Volume Control setting corresponds to how many db down (-db) from 0db at full volume?


I’ve wanted to be able to estimate how many watts my amps are driving as it corresponds to my preamp’s volume control setting. More specifically with a typical volume control having a rotational range from say 7:00 o’clock around to 5:00 o’clock, (my Croft preamp actually goes from Noon to 9:00), roughly where would it be -3db or -6db, etc. In one system with my 50 watt push-pull pentode amp, I rarely go past 10:30 o’clock. With my 25 watt triodes in my other system I rarely go past 3:30 o’clock on the Croft. Interestingly that’s roughly the same degrees of rotation!? Can I estimate approximately how many watts I’m operating at in each system simply based upon the volume control setting?
elunkenheimer
Short answer, NO. Lots of reasons that I'll let others w more technical background explain.
WAY too many variables, from system to system and from recording to recording.
You could get a watt meter.  These things sell for about $45 and are used by RC airplane fanciers and others that need to make measurements at modest power levels without affecting performance while taking the measurement.  A friend and I used his old fashion needle and dial watt meter to see how much power his speakers were drawing.  He rarely used more than .1 watts, but, then again his speakers are about 105 db/w efficient.
Thanks guys and I recognize there are many dependent variables. And thanks larryi, for I can put a steady 440 hz test signal from the Cardas record through the system at the db level equivalent to what I typically listen to music at as measured on my trusty Radioshack sound pressure meter. It wouldn't surprise me that when I'm listening though my 93db Brines Acoustics FT-1600 MkII's to music volume about 90+ db, that the draw is only a watt or 2 from my 25 watt triode monoblocks.  
No never good luck.

I’ve wanted to be able to estimate how many watts my amps are driving as it corresponds to my preamp’s volume control setting. More specifically with a typical volume control having a rotational range from say 7:00 o’clock around to 5:00 o’clock, (my Croft preamp actually goes from Noon to 9:00), roughly where would it be -3db or -6db, etc. In one system with my 50 watt push-pull pentode amp, I rarely go past 10:30 o’clock. With my 25 watt triodes in my other system I rarely go past 3:30 o’clock on the Croft. Interestingly that’s roughly the same degrees of rotation!? Can I estimate approximately how many watts I’m operating at in each system simply based upon the volume control setting?
elunkenheimer
I'm sure there is a clever way to do this estimation but it's not coming to me right now.
i'm thinking out loud here & if i end up writing gibberish please excuse me....
most volume controls are 40-45dB in attenuation unless you paid a premium for your unit or the volume control in which case you would get something like 60dB of attenuation. I also believe that you would have something like 24 steps in the volume pot going from full attenuation of -44dB --> no attenuation of 0dB.
so, now you can calculate how much you've attenuated the preamp signal going to the power amp.
even if you know this, you still don't know what amplitude signal is being fed into the power amp input. You could measure this at the inputs of your power amp (probably need to pop the top lid & attach a voltmeter (which will give you the average voltage). 
Even if you know this you don't know the transfer function of the power amp i.e. what input amplitude corresponds to what output power. It's not linear; it's most certainly logarithmic. 
One of my amps has a VU meter & when the volume knob is set to -48dB, I see that i'm using 0.1W (of 120W/ch) on average just eyeballing it.
 
I’m thinking out loud here & if i end up writing gibberish please excuse me.... Even if you know this you don’t know the transfer function of the power amp i.e. what input amplitude corresponds to what output power. It’s not linear; it’s most certainly logarithmic.
Hi Bombaywalla,

What you wrote certainly isn’t gibberish, but I’d feel pretty certain that to a very close approximation the transfer function of a power amp is linear. Notwithstanding the fact that it is often expressed as db of gain, with db being defined as a logarithmic function, as I’m sure you realize:

db = 20 x logarithm(Vout/Vin).

In other words while db of gain is computed logarithmically, Vout/Vin is a numerical constant (to a very close approximation), as long as the amp is operated within its capabilities, and assuming a constant load impedance. (The constant load impedance assumption being particularly important in the case of tube amps, since their voltage gain can be affected somewhat by the interaction of their output impedance with variations of speaker impedance over the frequency range).

Consider also that if an amp’s gain/transfer function were logarithmic, both distortion performance and dynamics would be severely compromised.

Regarding the mention of meters, before relying on any such measurements I would want to see a specification of both the speed and the bandwidth of the meter. First to assure that it is fast enough to capture the full amplitude of a high speed musical transient, and second to assure that it can capture the full range of audible frequencies.

It should also be kept in mind that recordings having particularly wide dynamic range, such as a lot of well recorded minimally compressed classical symphonic music, can require **enormously** greater amounts of power during brief dynamic peaks than the average amount of power that is required by the same recordings. For example a recording that is played at an average level of 70 db but reaches brief dynamic peaks of 100 db (and I have many such recordings in my classical music collection) will require 1,000 times as much power to reproduce those brief peaks than to reproduce the average level of the same recording.

Finally, if it is of any help I believe that the markings on the volume control dial of this Music First passive preamp represent db of attenuation, and I believe loosely approximate the attenuation vs. position characteristics of typical rotary volume controls on active preamps and integrated amps.

Regards,
-- Al

yeah, Almarg, you are correct - the xfer fn of a power amp is linear. don’t know what I was thinking when i wrote that??? ;-) thanks for catching this.

re. VU meters - the manual of my vintage Yamaha says that the response time is 1uS. I don't think it mentioned a bandwidth per se. I'll have to go back & re-read it....
guys, 
i was thinking about this topic this morning & tell me why this won't work: 
elunkenheimer should use a voltmeter & measure the average voltage on the speaker binding posts (of amp or speaker - better at the speaker as it will take into account any I*R drop in the speaker cable). 
Then to calculate power simply do V^2/R where R is manuf specified average resistance of speaker (4 Ohms/6 Ohms/8Ohms, etc). 
elunkenheimer can do this calculation at several (or each if he wants to do the work) preamp volume knob settings.
Speaker impedance varies vs. freq but this will be a rough estimate of power being delivered to the speaker.
Bombaywalla, yes, that will work provided the meter has adequate speed and bandwidth.  And in fact about 35 years ago I had a Radio Shack audio power meter which worked in exactly that manner.

But the larger question I would raise about all of this is what's the point?  A brief dynamic peak in the music that is just 10 db louder than the level that is measured will require 10 times as much power.  A peak that is 20 db louder will require 100 times as much power.  In most circumstances most of the power capability of an amp is probably needed just for brief dynamic peaks.

It seems to me that it would be more meaningful to do an approximate calculation of the maximum SPL that can be generated at the listening position by the particular speaker/amp combo.  In past threads you and I have both posted info about how to do that.  For example see my second post in this thread.

Best regards,
-- Al
 
thanks Almarg.

But the larger question I would raise about all of this is what's the point?
let's hope the OP chimes in here & let's us know what is on his mind re. this.....