Am I really listening to my F5 at 0.22 watt? - diyAudio
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Old 15th September 2012, 03:16 PM   #1
willamp is offline willamp  United States
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Default Am I really listening to my F5 at 0.22 watt?

Hi All,
I currently use a Pass F5 clone that I made and am considering building an F4 clone. As many of you are familiar the F5 has a gain of 5.8(15.2dB) and the F4 has a gain of 1. I currently use a passive attenuator in place of a pre-amp and wanted to know if I would need a pre-amp with the F4. Not that I would mind building one, just wanted to know.

My initial thinking was that when I listen to music my attenuator is set to -20dB and since the F5 has a gain of 15.2 I should be ok with the F4 and have a few dB to spare.

I then started to analyze the situation more closely to see at what wattage am I running the F5 and then could I achieve that same wattage with the F4. Much to my surprise I calculate that I am running the F5 at 0.22 watts! This volume is plenty loud enough for me and fills my listening room just as I like it. My current speakers are rated at 91dB sensitivity but my previous ones were rated at 89dB and my attenuator settings weren't too much different with them.

What I'm wondering is, it really possible that I'm running the F5 at 0.22 watts and getting complete enjoyment out of it? My calculations are as follows...

assume CD output is at rated 2Vrms (my phono stage is spec'd at 1.3Vrms but lets use 2v)
speaker spec'd at 6ohm nominal (reduces to 2ohm at 20khz)

-20dB attenuator setting is a gain of 0.1, so F5 input is 0.2Vrms
F5 gain is 5.8 so speaker is driven with 1.16Vrms

e=(V^2)/R
e=(1.16^2)/6
e=0.22 watts rms

Does anyone see anything wrong with the above?

Considering the F4...

attenuator setting to achieve the same 0.22 watts would be -4.37dB

So given the above I still have about 4dB to spare with the F4 which is consistent for my initial thinking.

Just was quite a surprise to me that my listening level which is quite loud in my room only requires 0.22 watts of power.

I do understand there are other things to consider such as instantaneous power and also the fact that my martin logan planer speakers do increase to 2 ohm load at 20khz. Not sure if these need to be factored in to my analysis if the F4 will be OK with no additional gain stage. Maybe I am looking at this whole thing too simply.

Phil
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Old 15th September 2012, 03:54 PM   #2
Mooly is offline Mooly  United Kingdom
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It's quite possible yes... and its easy to get a really good approximation too.

So how much power do you really need for domestic listening ?

and this one has test tracks you can download and use,

A Test. How much Voltage (power) do your speakers need?
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Old 15th September 2012, 04:08 PM   #3
willamp is offline willamp  United States
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Mooly, thanks for the pointers. Looks like an interesting thing to investigate. Maybe I'll get an spl meter too.
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Old 15th September 2012, 04:13 PM   #4
Mooly is offline Mooly  United Kingdom
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Your welcome

Give Pano's test a try sometime... think you'll be surprised.
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Old 15th September 2012, 04:20 PM   #5
tsiros is offline tsiros  Greece
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unless you are listening to pure sine tones for your musical enjoyment

there is no sense in talking about amplifier wattage.
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Old 15th September 2012, 04:20 PM   #6
Mooly is offline Mooly  United Kingdom
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Knew I'd posted it somewhere. The SPL meter

A Test. How much Voltage (power) do your speakers need?
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Old 15th September 2012, 04:26 PM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by tsiros View Post
unless you are listening to pure sine tones for your musical enjoyment

there is no sense in talking about amplifier wattage.
This is all discussed in Panos' thread
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Old 15th September 2012, 04:47 PM   #8
tsiros is offline tsiros  Greece
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So am i correct or am i wrong?
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Old 15th September 2012, 04:49 PM   #9
Pano is offline Pano  United States
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Willamp - your numbers look good, you've just overlooked one thing. 2V RMS is the highest level your CD player will provide. Or 2.82 volts peak.

But the average level of music is going to be about 15-16dB below that, or even lower on dynamic recordings. Given your overall voltage gain of about -5dB (-20 +15.2) the average voltage to your speakers are getting is probably about 0.25 volts.

So you may be listening to just 10 milliwatts, average. That ain't much! I think I run my horns higher than that. (I'll check).

You still need peak power, and that's going to be up around 1/2 watt on the peaks at your volume setting - right there were you figured it.
The answer to your question "Am I really listening to my F5 at 0.22 watt?" would be "No". You are listing to it at 0.01 watt average, 0.5 watts peak (per channel).
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Old 15th September 2012, 04:54 PM   #10
Pano is offline Pano  United States
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Thanks Mooly!

Quote:
Originally Posted by tsiros View Post
So am i correct or am i wrong?
Neither, really. You do want to know that you have enough power (voltage and current) not to clip your amp. And you'll want some headroom above that, too.
Knowing that the vast majority of recordings are mastered at an average level ~18dB below peak helps you know what's going on.

Will doesn't even have to measure because he knows the voltage of his source and the overall gain of his system.
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