Swordfishy/ASPEN FETZILLA power amp

[gaetan8888]

Hello Hugh

I did suggest a ferrite bed between the power supply cap center tap to the star hearth connection for those who may use a transformer with just two secondaries, like you suggested to Ron.

Bye

Gaetan

[AndrewT]

Originally Posted by Mooly

............... You would have to read the whole thread to get the gist of it but this dives in at the most relevant point.

3 stage LIN topology - NFB tappings?


Now I'm certainly not saying that the same issue is at work here but it's something to look at.

I was enjoying re-reading this and remembering many things I had already forgotten about.

That is, until Dx piped up in post50 with his usual physcobable.
How can reducing buzz and hum not improve the audio quality coming from the speakers?
I am serious read my question again if you need to.

I am glad he has almost bowed out of interfering in this Forum.

[RonR]

Thanks for the responses and ideas guys, I'll be following up on them as time allows.

I forgot to say in my previous post that my AKSA amp has a PSU very similar to the latest Naksas, using CRC and one bridge per rail. Sorry for the confusion.

The recommended PSU from the documentation thread was the first thing I tried, the only difference being a single transformer, with both +ve bridges fed from 1 winding, and both -ve bridges fed from the other (exactly as Hugh describes above). It hummed with both inputs connected

With the second set of boards I left off the bridges and smoothing caps and tried 2 "HackerCap" boards, using 3 x 6800uF per rail per board in CLCLC configuration, but still fed from one tranny. Slightly less hum but still

Finally I added 15000uF per rail between the bridges and HackerCap boards, this resulted in the least hum. I can only hear it from <1M from the speakers, but it's still there.

So it looks like the hum is proportional to the amount of ripple on the rails.

This weekend I think I'll go back to the recommended PSU arrangement and try it with 2 transformers, to prove that I've built the Fetzillas correctly (or not - we'll see!).

Cheers,

Ron

[AndrewT]

My FetZillas are noisy with the inputs shorted. Probably noisier than any amplifier I have ever assembled.

I have tried a stereo pair and ran into hum problems as well.
They sound OK, but I don't describe them as great.

I will have to re-read post888 in case there is something I have got wrong or missed or not implemented.

[PaulBysouth]

Ron.
re post #886. My FETzilla amps where initially noisy when connected to my CD player (an Audiolab 8200cdq). The solution was to remove the earth lift resistor, ie the 10 ohm R26 in the attached drawing. I suspect my issue was that the Audiolab 8200cdq is earthed to the mains, so I had in effect an earth loop despite the 10 ohms. Your FETzilla may have the same problem. Just cut R26 off the pcb, the back to back diodes are sufficient. With R26 removed the are very quiet.

My FETzilla amps are build as two separate monobloc type amps (in NAKSA-70 chassis).

I hope this helps,
Paul Bysouth

[AKSA]

Ron,

I support Paul's suggestion; cut the 10R float resistor across the two diodes on the ground scheme. This should help with hum. Of course, quiescent is 400mA on this amp, so ripple is difficult to suppress completely.

Paul's monobloc FetZillas are very, very quiet, BTW.

Cheers,

Hugh

[ajst2duk]

Yep my dual mono's are very quiet - still liking the sound very much - in fact it's sitting in our main 2ch music rig now. -AJ

[RonR]

Spent some time last weekend putting together some "Modules" and cables, so I could test various configurations of PS.

1st experiment - 2 transformers, 4 bridges, 4 CLCLC cap banks using 6800uF / 10uH. Result - silence. No hum at all even with my ear right next to the speaker cone, so it looks like the boards are correctly assembled.

2nd experiment - 1 transformer, 4 bridges, 4 CLCLC cap banks. Result - silence with one input connected, and hum when both inputs are connected. The only change from above was to remove the 2nd transformer and connect the 2 windings to 2 bridges each.

3rd experiment - 1 transformer, 2 bridges, 4 CLCLC cap banks. Result - silence with one input connected, and hum when both inputs are connected.

4th experiment - Removed R26 on both boards. Result - no change to the hum at all.

If I connect one input and just the outer shield of the other input, it's silent. I don't think that earthing's the problem here.

In fact, the more I listen to the Hum, the more it sounds like a "Buzz".

[AndrewT]

Ron,
you are confirming what I found. I cannot get a quiet stereo amplifier arrangement.
But I cannot get a completely quiet mono amplifier arrangement either. I wonder if these coincidences are linked?

I have asked Ajs to confirm his readings, but no reply yet.

[RonR]

Hi Andrew,

My dual-mono configuration is dead quiet. In fact I had to look at the LEDs on the boards to check they were powered on.
The trouble is I can't fit 2 transformers in the case!

Cheers,
Ron