Re: Tube amp with 2.5 Ohm speakers??

On Thu, 20 Mar 2008 19:40:43 -0700 (PDT), Bret Ludwig
<bretldwig@[EMAIL PROTECTED]
> wrote:

>ed 16/8/4 secondary which means only
>> > half the secondary is used for 4 ohm applications in the first place.
>> > Using a four winding series/parallelable output is an aid, of course,
>> > but a purpose designed 2 ohm output is highly preferable.
>>
>> OK Bret.  That's what I had in mind - series/parallel
>> windings with a 2.5Ohms option.  Thanks for the
>> confirmation.  But are there any other considerations?
>>
>
> One consideration with 2 ohm outputs is that the lower voltage,
>higher current needed for a given power means more copper is needed,
>or I2R losses will be higher. Also the turns ratio for a given
>reflected load will be more extreme, yes. From that standpoint for a
>given design the higher impedances are indeed better. But satisfactory
>results are indeed possible even down to 1 ohm or less if sufficient
>design parameters are fully accepted.
>
> The use of four or five secondary part-sections properly brought out
>does allow paralleling. IIRC the Manila Book gives the early, larger
>VTL designs (which based on Sowter's catalog comments I suspect are
>Radford design) as being sold for a 7.5 ohm nominal output with the
>alternate configuration as something under 2 ohms. I don't have the
>book with me.
>
> Use of a separate tertiary for NFB is highly desireable in such an
>amp to avoid a lot of ad-hockery and to float or ground the output as
>desired.
>
> Even for solid state amps, the question does remain, why would one
>particularly want to design a 2.5 ohm speaker? Or is this some kind of
>exotic dipole or a motor or shaker load?

Where I've seen them employed it was to get more power from low
voltage supplies, like the TDA2002 that's rated 8 Watts into 2 ohms
using 14.4V... straight off the battery.