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Keantoken's CFP cap multiplier
Keantoken's CFP cap multiplier
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Old 10th September 2013, 03:16 AM   #511
keantoken is offline keantoken  United States
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Thanks for the info. I've seen diode bounce on the scope when there was a major resonance in or around the transformer, but I haven't seen that once the resonance was damped properly.
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Old 10th September 2013, 03:24 AM   #512
BenY is offline BenY
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Originally Posted by keantoken View Post
Thanks for the info. I've seen diode bounce on the scope when there was a major resonance in or around the transformer, but I haven't seen that once the resonance was damped properly.
Yes,
Dealing with this by dampening the diodes is very good.. but still a partial solution.. The regulation margin of a transformer introduces non linearty that has an impact on the sound.

Last edited by BenY; 10th September 2013 at 03:41 AM.
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Old 11th September 2013, 09:23 AM   #513
bravi is offline bravi  New Zealand
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Default Regulation margin

How does one size the load resistor?
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Old 11th September 2013, 01:53 PM   #514
BenY is offline BenY
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Transformers generally have between 5% to 10% margin.
The load should be at least as much as the actual margin..
This may not be reasonable for a power amp but for a preamp it`s quite usefull.
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Old 11th September 2013, 07:50 PM   #515
RCruz is offline RCruz  Switzerland
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Keantoken's CFP cap multiplier
Quote:
Originally Posted by BenY View Post
Transformers generally have between 5% to 10% margin.
The load should be at least as much as the actual margin..
This may not be reasonable for a power amp but for a preamp it`s quite usefull.
Please elaborate...

Is there a simple formulae to use in this case ?
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Old 12th September 2013, 03:45 AM   #516
BenY is offline BenY
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Years ago Mr Erno Borbely suggested to me a 10% resistive load on the seconderies as a rule of thumb.
If you want to custom tailor a load for your specific tranny.. Say if your transformer is 50VA , test it with several loadings starting from zero load to about 15% and plot the results into a graph, This way you will find accuratly to what point your particular transformer should be loaded.
Transformers voltage output sag faster for the first 5 to 10% of their power rating.

But as Mr Borbely said "You can`t go wrong with a 10% load".

Last edited by BenY; 12th September 2013 at 03:59 AM.
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Old 12th September 2013, 06:22 AM   #517
RCruz is offline RCruz  Switzerland
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Keantoken's CFP cap multiplier
Hi ben, should this test be done before rectification or after ?

Should I measure voltage drop due to increased current in AC mode or after rectification in dc mode ?
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Old 12th September 2013, 06:52 AM   #518
keantoken is offline keantoken  United States
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Thanks a ton for this information! I wasn't sure myself what you meant because transformer regulation is at full load, whereas according to you the resistor needs to be sized at around 10% the VA of the trafo, and I don't see a connection between that and regulation.

So if you have a 50VA transformer, you need a resistor across the secondary drawing 5W. Since secondary output is roughly a sine, you'll need to convert to RMS. So if you have a DC output of 24V, 24*.707=17Vrms. 5W/17V=294mA. 17V/294mA=58R. That will be a lot of heat.

However if we use a cap here, it will provide the necessary load without dissipating any heat inside the chassis. For this we choose a capacitor with a reactance of 58R at 60Hz. However, since saturation losses are mainly 3rd harmonic and above, I think we could use a cap set for 58R reactance at 180Hz, the 3rd harmonic. This would reduce idle power consumption and importantly, the size of the cap.

For a 50VA transformer with 24VDC output, I get 15uF for the right size secondary cap. This would draw about 1.7W idle. For a 500VA 24VDC trafo then, you'd want 150uF which would draw about 17W idle. Technically, the capacitor isn't drawing any power (thus no heat), it's just shunting current, but the power company will charge you anyway because they leave phase out of the calculations.

At 15uF you may find a suitable lytic cap but at 150uF I don't know if a suitable lytic would be less expensive than a film cap, or last long enough to be worth considering. I've looked for film caps at this size and they go for about $20. An amp will get the same measured performance with or without the cap, so there's no need for it per se but it may reduce inrush for toroids.

The right size cap may actually reduce idle consumption from saturation losses but this would require experimentation.
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Old 12th September 2013, 06:55 AM   #519
keantoken is offline keantoken  United States
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Originally Posted by RCruz View Post
Hi ben, should this test be done before rectification or after ?

Should I measure voltage drop due to increased current in AC mode or after rectification in dc mode ?
VAC readings can be misleading because of RMS. For instance a square wave and a sine wave with the same peak voltages actually have a different RMS voltage reading. So measure voltage after the rectifier.
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Old 12th September 2013, 07:34 AM   #520
RCruz is offline RCruz  Switzerland
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Keantoken's CFP cap multiplier
According to your calc, using a 120VA tx at 50Hz and outputting 36Vac unloaded I would need a 20uF cap in the secondary to produce a 470mA load.

I will try this and report subjective listening impressions.
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