Jim Lesurf wrote:

In article
,
Andy
Evans wrote:

For all that, if I wanted the best sound possible I'd buy some big full
range ribbon speakers - the bigger the better. I happen to know of a
pair of Apogee Caliper Sigs coming up for sale on ebay and you might
just get them before they're listed by phoning Rupert on 07920054344 in
the next couple of days.

You'd need at least 15 watts to drive them properly - push pull 300b
should do it.

You could use the Avantics for the treble and bi-amp, but I'd be tempted
to sell them for a good price and get a push pull 300b amp instead. All
that assumes a preference for good sound over tasty retro amps - a big
gimme.

Makes me wonder about situations where the loudspeaker impedance may be
lower than the output impedance of the power amp... :-)


What needs to be examined is the speaker sensitivity and ohms when
considering a tube amp.

Most 300B PP amps will have a load on the anode circuit of about 5k. The
300B anode resistance total = 1,600 ohms and if the sec load outlet
match "is for" 6 ohms, then the OPT ratio = 5,000 / 6 = 833 : 1 so the
Ra-a appears as 1,600 / 833 = 1.92 ohms to which we add the OPT winding
resistance of perhaps 0.2 ohms so Rout of the 300B PP amp = 2 ohms.

If 12dB of global NFB is used the Rout will become about 0.5 ohms giving
an DF better than 10 with a 6 ohm load.

Using 300P in SE, or PP or using triode connected KT88, 6550, KT90, the
Rout without NFB is around the 2 ohm mark and will NOT be higher than
the speaker impedance unless some fuctard designed them with Z dipping
to very silly low ohm values.
In that case, if there is a tap on the OPT to give a load match to 3
ohms, the Rout without NFB will be 1 ohm, and loads below 4ohms nominal
will be tolerated OK.

25Watts class AB1 is easy from 300B and if speaker sensitivity is above
88dB/W/M, sound is OK but the higher the sensitivity the better.

What's so great about ribbons though?

Patrick Turner.


Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html