the B+ rectifier and the ground buss connected to it. The negative side of the 1650uf 800v capacitor is connected where the B+ rectifier makes it's connection to the main ground bus.
If you follow this ground-bus coming away from the B+ rectifier, the cathodes of each 807 are grounded to this bus at different points along this bus as you move away from the rectifier, as also do the trim pots for each bias adjustment for each 807. The negative side of all four 807 filiments and one of the 12AT7 filiments tie to this ground bus at various different points too, and also the two cathodes of the 12AT7 phase inverter. Moving still further away from the rectifier, the ground bus turns a corner where we encounter the grounding point of the second filter capacitor (100uf @ 330v). The ground buss moves away further still, until we come to the point where the input cable shielding is connected to this same ground bus. Moving beyond the grounding point of the cable shield and still further away from the rectifier, we come to the point where the other 12AT7 is grounded to this same bus. At this point, this ground bus splits off to the left and to the right, forming a "T". If you follow this "T" out to the right, we find the ground point of another 100uf 330v capacitor for the large B+ supply. If you follow this "T" out to the left, we find the ground point of two different capacitors, both of which are 100uf 330v to be used for the smaller and separate supply that runs B+ for the first 12AT7, as well as the grounding point for the negative side of the rectified filament supply for this same 12AT7.

So if you draw out a schematic that follows the description I just gave above, we find doosey on top of doosey on top of doosey.
1) There should have been separate ground buses for the negative side of each capacitor and the circuits that they filter, instead of having one common buss for each capacitor to connect to along different points of that same ground bus.
2) The filament should never have been grounded to the same bus where the lowest-level signals come in. If there is going to be surge current, the filaments draw the highest amount of current, and so they should have had their own bus.

So whether by filament current or by power supply filter capacitor, tying these ground points along the same ground bus causes a voltage gradient to be set up across this buss at different points. Sure this ripple current may induce a gradient of only a few micro-volts, but then that voltage gets amplified from state to stage, until it presents it's self at the output as a hum - a hum that is too loud and present to be ignored.

I was very successful in solving the problem but before I give you my solution, let me clarify that indeed this amplifier did come from the factory with this horrible grounding topology. I didn't want to believe it either, but yes - this amplifier no doubt always did have this problem and it's a problem inherent to the design, a series of mistakes made by the manufacturer, and I honestly do not know how they managed so tell these amps to discriminating audiophiles the way that the ground topology was made, and it's subsequent inevitable hum.

For the record, I do wish to say: Other than this terrible ground bus topology, I find everything else about the design and build quality of these amplifiers to be rock solid, and too compellingly good to NOT own them and make the effort to re-design them for the better. So here was my solution:

At different points along this ground bus, I used a razor blade and cut small sections of trace out of the bus, in order to "break up" this bus into different zones, each electrically isolated from the other. I then ran 8awg copper wire from the ground side of each capacitor to the negative side of the rectifier that feeds it's positive side. At no time do the new ground busses intersect, other than the same connection point where the rectifier joins it.
I also removed the filiment grounds from that bus, and ran a separate ground bus for them, so that filiment current doesn't pass through the same medium as the signal ground.

The result is a VERY clean power supply for a VERY strong and VERY clear sounding pair of VTL Deluxe 120 amplifiers.
The work was worth it, and the results are incredible!


Regards,
Jim Butler
 

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RichardO

Joined May 4, 2013
2,271
It's been quite a while, so I wanted to post a follow-up to this thread.
YES INDEED, I have found the causes of this amplifier's hum problem and corrected them.

........

At different points along this ground bus, I used a razor blade and cut small sections of trace out of the bus, in order to "break up" this bus into different zones, each electrically isolated from the other. I then ran 8awg copper wire from the ground side of each capacitor to the negative side of the rectifier that feeds it's positive side. At no time do the new ground busses intersect, other than the same connection point where the rectifier joins it.
I also removed the filiment grounds from that bus, and ran a separate ground bus for them, so that filiment current doesn't pass through the same medium as the signal ground.

The result is a VERY clean power supply for a VERY strong and VERY clear sounding pair of VTL Deluxe 120 amplifiers.
The work was worth it, and the results are incredible!
Wow!
That was a lot of work to get rid of the hum. I am glad it was worth it.
 
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