has anyone here listened, owned, or know a velleman kit form K4020 MOSFET amplifier?

[pkgum]

has anyone here listened,know or own a velleman K4020 MOSFET kit amplifier?

heres the link to it: http://www.qkits.com/serv/qkits/vell...ages/K4020.asp

WHAT ARE YOUR COMMENTS ABOUT THIS AMP?

[walker]

Pkgum, unless any here has built this amp it will be hard to tell. Do you have access to a schematic? It is the first variable bias amp kit that I have seen, (if memory serves me right). It sounds like an interesting amp, if it ever makes it down here to WA, (Wait Awhile (West Aust) I'll have too give it a go.

You got to love an amp with these specs though; 300W output, 70% efficiency, power consumption 220W? CLASS A!

But seriously it sound like it could be a great amp, who knows?

Regards WALKER

[Dirk]

This amp was reviewed in Audio Amateur in 1999. Seems like 300 W was a bit optimistic. Since I'm about to move, all back issues are in boxes, so I can't dig the article up easily.

Regards,
Dirk

[djk]

" This amp was reviewed in Audio Amateur in 1999. " I recall that review. They had problems with the amp oscillating in the megahertz region.

[pkgum]

"They had problems with the amp oscillating in the megahertz region." Can you explain this in simpler words? Is this a really bad problem?

IF ANYONE HAS ANY COMMENTS ABOUT THIS AMP PLZ POST IT

[walker]

Pk, high frequency oscillation is a common problem with some amplifier output stages, (especially those with high speed output devices-read MOSFETs). It is often referred to as parasitic oscillation and can smoke your output transistors! It is not a good thing BUT usually easily cured by limiting the output stages operating speed. A change to the output filtering or small capacitors or ferrite beads on the transistors usually does the job without effecting the amps performance.
What is concerning is that the original design was, allegedly, not unconditionally stable. These days that should not be a problem. I would like to know under what conditions it was found to oscillate. For instance was it only with electrostatic speakers that it became unstable? It would not be the first amp to have trouble driving capacitive loads.
To get the best performance out of control loops they are usually tuned to be as fast as possible, to the point of oscillation and then backed off a little. Amplifier design often follows a similar strategy.

I don't think that you should reject the amp without more information. I'm still be interested in the design, does anyone have a schematic?

Regards WALKER

[haldor]

Based on the specs it is a 100W amp into 8 ohms, 150 W into 4 ohm load (with the extra $45 of caps). Forget the "Music" power, the only thing that means anything is the RMS rating. I would also take the "Class A" claim with a grain of salt. The website terminology makes me think they are using variable rail voltages to keep the dissipation in the output devices down at low level settings (isn't this class G?).

Phil Ouellette

[walker]

Haldor, don't get fooled by RMS figure either. Some amps can produce say, 100W rms but if you blink you'll miss it. They often come with 60VA power supplies, don't you just love to hate those marketing departments.

I should start a new thread on marketing con's.

Regards WALKER

[haldor]

I guess thats like those 200 Watt PC speakers that use a 300 mA wall wart power supply.

Phil

[pkgum]

so the things not class A? look at the link below...it claims it is pseudo class A: http://www.snippets.org/ldsg/sect-19.php3

ANY PROBLEMS?