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In article , Stewart Pinkerton
wrote:
On Sat, 04 Oct 2003 17:56:59 +0100, Jim Lesurf
wrote:

In article , Stewart Pinkerton
wrote:

Note that I'm referring to pretty poor amps here - but then you've
pretty much confined the field to such amps, with your
pre-qualification of extremely saggy power supplies. I take it that
we're not dealing with Class G multi-rail amps here?

No. I'm really meaning the deliberate choice of designing the PSU so
the rails do fall under sustained demand, but hold up for musical
transients.

That brings me back to my previous point. Since the reservoir caps are
recharged 100 times per second from the mains supply, you *can't* have a
rail which 'gradually' sags over a few seconds.

A few seconds would be quite a long time. However when the transformer is
going into saturation and/or resistive limiting, then the fall can extend
over a number of half-cycles of mains. The reason being that each
half-cycle does not now always recharge the caps up to the same peak
voltage level you'd get on minimal load. Hence you can arrange for a
response time for this which is somewhat longer than a half-cycle of mains.

By using fairly 'large' caps you can control the rate of fall in between
rechargings, and then use the transformer behaviour to control how any fall
under heavy load proceeds from one half-cycle to the next.

As an illustration, published 'tone burst' tests always take place in
less than one cycle of the mains, typically being three or four cycles
of 1kHz tone.

Afraid I can't now recall all the details of the tests I used to do as it
was before the flood. ;- However I think I tended to use 100ms and 500ms
sinewave bursts for mid-term bursts[1], and 'short' burst (i.e. about a
cycle of 20kHz) to test for more 'transient' effects.

Slainte,

Jim

[1] No idea now what frequency, but I suspect it was a few hundred Hz as
opposed to 1kHz as I tend to feel this might be more representative of
music. However i'm now guessing beyond my ability to recall... :-/

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Jim Lesurf wrote:

Also, 2,200 uF is perhaps a tad on the small side if you want to support
sustained mean currents of the order of over 30 Amps per channel (i.e. over
60 Amps total for stereo), particularly using a non-SMPS! The resulting
voltage ripple may be too high.

Where is all this current going?
Does this hypothetical amplifier provide 25kW per channel into 8ohms?

Roger.

Old Fart at Play wrote:
Jim Lesurf wrote:

Also, 2,200 uF is perhaps a tad on the small side if you want to support
sustained mean currents of the order of over 30 Amps per channel (i.e.
over
60 Amps total for stereo), particularly using a non-SMPS! The resulting
voltage ripple may be too high.

Where is all this current going?
Does this hypothetical amplifier provide 25kW per channel into 8ohms?

Roger.

FWTW 30amp into 8 ohm is 7.2Kw.

What did shock me first time I worked it out, was what the charging
current was with a capacitive input power supply. I found the DC supply
to the two input valves in my amp (600ma at 6.3v) was providing about
7amp peak current.

--
Nick

In message , Nick Gorham
writes
Old Fart at Play wrote:
Jim Lesurf wrote:

Also, 2,200 uF is perhaps a tad on the small side if you want to
support
sustained mean currents of the order of over 30 Amps per channel
(i.e. over
60 Amps total for stereo), particularly using a non-SMPS! The resulting
voltage ripple may be too high.
Where is all this current going?
Does this hypothetical amplifier provide 25kW per channel into 8ohms?
Roger.

FWTW 30amp into 8 ohm is 7.2Kw.

What did shock me first time I worked it out, was what the charging
current was with a capacitive input power supply. I found the DC supply
to the two input valves in my amp (600ma at 6.3v) was providing about
7amp peak current.

The PSU simulator available from www.duncanamplification.com site is
very good at showing the voltages and currents.

The peak current into a capacitor filter is the reason that all valve
rectifiers have a certain maximum capacitance that they can work into.
--
Chris Morriss

In message , Chris Morriss
writes

The PSU simulator available from www.duncanamplification.com site is
very good at showing the voltages and currents.

The peak current into a capacitor filter is the reason that all valve
rectifiers have a certain maximum capacitance that they can work into.

I always get this wrong! It's www.duncanamps.com

sorry.
--
Chris Morriss

Nick Gorham wrote:

Old Fart at Play wrote:

Jim Lesurf wrote:

Also, 2,200 uF is perhaps a tad on the small side if you want to support
sustained mean currents of the order of over 30 Amps per channel
(i.e. over
60 Amps total for stereo), particularly using a non-SMPS! The resulting
voltage ripple may be too high.

Where is all this current going?
Does this hypothetical amplifier provide 25kW per channel into 8ohms?

Roger.

FWTW 30amp into 8 ohm is 7.2Kw.

Yes, but we're playing with ball-park figures
and so for the PSU to average 30amp it is giving 60amp for half the time
then the other rail is giving 60amp for the other half.....

So it's four times the power you calculated.

Roger.