P3A Comparison table ( long .... )

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#583
If both transformers are dual secondary type and you use dual rectifiers on each transformer then it is simpler to avoid a hum problem.

Using two centre tapped transformers requires the centre tap on each channel to be connected to the audio and power grounds and to the Chassis. You end up with hum loops.
There are solutions, read Joffe.
 
#584
royville said:
Any way I'd be grateful if you could have a look over my boards and let me know if there's anything amiss.
Also, does it seem okay the way its laid out, or is there a better way?
Click to expand...

Looks pretty good to me. If you want some nit-picking I'd offer the following:

> would be nice to clean off the solder flux around the leads of the Nichicon MUSE rail caps. Nice choice of capacitor, I recommend these type.
> the large red input caps aren't really designed to be mounted partly off-board like that, I would worry about stress/strain on the lead soldered into the board and would add some glue to hold them firmly to the board. Same comment applies to the rail capacitors, put something around their base to hold them on the board more firmly since they are not mounted as the manufacturer intended.
> missing insulation from the rail capacitors on the left-hand board near the power quick fit connectors
> I thought the copper heatsink was a neat addition - like others I would have put the Vbe in contact with it but I've also heard that Rod had a reason not to do so and maybe it's just fine as-is.

I haven't checked the resistor codes for you

As for transformers - well if you have them already why not use them ? I have never quite been convinced that dual mono transformers was necessary but then again I have no personal direct experience in comparing the two options. Is a single trafo big enough (VA rating) for two channels ?

If using only one trafo I don't see much need for two rectifiers, they'll be both 'on' at the same time so no benefit to channel isolation and they are beefy enough that one of them alone can serve both channels. If one trafo is big enough I'd use just one, one rectifier too. But having separate capacitor banks does provide some isolation between the channels - be sure to use CRC to achieve this separation and to improve filtering.

Layout looks reasonable to me - of course usually better to put the amplifiers in the centres of the heatsinks. Position things to keep wiring short, keep signal wiring away from output wiring and mains wiring.

I'd also leave space in the chassis for speaker protection boards in case you wish to add them later (plenty of them available on eBay).
 
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#585
@ Andrew T
We have been there a million times before , please don't do that . Center Tap doesn't have to be connected to the Chassis ... Mains Ground does ONLY

Constructing a machine that has CT connected to ground is the NO 1 reason to have a system that will create ground loops , and its not going to be able to co operate with a Lap top or any other source that might have common ground to line outputs and mains supply .

@Roy
Having 2 transformer and go with Twin Mono design can and will make a better amplifier Honestly your approach is very good : Go with one stay with the amp , observe , listen , evaluate , and then go with a second and you will find out and have an opinion on your own .

Kind regards
Sakis
 
#587
east electronics said:
@ Andrew T
We have been there a million times before , please don't do that . Center Tap doesn't have to be connected to the Chassis ... Mains Ground does ONLY

Constructing a machine that has CT connected to ground is the NO 1 reason to have a system that will create ground loops , and its not going to be able to co operate with a Lap top or any other source that might have common ground to line outputs and mains supply .
Click to expand...
Have you chosen to ignore all the exposed metal contraptions on your amplifier that could have Mains voltage on them and could kill operators of your equipment?

If it takes a million and one times repetition of this safety issue then I will do it.

We cannot ignore operator safety.
 
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#589
Our amps are class I apparatus most of the time, given that they're often built in a metal chassis. Appliance classes Class I says that the metal chassis, the outside needs to be grounded to earth and it it is to be used with a 3-prong connector. It doesn't say anything about attaching internal grounds (any reference of the double isolated circuit (after transformer secondaries) really) to earth.

It may offer additional safety when working on it with the lid off, but that's a different circumstance all together.
 
#591
Andrew the reason of having a metallic Chassis and all metallic parts of it connected to mains ground is safety

So anything with a reasonable voltage touches inside or outside the Chassis will short circuit with ground and blow fuses or trip safety relays on the electric installation ...

The CT of the secondary of one amplifier has no reason to placed on the mains ground ...
 
#593
All nit-pickings welcome here!

All nit-picks taken on board and will be duly processed.

Can I use that 'hot glue' stuff for securing the caps?

Totally forgot about the little sleeving - its on my bench & cut to size. 😱

I only have the one TX at the moment (borrowed from another amp) so I'll finish the amp as is, then as suggested by Mr Sakis I'll double up on the caps/rectifiers, - listen, then finally two TX's, and listen again. Of coarse that's assuming it doesn't morph into a dragon and go - "Up Up Up In A Puff Of Smoke". :bawling:

Does the CRCRC mean I need to put resistors between the PSU Caps? If so what value do I need to use?
I think I understand the CT arrangements and Ill post a diagram to confirm later (at work presently).

Thank you so much to every one for your help and advice, Its really appreciated. - Please keep it coming.

Kind Regards
Roy.
 
#594
I much prefer clear silicone window sealer to hot melt glue for securing cool components.

I use clear silicone window sealer as a gasket in engine and gearbox assembly instead of the standard paper and two layers of gasket goo. It works very well, in the joint, in the presence of high pressure oil, low pressure hot water, petrol and methanol.

I started using it to "glue" electrolytics to the chassis instead of the metal clips.
Now I use it to fix relays to side panels, to fix brackets to floor and back panels to then mount PCBs on = no extra external screw heads.
 
#595
royville said:
..................Does the CRCRC mean I need to put resistors between the PSU Caps? If so what value do I need to use?...................
Click to expand...
Yes the R = resistor.
The value depends very much on what peak current and what average current passes through.
The resistance of the secondary and the reflected impedance of the primary and the source feeding the primary add up to a further resistance that is missing from your question.
The filter effectively becomes
rCRCRC.
The first C takes the brunt of the mains ripple. That C must be rated for the very heavy ripple that will be imposed on the capacitor.
The last C supplies most of the varying current passed to the load.
The amplifier sound quality is affected by this last C. This effect may be reduced by having adequate, or more, local decoupling at the varying load.

Typical R values
preamp drawing continuous 100mA with lots of spare voltage overhead, could afford to lose 1V to 10V of drop across all the resistors
For 1Vdrop the total R would be 10r i.e. 4r7 per R. Dissipation = 47mW (any 250mW or more resistor will do.
For 10Vdrop each R could be 47r. Dissipation = 470mW. Use rating >=1W or use 10r||10r, 500 or 600mW resistors in each position.

Power Amplifier drawing a quiescent current of 100mA and a peak current of 5A with little spare overhead.
try 0r1 for a Vdrop of 10mVdc. Dissipation is more difficult to calulate due to the last C supplying most of the varying current. I'll assume the second R passes only half the maximum current, i.e. 2.5A
Dissipation becomes 625mW. use three 0r33 500 or 600mW in parallel or 0r1 2W
 
#599
The CT is not an exposed part.
But any conductive part connected to the CT, that is exposed, MUST be connected to the Chassis.

A simple example is the Signal Return Barrel of a RCA/Phono socket.
This is a metal part. It is conductive. It is exposed.
It must be connected to the Chassis.

BUT !!!!
we are told repeatedly to insulate the RCA from the Chassis. Then we connect the internal grounding solder tab to main audio ground via a series of other wires and traces.

Some where along that route of tab+wires+traces we must make a connection to Chassis.
That same Signal Return will be connected somewhere to the PSU Zero Volts. That PSU Zero Volts will be connected to the transformer CT.
BUT NOT IN DUAL RECTIFIER DUAL SECONDARY. There is NO CT. That is what makes it different.
One has the CHOICE of where to make the two channel connections to the Chassis. That choice when wisely made can avoid another LOOP.
 
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