Moving PSU-caps off pcb?

[Brillegeit]

Hi guys!

I want to upgrade my Abrahamsen v2`s psu-caps, but the pcb has no space for extra caps, and I can`t find any higher capacitance that will fit the footprint of the original ones. The original ones are 8200uF/80V/105C. 35x65mm, 2 in each channel, 4 in total. The amp has separate rectifiers/caps for each channel with a common toroid.

This upgrade is available from the factory, but off course at a premium!
The factory website says the upgrade is doubling the capacity of the psu-caps, resulting in noticeable sonic improvement.

Now to my question; would it be ok to move the caps off the pcb and just connect them to the pcb with wires? The amp has plenty of free space inside(see picture). I was thinking of buying 4 more similar caps, doubling the the total capacitance.

And how about some film bypass-caps for the electrolytes? What value?? I`ve read on here a few different things, some say 10% of electrolytes, other say 100times less than electrolytes, what would be good values to start with?

Also, should I be adding several bypass-caps? ie; one 220uF and one 47uF? And maybe a power-resistor for discharging the caps when amp is turned off?

What would be the best placement for bypass-caps, straight across the main-caps, or should I solder these to the pcb?

Cheers

- Bjørn

[h_a]

If you add caps, leave the original ones on the pcb (you can also replace them, but keep the pcb populated); the closer the caps are to the load (power transistors) the better.

For bypass recommendations I'm sure others will chime in ;-)

Have fun, Hannes

[Brillegeit]

Quote: "the closer the caps are to the load (power transistors) the better."

That was what I thought. Had to ask though..

So would you recommend just connecting wires to the underside of the pcb(leaving original caps in), and connecting these to the new/additional caps?

would it alsobe a good idea to have the bypasses also connected directly to the pcb then? I don`t think there is space for caps underneath the pcb though..

Cheers

- Bjørn

[Brillegeit]

Just thought I`d upload some more pictures of the inside, so you guys can see what I mean about space on the pcb. :-)

Would it be ok to place 4 extra capacitors+ bypasses in the vast open area() below the pcb, to the right of the toroid?

Also, I have some sheets of copper, should I try to build a divider/shield between caps and toroid? to minimize interference? Maybe shield the wires coming of the toroid aswell? I`m guessing those wires are a source of 50Hz- hum...

Any answers much appreciated!

cheers

- Bjørn

[h_a]

As to wiring the additional caps I can't say much, you have to see how you can do that; thick wire of course prefered.

Bypasses should reduce impedance at higher frequencies, so it's a good idea to place them directly onto the pcb (top or bottom equally fine as long as their leads are not shorting with something), as otherwise leads kill all benefit.

What you can try is to fit a copper 'waist' band around the toroid (around the outside! not through the center as that would form a shorted windind!), though copper is not really suitable to shield 50Hz magnetic fields. But you can try and see what it brings.

By the way, how do you know it's 50Hz humm? Often humm is 100Hz and that has an entirely different cause.

Have fun, Hannes

[Brillegeit]

Thank you for your input hannes! :-)

about the bypasses; I`ll try to fit them on the pcb somehow, off course without shorting anything. I am an electrician, so I understand electrics pretty good, electronics is not my strongest field though..

About the shielding of toroid and wires; there is no hum at the moment, I was just thinking that if I placed the caps next to the toroid, they might pick up some noise coming of the toroid and wires?
The reason I was thinking 50Hz-hum is because that`s what the mains of the toroid is running at. 230V/50Hz. Probably just me being paranoid about noise...

The wires from pcb to caps, should they be twisted maybe? Would it be any use running a common ground from the extra caps? instead of 4 wires each side, just three?

Cheers

- Bjørn

[Mooly]

My own opinions for what it's worth,
Any mods have to look original... if it ends up with wire's all over the place and bit's stuck here there and everywhere you won't like it
And yes there is a possibility of inducing noise into the wiring from the radiated field from the toroid..

More serious stuff. It's a Class B amp going off the size of the heatsinks, so 8200mfd on each rail should be plenty. If you increase this value make sure any switch on surge is within the rating of the rectifiers.

Adding decoupling can in some instances be worthwhile in an attempt to reduce the impedance of the rails at HF. I would add something like 0.1uF across any small electroylitics on the rails, depending on voltage ratings surface mount caps can be ideal and fit directly on the print on the PCB.

Power resistor to discharge the caps... no point whatsoever in that imo It will generate heat, theoretically it will increase the ripple on the rails, which at low volume is just what you don't want... although it should not be audible if the amp has a decent PSSR (power supply rejection ratio). It may also interfere with any designed time constants within the amp (unlikely) that ensure you can switch off and on quickly without confusing the switch on time delay etc.

[peranders]

What is your actual supply voltage? Maybe you can use 63 V types?

[Brillegeit]

@molly; your opinions are much appreciated! :-) I always do neat wiring, it`s my job.. But I get your point about it looking like a bowl of pasta.

The amp runs at A/AB, I don`t know when it starts running AB though..

This upgrade is also performed by the factory, it doesn`t say anything about changing of rectifiers, so I reckon they should handle it. I`ll check type and current-rating anyway, just to be safe. :-)

Guess I`ll skip the resistors. Thank you for your input on this one!

@peranders; I haven`t measured rails, but I will check on this. The original caps are Nippon 80V 8200uF 105C, won`t it reduce the lifetime of the new caps if I choose 63V?

I`m thinking about just replacing the original ones with 10000uF BHC slitfoil, and just some film bypasses. Will be much easier, and won`t look like a bowl of spaghetti! Then maybe at some later point, I can try adding the original ones, to see if it makes a difference.

Cheers

- Bjørn

[Brillegeit]

Checked the rail voltage; +-36V! Then the 80V caps are a bit overkill, don`t you think?

If I go for 63V, I can fit bigger than 10mF as well so that`s a bonus! :-)


Cheers

- Bjørn