Q for scott & arny... crown xs500 innards

"geezer" <[email protected]> wrote in message
news:[email protected]
> here's a photo of the inside of a Crown xs500 I borrowed.
> the crown has a SMPS coupled to a bipolar output section,
> and the cheesiest means of mounting cooling fans I have
> ever seen... A nylon tie strap??? SHEESH
>
> http://picasaweb.google.com/liveoak.nine/Xs500Innards
>
> It has 6 + 2 caps, all 1800uf 200v. Is this typical of
> most SMPS?

IME, that is relatively large. I'm used to seeing more like 180 uF.

8 x 1800 uF is like 7,500 uF per side which is on the lower side of analog
power supplies in regular power amps.

> Now that is a lot less storage than the caps
> in my adcom 555, but is it sufficient to assuage the
> problems that Scott and others feel are inherent in SMPS.

I suspect that power supply cap energy storage is more of an issue under
suboptimal operational conditions like on the end of a too-long, too thin
extension cord.

> If not, what size caps should be used? This amp does 3
> or 4 hundred watts into 8 ohms (It's rated at 400, but
> only midband, so I'mm assuming closer to 300 full
> bandiwdth).

I guess that depends on what sort of operational environment you are trying
to cope with.



> Arny, you're the ABX guy, right? Ever done any
> comparisons between switch-mode amps (power supply,
> output, or both) and linear designs? I'm curious if the
> diffeences would be apparent under an ABX test.

The most audible characteristic of switchmode amps is usually a rising
output impedance starting within the upper part of the audio band, leading
to frequency response in speaker loads that is dependent on the impedance
curve of the tweeters.

Most tweeters have a rising impedance at high frequencies, which makes many
switchmode amps tend sound like they have more "air".

geezer <[email protected]> wrote:
>here's a photo of the inside of a Crown xs500 I borrowed. the crown
>has a SMPS coupled to a bipolar output section, and the cheesiest
>means of mounting cooling fans I have ever seen... A nylon tie
>strap??? SHEESH
>
>http://picasaweb.google.com/liveoak.nine/Xs500Innards
>
> It has 6 + 2 caps, all 1800uf 200v. Is this typical of most SMPS?
>Now that is a lot less storage than the caps in my adcom 555, but is
>it sufficient to assuage the problems that Scott and others feel are
>inherent in SMPS. If not, what size caps should be used? This amp
>does 3 or 4 hundred watts into 8 ohms (It's rated at 400, but only
>midband, so I'mm assuming closer to 300 full bandiwdth).

I don't know, and as you'll note, I am only making a guess that the issues
I hear are related to the reservoir size. I haven't done any actual testing,
but it would be really interesting to do so.

>Arny, you're the ABX guy, right? Ever done any comparisons between
>switch-mode amps (power supply, output, or both) and linear designs?
>I'm curious if the diffeences would be apparent under an ABX test.

They would be, BUT different isn't always better... and a switching supply
WILL be stiffer than a typical linear supply at low frequencies because
of the regulation. This should be a good thing.

The way to do it is to take an amp like this, split the power supply off.
Compare the stock supply, an outboard linear supply, and then the stock
supply with an added capacitor bank.

I bet folks will hear differences but I'm not going to put money on the
table regarding what they are.

>As always, I'm very appreciative for the tutelage i receive here on a
>regular basis.

Now is your chance to write an AES conference paper!
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."