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Old 16th March 2005, 01:32 AM   #1
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Default a speaker whose load varies too much for an OTL

If you have a speaker whose load varies too much for an OTL, can a Zobel (an/ or something else) Ďcureí the problem?

Thanks
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Old 16th March 2005, 05:42 AM   #2
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What is too much, and why should this be a problem especially with an OTL amp? Generally SE amps have much higher output impedance than OTL`s and sometimes a SET amp will give unwanted effects when used with some speakers but normally it is not a big problem.

OTL`s in general have lower output impedance than SET amps so they should have even less problems, as an example my SET amp have ~2.5ohm out and my OTL 0.3ohm both built with the same kind of tubes, 6C33C.

Regards Hans
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Old 16th March 2005, 05:53 AM   #3
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Hans

Iím so far just planning the system, having heard numerous favourable comments on OTLs, but with not much knowledge.

> What is too much?
I donít know: Iíve read eg Mohan and I think Frank saying that if the resistance of the speaker fluctuates too much, an OTL will not perform so well, they didnít quantify a problem level.

> Why should this be a problem especially with an OTL amp?
Iíd like to know too.

> Generally SE amps have much higher output impedance than OTLs
Yes

> OTLs in general have lower output impedance than SET amps
The converse is true.

> so they should have even less problems
Apparently not, Iím just wanting to learn exactly where and why too.

Cheers
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Old 16th March 2005, 08:05 AM   #4
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Quote:
I donít know: Iíve read eg Mohan and I think Frank saying that if the resistance of the speaker fluctuates too much, an OTL will not perform so well, they didnít quantify a problem level.
As I wrote I don't see a problem with OTL's that is worse than with SET amps regarding Z out but maybe Mohan and Frank was refering to the fact that the output power from an OTL gets lower when loaded with lower speaker impedances. My OTL gives 25W in 8 ohm and 20W in 4 ohm, I haven't measured with lower impedances but it will be lower still at for instance 2 ohm.

That power goes down is expected as the voltage drop over the output tubes will increase at higher current but there is also a risk that there will be a more severe drop in output power at some point if the saturation current level in the output tubes is reached, this is one reason why it is usually a bad idea to use an OTL with a speaker that drops extremely low at some point, (some commercial speakers drops well below 2 ohm even though the nominal impedance is 8ohms). It all depends how the OTL is designed and what output tubes are used, some tubes do not have a very sharp saturation current level but some do.

Regards Hans
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Old 17th March 2005, 12:52 AM   #5
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Hi,

Quote:
If you have a speaker whose load varies too much for an OTL, can a Zobel (an/ or something else) Ďcureí the problem?
Most olden days OTLs were designed with 16 Ohm speakers in mind and in those same old days woofer impedances could go off the mark quite wildly especially with a horn type enclosure.
You almost invariably saw an RC network across the speaker output clamps of the OTL amps in order to even out those variations. With some degree of success but I digress...

The reason I mentioned this before was that with some, otherwise excellent, woofers such as big Altec 416s in a Voice of the Theatre type enclosure the impedance peak was almost impossible to control with most any other amp, especially not a SS one.

Yet, as the OTL amp puts out more power into a rising load impedance they were the amp of choice as they managed to take control of the speaker to such an extent that almost all of it's shortcomings disappeared.

Modern woofer units are much better behaved and if the designer of a multiway speaker takes countermeasures, an impedance correction network is often fitted right across the speaker terminals.
Which is where it should be as it's the most effective place to put it.

So, don't you lose any sleep over it..........

Cheers,
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Frank
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Old 17th March 2005, 08:07 AM   #6
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Thanks again Frank . . .

I had no idea with a rising load impedance they may be the amp of choice - (I heard it was the other way around).

Iíll try to relax

Cheers
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