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Gluca 12th April 2006 10:14 AM

Ripple and chokes
 
I am on my way to finish my new amp and ... yes ... I threw the parts with care and caution on the breadboard and everything seems to work fine.

PSU is: hybrid GZ34/SS bridge + CLCLC and 750V at the output. I am using hammond chokes for the time being (I have an option to upgrade them in the near future) and to be on the safe side I wired them on the ground leg (those chokes are rated at 500 or 600V max). I measured the ripple and found it to be 500mv (RMS). I was not that happy so I rewired the stuff and put a choke on the upper leg (exposed to the B+ high voltage) ... well the ripple fell to 20mV or so. Quite an improvement!

Here and there I read it is quite a common practice to use chokes on the ground side when they are rated for low voltage use but ... hemmm ... results seem to suggest this practice is not going to give you the best results.

Any comments? I'd like to undertand the facts in a better way.

Here is a schem.

Ciao
Gianluca

John Bartley 12th April 2006 11:20 AM

Quote:

Any comments? I'd like to undertand the facts in a better way.

Here is a schem.

Ciao
Gianluca
Just a comment and I'm looking forward to seeing the other replies ... my understanding about chokes in B- lines is that this was commonly used when various B- values were required for multiple bias requirements (my experience is with tube radios).
It seems to me that if you are putting them in the B- line, then the total currrent requirement for the amp will flow thru' the chokes. It also seems to me that it shouldn't matter whether the chokes are in the + or -, and smoothing at the end of the filtration line should be the same.

ps : I didn't see any schematic

EC8010 12th April 2006 11:38 AM

It makes a difference whether the choke is in the + or - rail once you consider stray capacitance from the choke's coil to chassis. If it's in the + rail, that stray capacitance helps filter, if it's in the - rail it short-circuits the choke.

Geek 12th April 2006 02:28 PM

Same as John... I've used them in the negative leg for transmitting amplifiers because of the voltage rating considerations and also get a free V- bias supply to boot :)

Gluca 12th April 2006 02:55 PM

Schematics

oooops ... missing pic. uploaded onto imageshack (I hope it works now).

I moved the 10H choke on the B+ leg.

Yes, I had your same ideas ... but my tests showed an unexpected result. I am pretty sure I took the measurements properly and I repeated it a numebr of times. I'll try again.

How much the stray C can help? The difference seems to be huge?

Any idea?


Ciao
Gianluca :confused:

John Bartley 12th April 2006 04:38 PM

Having seen the schematic, I will confess to being somewhat confused regarding the the +- AC voltages? and by the seemingly misnumbered pinout on the rectifier tubes? The rest looks pretty straightforward.
So, being very ignorant on things Audio (and being new here), I look forward to more discussion about this PSU schematic.
The inherent capacitance in the inductor is something I will play with and learn about as I head towards building an amp.

cheers

EC8010 12th April 2006 04:44 PM

Another possibility...
 
It might depend on how the measurement was made. If an oscilloscope was used and that diagram accurately reflects reality then the chassis connection at the reservoir capacitor means that the oscilloscope wouldn't see the chokes at all.

Gluca 12th April 2006 06:04 PM

Pins etc ... yeah ... but that's the sw I use, +- are just a reference (for the sake of clarity) to my power tranny.

@EC: I used the very method you suggested time ago (was it dec last year?) ... I mean 0.1uF wired to B+ and to a large resistor (240k, you said 1M) tied to ground (B- actually or signal ground if you like, does it make the difference? shall I ground the probe to the earth?) and my oscilloscope...

I couldn't listen to the amp (power stage to be completed) so I can't report on any audible hum ... just seen the traces on the o/s.

Ciao
Gianluca

EC8010 12th April 2006 06:21 PM

Your problem is that oscilloscopes measure between input and earth. If you move the point at which your 0V connects to chassis (earth) from the reservoir capacitor to after the last choke you should get a more sensible measurement.

Gluca 12th April 2006 07:03 PM

Mumble-mumble got it. Thanks. I was just measuring the charge and discharge of the last reservoir cap. Wasn't I?

But ... shall I return all the cathods (and reference the signals) to what I called signal gnd on the scheme or to earth? Star grounding here.

Gianluca


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