Amp PSUs - Joule per Watt?

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Amp PSUs - Joule per Watt?

Postby soundbrigade » Mon Jan 01, 2018 3:53 pm

The Russian guy that once sold me the Russian monoblocks talked about a "recommended" ratio regarding the energy that the PSU (caps) could store versus the wattage of the amp.
Have anyone heard about such figures?

There's a controversy in our sector, where people claim the more - the better, whilst other argue that extra Farads do no good or even can make things worse. :/

I have a faint memory, but I have to consult that booklets that came with my amps to be sure, that our Russian friend talked about a 1 Joule per Watt. This could also propably be some sort of lower limit, but I am a bit lost. (???)




PS. The reason I ask is that I recieve mails from a guy who know nothing about electronics and happily hooks up his high-Z headphones to his tubeamps low-Z speaker connectors and no says it will be too expensive to replace the faulty 10000uF cap in some amp (he gives them strange names, like Angle for Musical Angel) so he will use a 470uF ... =:o
Besides, it could be helpful when designing PSUs if there are some kind of rules of thumb. (There's the PSU Designer, but ...)
Magnus

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Re: Amp PSUs - Joule per Watt?

Postby TomMcNally » Mon Jan 01, 2018 5:28 pm

Good question - I don't know the formula. My day job is working on BIG transmitters, I think the 35,000 watt FM (tube is a 4CX20000C in Grounded Grid) with a plate voltage of about 9500 volts at 3.5 amps, uses one 2 MFD capacitor and a choke. Old audio amps had 10 and 20 mfd, but that was about as big as they came.

Happy 2018 !

... tom
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Re: Amp PSUs - Joule per Watt?

Postby Geek » Tue Jan 02, 2018 3:18 am

Happy New Year, Magnus to you and your lovely family! :))

soundbrigade wrote:There's a controversy in our sector, where people claim the more - the better, whilst other argue that extra Farads do no good or even can make things worse.


Actually, it does make things worse.

Tranformers can overheat with peak currents and diodes can fail. Just have them take transformer measurements and run Duncan Amp's PSUD if they don't believe it.

The "over-read and under educated" make all sorts of foolish "profundus maximus" on audio forums :/

I have a faint memory, but I have to consult that booklets that came with my amps to be sure, that our Russian friend talked about a 1 Joule per Watt. This could also propably be some sort of lower limit, but I am a bit lost. (???)


The ARRL Radio Amateur's Handbook has the formula for ripple for current draw. They fixed the formula for 60Hz, but can easily be tweaked for 50/100 60/120Hz.

...I recieve mails from a guy who know nothing about electronics and happily hooks up his high-Z headphones to his tubeamps low-Z speaker connectors and no says it will be too expensive to replace the faulty 10000uF cap in some amp...


Yeah, my killfile for those is pretty darn big :/

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Re: Amp PSUs - Joule per Watt?

Postby paart » Thu Jan 04, 2018 9:31 am

Magnus,
Increasing power supply capacitance in a class AB amplifier will in theory, improve its performance. However, it is not just a matter of installing larger power supply caps and hoping for the best. There’s a lot more to it. As Greg noted, a large increase in power supply energy storage will eventually cause serious issues if certain precautions are not followed.

I am unaware of any way of determining the “best” amount of energy storage, but at some point, the rule of diminishing returns takes effect. Increasing the size of power supply capacitors lowers the output impedance of the supply, which is desirable, but at some point, the contact resistance of the capacitors (if screw type) and even the resistance of the wire from the supply to the output devices, or output transformer becomes the limiting factor.

This was addressed in an article that I wrote some years ago about modifying a Dynaco ST-35. If you would like to read more about the details of increasing power supply energy storage, follow the link below:

http://www.tronola.com/html/ps_mods_for_st-35.html


The changes to the power supply begin at page 3.

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Re: Amp PSUs - Joule per Watt?

Postby kheper » Thu Jan 04, 2018 1:58 pm

Patrick Turner published an equation, regarding the recommended minimum power supply capacitance for high-fi amps. However, it's more of helping-hint than a mathematical truth.

C = W / E^2

The minimum capacitance in Farads equals the output power of the amplifier divided by the square of the B+ voltage

http://tubes.nekhbet.com/power.shtml#fil
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Re: Amp PSUs - Joule per Watt?

Postby Geek » Fri Jan 05, 2018 12:56 am

Good to note what he follows with:

However - when large capacitances are used, there's a definite risk of a "voltage doubling effect" should the amplifier be switched off then back on again - in a short period of time (less than a minute). The resulting surge could exceed the maximum voltage rating of the filter capacitors, frying them.


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