Fetzilla Build Documentation

[AndrewT]

I suspect air flow over the semiconductors are changing the parameters due to small changes in device temperature and that changes the offset.

[meanman1964]

Test with some kind of covering over the amp?
Edit : With covering still the same result

[meanman1964]

You can read at the schematic the following "Adjust R12 to set T3 Vds to aprox 8 Volts"
Is this important and how can I measure this(probes placing) ?

[AndrewT]

Yes, this is important.
There are three presets. All need to be set to instructions.

[meanman1964]

Bias is set at 380mV, bootstrap voltage set at 24,8 Volts(24Vac toroid) but How can I measure the Vds of T3?
Probes connected to Drain and Source of T3 correct?

[AKSA]

Yes, MM, that's it, across Drain Source of T3.

Offset adjustment on this amp is not wonderful, but it's not bad either, and while it will drift up and down a few mV, this is not at all critical AND HAS NO IMPACT ON SOUND QUALITY WHATEVER.

Noise is more of an issue, however, I do concede that. If you can hear white noise from the speaker at distances more than 1m then there is something wrong. I did not find my FetZilla noisy at all, thought it very quiet actually. Not as quiet as the NAKSA, but damn close.

Andrew, this amp was designed to reproduce sound in a pleasing way. Does it do it for you? Do you have any comments on the sound quality, or is this a verbotten topic for fear of ridicule?

Cheers,

Hugh

[meanman1964]

I realy don't hear any noise at all.Testing with a vintage speaker the DM14 from B&W which has a DC and overload protection and it doesn't kick in.My other Dc protection unit is maybe a little too sensitive.What could be reason of this drifting?

[AKSA]

MM,

Offset drifting is due to the use of a singleton rather than a long tailed pair, and the variation in 1st stage current due to thermal cycling of the voltage amp, the ZVP, which changes its Vgs with temperature.

The method of powering the input stage chosen here necessarily involves some compromises in offset control, never a trivial issue, but this was done to keep noise to an absolute minimum and to create an even harmonic profile. The offset variations are sluggish, predominantly infrasonic, so they don't actually create audible noise. There is no doubt the LTP, the standard diff amp, used on 95% of designs is superior from an offset POV, but it has other problems, not least being linearity at large inputs.

If you don't hear noise then the object of the exercise is met, and God is in His Heaven!

To keep the ZVP at a constant temperature, it's heatsinking on the board is carefully worked out to run it at a highish but consistent heat. You should wait until it reaches this temperature before setting the output offset. Same applies to quiescent, of course, although the tempco of the laterals is very much in our favour here......

Andrew,

Do you think you might answer my simple question, civilly put?

Cheers,

Hugh

[meanman1964]

Originally Posted by AKSA

MM,

Offset drifting is due to the use of a singleton rather than a long tailed pair, and the variation in 1st stage current due to thermal cycling of the voltage amp, the ZVP, which changes its Vgs with temperature.

The method of powering the input stage chosen here necessarily involves some compromises in offset control, never a trivial issue, but this was done to keep noise to an absolute minimum and to create an even harmonic profile. The offset variations are sluggish, predominantly infrasonic, so they don't actually create audible noise. There is no doubt the LTP, the standard diff amp, used on 95% of designs is superior from an offset POV, but it has other problems, not least being linearity at large inputs.

If you don't hear noise then the object of the exercise is met, and God is in His Heaven!

To keep the ZVP at a constant temperature, it's heatsinking on the board is carefully worked out to run it at a highish but consistent heat. You should wait until it reaches this temperature before setting the output offset. Same applies to quiescent, of course, although the tempco of the laterals is very much in our favour here......

Andrew,

Do you think you might answer my simple question, civilly put?

Cheers,

Hugh

For me this amp is death silent no hum or what so ever,the bias is rock steady,bootstrap voltage set on 24,8 Volts(can't set it lower) and the offset is drifting but stays below the 10mV.And yes,I also noticed when the amp is warmed up it's a bit easier to set the offset.Now move on to the next channel and after that's done compaire the Fetzilla with my Blame ST.One thing I do know these are two total different sounding amps.

[AndrewT]

Originally Posted by AKSA

................Andrew, this amp was designed to reproduce sound in a pleasing way. Does it do it for you? Do you have any comments on the sound quality, or is this a verbotten topic for fear of ridicule.............

Originally Posted by AKSA

.........................Andrew,

Do you think you might answer my simple question, civilly put?
...............

Is your question aiming to "ridicule"?
Are my previous "sound" reports" good enough?
It appears not.
You said in response to one of my reports

Sorry Andrew,

Your findings, not mine.

giving the impression to other Members that I should just be ignored.
Was that "civil"?