ebay GainClone PCBs

[portreathbeach]

Can anyone tell me if they have built a GainClone amp using these boards:

PCB for Audio Amplifier LM3875 HiFi Gainclone Amp Qty:2 on eBay (end time 24-Feb-10 00:05:21 GMT)


There are several options with these boards, inverted and non inverted and plenty of places for components. Just wanted to know if anyone has used these and what the sound quality would be like, as it doesn't seem like there is a separate star ground for the power and the signal.

Thanks

[linuxguru]

It has a passable star-ground, though not the best layout possible. The bigger concern is the absence of a lifted-ground (ground-loop breaker), though one can perhaps be kludged in by lifting a pin or two and soldering a 10-ohm resistor suitably. Is this also available on EBay US, or only on EBay UK?

[portreathbeach]

Thanks for the quick reply linuxguru.

The PCBs were advertised on ebay uk, but they are from the US.

What do you mean when you say lifted-ground?

[linuxguru]

Originally Posted by portreathbeach

What do you mean when you say lifted-ground?

Lifted-ground is a separate signal ground, isolated from the ground plane with a small resistor (say 10 ohms). The shield ground from the input connector is connected to the lifted ground, and so is the input bias resistor and feedback network, but nothing else.

It's also known as a ground-loop breaker, and it's main purpose is to eliminate 50/60 Hz hum pickup.

[katana2112]

Isnt that Peter Daniels? Audiosector?

[Peter Daniel]

Originally Posted by katana2112

Isnt that Peter Daniels? Audiosector?

I am not associated with this ebay seller in any way.

[katana2112]

My apologies, i should have checked with you first...

[portreathbeach]

I have another question.

My end goal of my audio project will be a 4 separate aluminium enclosures. 2 of these will be Gainclone amps (bi-amping my speakers), 1 will be a power supply for the Gainclone amps and the last one will be a pre amp which will be IR controlled and use a PIC microprocessor and relays to carry out the switching etc.

The pre-amp will have it's own small power supply built into it fed via 230v, transformed done to 5V to run the microprocessor, the enclosure will obviously be earthed as will the Gainclone power supply enclosure.

The 4 enclosures will be stacked, so technically all of the enclosures will be at the same potential, but I will be earthing the Gainclone enclosures with an extra cable from the power supply.


Now, you say that I need to ground the signal ground via a 10 ohm resistor, where should I put this resistor? Should I put 1 in each enclosure?



Any help would be great.

Thanks

[portreathbeach]

Just another quick one...

If I were to tie the signal grounds to the chassis in each gainclone amp in my system, then there would be a parallel ground (2 x 10 ohm in series), so the start ground of the signal would be 5 ohms difference from main ground? And again, if I were to add a 10 ohm resistor to the pre-amp, I would have an even lesser resistance as there would technically be 3 x 10 ohm resistors in parallel!

Also, do I ground the power supply (+PGND and -PGND) in the power supply enclosure, the Gainclone amp enclosures, or both.

Thanks again for all of your help

Craig

[linuxguru]

Originally Posted by portreathbeach

If I were to tie the signal grounds to the chassis in each gainclone amp in my system, then there would be a parallel ground (2 x 10 ohm in series), so the start ground of the signal would be 5 ohms difference from main ground?

When you tie the signal ground to the chassis ground on each chassis, there is a potential for ground loops to be created. That's because there may be two separate shields in the signal cable between the source and the amplifier, as well as a possible common chassis ground (through earthing) between the source and the two amplifiers. Ground loops are the most common source of 50/60 Hz hum - they can inductively pick up line frequency interference from transformers, AC lines, etc. It's pretty difficult to eliminate all sources of such interference.

It's easier to break ground loops at the input to the amplifier. The most common way of doing this is to separate the shield ground from the chassis ground (inside the gainclone enclosure) with a 10-ohm resistor, paralleled with two back-to-back rectifier diodes like the 1N4002. When this happens, there is no longer a low-impedance ground loop, which helps reduce hum substantially.

The exact value of the resistance isn't that important - anywhere from 2.2 to 10 ohms is commonly used.

Also, do I ground the power supply (+PGND and -PGND) in the power supply enclosure, the Gainclone amp enclosures, or both.

Most probably better to ground them together in each gainclone amp enclosure at the star ground point only. This should be close to, or at, the terminal of one of the filter capacitors on the gainclone board.