Re: One a Minute ?



Phil Allison wrote:

** Hi to all.

Aussie electronics magazine " Silicon Chip " has just published a technical
review of this amp:

http://www.fountek.com.au/products/a3500-ss/

They verified most of the specs OK - modest though they be (ie 0.63% THD).

They added one new one, the damping factor was only 1.6 with both 8 and 4
ohm loads - which is typical for guitar amp output stage !!! Even by
early 1950s hi-fi tube amp standards, this is piss poor indeed.

Guitar amps are usually always set up in pentode mode with screens taken
to
a fixed B+.
So, with a pair of 6L6 with Ra-a = 66k ohms and ZR of OPT 5k : 8 ohms,
the transformed
Ra-a at the output secondary becomes 105 ohms.
After adding in say 1 ohm of OPT winding resistance, Rout = 106 ohms.
The amount of GNFB used in guitar amps is very low; many don't use any
at all
so the amp operates as a current source with poor DF but
it doesn't matter; the guitar amp has its own speaker and a non flat
rising speaker output
as F rises is all part of the mystique, along with high THD et all.

The Chinese amp quality is visually impressive.

But when you dig beneath the surface, you will often find the
schematic to be attrocious.
The fucking crap has a printed circuit board so changing the schematic
to something decent means ya gotta rip out the whole guts
and put in tag strips and rewire all if it.

Notice how the Fountek site doesn't show a schematic?

They are so ashamed of themselves they keep the real nitty gritty
technical content to
a minimum.

The amount of NFB is usually very low BECAUSE the if they tried to use
more
the darn thing would oscillate and that's usually BECAUSE the chinese
don't show any ability
to understand the principles of critical damping of an amp with NFB.

With low NFB, the damping factor is poor, and THD is high at full PO.

ALL the Chinese amps I have seen are poor copies of amps which were poor
designs
and made in the west in the 1950s.

The amp is most likely configured in 40% UL which gives a typical
DF = 1 without NFB because the Ra-a of the output tubes
is much reduced just by the UL connection. The UL Ra-a divided by the
OPT ZR
is about equal to the speaker loading.

So perhaps the NFB is only 4dB, which might give a DF = 1.6.

HOWEVER, most people in fact will not notice the poor DF and they'll
maybe even get a welcome peaked bass response which is great for
boomchikkaboom "music".
There are sensitive Chinese speakers equal to generics made in 1960,
and these work fine with the Chinese amps.

Piano is largely unaffected.

Levels used by most are at 1W or less average, and THD is much lower
than the 0.63% which would be at 1dB below
clipping at 32 watts probably.

Loverly when ya plug in ya i-pod....


Models sold by Quad and Leak well over 50 years ago delivered THD figures of
0.1% and DFs of around 10 to 20.

These old amps had much greater amounts of NFB.

This Chinese made 32wpc power amp retails here for a cool A$1900 .......

Bet they make 'em one a minute, for the legendary market with that same
birth rate.

...... Phil

The cost ex the Chinese factory excluding bribes and BS is about $200.

The Fountek is expensive; there are others selling by direct sales
on the Internet for $700, and 2 x KT88 per channel.
I had to re-wire a pair of them a couple of years back to stop them
smoking
and force them to make music.

Mediocrity attracts far more customers than excellence.

Patrick Turner.
.

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