SET amps and speaker sensitivity.

This, Maria, can be a proverbial can of worms, but here goes...

Regarding speakers, 'high-sensitivity' MIGHT be defined as anything at or above 93dB, but there's NO concensus here. As the 'typical' or 'average' speaker is rated around 90, I figure a system requiring half the power for the same output is high in sensitivity. 'High-sensitivity' goes WAY beyond that, however; speaker systems intended for home use with sensitivities approaching 110dB are not uncommon.

The higher the sensitivity, the less power the system needs for a given acoustical output. IF, and it's a BIG 'IF', an average system uses 90dB speakers and 100 Watts per channel and fills a room to satisfying levels, then with 93dB speakers one would need only 50 Watts, and with 96dB speakers only 25 Watts, and with 99dB speakers 13 Watts, etc.

An individual's need for loudness and dynamic 'liveness', and the room volume, are important factors in this complex issue. I'm using 97dB-sensitive speakers with 2.5WPC SETs on the midrange and am getting highly satisfying loudness and dynamics in a largish (3200CF) room while playing large-scale orchestral music. Yet some dideebopper may determine that my system doesn't get NEARLY loud enough since his ears aren't yet bleeding.

Perhaps a rule of thumb might be that if you want to use single-digit-power SETs, you'll need speaker sensitivity at least approaching 100dB.

I expect others will add more to this discussion.
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Through my experience, I would not match a typical low powered SET amplifier with any speaker less than 100+ db sensitivity. While the amps may drive the speakers, they most likely will not drive them as the speaker manufacturer intended. You may hear wonderful midrange, but bass and dynamics will suffer greatly.

Please also take into consideration room size. If you are in a very small room, you might be able to go with a little less sensitive speaker.

Most importantly is to select a speaker that has a benign impedance curve across the frequency range. This is true for any tube amplifier, not just SET amps.

Then depending on the rated output of an amp, you can look at the sensitivity number of the speaker. I use speaker with an 88db per meter sensitivity on a 12wpc rated amp. Most folks will recommend a slightly higher output amp, but I find it more than adequate.

Note, if your speaker does not have a benign impedance curve, no tube amplifier will drive them properly. Amplifier watts will not mask a bad speaker choice.

Regards
Paul

Suggest you go to Musical Fidelity's web site and read up on the subject. They have an outstanding article explaining how much power you really need to drive different speakers.

Read "Some Insight into Proper Speaker Selection" on Welbourne Labs website. See:http://www.welbornelabs.com/recomendspeaks.htm. You have to scroll down a bit to see it.

A good rule of thumb for watt/dB combinations is 4w/93dB where every 3dB decrease in speaker sensitivity requires double the power wattage from the amp. Hence, 8w/90dB, 16w/87B, 32w/84dB and so on.

This was taken from a review of the Dared 845 amp. The reviewer was using the 18W Dared SET 845 amp with 84 db sens.Apogee minor speakers and had no problems at all with power.

http://www.6moons.com/audioreviews/dared/845.html

Of course, you must also take into consideration listening room volume, dynamic range of the music you like, impedence curve of speaker and ability of amp to deliver current to that load. For example, I can get ear splitting volume from my 1w Berning Micro-zotl in my small study (70 sq ft; 500 cu. ft.) with 93 dB speakers, but that is not be the case in my living room (300 sq. ft, 2500 cu.ft. with opening into large hallway).

Ok, thanks guys. So the higher the the dB rating on a speaker, the more sensative it is? 93dB is more sensative than 90 dB and so on?

peter

That's right Peter. And dB is a logarithmic scale, so a 10 dB increase is twice as loud! Also FYI, 3 dB is perceptible as an increase in loudness (less than that is often perceived as better but not necessarily louder). So you need to double the power output of an amp to get a significant increase in volume. FYI, sensitivity is generally in units of dB/1 watt/1 meter; 1 watt (or 2.8 volts) of output, producing X dB of sound pressure measured at 1 m from the speaker.

There is no hard and fast "right answer" but there are common guidelines. As swampwalker suggested, space/volume will have an effect. Speaker impedance (and how it varies by frequency) could also play a very strong role. Second, not all amps of X watts are created equally (power supplies may affect amp response to speaker load).

That said, I find the article to which Kehut refers above to be at best oversimplified and misleading, because I have think most speakers with 2x that sensitivity (87db/6ohms) would present trouble with the amp being reviewed with music which had real "needs" (Holst Planets' Saturn, Reger organ music, and the list could obviously go on). 845s are supposed to have more oomph at the low end than other SETs but my personal experience with SETS and relatively efficient speakers (96db on the bass, far more on the mids/tweets) is that one needs to test even higher-efficiency woofers with more power than recommended to see if one is getting the best out of the speaker. My experience with 96db woofers and 13 watts (despite a power supply I rate quite highly) suggested that to get real clarity out of the bass (and bass that goes deep with clarity makes the the rest of the spectrum stand out that much more - positively shiver-inducing in my book) I needed probably double or quadruple that at times. 100% of the time while listening to Mozart chamber music I would be fine. And even most of the time on orchestral music I would be fine, but when push comes to shove comes to tympanies pounding, cymbals crashing, and the organ in the background grinding out the low notes, I needed more oomph.

Of course, that and a dollar will get you a dollar cup of coffee.

You also have to consider the actual impedence of the speaker. Assuming a benign impedence, then 60 tube push pull watts for a 95db sensitive speaker. 30 watts for a 98db push pull amp and 15 watt SET amp for a 101db speaker. If I had a pair of Lamm ML2 I would be looking at a speaker of at least 100db sensitivity.

If I told you that I could carve a turkey with my $30k pocket knife I would not be lying. Just as SET amp maker will say that their 18 watt SET can drive a 95db speaker. Sure it might play loud, but the question is would it sound better with more power?

In SET amps as with many others, all watts are not created equal. Other aspects than simple output figures determine abilty to drive real speakers. In particular, the size and quality of output transformers is a factor. Now they are heavy and expensive, so in simplistic terms, the more you pay for an amp, the better it will drive speakers for a given output.
AS Ultrakaz says speaker impedence is very important for SET's, the speaker may be quoted at 8ohm impedence, but have nasty dips to 2ohms in the frequency range. A SET won't respond well to that.
The answer, try your speaker and SET together preferably in your own rooml, see how they match.

Wow! great to have such an overwhelming response. The more I know, the more confused I get. I guess I am realizing that I might not even have a clear understanding of what an SET amp is.

I am assuming (again an assumption) that not all tube amps are SET amps. Could some of you give me some examples of SET amps. On that note, could some of you give examples of good speaker SET amp combinations that you have run across in your expereinces. Truly I am a novice here.

On another note, my room size is 22L by 13W by 8H. That in feet. Is this big enough to put a floor standing speaker in? I've read on other forums that this is a common mistake, trying to fit too big a speaker into too small ar room Any feedback would be appreciated.

And lastly, just to give polks and idea of what I am looking into for the distant, I mean far distant future, would be something like matching the audio aero prestige monoblocks or the lamm ml2.1's with a floor stander from focal like the alto ot nova utopia be. Any comments?

Thanks all,
Peter

Basically, unless your speakers are 93db or more efficient, forget the normal SET. I grant that there are more powerful and expensive output tubes that could be used with less efficient speaker, but few rival the sounds of 45, 50, 2A3, or 300B SETs.

In my experience there's a huge range of preferred loudness levels among different listeners.

Whether you listen at 70 dB average SPL or 90 dB average SPL will make a hundredfold difference in your amplifier power requirements.

So in my opinion, "this first - know thyself".

Duke

SET- Single-Ended Triode amplifiers are a special class of tube amplifier. They are all class A. Usually the circuits are quite simple- the meat of it is the output transformer, which transforms the impedance of the output tube to that of the speaker.

Since there is usually no feedback, the amps are best off driving speakers that have a benign (flat) impedance curve, failing that at least a curve that has more high impedances than low as the amp will attempt to make constant power into higher impedances. Having no feedback means less in the way of high frequency sheen or hardness.

SETs are *particularly* good at low distortion at low power levels. Unlike regular push-pull tube amps and transistors, the lower the power the less distortion- so you get a lot more low level detail. This comes at a price- low total power output and often limited bandwidth. To really appreciate what they do, high efficiency speakers (97+ db) are *mandatory*! -so that the amp is able to work in the low distortion region of its envelope.

If you use lower efficiency speakers you simply will not realize the benefits of SET unless you have a very small room/nearfield situation or the like.

Due to the nature of the bandwidth issue associated with the output transformer, the smaller you make the amp the better they sound. 15 years ago the 300b was the insider story- 7-8 years ago the 2A3. Today its the 45- only good for 0.75 watts (in truth...). So to take advantage of such low powers a speaker of 103 db or more is recommended.

Great thanks folks. I have a lot to learn. So is this where bi-amping comes into play. Can you bi-amp two lower output SET amps and pair those with less efficient speakers?

Cheers,
Peter

Peter,

I never think of biamping as a means of delivering more juice. Rather, I find that the biggest advantage derived from biamping is a significant increase in liveliness and microdynamics at lower volume. But, biamping is quite a complicated subject and good results can be quite tricky to achieve. I personally like the results I've heard from active biamping (a dedicated crossover that splits the signal at the line level before the signal is amplified by separate amplifiers). But, even when this was being done by someone who is experienced, it took a lot of experimentation and changes to the design of the crossover and even the amps to get optimal results. When not done right, the drivers don't blend correctly.

I think the better first course is to get amps suited to the particular speakers that you like/own. I would be less concerned with the operating type -- SET, or pushpull, etc.. I own an expensive SET, but, I don't think they are necessarily the best amps, and this is the case even with the fairly high efficiency speakers I own. I have heard some fantastic pushpull amps (e.g., 60-year old Western Electric amps) as well as several fantastic OTL amps (incredibly lively and upbeat sounding).

I know that getting enough power to properly drive a system is an important consideration. But, sometimes too much emphasis is placed on power. I have seen people pursue more and more power because something is lacking in the sound and they do not quite connect completely with the music/performance; these people think more power will do it. I have found that whenever I have made the right choice in improving my system, I tend to listen at lower volume than before -- I don't need volume to compensate for something else that is lacking. Good systems resolve musical detail, harmonic structure, etc. at low volume.

Maria

Forget bi-amping. The more simple a SET setup is, the better it is. SETs fall in the less-is-better genre.

I disagree with larryi in the sense that one needs to match speaker to an amp - select a SET amp and then find the speakers that will work with them.

Regards
Paul

Paul,

I agree that one does need to match speaker and amp (as well as linestage for that matter). But, there are so few speakers that work with some SETs, particularly 45s, and MANY high efficiency horn systems and single driver systems have very distinct colorations. If one is interested in SETs, I would still suggest first auditioning a variety of high efficiency speakers, and if one finds a speaker to one's liking, then finding an appropriate amp. Personally, I have heard so few currently-available high efficiency speakers that I could live with and a whole lot more amp choices that are acceptable. I personally don't HAVE to have a SET amp, although I really do appreciate the virtues of low-powered SETS; I just happen to have a speaker that does work with low-powered amps.

By the way, I heard a direct comparison of my Audionote Kageki amp with the Western Electric pushpull amp I mentioned above, and frankly the Western amp was dramatically better, but then again, it's four times as expensive.

My concern for someone who is just exploring this territory is that they not lock themselves into a particular technology to the exclusion of a broader exploration.

Here is a simple rule of thumb. Sensitivity of speakers is in dB/watt at a particular imoedance e.g. 8 ohm.
For every increase in 3dB, you need to double the power of your amp in watts.
E.g. Say you have a 91dB/w speaker. This means this speaker will putput 91dB at 1 watt and at 1 meter distance. To get 3dB additional output i.e. 94dB, you will need 2 watts. For 96dB, you will need 4 watts and 99 dB, you will need 8 watts etc.
Of course, you also have to look at the speakers impedance as this will vary with frequency. A speaker that drops to 2.ohm at low frequency will often sound bass light because SET amps usually have trouble driving below 4 ohms which mean that the power from the amp drops with low impedances.