Aikido driver into an Interstage in Parafeed

[James D]

I find the most important ring to snub is the one caused by the psu trannie secondary...

I snub the diodes when using 1N1007s, say, then the psu secondary, then the choke - but it depends where the most ringing occurs I guess. I tend to ultrafast soft recovery diodes and then snub the psu trannie if needed and that cleans things right up. I know Morgan Jones recommends snubbing the chokes but I've not found any benefit with the chokes I use - which are fairly high dcr so probably self damped to a degree

James

[NickG]

I found that when I first built the 211 supply and only had 10nf before the first choke it was very unhappy, changing to 0.22uf calmed things down. That may be a bit of a extreme example though, a relativly low DCR 50H choke with 1500v hitting it.

[Andrew]

Originally Posted by James D

I find the most important ring to snub is the one caused by the psu trannie secondary...

James

Not thought about that! What cap size do you think is most appropriate?

I generally follow MJ and use 220nf around chokes. I also put a zobel of 10nF and 6k8 before a vacuum diode (plus 220R on each anode).

cheers,

-- Andrew

[James D]

I usually end up around 10-50nF accross the both windings i.e. differentially applied. DHTRob has a good psu article here.

James

[Andrew]

Hi James,

Is this something you have to subscribe to? I just get http://subscribers.broadcastengineer.../subscribe.cfm when I click your link.....

cheers,

-- Andrew

[James D]

Damm! Sorry!!! Wrong link off the clipboard!!!

Here is the right one and I'll re-edit the post too.

http://www.dhtrob.com/overige/pdf/dhtrob_psu.pdf

James

[IslandPink]

Thanks James, I got that link off the other thread and printed out the relevant pages. I've been holding back reading them till I get home, with a cup of tea, in front of the Stereo . Looks good . I was on PSUD earlier in the week , trying a few things , so it's very timely and relevant .

Mark

[Andrew]

Originally Posted by Andrew

I generally follow MJ and use 220nf around chokes. I also put a zobel of 10nF and 6k8 before a vacuum diode (plus 220R on each anode).

cheers,

-- Andrew

Doh having Homer moment - looking back at my notes - the purpose of the 10nF in series with 6k8 all in parallel with the secondary and placed before the rectifier is to tame any ringing in the secondary.

Of course it could be completely useless but that was why I did it.

cheers,

-- Andrew

[dave dove]

james

being an electronics imbecile
does differentially applied
mean one cap per 1/2 CT secondary?
hadn't thought of that ringing....

what i don't get is that MJ snubber
isn't isolated from decoupling caps
and seems to become
just a slightly larger decoupling cap
whereas traditional snubber
not going directly to ground
works only on the choke....

does that make sense?
how's my vision?

btw
that article

i tried it with low current preamp psu
and high current power amp psu
and found that high dcr tx
created with resistors on rectifier
caused rise time to slow considerably
and you could hear it
one has to be careful
not to go too far

does the ?zobel? work?

dave

[James D]

Hello Dave,

Differentially applied in this case means between the two hot winding so not connected to the centre tap.

The MJ snubber works by being a short to ground at rf applied directly to the choke terminals. The high value cap that it is also bypassing is a likely to be significantly higher impedance at rf and is likely to be physically distant from the choke terminals the MJ snubber needs to be right up against the choke terminal and with a short direct path to the earth star point i.e. not in series connection with the other caps... The other way of doing it is to connect a cap just across the choke terminals and this effectively shorts out the choke at rf so it can't ring but it lets any rf noise straight passed the choke and into the amp circuit... not a good idea. MJs method is far superior.

Absolutely you have to be careful not to go to far with the anode resistance... You have to keep the design in balance and that isn't easy as there are no hard and fast simple rules that always work. The thing I get from the article is that it is better to have higher resistance in the trannie secondary of rectifier anode circuit than it is in the dcr of the chokes... Not something I usually do. I usually over spec my trannies and use ss diodes so I have low resistance in that part of the circuit and use moderately high dcr chokes (they are much cheaper).. now I've got to try the other way around... damm!

The zobel across the secondary is probably more effective than just the bridging cap but it is harder to calculate - you need to know the leakage inductance and capacitance of the trannie to work out the right values of the zobel to kill the resonance before it starts... the cap just shorts the trannie at rf so any resonace doesn't get out of the beastie and any triggering rf pulses don't get into it... cruder but much less critical in value.

James