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In Reply to: List speakers suitable for OTL amps. posted by The Killer Piglet on September 23, 2006 at 12:10:51:
Recently I've been working on designing a speaker specifically intended to work well on a high output impedance OTL amplifier, and it might be useful if I shared some of my observations.The amp I'm working with is the Atma-Sphere S-30, which puts out 30 watts into 8 ohms and 45 watts into 16 ohms, and Ralph recommends you not use a speaker whose impedance goes below 7 ohms in a region where there's lots of music energy. So right off the bat we come up with our first rule of thumb for matching speakers with high output impedance OTL amps:
1. All else being equal, high impedance speakers work better with high output impedance OTL's than low impedance speakers do.
Sixteen ohm woofers are pretty scarce, especially with the other characteristics I wanted. I had a pair of prototype 16 ohm woofers built for me, and while they were promising on paper and measured pretty well, they didn't sound as good as I hoped. A couple of 8 ohm woofers I had on hand sounded better, presumably because the manufacturers took more time to optimize them. So we come to the second rule:
2. A good 8-ohm speaker will sound better with a high output impedance OTL than a mediocre 16 ohm speaker will.
In addition to the Atma-Sphere amp I have several other amps on hand, including a single-ended triode amp and a couple of solid state amps. I tweaked the speaker until it sounded balanced on the Atma-Sphere amp, then tried it on a very low output impedance (high damping factor) solid state amp. To my horror, the measured frequency response changed by about 4 dB in the midrange! At that stage in the design, the impedance curve had a huge peak in the midrange. When I switched to the solid state amp, the result was a dip in the frequency response where that impedance peak was because I had optimized the design for the OTL amp. So I learned the hard way, and am now paying much closer attention to the impedance curve.
Now you won't find any published frequency response curves measured with OTL amplifiers - the magazines that do serious testing (Sterephile and SoundStage online) use solid state amps. But they do publish impedance curves, and that will help you to evaluate what effect an OTL amplifier will have on the sound.
At the link below you will find SoundStage's measurements of the Coincident Total Eclipse, which is a speaker that works well with high output impedance OTL and SET tube amps. The frequency response curve shows the Total Eclipse to be a bit weak in the bass region, and there's a large dip in the region between 1 and 3 kHz.
Now scroll down and look at the impedance curve. Notice that there's an impedance peak at 50 Hz, and another fairly broad peak centered on 1.5 kHz. So if you connect this speaker to a high output impedance OTL or SET amplifier, the frequency response curve will be boosted at the impedance peaks! Israel Blume has put the impedance peaks right where they will definitely improve the speaker's frequency response with that type of amplifier. Israel if you read this, nice job!
Which brings up to the third rule:
3. By looking at the measured frequency response AND impedance curves, you can make an educated guess as to how a speaker will work with a high output impedance OTL amplifier.
Hope this helps.
Duke
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Follow Ups
- Three rules derived from an OTL-friendly speaker building odyssey - Duke 21:09:30 09/28/06 (2)
- Re: Three rules derived from an OTL-friendly speaker building odyssey - Brian Walsh 22:19:19 09/28/06 (1)
- Re: Three rules derived from an OTL-friendly speaker building odyssey - Duke 01:21:37 09/29/06 (0)
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