Keithr wrote:
Patrick Turner wrote:
GerryE123 wrote:
Hi:
I recently acquired 4-ohm speakers and re-wired my SE 45 amp to use the
4-ohm OPT tap. There's an autoformer on the home audio market that
effectively multiplies the load your amp "sees" by 2x, 3x or 4x.
Is it possible or even likely, that my system could sound "better" using
this autoformer and re-wiring the OPT impedance tap to 8 or 16 ohms? I
realize this question may be more complex than it sounds (many variables).
I have a VERY simple, but high quality system. The thought of running my
signal through a large coil of wire (the autoformer) is not that appealing,
but if it could sound better...
Gerry
P.S. I know I could just add a resistor, but that has it own set of
problems.
A company or bloke runs a business called Zero Impedance and makes
toroidal transformers rated for maybe 100W for allowing a 4 ohm speaker
to appear to an amp as a 16 ohms speaker. Or vice versa, something 16
ohms can be made to appear as 4 ohms, 8 ohms, whatever.
I measured one of these and found bandwidth to be extremely wide and
distortion far less than that occurng in the tube amp concerned
and winding losses to be negligible.
Two guys here have such transformers to optimise the load match between
amp and speakers.
Both said that if they used the transformer to make a 4 ohms speaker
appear to the amp as an 8 ohm or 12 ohm load, the music sounded better.
That's because the distortion becomes lower and damping factor becomes
higher.
Exactly how does the damping factor become higher? If you use say the 8
ohm tap on the OPT, then an autotransformer to step this down to 4 ohms,
provided both transformers do what they say, nothing is changed from
using the 4 ohm tap in the first place. The output stage sees exactly
the same load impedance and the damping factor is unchanged because the
speaker still sees the same source impedance.
Suppose a tube amp has a DF= 8 at the outlet labelled "8 ohms"
This would mean the Rout of the amp is 1 ohm.
With 4 ohms used at this outlet, the DF is only 4.
With a 2 : 1 impedance matching transformer with turn ratio of 1 : 0.7,
the 4 ohm speaker is made to look like an 8 ohm speaker load at the amp
and DF is raised to 8. If the ZR = 4:1 with a TR = 2:1, the 4 ohm
speaker becomes a 16 ohm speaker and DF = 16.
Suppose you need 2.83V for 1 watt of SPL with an 8 ohm load at the amp.
With a 4 ohm speaker of the same sensitivity you'd need only 2V to get
the same sensitivty, but there will be some increase in THD.
If you make the 4 ohms speaker look like a 16 ohm speaker, the amp needs
to make 4V to get that watt of sound.
Usually the THD will be a bit lower because of the higher load despite
having to make more voltage.
But you can see that you'll run out of voltage headroom sooner with a
step down voltage transformer.
if you step up the voltage, you'll run out of current headroom.
More info at
http://www.zeroimpedance.com/faq.html
The down side is that they run out of headroom at a lower level of
*power*. With most music this doesn't matter if the amp has a high
enough ceiling, but an SE 45 amp hasn't got that unless you have horn
loaded speakers.
To get the best sound from SET, you really do have to know what the
anode load is and do the required loadline analysis to make sure what is
the right load for symetrical clipping. Using a load below this critical
load breeds much more distortion and using a load above it
brings less distortion. But either side of the critical load for
symetrical clipping, SE power ceilings diminish.
Ya gotta know what ya doin.
These transformers may be fine if you do not have an output tap to match
the speakers, but it is hard to see what they bring to the table if the
correct tap is available.
Most modern tube amps are configured to make their maximum clipping
power when an 8 ohm load is connected to what is labelled an 8 ohm
outlet. Some have a 4 ohm outlet which is usually a tap at 0.7 times the
OPT secondary turns meant for 8 ohms.
So if the OPT sec turns were 100t for 8 ohms, the 4 ohm outlet is at
70t. If you wanted a match to 2 ohms, the tap would be at the 50t point
or what is the CT of the 8 ohm winding.
Unfortunately, most "8ohm speakers" are often only 8 at maybe 1khHz but
have a big dip in Z between say 100Hz and 300Hz because of the crossover
filters between bass and midrange which is the middle of the band where
most audio music energy is located. Any tube amp which is configured to
give maximum PO at 8 ohms tends to have very poor performance when 4
ohms is connected so if there is "4 ohm" labelled outlet, ie, a tap for
the better load match, it should always be used with an 8 ohm speaker.
Most modern tube amps are PP amps set up so their makers can claim a
huge power level to get sales.
So we will see where someome has 2 x EL34 but PO = 50 watts, class AB1.
If you test many PP EL34 amps and plot the PO against load at 2% THD,
you get a peak in absolute maximum sine wave power at the onset of
clipping (2% thd) maybe at 50 watts. At whatever load is required to
produce this PO the amount of class A in the total AB PO is very small,
a few watts at most, and THD is high at all levels.
A sensible Hi-Fi amp designer will make the maximum PO of say 50 watts
be produced when the load at the OPT secondary is 2.5 ohms.
The OPT turn ratio is selected to give the load match so that 2.5 ohms
is where maximum PO is produced and loads either side of 2.5 ohms have
less power at clipping. When 8 ohms is used in such an amp, you'll find
that max PO might only be 30W class AB. But the DF will be high and the
THD low at all levels and the amount of class A = 12 watts before the
action becomes class AB. If 16 ohms is connected to this amp you'll find
only 20 watts might be produced but its all class A1 power, and if 20
watts is more than enough PO for most folks listening levels then that
20Watts has a chance of sounding very nice because DF is high, is low.
With many amps made by major brandnames, ARC, Manley Labs, the music
benefits with an impedance matching tranny to achieve what the makers
should have provided in their creations, ie, a better match for Hi-Hi,
not a load match which you'd only use if you were making a PA amp where
loudness is the most important consideration and not fidelity.
Many modern tube amps are merely PA amps with moderate global NFB.
Bloody awful. No Finesse. Sold by promoting watts, not what they do
with music.
When you consider SE amps with a lone 45, 2A3, 300B etc, the load
matching becomes even more critical than that for a quasi class B PA amp
with lots of power ability, if you have low sensitivity speakers.
With high sensitivity horn loaded speakers then 4 watts from a 2A3 is
more than enough. But for best sound you still need to load match
properly. I've devoted some of my web pages to load matching but the
message doesn't get through easily.
Load matching is like using the right gearbox ratio to match an engine
to wheel speed so the vehicle can be driven up a hill with the family
and luggage without the engine stalling. Or its like riding a bicycle
using the right gearing.
One of the very worst offenders in brandame amps where they got the load
matching appallingly wrong was in a CR Developments PP amp model known
as the "Woodham".
http://www.crdevelopments.com/ I had one here 18mths
ago for a reapir/rebuild. This utter POS had a reflected load anode to
anode = 3.2k, but with a 600V anode supply, and a poor copy of a
McIntosh output stage using small toroidal OPTs. It cooked output tubes
regularly. THD was appalling. DF was ****house, and the only good thing
was the metalwork.
I totally stripped it out and re-wired it to my own design and lowered
the B+ to 420V with a choke input B+ filter, then adjusted the output
transformer outlets so there was only a 4 ohm load match, ie, the RLa-a
was 6.4k instead of 3.2k, and the PO was the same as the original amp
but THD far lower, DF far higher, and no more fried tubes every month.
Many brandname designers have completely forgotten how to design and
manufacture a decent tube amp. CR Developments is such a brandame.
Silk Amps from Thailand is another brand, quite a few made in China,
Audion is another, labelled "Made in the UK" like the CR, but utter
crap,
and made to match the worst from China in a very determined manner. The
fuctards making all this torrent of crap give tube amps a bad name and
cause untold grief amoung music listeners. I hope they all go broke
during these hard times of economic hardship.
Patrrick Turner.