Rerurn to Romy the Cat's Site


In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: Approaching the LF DSETPosted by Romy the Cat on: 12/22/2015
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?
   
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.

Rerurn to Romy the Cat's Site