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Romy the Cat
Boston, MA
Posts 10,184
Joined on 05-28-2004
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Post #:
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63
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Post ID:
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22334
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Reply to:
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22331
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fiogf49gjkf0d  anthony wrote: | Hi Romy,
So it is time to think about the bass output
transformer in my DSET. My bass channel will be 8 x 25W/8565-00 each side
set in a sealed box and will cover the frequency range below say 100Hz.
These drivers can be wired anywhere between 1R and 64R which gives quite
a bit of flexibility for adjusting the amplifier/speaker system damping factor.
Reading your threads about your Scanspeak arrays it is evident that the
system damping factor is important for the bass channel and is influenced by
the OPT decisions, how the 6C33C is loaded and most likely by how the speaker
is wired.
I propose to wire the drivers in parallel so
that they present a nominal load of 1R to the amplifier which in effect will
maximise the system damping of the drivers, which from what I take from your
writings could be a good thing. Then I will have a bass OPT wound with a
secondary of 1R or 1.1R that will pass 450mA and cover say 5Hz - 800Hz before
rolling off.
Romy, given your experience with the amplifier
and the drivers, are you able to offer any advice regarding the OPT or wiring
of the drivers before I give the transformer winder my final specs?
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Anthony, this is a very good
and a very complicated question. The
question is boil down to the following: to connect in parallel and drive low
impedance or to connect to series and to drive high impedance. There is no
right answer to it in my view as not one did a methodologically clear
experiment and got the empirical answers. I would outline for you my thoughts
on the subject and what I would do, under no circumstances take it as “right
way” to do it as there is no right way. Examine my reasons yourself and with
your other advisers and make a decision for yourself.
There are 5 main points that
shall be taking under consideration when you think about the subject: output
tube, transformer, cables, drivers, usability. Let me to describe each of them.
The Output Tube. I
personally do not think that there is any difference for the tube to drive low
or high impedance behind a proper transformer.
Let pretend your tube wants to “see” 1.500R. If you have on secondary 0.5R or 64R it would
not be any difference for tube with appropriate transformer that converts 0.5R or
64R to 1.500R. It would be a different transformer ratio (and to a degree a
deferent transformer design) but I doubt that tube will see some kind of
“reactance” on secondary. Consult on the subject with some smart tube amp
designers (good luck to find them) but my vote goes to the statement “tube does
not care”.
Output transformer. Here is
what the thing become complicated. Of you drive ultra-low impedance with step
down transformer then your secondary turns will decrease. They primary will
have whatever they have for all case but secondary will go lover and lower as
load impedance drop and might even become foil-made. That is for sure will make
the transformer faster and better for transients BUT your need LF DSET
transformer it means you need inductance. As you will be losing your turn on
secondary you will be losing inductance and the only way to get it back would
be to increase the core size. You need
to put your transformer maker on the task and see if you will be able to obtain
a transformer core for half amper gap, no secondary turns and 5Hz at full
power. That will be a small refrigerator size piece of iron and it might be
very expensive.
Cables. Driving 1R you will
be driving current. Driving 64R you will be driving voltage. Some of the cables/connectors
behave differently under voltage vs current conditions. Also if you drive 1R
with low voltage then you might pick some on like interferences.
Drivers. If you go with 8 x
25W/8565-00 then make sure that your make the right boxes and over dump them. I
would use 4 individual boxes by 2 drivers each. I do not think that drivers
would care how they are connected. The difference would be in case of driver
mismatch. In real world all drivers will be slightly different. You are not a
large speaker builder and you won’t be able to pick among 100s driver the best
matching drivers. So, if one driver has Fs 20Hz and another 24Hz then the
question is: would it be betters for them to be connected in parallel or not. I
do not know the answer and I am not informed enough to make a guess. You need
to consult with somebody else, BTW, if you do them post a received answer.
Usability. That is bitch.
First thing first: how do you know how much your tube need to be loaded?
Pretend that it is 6C33C and pretend that you need to load it with 600R to get
a balance MF sound. Idle tube more you dive you less harmonics, faster sound,
in a way over-burned transients. Loading the tube more five you “slower”, more
euphonic sound and more power. That would be true for MF but at LF you will have
much less negative impact of tube idling and you might find VERY beneficial to
load your 6C33C with 1 kR or 1.2kR. That are not exact number but I just give
you an idea. Also, the differences at 1R load are huge. Each change of 0.1R
loading will give you a large difference in sound. You will need to hit very
prissily and you as you start will not know where to aim. I can only presume
that you will be in the realm of 1200-1500R, or perhaps higher but with “odd”
transformer and a few other variable it is very hard to predict anything. Do
not also forget that with limited wiring on the secondary you will have very
limited opportunity to have multiple taps on your transformer as the filling of
the barbing will be suffering.
Usability continue. There
are more ugly things: as you ideal more your bass tube and get better sound (to
a degree) you will be losing power. Even with 8 drivers and let say a single
6C33C you will need a sealed room of let say no more than ~300sq feet or you
will be driving your 6C33C into A2, which is a clipping for this tube. If you have a normal listing room of 600+
with open floor plan then you are way out of your DSET capacity with 6C33C
unless you super load the tube and then you have shity sound. Even if you did
resolve your problem with power somehow then the ugly truth that it will take
time for you to understand what type of bass you need. My estimation that it
would be good to stay with properly doe bass for 2-3 months in order to “get”
where/how to move next with bass.
So, from all that I said
above if I were in your position I would not go final DSET solution with bass
now but rather did it incrementally. Make your bass sections, connected them conventionally,
let say 16R each and drive them with some kind of SS amp. Then take the SET
that you would like to use as DSET and drive it into the bass channel. Play
with loading, find the best configuration and make a judgment about sufficiency
of power. Then go for a DSET that would be against a conventional load. 4x16R in
parallel = 4R. That would be a conventional load and you can use many different
amps to try, including your SET, If you test everything with your SET, find a proper
loading and get conformation about power then you can converts the found transformer
ratio to any another load.
That is basically what I did
over the time and this is what I would do if I do it again. Do not forget that
your situation might be very different from what mine was in my old city home.
Your bass channel will be running under 100Hz and your room dissipation at
100Hz might be very different then at 40hz, so you might need much more power than
you might think. It is nice to learn about it experimenting with SET then to
pay a lot of money for DSET OPT and then learns that you have not enough DC gap
to add another tube in parallel or that you need to lose another 10% turns on
secondary and the cost of shipping of the transformers will cost more than a
pair of new one.
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
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