[FullRangeMan]

This is a very useful tube amp power guide based on amp Watts and speaker SPL created by Paul Joppa and posted in a forum in 1999:
Posted by Paul Joppa on August 05, 1999 at 21:08:11:
While I certainly don't want to suggest that science is everything, there are two points about SET amps that are highly predictable:

1) They are usually low power.
You want typically 102dB capability for fully satisfactory sound.
The amp adds 10*log(watts) dB to the speaker sensitivity. (base 10 logarithm) Some examples:

     45 tube 2W = 3dB wants 99dB speaker
     2A3 tube 3.5W = 5dB wants 97dB speaker
     300B amp 8W = 9dB wants 93dB speaker

 88dB speaker wants 14dB = 25W
 90dB speaker wants 12dB = 16W

You can have lots of fun with less power, if your room is tiny and/or you like chamber music or little-girl-with-acoustic-guitar music, but this is the standard minimum.

2) Most SET amps are without feedback. Triode amps without feedback, SE or push-pull, have a damping factor of 2 or so, not the 50-1000 of heaviily fed-back sand amps. This means the bass resonances of the speaker are "loosened up" a little - expect the bass to change! Speakers whose bass is a little too quick and tight with a feedback amp are likely to come alive with a no-feedback amp, while flabby bass will just get worse.

[FullRangeMan]

So continuing the Paul Joppa sequence we have:
 88dB speaker wants 14dB = 25W
 90dB speaker wants 12dB = 16W
 92dB speaker wants 10dB = 10.2W
 94dB speaker wants   8dB = 6.55W
 96dB speaker wants   6dB = 2.35W
 98dB speaker wants   4dB = 1.51W
100dB speaker wants  2dB = 0.96W

[maty]

http://www.doctorproaudio.com/doctor/calculadores_en.htm#calc_spl

Calculators / Audio / Acoustics / SPL

An example,

With 3m -> 90.5 dB

With 4m -> 88 dB


I always use these parameters: 80 dB and DR17 -> 97 dB peak

to ensure that an excellent recording with high DR is not a problem for the amplifier.

Modern commercial music < DR10 -> 90 dB peak


Note: 80 dB is a high volume!

[FullRangeMan]

Seems an result similar to Paul Joppa:
Doc calc: for 94dB/1 driver per box it showed 93.5dB to a 8W amp

The PJ calc: 300B amp 8W = 9dB wants 93dB speaker.

[maty]

With true sensitivity of 94 dB.

2 watts

2m -> 94 dB

3m -> 90.5 dB

4m -> 88 dB


4 watts

2m -> 97 dB

3m -> 93.5 dB

4m -> 91 dB


8 watts

2m -> 100 dB

3m -> 96.5 dB

4m -> 94 dB


16 watts

2m -> 103 dB

3m -> 99.5 dB

4m -> 97 dB


1m = 3.28 feet

2m = 6.56 feet

3m = 9.84 feet

4m = 13.12 feet

[FullRangeMan]

Thanks Maty.

[JLM]

Maty, what is 'DR'?  Agree with regards to 'true' sensitivity as many manufacturers are 'generous' with their specifications.  How are the calculations adjusted for room boundaries?

FRM, what is the basis for a 102 dB goal?  I assume since you're discussing tubes that it is an average sound pressure level, if so I agree that it is a good number (a bit low for rock but plenty high otherwise).

[roscoe65]

Presumably DR = Dynamic Range.

I disagree with this position.  Echoing JLM's question:  where did 102dB come from?  Who decided that 102dB peaks were sufficient?

THX Certification (for what it's worth) specifies 105dB peaks with 20dB dynamic range, setting the RMS level at 85dB.  I agree with these numbers as a minimum objective in a home audio environment.  20dB peaks are not uncommon in classical music, and 85dB A-weighted is the maximum 8-hour safe SPL.

However, simple arithmetic does not give us all the answers.  Listener preference, music choice, and room design will also determine power requirements, as will the design of the speaker itself.  Some speakers are very efficient yet demanding on low power amps (complex crossovers, impedance dips, weird phase angles) while others do well on much more power than they should "need" (I'm looking at you Harbeth).

There are also speakers that are very efficient but simple need a little more power to get up and go.  My Omega RS5 drivers are 94.5 dB efficient and my Alter 414-8Cs are 96 dB efficient.  However, the Altecs have about 8-10 times the cone mass of the Omega, and despite their higher stated (and actual) efficiency simply need a little more power to get moving than the Omega's do.  On paper, the little Omega shouldn't work with 2wpc, but it's low cone mass allows this low power use.

I have found at the end of the day even with most high efficiency speakers having a minimum of 5 wpc is necessary to avoid compromises.  In reality, as long as your speakers are an easy-to-drive 90dB or more and your power is an honest 20wpc or more you'll have sufficient headroom for more domestic environments. 

[BobRex]

The other thing to consider is how the speaker loads a room.  The decrease with distance is different between point source, line source, and horn. 

[Wind Chaser]

Some speakers are very efficient yet demanding on low power amps (complex crossovers, impedance dips...

And just as consequential, impedance peaks. Anything but a smooth curve is a no go with a SET amp, which really limits the choice speakers.

I other words just because a speaker is very sensitive, that doesn’t mean it will match well with a SET amp.

[fredgarvin]

I know that in my room, roughly 650 sq ft, my 6.5 watt SEP gets far more volume than I want with my 97.5 db Quartets. Usually seated about 10 ft. off. It hangs very well with my sand amp at 200 watts. However I wonder if that would still hold true in a home theater setup.

[maty]

This is with true nominal 8 Ohms. It is very important to know the impedance graphs. At least the value of minimum impedance.

Only I have two cuts with DR19. Between 10 or 20 cuts with DR17. A lot of with DR15.

Just now:

Mahler, Klaus Tennstedt, London Philharmonic Orchestra ‎– Mahler Symphony No. 1 (1978), vinyl

https://www.discogs.com/Mahler-Klaus-Tennstedt-London-Philharmonic-Orchestra-Mahler-Symphony-No-1/master/635355

 DR      Peak      RMS      Filename
----------------------------------------------------------------------------------------------

DR16      -0.40 dB    -23.19 dB    A1 Symphony No. 1 In D Major 'Titan' - Langsam, Schleppend - Im Anfang Sehr Gemaechlich.aif
DR15      -1.89 dB    -22.96 dB    A2 Symphony No. 1 In D Major 'Titan' - Kraeftig Bewegt, Doch Nicht Zu Schnell.aif
DR16      -8.15 dB    -30.25 dB    B1 Symphony No. 1 In D Major 'Titan' - Feierlich Und Gemessen, Ohne Zu Schleppen.aif
DR15      -0.67 dB    -21.06 dB    B2 Symphony No. 1 In D Major 'Titan' - Stuermisch Bewegt.aif
----------------------------------------------------------------------------------------------

 Number of files:   4
 Official DR value:   DR16


With 80 + 17 dB you are not wrong. Usually we listen to music about 60-65 dB. Home audio off course.

[maty]

And we do not want that the poweramp works at max power -> distorsion.

Btw,

* Power with tubes at 1% THD

* Power with SS at 0.1% THD

[roscoe65]

And just as consequential, impedance peaks. Anything but a smooth curve is a no go with a SET amp, which really limits the choice speakers.

I other words just because a speaker is very sensitive, that doesn’t mean it will match well with a SET amp.

This is a very good point about the amp/speaker interface.  Different amplifiers react to impedance differently.  I own a  SET amp as well as two SEP amps.  These tube amps really like higher impedances.  The 421A SET puts out great bass into rising impedances.  My SEP amps don't seem to care what impedance the speaker is and put out almost the same power from 2-16 ohms.  I also own a Firstwatt F3 clone that puts out 15 wpc in 8 ohms or 10 wpc into either 34 or 16 ohms.  Yet another solid state amp (RWA Signature 16) doubles power into 4 ohms (16 into 8, 32 into 4 ohms).  Of the low powered amp I own, the "conventional" class AB mosfet RWA is most sensitive to impedance.  I cannot use it with one pair of speakers (Omega 1.5 way) because the reduced impedance in the midbass results in an unwelcome hump.

It has been my experience that it is far easier to deal with a high impedance with a tube amp than low impedance.  I've also found that the "8 ohm" driver in your speaker is usually only 8 ohms over a very small range.  When you start adding filter networks, you may completely change the impedance profile.  In a recent example we were discussing using a first order horn tweeter crossover at 6.5khz.  With an 8-ohm driver we would use a 3uF series cap to roll off the driver.  But we are padding down the driver with an autoformer, so with 12dB in attenuation the 8 ohm impedance of the driver is reflected back to the amp as 126 ohms.  So we need to either use a 0.2uF capacitor instead (which ain't so bad if you have a nice fancy cap you want to use) or add a swamping resistor in parallel (maybe 8.2uF) to stabilze impedance the load the amp.

Speakers almost never act like resistors, and some amps care more than others.

[Tyson]

It also helps a lot if you have separate, active bass, and only use an SET amp for the mids/highs.  Even a 45 tube with 2 watts is able to do Mahler at crushing levels.  Of course it helps that I have 600 watts driving 4 subs from 20hz to 200hz. 

[fredgarvin]

Generally we say that higher impedance rated drivers increase a tube amps power, the opposite with solid state. Tekton has started releasing 16ohm speakers in some of his large models now, standard. Of course, he changes things around very frequently.

[maty]

Just now,

Antonín Dvořák / Wiener Philharmoniker, Lorin Maazel ‎– Symphonie N.9 >>Aus Der Neuen Welt<< From The New World (1983), vinyl

https://www.discogs.com/es/Dvo%C5%99%C3%A1k-Wiener-Philharmoniker-Lorin-Maazel-Symphonie-N9-Aus-Der-Neuen-Welt-From-The-New-World-Karne/release/3626699

 DR      Peak      RMS      Filename
----------------------------------------------------------------------------------------------

 DR15      -0.72 dB    -21.70 dB    A1 Symphonie Nr. 9 E-Moll Op. 95 'Aus Der Neuen Welt' - Adagio - Allegro Molto.aif
 DR19      -1.82 dB    -28.32 dB    A2 Symphonie Nr. 9 E-Moll Op. 95 'Aus Der Neuen Welt' - Largo.aif
 DR15      -0.61 dB    -22.55 dB    B1 Symphonie Nr. 9 E-Moll Op. 95 'Aus Der Neuen Welt' - Molto Vivace.aif
 DR14      -0.62 dB    -20.15 dB    B2 Symphonie Nr. 9 E-Moll Op. 95 'Aus Der Neuen Welt' - Allegro Con Fuoco.aif
 DR15      -0.40 dB    -21.55 dB    B3 Karneval-Ouvertuere, Op. 92 - Allegro - Andante Con Moto - Allegro.aif
----------------------------------------------------------------------------------------------

 Number of files:   5
 Official DR value:   DR16

[FullRangeMan]

Maty, what is 'DR'?  Agree with regards to 'true' sensitivity as many manufacturers are 'generous' with their specifications.  How are the calculations adjusted for room boundaries?

FRM, what is the basis for a 102 dB goal?  I assume since you're discussing tubes that it is an average sound pressure level, if so I agree that it is a good number (a bit low for rock but plenty high otherwise).

You want typically 102dB capability for fully satisfactory sound.
According Paul the speaker should offer 102dB to a suited DR in a home, I suppose for movies it must be more for HT setup.

[roscoe65]

It also helps a lot if you have separate, active bass, and only use an SET amp for the mids/highs.  Even a 45 tube with 2 watts is able to do Mahler at crushing levels.  Of course it helps that I have 600 watts driving 4 subs from 20hz to 200hz.

I have stereo Rythmik F8’s, so I hear you.  600 watts a side up to 250hz if I want.  It lets me smoothly blend with just about any satellite, from minimonitors to open baffles.  Right now I swap out a couple different Omega speakers.  I’m soon putting an Altec 414/802 into rotation.