Luxman SQ38D
But she's been unable to focus much on her audio hobby
interests because of her heavy committments to well paying job at a
university.
** The uni's workshops must be where she gets all that nice metalwork and
cabinetry done.
* * Foreign orders anyone ??
I doubt it. Unfortunately, perhaps you don't know Susan as well as I
do, and bearing in mind your own difficulties with females and your
sprinklings of negativity I can only smile at your comments, and I
hope in that in the fullness of time you would find a wealth of
personal pleasures to be had where you wouldn't dream of dissmissing
so many people because you can gain access to their wit, their
determination to genuinely discuss things, and other wondrous facets
of human bevaiour so that you benefit as they might, without duress,
or verbal scaldings of boiling oil when there are inevitable
dissagreements, as indeed there must be for any of us to attain
wisdom.
Well you may complain about the costs of building a minimamlist
amplifier with just an input tranny and output tranny and 2 N mosfets
in source follower mode and class A. Ah, but the cost you say -
hopeless silly effort. Indeed cost is high, but all hobbies are all a
waste of time and money when that time and money could be put towards
fine charitable causes, or medical research, or R&D for a company
wanting to make 1 square metre solar panels that are 50% efficient for
$2 each, with lightweight batteries to go with them. Surely the
persuit of listening to music with valve amps is a true monumental
waste of human resources, and you'd prefer them all to beat a path to
your door to get advice on how to make an amp for $3, weighing less
than 100 grams and 99% efficient. Of course that's exactly where the
audio industry had headed with PWM amps fed by digital audio signals,
so the world beat you to it. Its another huge tragic waste that the
world wants 6.1 home theatre, with screens 10 feet wide and suburban
houses which compete with those owned by Louis IXV, and of course
everyone needs 2 cars each and they want to each eat enough to feed a
family of 16 Africans, and only vote for parties that give out yet
more Guvvy assistance. They then spnd hundreds of thousands on
expensive messy acrimonious divorces, and while learning nothing,
repeat it all. So I see your point in complaining about the cost of
Susan's minimalism approach. Don't worry, she more than compensates by
not having children and living a very simple life while owning a push
bike, while battling on in a maddening world.
I'm left wondering also about what became of your website? Where has
it gone? But then it dawned on me that perhaps you never got around to
inviting the world to your mind, so they may see how good audio could
be achieved. Was it all too vain and selfish to make a website? That's
a real pity considering you have vast knowledge. Nobody I know, except
possibly someone who might be your twin, would frown on having a
website.
In 1990 I frivolously built a pair class A amps with 4 mosefts with an
OPT. Mosfets are driven by LTP with 2 garden variety bjts in LTP and
one for a CCS. It has cap coupling, and works just fine, and gives 40
Watts in class A with THD 0.3%. I've never found anyone to say it
sounds worse than a tube amp with 4 x EL34 etc, and one may wonder why
I made these things but I enjoyed the experience and it taught me
more. One of these days I may convert it to Circlotron and have a pair
of EL84 as input/drivers, but I may never find the time.
Susan's amp has a step up input tranny with 600 ohms impedance. The 2V
input must be raised to roughly 30V output for gate to gate drive, and
to get input Z = 600r, the sec load must be 136k, and if Sec C load =
100pF, then Cin at Pri = 0.022uF. I'm guesstimating here but HF roll
off would trouble many people unless thay had a very low drive source
powering the IPT and if they didn't have a decent IPT.
Sue just uses a power mosfet as source follower to drive the IPT. The
lower the drive source Z, the less the characteristics of IPT and OPT
will cause distortions. Of course many say its a waste, but it matters
not, and the status quo is stained by productions governed by
accountants, and anything other than what mainstream industry does
becomes invalid.
Anyway, using a pair of EL84 in triode with Ra = 2k5 to drive perhaps
50pF of input C at gates seems a reasonable solution. Having mosfets
in circlotron with simple OPT means you have matching devices, and if
Vds on each mosfet is 20Vrms, then gate drive = about 10.5Vrms Vg-0V,
and the amp Rout is low enough and because class A, and ß = 0.5 and
open loop mosfet A = about 20, then THD is low, bandwidth vert wide,
and so you get hi-fi, fairly simply, without GNFB, or an IPT. The OPT
is probably best if an auto tranny, and can have 8 ohms input end to
end for 50 Watts of output with 20Vrms. If OPT core = 50mm stack of
50mm tongue E&I with partial air gapping for µe = 3,000, then wire
size is large and its quite easy to wind. On each side of PP circuit I
think a good load is 32 ohms for each of 4 class A mosfets, so that
all four drive the OPT primary of 8 ohms. Taps for 4 ohms and 2 ohms
are easy but I'm still not sure of HF performance because with say 2
ohms, using 1/2 the total auto tranny turns, the leakage inductance
affects the HF pole.
I'm trying to figure out a decent winding set up that allows many Z
combinations so as RL is lowered, the class A action remains and the
amp doesn't move into class AB.
There doesn't seem to be very much online about making audio auto-
trannies with very low winding losses and HF losses for Z range
between say 1 ohm and 64 ohms. Having an auto tranny which gave say 64
ohms to 4 ohms would be a boon to owners of tubed OTL amps. They tend
to become dissolutioned by OTL tubes suffering bias failure from
overheating due to the inevitable very poor load matching even with
multiple 6C33c output tubes. The load line analysis I've done on 6C33c
indicates good class A is available if Ea = +160V and RLa = 250r, and
Pda = 60W with Ia at around 370mA. 4 such tubes in class A can give
40Watts, and OPT primary RL can be 62.5 ohms. To my mind, this is
superior to using the same tubes hooked directly to 8 ohms and forcing
tubes on each side of PP circuit with low bias, and in class AB2, to
each produce say 1.4 amp peak current. 40W into 8 ohms is 3.15 peak
amps, so at least 6 x 6C33c are wise, and Pda with continuous signal
invites smoke, and things get worse with 4 ohms. Auiophile say they
like the transparency of OTL, but all OTL I've seen have a very large
amount of NFB applied, and its NFB that improves the sound, rather
than the absence of OPT.
But I digress.
I hope Trevor has by now worked out what is the best tone control with
tubes, IMHO, ( apart from scrapping tone control completely and
loudness function ). It is achieved with cathode follower input
driving a Baxandal type R&C circuit within a unity gain shunt FB
network So at least 2 halves of 12AX7 are needed for 1 channel.
Usually such unity gain tone control stages do not color sound and
nobody I know can tell when they in the circuit and not being
bypassed. Such
circuits may be left in circuit un-bypassed and they provide a low
Rout drive to volume control pot immediately before power amps. But
line stage gain and power amp gains need careful study. Most ancient
amps have high sensitvity because line-in source levels were 200mV, or
less, like Leak, with 100mV, because ppl hated paying more the extra
tube needed to get phono amp output higher when using a lone EF86 per
channel, etc.
Lord knows what might be in the SQ38D Luxmam, but typical tone control
circuits in 1960 used 1 x 12AU7 to raise signal by 22dB, then feed
that to a passive high value R log pots and C etc with very little
overall gain. Such old amplifiers with such tone controls usually
suffered C value drift and pots became noisy, and 12AU7 has no NFB so
it puts in 2% THD, and I am no fan of ancient stuff this way and they
are best reformed with unity gain with FB and linear pots. It means
gutting and re-wiring, and many hours of work. Usually the rest of all
such Old Junk amps also need gutting, re-designing, then re-wiring. I
have several here awaiting My Treatments. One has to end up doing what
bean counters dissallowed in 1960. Such old junk then sounds at its
possible best, which can be "brilliantus maximus", with quite low THD
and noise, with both channels having equal gain and phase shift so
image is good.
Its very rewarding work to do. I used to re-engineer some SS amps, eg,
Phlame Linear, but after awhile it seemed like cleaning toilets, I
dunno why, but at least I reduced the Phlame factor and smoke.
I have a pair of ancient 30 Watt RCA amps in the pile, 2 x EF86 + 2 x
KT66. Hmm, a delicious opportunity to make 'em much better, but I
don't know whay they were parked by owner 30 years ago - maybe new
OPTs are needed! Trevor would be unable to persue me for a better
mission plan than the one I might use myself - Draw up the existing
schematic, think about it for a couple of days, decide what to change,
check available space for parts left out in 1960, draw up everything
properly, leave room for flexibility for changes during the process.
Pay attention to switches on front panel. Typical 1960 amps were
riddled with Do-Dah switches for tape monitor, hi-lo cuts, and pahse
of signals. These are all best omitted from the new design, and often
I have had to make a new front panel with less stuff&junk on it. Often
more tag strips need installing, plus far better electros and new Si
diodes to replace tube rectifier/s. Often IEC socket plugs are wise
plus all new speaker terminals allowing 4mm banana plugs, and all RCA
sockets need changing. The list of things needing to be done would
fill a couple of screen fulls. No wonder I found I could not earn much
as an audio tech on this type of work for owners loathe to pay a
socially just wage.
But for me, since I took deep interest in audio maters in 1993, it was
about the wonderment rather than the dollars. Because I worked my arse
off when young, I have escaped paying rent, or upkeep of ex-wives etc,
and i could afford change from the building trade to audio trade. Its
no wonder that so few ppl try to fukkabout commercially with tube
amps, but I didn't fancy learning about computers, or fixing
toasters.
Patrick Turner
.... * Phil- Hide quoted text -
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