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Old 3rd April 2005, 11:57 AM   #1
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Default OTL for Raven R2

Hi!,

I am still desiding what to build for my raven R2 as part of my 3way speakers system. I have Transcendent T8 which would drive the mid (C-quenze) OK. But Raven has the following spec

Impedance 4 ohms
Sensitiveiy: 98dB/2.83V/1m
Nominal power handling: 15W

And at times, the impedance can get as low as 2 ohms. I am planning to build a stereo T8 for it but worried that the output impedence may not match.

Any comments?
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Old 3rd April 2005, 11:07 PM   #2
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Most OTL amps are quite unhappy at 4 ohms, even less so at 2 ohms.

But, I suppose that it is possible to build a special purpose (bandwidth limited) OTL just to drive the Raven, if you pick the right tubes and the right number of tubes for the output section.

Also, you need to look at the impedance curve and remember that the freq resp is likely to look like the inverse of the response using a big solid state amp, since the OTL will have an output impedance *above* the driver's so when the impedance dips the power output will also dip... if that looks like it will be ok you may consider proceeding.

The other factor is that you'll be throwing away a whole lot of plate dissapation in the OTL for just a few watts of tweeter power... but if it sounds good, so what!

Ur probably best off using a number of the large Russian 6C33C tubes for an amp like this.

The choice of OTL circuit will have a large effect on the result too...

But, I would have thought that a 300B amp & Permalloy tranny, (maybe one of those capacitive coupled output transformer circuits) would be a more likely candidate for sonic nirvana on a ribbon tweeter?


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Old 4th April 2005, 11:27 AM   #3
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Quote:
Also, you need to look at the impedance curve and remember that the freq resp is likely to look like the inverse of the response using a big solid state amp, since the OTL will have an output impedance *above* the driver's so when the impedance dips the power output will also dip...
That is not exactly true, the output impedance of an OTL can be as low as for any other amp depending on the amount of feedback that is used, as an example my OTL have a measured output impedance of 0.3 ohm with a moderate amount of feedback, it can be lowered at least by a factor of 10 but for I dont see a need for lower out impedance than 1/20 of the speaker impedance, (I use 8 ohm speakers).

What is the possible real problem with an OTL and low speaker impedances is that the maximum power is lower for lower speaker impedances, for instance my amp gives 25W in 8 ohm and 20W in 4ohm and I expect that it would give even lower power for 2ohm, however this is only a problem when playing close to clipping.

My recommendation was to build the speaker for 8 ohm instead of 4 but I found out that the Raven driver can only be had in 4 ohm which is unfortunate.

BR Hans
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Old 8th July 2005, 03:21 PM   #4
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Thanks! Would putting a resistor in series solve the problem?
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Old 8th July 2005, 04:22 PM   #5
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Quote:
Would putting a resistor in series solve the problem?
Not really. Although the amp will be happier you'll simply be losing power on the resistor. 50% if you choose a 4ohm resistor.
OTLs and ribbons don't mix. Sorry.
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