Swordfishy/ASPEN FETZILLA power amp

[ajst2duk]

Originally Posted by ratbagp

Last weekend I visited Greg (aka Swordfishy and GregH2). I took at few photos that you can see at
Bottlenecks Audio: Visiting Swordfishy (aka GregH2)

It's always interesting to meet people who you know from forums and my experience is that they are always friendly and willing to share their experience and knowledge. Greg is no exception.

ray

Looks like you had a good time - I think it's time for another round of group buys when Hugh gets better !!
Great design Greg, this was still the most rewarding hifi project for me yet.
AJ

[syklab]

Hi,
Finally got sometime to finish off the Fetzilla today, fixed at 24V across R12, quiescent at at 37mV and offset is less than 2mV. My transformer were marked as 26VAC but measured to be 29VAC, a bit hight. The initial listening is very sweet sound, detail and deep sound stage and I will burn in a bit more tomorrow. The only thing I not sure is the temperature is a bit high at 55 degree C without playing hard and the room is less than 28 degree C. Is this normal? Is there anything else I should check?

[keantoken]

I ant to say something I realized recently. I as told to try different caps for the bias generator bypass. I've observed many times in the amps I've designed that simply touching this cap changed the stability of the amp drastically. This makes sense. The VAS output is a sensitive node ith large signal. Any small capacitance or nearby trace can have an effect. The bypass cap itself has a large surface area, usually most of the surface area of the VAS output trace comes from this cap. So the cap is essentially a radiator or antenna. The size of the cap you put here ill determine a lot of hidden factors such as radiation to other traces, and total capacitance to ground. Furthermore RF coupling through EM aves may turn it into an antenna.

The Fetzilla has FET outputs so the capacitance from this node is already high. The capacitor still radiates hoever. I find in my latest prototype I had to sanditch some aluminum foil in some masking tape and shield nearby traces from this capacitor in order to achieve stability comparable to simulation.

In any case, I thought it may be a good idea to solder a ire to ground and rap it a fe times around the bias generator bypass cap to short out its large EM field. I used a strip of foil stuck in tape, but I think the ire should be sufficient.

I ould try this only if you're experimenting, the official Fetzilla is fine as it is.

[syklab]

Originally Posted by keantoken

I ant to say something I realized recently. I as told to try different caps for the bias generator bypass. I've observed many times in the amps I've designed that simply touching this cap changed the stability of the amp drastically. This makes sense. The VAS output is a sensitive node ith large signal. Any small capacitance or nearby trace can have an effect. The bypass cap itself has a large surface area, usually most of the surface area of the VAS output trace comes from this cap. So the cap is essentially a radiator or antenna. The size of the cap you put here ill determine a lot of hidden factors such as radiation to other traces, and total capacitance to ground. Furthermore RF coupling through EM aves may turn it into an antenna.

The Fetzilla has FET outputs so the capacitance from this node is already high. The capacitor still radiates hoever. I find in my latest prototype I had to sanditch some aluminum foil in some masking tape and shield nearby traces from this capacitor in order to achieve stability comparable to simulation.

In any case, I thought it may be a good idea to solder a ire to ground and rap it a fe times around the bias generator bypass cap to short out its large EM field. I used a strip of foil stuck in tape, but I think the ire should be sufficient.

I ould try this only if you're experimenting, the official Fetzilla is fine as it is.

Does your Fetzilla runs hot? Which bypass cap you were refeering to?

[keantoken]

Bias generator bypass. Bypasses the bias pot...

My fetzilla ran fine. I just thought I ould mention it for those to hom it ould be useful to kno.

* My key does not ork.

[smartx21]

Originally Posted by keantoken

Bias generator bypass. Bypasses the bias pot...

My fetzilla ran fine. I just thought I ould mention it for those to hom it ould be useful to kno.

* My key does not ork.

Try ALT+87 for "W" and ALT+119 for "w".

[keantoken]

You are the third person to suggest that! It doesn't ork for me for some reason.

[syklab]

Originally Posted by keantoken

Bias generator bypass. Bypasses the bias pot...

My fetzilla ran fine. I just thought I ould mention it for those to hom it ould be useful to kno.

* My key does not ork.

Is it C9 22uf that you are refeering to?

[keantoken]

Yes.

[syklab]

Originally Posted by keantoken

Yes.

Hi,
How hot does your Fetailla gets?