On Nov 8, 2:32*pm, Bret L wrote:
However, In the US many would buy a Hammond to get 16W of SET power
and the OPT might be only $100, and freight lower, so there is always
going to be cheaper than Monolith.
Price isn't everything the same to everyone.
Perhaps the Monolith has better *winding* quality than the Hammond,
which BTW could be improved if they added about another 30 minutes of
labour time, and then the quality difference would be negligible.
*The new transformer line looks like a winner, the proviso being I
just don't think SE makes sense for hi fi.
Many ppl would say SE amplifiers have too many shortcomings to be
considered adequate to produce a genuine hi-fi experience.
But the parameters within which hi-fi can be produced is where the amp
has enough headroom for desired dynamic range, the bandwidth is
between 14Hz and 30kHz, -3dB, damping factor is at least 10, and THD +
IMD is under 1% at 1dB below clipping, and declining at least with
output voltage or better, and SNR at wanted average power levels is
better than -75dB, unweighted.
Unfortunately, it is almost impossible to conveniently make an amp
comply with ALL the above requirements without using a considerable
amount of loop NFB of some kind in addition to having triode
operation, or local NFB from OPT.
And many audiophiles state a dislike for any form of loop NFB, so they
must put up with a compromise here and there unless they make the amp
extremely inefficient by using triodes with a very high OPT P to S
turn ratio.
For example, one EL34 in triode at just under clipping may produce
6Watts into 8 ohms, 6.9Vrms with max THD at 5% into load X. Bandwidth
is easy to achieve, and perhaps low noise, even without loop NFB.
There will be enough headroom if horn speakers are used, or the
speaker is a metre away and has say 93dB/W/M efficiency. But the DF
might be poor.
At 0.25W, the VO = 1.41Vrms and we could expect THD = 1.0% and the
resulting IMD about 3% where there was a 4:1 ratio between a bass
signal at say 100Hz and treble signal at 1kHz. So even at 0.25W, THD
+IMD exceeds the 1%.
But with horn speakers the VO level could be 0.35Vo, with THD = 0.25%
and IMD at 0.75% and we have THD+ IMD under 1%.
But suppose we have 10 x EL34 in parallel to make say 10W. Then each
one is producing 1W and we would the RL for each at maybe 5 x the
value where only one tube is used. THD should be about 1.5% at 9.9W,
( 8.9Vrms/8 ohms ) so THD + IMD = 6%,
so at 0.25W at 1.414Vrms and 0.25W THD+IMD = 0.95%. With 10 tubes in
parallel and a very high P:S TR, a low DF could be achieved and we
have what is officially called hi-fi. About 97% of audiophiles refuse
to use multiple parallel triodes, and they still very much like their
results.
One could achieve similar results with solid state devices but only if
some form of loop FB was used, such as the emitter follower or source
follower connection.
I doubt many amps have been made using say 10 x power mosfets as
parallel SE source followers to drive a speaker load, but it has often
been done in PP, and without loop NFB using a pair of matched NPN or a
pair of PNP devices and with transformer input and output coupling.
See the website put up by Susan Parker, and her Zeus amplifiers,
http://www.susan-parker.co.uk/zeus-toroid-amp-1.htm
Having said all that, I know a guy who has single 300B tubes for each
channel to drive Tannoy dual concentrics, 96dB/W/M in 7Cu.ft boxes and
there is no loop NFB and regardless of the probable measurements might
be if made, the sound IS extremely good, and definately Hi-Fi afaiac.
I use a single EL34 in triode for my kitchen radio with a separate
speaker box and to which I feed AM and FM tuner signals.
Again, the sound is fabulous.
Putting the filaments and the HV on separate cores is a Good Idea, so
is having the bias winding on the fil xfmr.
Just why would using separate PTs be such a Good Idea?
I've never found any need to have separate PTs for the various supply
voltages needed. Its not a completely bad idea to have separate PTs
because if one fuses the HT tranny, then perhaps a replacement can
more easily be found, but the weight and size of the amp will be high,
and neither high weight or large size can necesssarily improve the
fidelity.
For example, suppose you had a 200VA rated 110V : 230V HT transformer.
Not hard to find such a thing in the US.
Isolation trannies of 230V:230V are easy to find where mains V is
higher than the US.
Then using a voltage quadrupler, the 230V winding easily can make
1,240Vdc to power a couple of 845 each able to make 28W of pure class
A in SET mode if one knows how. Without NFB, the DF would be about 3,
because the anode load would be about 7k0, and Ra = 2k2. But suppose
we add a mild and blameless 10dB of global NFB. The the DF would
easily be better than 10, and so the NFB saves us from using 3 x 845
where one is plenty.
Patrick Turner.