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I am aware of the need for higher efficiency speakers , usually stated as a db. rating , in the high 90's to low 100's . Further , I am aware of the need for a fairly flat impedance curve and a higher average rating in the range of 10 , 12 , 14 ohms etc.
But what measurements depict the ability , of a speaker , to work with the high impedance output of a SET amp ? Am I missing something here ?
Thank you .
saki70
Follow Ups:
First of all... listen to Duke! He has been there and done that more than most of us. He knows what he is talking about.
Second, this article by D.J. Tomcik of Electro Voice in the 1950s can be tough sledding, but lays bare all of the relevant concepts. Spend some time with this one...
http://www.dissident-audio.com/Loudspeakers/CriticalLSDamping.pdf
I routinely design speakers to work well with tube amps having an output of 4 ohms or greater (damping factor = 2 or less). Here are a couple of things I've found to work well:- Shoot for a relatively high and smooth impedance curve, along with suitably high efficency, things you're already well aware of.
- When designing the bass section, use the effective woofer Qes that results from the amp's high output impedance. I like to incorporate user-adjustable bass tuning, to cover a relatively wide range of amplifier types and/or room acoustic situations. Comparing the same box with the tuning optimized for either low-damping factor tubes or solid state, we can get up to 1/3 octave deeper bass extension with the optimized low-damping-factor tube setup. Unfortunately I don't think any of this will be apparent on a spec sheet, unless the manufacturer spells it out.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
Edits: 09/01/15
and it ought to be a factor for me to experiment with the three sphere vertical point-source array we have discussed. ? Using a WR driver (with fill-in mid-tweeters in parallel to flatten the room/power-response as the main WR diver's dispersion narrows.The bass spheres are QB3 critically damped, with Fb at 38Hz. They work well with room-gain.
Using tube amps 20wpc pentode from 170Hz down fed by line-level xovers. Into an 8 ohms test load and from an 8 ohm tap, the output impedance is 1.5 ohms at 20 Hz. I can try 4 ohm or 16 ohm nominal loads, with 4/8/16 ohm taps.
I/we will listen and measure* and may find that it will depend on the recordings. Close miked kick drum and eq'd bass guitar probably won't need an under-damped set-up, but distantly miked acoustic music just might. ? ;-).
*I can now get access to a calibrated mike and RTA, and expert help, probably several separate visits. Which is vital, and a relief! A measurement chamber is a - remote - possibility.
Please email me?!
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 09/03/15 09/03/15
Thanks for the post. Not many do this, and I feel the few that do are not well publicized. Please let me encourage you to "spell it out" more forcefully! I totally support such an effort.
That's because very few have Duke's 'skills'.
:-)
Ok , thank you to the both of you .
Happy Tunes
saki70
The impedance issue is an amplifier issue, not a real speaker issue, and often not an actual issue at all. There are only a few specific and unusual circumstances where a higher impedance per se is better.
Modern speakers (those designed in the last 40-50 years...) are usually designed for low output impedance amplifiers. A relatively flat impedance spectrum reduces the response changes cause by high source impedance. This is most evident in the deep bass, where the impedance fluctuations are hard to reduce, and cause lumpy bass. Some speakers are overdamped, and work well with SETs which reduce damping; a very very few are actually designed around typical SET output impedance.
These two problems meet in the middle, where using a 16-ohm speaker on the 4-ohm tap of the output transformer reduces the source impedance. Unfortunately it also reduces the available output power.
Hi Paul,
What if the nominal speaker load is at 4 Ohms and never dips below 3 Ohms and in the mid-90's dB efficiency, and paired is with a zero fb SET amp of let's say 6W that only has 8 or 16 Ohm taps on the OPT. So only either taps could be used... What other implications are there besides being relatively underdamped in the response? Thank you.
If it has 8 AND 16 ohm taps, connect the 4 ohm speaker between those two taps....
If you only have one tap (2 terminals), then your max power capability will be cut in half at least and your damping factor, too.
Hi,
Thanks for the reply. I guess It would be the worst case scenario on paper (your last sentence.) I just have to do empirical testing to subjectively assess things.
The output tap is chosen to get the best power/distortion combination. A 4 ohm speaker on an 8 or 16 ohm tap will have higher distortion and 2 or 4 times worse damping; it might have greater peak power - or might not - depending on the design choices.
Thanks Paul.. I guess it also boils down on the OPT design especially on the secondary windings.
Best regards..
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