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In Reply to: RE: OTL amps and high impedance speakers. posted by Cougar on April 22, 2017 at 13:55:07
was not entirely accurate if it was quoted correctly. There's a pretty big difference between four and eight ohms let alone sixteen!
As a generalization there are problems! A lot depends on the individual OTL. It would be a pretty low power OTL to put out more power into 72 ohms as opposed to 16 for example.
Our smallest amp is the S-30 which makes its highest power into about 25 ohms 50 watts) and it's actually less power into 72 ohms! So the comment doesn't really apply to our amps.
Generally speaking our smaller amps, the S-30 and M-60, are both quite happy on 16 ohms and both do well on eight ohms too. Even with a custom speaker I can't imagine why you would want to go over 32 ohms.
Our larger amps are fine with four and eight ohms; the biggest puts out its maximum power into three ohms (over 500 watts and as far as we know is the most powerful OTL made).
As a non-techie person, this issue has caused me some consternation. Obviously, the power output of any amplifier has to go down, eventually, as impedance of the load rises. Otherwise, crossovers would not work as advertised. But it also occurred to me that tube amplifiers, and OTL tube amplifiers in particular, may drive crossovers more efficiently than do, say, solid state amplifiers with either very low output impedance, very limited voltage capability, or both. Thus, it could be said that the speaker crossover ought to be designed with the type of amplification in mind. An OTL might sound different from an SS amplifier on a given multi-way speaker, for that reason alone.
Ralph, can you comment? Thanks.
The issue is that the output impedance of some amps is high if they don't run feedback, and low if they do.
This can affect how the crossover works.
More at the link below.
Ralph, I was hoping you would answer this question. Thank you for the explanation and info, it answered a lot of question I had.
i saw that the Zero Autoformers were used with your amps. Are they as good as what I have seen written about them?
I have also been looking into your M-60 kit as well as the Transcendent. I currently have a Latino VTA-120 that I built from a kit and I'm happy with but I have been hearing good things about the OTL's for a while and now have the bug to build one. I was looking to do a Beast or the M-60 in kit form but this thing of proper speaker loads came up.
Thank you for the help.
I have written about this before, maybe many times. I modified my Sound Labs 845PX speakers, which in stock configuration had a nasty impedance dip at critical midrange frequencies, down to below 4 ohms, such that the impedance after the modifications is a fairly flat 20-30 ohms, from around 100Hz to 5kHz. Impedance falls gradually above 5kHz to 2 ohms at 20kHz (nearly unavoidable with a large ESL like mine, due to the capacitative nature of the speaker itself). In any case, the improvement in both speaker efficiency and in the capacity of my Atma-sphere MA240 amplifiers to drive them, and hence the pleasure factor, was nothing short of phenomenal and continues to gratify me every time I fire them up.
(Efficiency went up as a result of the mods independent of the fact that impedance went up, because the low impedance of the OEM configuration is in large part due to the fact that the crossover places a rather low value resistor, anywhere from 5 ohms to 10 ohms depending upon the year and model of the SL speaker, in parallel with the output of the amplifier. The resistor inevitably sucks amplifier power and converts it to useless heat. That resistor is now in the trash.)
I feel certain that any 50W tube amp would drive the modified SL speakers to ear-splitting levels with no strain.
They are a problem solver; when a person wants to use a low impedance load the ZEROs are often helpful for getting many amps to sound better on that load, even transistor amps.
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