Discussion flowed thus..................
I have a 50 Watt solid state amplifier that is rated at 75 Watts into 2.4
Ohms.
** That's a bit of an oddball ?
You seem to be saying that I can't use a "multi-tapped auto tranny" to
deliver
75 Watts, how can I match an 8 Ohm speaker to 2.4 Ohms?
** You better not use that Weber unit unless your 75W amp has VERY good
overload and short circuit protection - ie VI limiting.
An auto tranny is not out of the question, but it would be wise to add a 50W
rated series resistor of about 3 ohms on the 2 ohm input and bypass that
with a pair of 1000uF, 50V electros wired back to back. Ideally, the core
ought to be toroidal too - for way less losses and better bandwidth.
The 8 ohms tapping needs to be rated at 50Vrms at 60Hz ( so 25Vrms at 30Hz)
for true hi-hi.
A simple centre tap should be close enough 2.4 ohms.
..... Phil
Unfortunately, the red boxed Weber LV referred to has no specs except that there are 50W and 100W versions, and very cheap, and probably made in Asia and probably it saturates at maximum applied voltage for power at 30Hz. There are no claims made for power bandwidth, so assume the worst. But probably it will work fine with a guitar amp in which a bit of saturation at 50Hz and HF extensing to only 6kHz might be fine, and there's no worry about winding resistance losses which could easily be 15%.
The matching tranny at my website is a hi-fi item, with very low Fsat at full PO and very wide BW from 10Hz to 200kHz. It has E&I lams, to suit DIYer attempts at making such a thing. The size of the GOSS core Afe of my photographed 100W auto-trans is 44mm x 50mm, and Np = 204t. Core is partially air gapped to make the Lp appear as an inductance rather than saturable reactance at LF. If one were to use a toroidal core, and Np was 204t, then the Afe section of toroid will need to be same area, 44 x 50 = 2,200 sq.mm, which means its going to be a big toroid, about equivalent of a toroid used for a 500VA mains trans maybe, to get the same low Fsat with 204t. But a toroid without any air gap with GOSS may have µ = 40,000, and its saturation behavior is more sudden at very low F. Partially air gapped toroids are difficult to find or to make, so E&I will do fine.
I've never seen an SS amp designed specifically for 2.4 ohms and 50W.
It means it generates 10.95Vrms into 2.4r, and maybe it makes 13V into 8r which is only 21W. But there's no reason why 4, 8, or 16 ohms can't be used with an amp that is specified to handle 2.4r, if the volume level is high enough without clipping.
Matching trannies are needed where an amp struggles with say 4r and you have a 2r speaker, or you have an amp where maximum AB PO is say 100W at 4r and you want the amp load to be 16r, so that PO may only be 30W but it is all pure class A, and with better BW, lower THD, better DF etc. So if the speaker was 2r, then you'd need a Z ratio 16r : 2r, ie ZR = 8 : 1, and TR = 2.83:1, which may be difficulr to get exactly it taps on a single winding are to be positioned at ends of layers in an E&I type tranny. But 3 : 1 is easily configured, and would be close enough.
I have used an OPT to couple mosfets to speaker.
http://www.turneraudio.com.au/solids...ono-mosfet.htm
The use of mosfets may be better than bjts with transformer coupling. Bjts may indeed need carefully arranged V/I limiting.
I'd suggested that if pp fear using an IST with SS amp then maybe 10,000 uF cap would be OK. Phil's idea of say 3r0 plus 1,000uF in parallel is probably better, ( certainly for SS guitar amp ) and XC = 3r0 at 53Hz, way above what Fsat will probably be for any audio tranny connected. For 10Hz, tranny may saturate, but XC = 15r9, so the C+R network has become mainly resistive and current spikes from core saturation are limited by value of V0 / 3r0.
For hi-fi, most audiophiles would cringe with horror at using the red boxed Weber and they expect to never have to put any damn series FILTER thing between their amp and speaker. Its bad enough having to use an auto trans. But well designed auto trans will be fine, and work just as well as say the step up transformers used in most ESL speakers, where the tranny has an input winding not unlike the secondary of a normal tube amp OPT, while the ESL secondary has many thousands of fine wire turns to get several thousand volts to be applied to ESL stators.
Some ESL step up trannies are horrible items. I've tested a few. I've seen one which was included in a poor quality kit where Fsat was 32Hz at moderate volume, and this could kill SS amps. To make the Fsat at 10Hz for the same applied voltage, turns must be trebled, or AFe trebled, or both Np and Afe increased a lot. This always means turn length must increase so wire size must be much increased, so you end up with a far heavier item for real hi-fi than the crap mostly sold. Kit maker morons like to shove crap on the public at huge price while avoiding high costs of parts supplied.
Patrick Turner.