Re: Amp power ratings|Luxman|different can of worms (short)

From: Mark D. Zacharias (mzacharias_at_yis.us)
Date: 10/15/04 

Date: Fri, 15 Oct 2004 04:22:38 -0500

I think I would have just used the Fluke on an AC range. Scope readings are
rather approximate. Run the amp right up to clipping, as viewed on the
scope, measure with the Fluke. At the least your reading will have an
accurate numerical value.

(Boy, you can tell I don't have a newer scope with readout, can't you?)

Mark Z.

"JURB6006" <jurb6006@aol.com> wrote in message
news:20041014225017.05710.00006239@mb-m14.aol.com...
> Hi;
>
> Does anyone know for sure the power rating of the Luxman M117 ? I saw it
> from
> Google as 200WPC RMS @ 8 ohms, 400@4 and 700@2. While it looks like the
> amp can
> handle 2 ohms (three pairs 2SC2922 and its comp), my observation is
> different.
>
> I fixed one today. With an amp like this I run a full power clipping check
> under load. The load I use is resistive. If I did more audio I might setup
> a
> complex dummy load/power soak, but I usually do RPTVs. In a discreet
> component
> amp, the VERY least is the full power check under load.
>
> Anyway, I used the clipping level on the scope to calculate. It was 54
> volt
> peak, that is 108 V P-P. At 54 V peak my calculations yield about 182 WPC
> at
> clipping. By this I mean a sine wave will fit there within the rails.
>
> The method used:
>
> (54*.707)^2/8
> and with SMAD, you divide last.
>
> So where did I go wrong, RMS means RMS voltage right ? If you square it
> first
> you get a bigger number in the end, but to my knowledge that would not be
> the
> correct number.
>
> I didn't check the line voltage at the time, but realistically:
>
> 1. Our shop used to have a machine shop in the back, it used to be an auto
> parts store, so the wiring is probably more than adequate.
>
> 2. The last time I was in any question about it my Fluke (TRMS) reported
> the
> line at 125VAC.
>
> 3. If it's actually rated 400@4 how could there not be any more inherent
> losses
> in the power supply. An amp that does 700 into 2 ohms would have to have
> some
> clipping headroom on an FTC rating at 8, or am I wrong ? (actually not
> FTC, but
> what Stereo Review and such used to test)
>
> In this can of worms we are not talking about a box under your dash that
> says
> 200W that has a 4 amp fuse. This is a discreet power amp from Luxman, very
> expensive and possibly even THX licensed. (I didn't look into it)
>
> I remember the old way, they gave continuous, and clipping and dynamic
> headroom
> in dBs. Actually 3 dB of clipping headroom meant it had alot more power
> than it
> said, but may have crossed over a line in THD or IM that was unacceptable.
> Dynamic headroom referred to the time after a relatively queiscent period
> there
> would be a very loud sound, before the main filters discharged a bit. It
> was
> measured with a burst.
>
> I know you'll never hear the difference between 182 and 200 WPC, but if
> the
> numbers are to mean anything they need to be accurate.
>
> Quick question about that amp, I got .002V on average across the emitter
> resistors (cold). This is on the unblown channel, I set the exblown
> channel to
> that, but it seems quite low to me. Of course there are three sets in
> paralell.
> I'd guess Luxman would have you use a watt or current meter on the AC line
> and
> adjust for a certain increase. Setting it to within a hundredth of a volt
> it'll
> be fine, but what if it both channels were blown or it was a monoblock ?
>
> In that case I do have a foolproof method using a scope to set the bias on
> any
> audio amp. I will not delve into that now because if I did I'd have to
> remove
> the "(short)" from the subject line. Thing that gets me is that I almost
> ALWAYS
> see an amp spec out higher on the scope than the rating in power, as long
> as
> it's rated right, in RMS. What gives here Luxman ?
>
> JURB

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