[smccull]

I am wondering if there is a way to measure the actual wattage I'm using when driving my speakers at actual listening levels.

I have a pair of VMPS RM30s and am driving them with a 220watt Butler TDB2250 amp. I'm thinking of going pure tube and am wondering how many watts it's currently taking to drive these speakers to the level I'm driving them currently. If I can measure that, it'll go a long ways in selecting a new amp.

Any ideas?

Thanks!

Steve

[YoungDave]

I would suggest running pink noise (easy for a voltmeter to measure) at a good loud level.  Using an AC voltmeter - a simple analog scale multimeter will be fine; a nice true RMS voltmeter even better - measure the voltage across the speaker terminals.  Power, simplified to ignore power factor and reactive loading, is E *E /R, that is, voltage squared divided by resistance.  The resistance may be considered the average impedance of the speaker.  So, if you read 5 VAC across a 4 ohm speaker's terminals, power is 5*5/4, or 6.25 watts.  I think you will find that 5 volts is pretty darned loud.  By the way, a 220-watt amp will make 30 Volts across that 4 ohm load.

The trouble is, this does not account for musical peaks.  I do not know how the peak-and-valley nature of musical dynamics translates into steady-state RMS equivalent.  If we consider the dynamic range of music to go up to twice as loud on the peaks as your average listening level, you will be looking at a 10 dB increase, which also happens to be 10 x the power - so your 6.25 watts average needs to make 62.5 watts for the loud bit.  But how many loud bits occur, and how long do they last?  I'm not really sure that a millisecond-length musical peak matters when compared to the continuous RMS power capability of the amp - up until it clips against the the rail voltage, anyway.

Another thing you might try is looking at a fast meter - VU response - or a peak-reading meter during a loudly-played piece of actual music.  See what the voltage peaks up to during the loudest bit, if you can look fast enough.

Some amps will easily make a 30-volt output (225 watts @ 4 ohms), but will overheat if driving a 4-ohm load at 30 volts for an extended period - multiples of minutes, that is. After all, 30 volts across a 4 ohm load will run 7.5 amps of current through the load.  That's a LOT of current running through an amplifier's output stage. Your speakers would melt before the amp in that case, though - long before the amp.  I think that's why amplifier makers no longer use speaker fuses (an anachronism from the early 70's, when nobody had made high power amps before).  A speaker, tweeters especially, will conveniently blow to protect a fuse, and not the other way 'round.

This is pretty simplified and un-technical, so I may have ignored some of the complicated engineering bits, but I hope this helps you  measure what you need.

I'm kind of interested in this myself, and would really like to read  others' ideas on the subject.

[James Romeyn]

The bass range is the least senistive.  The bass range is 4-Ohms (the mid/treble are higher).  So compute 4 as your default impedance. 

P = power in watts
E = Voltage in AC volts across the speaker terminals
Z = impedance in Ohms (4 in this case)

Put an AC volt meter across the speaker terminals. 

Play your most difficult musical passage at the loudest level to check for power draw.  Set the level while you listen at the desired area, for the loudness you want to achieve.   

P = E squared/Z

Z is 4 as mentioned above

Leave the level at the appropriate setting.  Re-play the loudest passage while you look at the VM to confirm the highest peak.  Then do the above math.  For instance, if the highest V peak is 15: 15x15/4 = 56.25W 

I like to power the bass w/ the 1000W table model PE sub amp + eq + xo (xo pot must be swapped to increase the pole frequency), about $450.

Then your choice of tube/hybrid for the planar array, 50-100W.  Hard to beat the Pathos Classic One Mk III, 85W into the 7-Ohm planars, about $2800.  Just forget it's an integrated & use it as a power amp if preferred.  It's level control is good & might be preferred to the present level control, compare for yourself.  The Bruce Moore 50W tube monos are nice, $5k/pr.  The Pathos may be equal in overal quality, though no AB comparison.   

I've had two, both stone reliable. 

Something that made no sense is that Bill Berndt's little $650 Mapletree tube preamp improved the sound between my CDP & the Pathos.  No idea why.

[zacster]

Something that made no sense is that Bill Berndt's little $650 Mapletree tube preamp improved the sound between my CDP & the Pathos.  No idea why.

It's the drive that an active pre provides.  I've been playing with both active (Aikido) and passive (Welborne remote) and while the passive does have a somewhat cleaner sound, I would also describe it as leaner.  It definitely lacked bass.  When I put the active pre in the bass is much fuller and rounded, but the highs tend to be a little brittle.  And of course I couldn't get things loud enough with the passive as my CDP has low output for a CDP. 

Rather than picking one, I'm combining the two into one box and have the output from the Welborne feed both the Aikido circuit and the output jacks directly, controlled by a switch. 

YMMV etc...