|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
194.208.88.198
In Reply to: Which PS would you rather listen to? posted by dave slagle on May 7, 2007 at 08:10:23:
Hi Dave!What might even be more educational is to monitor the secondary voltage of the power transformer. This shows quite interesting things.
Whilst we like the advantage of the quite constant current draw of choke input supplies, your simulation shows that there is an instantaneous reversal of this current in the power transformer every cycle. Difficult to believe that this does no harm.
I always tame the spikes generated by this current reversal by a small cap before the first choke (in case of single phase supplies). I monitor the secondary voltage on the scope and size the cap such that the waveform looks like an undamaged sinus again.
This problem with choke input filters is even worse with silicon diodes. I think at the point where the current reverses there is a small amount of time in which both diodes fight against each other.
This is a bit different with three phase supplies. The current is not forced to reverse in the secondary but is handed over from one phase to the next. Still at this hand over point there is a nasty spike while the two diodes fight. While the first one does not "want to let go" the next phase's diode want's to 'take control'. In a three phase supply this can be avoided with a very small series resistor in front of the diodes of each phase. I size this resistor such that each diode cuts off a small time before the next one starts to conduct.
Best regards
Follow Ups:
I think at the point where the current reverses there is a small amount of time in which both diodes fight against each other.I think it is exactly the opposite: a small amount of time where none of the diodes conduct. The secondary of the PT causes the voltage peak because the current is suddenly cut off.
Hi!That may well be. I wrote "I think" because I'm not sure. It's been a while since I played with this since I switched to three phase years ago.
Should be easily verifyable by monitoring the volatge across both diodes and see if there is a overlap or a fragment when none conducts.
Then there would a period in which the voltage across both diodes is above the "knee voltage" at the same time.I thought a mechanism like this is in play here: The choke tends to keep the current flowing. Thus the voltage at the end connected to the diode is pulled lower and lower, so the diode keeps conducting. When you scope the voltage at the input side of the choke (or after the rectifier) you will see that it is not just a rectified sinus going from 0 to the crest voltage, but it also sags below zero, which would indicate that the diode would still conduct even if it's input voltage approaches 0. But at a certain point the other diode will start to conduct too.
A scope picture of the secondary voltage points to this direction. It seems as near the zero crossing, the voltage "sticks" for a while. I assumed that is the periode when both diodes are conducting.
But again. I'm just wildly guessing here.
This certainly is the mechanism with three phase supplies. There the insertion of the resistor helps. This causes one diode to stop conduction just before the next begins. Although with this "fix" the current also ceases flowing for a moment, no voltage spike arises.
Best regards
All this talk about trees, how are you implementing three phase?!?
Thermionically addicted.
Hi!There is an article in Sound Practices 17. This issue was only released on the archive CD.
There is nothing special about it. Three phase schemes can be found in many electronics books. For example in the first chapters of the RCA transmitting tube manuals.
There's also an article in wikipedia. This is the scheme I typically use:
http://en.wikipedia.org/wiki/Image:Three-phase_bridge_rectifier.jpg
only TV dampers instead of the silicon diodes.
Let me know if you have specific questions.
Thansk Thomas. I understand the use of 3-phase and its advantages but are you using direct 3-phase service in the home or synthesizing it from single phase?
Thermionically addicted.
Hi!3 phase service right out of the fuse box available over here :-)
I remember discussing the 3-phase supplies with you quite some time ago. Short of running a rotary converter it isn't an option for me. But I might have a chance to pick up a bunch of SLA batteries from a UPS that is being replaced. That would give me a 480VDC supply (but I am a little concerned about safety!).So have you ever tried a battery (only) supply for B+ and if so, how did it compare?
Hi!No I haven't. I plan to run a phonostage off batteries. But that's a future project. A friend of mine is using a battery powered phonostage and likes it very much (he uses 3 phase for most of his system tto)
just to see what happens. I'll post the results when I'm done.
Naz
Thomas,
I'm sort of with the "there is a small amount of time where the diodes fight each other" school.Actually what happens is that this:
A diode when conducting has a depletion region of a certain width with a electrostatic field across it - that is it has a capacitance. When reversing the applied voltage you have to discharge that capacitance (the physics bods talk about sweeping out the minority carriers but its easier just to think in terms of discharging the cap). This results in a sharp peak of current FROM the power supply capacitors BACK INTO the diode before it turns off. The size of this "splat" of current depends upon the size of that intrinsic capacitor or more exactly the amount of charge in that capacitor which must be swept out to turn the diode off.
This charge is called Qrr the "reverse recovery charge" and it will not be a surprise to most of you that typical values are:
standard silicon diode - 500nC
Ultrafast Soft Recovery - 100nC
Schottky Diodes - 50nC
Silicon Carbide Diodes - less than 20nC
Tube Rectifier - 0nC
Thomas> > When you scope the voltage at the input side of the choke
> > (or after the rectifier) you will see that it is not just
> > a rectified sinus going from 0 to the crest voltage, but
> > it also sags below zero, which would indicate that the diode
> > would still conduct even if it's input voltage approaches 0.When the diode stops conducting it doesn't care
what the choke wants - it isn't conducting.
Now you have the inductor drawing current that
the diode isn't supplying so then you get the
lowering of the choke's supply side voltage.
The choke is sucking every available electron from
the wire and causing a voltage spike which only
discourages the diode from conducting all the more.
...add a resistor ACROSS the secondary winding of the traffo (before any diodes) so it's current isn't completely cut off when the diodes go into their "dead band" near zero crossing. It's the sudden change from some current to no current that creates the voltage spike. It helps on both cap and choke input supplies.I size this R by looking at an unconnected low voltage winding (heaters etc) on the same traffo with the delayed sweep on the scope, and decrease the R untill the switch-on and switch-off spike is vanished, or at least majorly reduced and turned into a soft ripple rather than a spike.
Sure this R uses some power - but with your wall of rectifier tubes I think you wouldn'd notice the extra heat!
I also tried using the same resistor on the HT but it uses MV rectifiers. I couldn't really hear any improvement so did away with it but on the LT (a seperate transformer) it was a significant improvement. Cheers Allen!
Hi Allen,
Am I right in thinking that this resistor can be on ANY of the secondaries since i guess what we're worried about is what the core sees? AC-heated filaments make a nice resistor. Is this one of the reasons that AC-heated filaments typically "sound better" than DC? In other words, more to do with the transformer than the tube. Interesting. Typical values?
Hi Allen, What sort of current is needed through these resistors ahead of the diodes? Is it a few ma? Or a few tens of mA? Is it proportional to output DC? What is a good starting point? Thanx.
Hi Allen!neat trick with that resistor across the secondary. Never thought of that. Thanks for pointing it out. I will give it a try.
Will you be in Munich during the High End fair ? There will be a small meeting at my place. We will try some Goto Tweeters with my speakers. It will be on the 19th in the evening.
N/T
| ||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: