The model 260 was designed to drive 4 pair of 8-ohm speakers' directly without impedance matching volume controls. It was the first 2-channel amp designed for the rigors of distributed audio and started shipping in 1991.
The 260's power supply and output stage were considerably overbuilt for a 60 watts/channel amplifier. A stereo amp designed to run a single pair of 8-ohm speakers could easily get by with 15,000 microfarads of filter capacitance and one pair of output devices per channel. The 260 had 30,000 microfarads of filter capacitance and 2 pair of high-speed Sanken metal case output transistors per channel. It also had a special power transformer with a 3-to 1 turns’ ratio and heavier gauge wire to provide higher current into low impedances. It was actually designed to drive a 2-ohm load and was reverse-rated to get an 8-ohm rating.
What we sometimes found was that because of its reputation and the need to cut some financial corners, some dealers’ were connecting not 4 pair, but 5, 6, sometimes 7 pairs of speakers. They would then turn-down the front panel recessed potentiometers below the trigger level for the protection circuit. So the amp was actually driving this insane load while sitting there 'cooking' for several years. After 7-9 years, we started getting them back for service with descriptions such as, 'No Output' or 'No Sound'. Service would disassemble them and find resistors that had entirely changed color and a failed bridge rectifier. Until recently, service would do a rebuild and send them back into the field with instructions to 'please connect no more than 4 pair of 8-ohm speakers'. Incidentally, the 260 can still be repaired outside of our San Clemente facility; if you need the schematic, send me an email and I'll gladly forward it.
We took all the lessons learned from over-burdened 260s, upgraded the parts and incorporated them in the Sonamp 275SE. The 275SE has double bridge rectifiers and they’re each mounted on their own custom heat-sink. The number of output transistors increased from 4 to 6 per channel and we added 35% more heat sinking for improved cooling. We didn’t need to go this far in upgrading from 60 watts/channel to 75 watts/channel. But the goal was a beefed-up workhorse amplifier with a long projected service life. The 275SE has been shipping since 2003 and they’re not coming back ‘hot and over-cooked’ after several years.
That’s not to say every 260 was ‘abused’ in this fashion. The good news here is if it was used within it design limits, it may still have years of trouble-free service in front of it. And, if it does need service, parts are generally still available. The 260 should sound very good in a straightforward stereo speaker set-up. If both channels work properly, it would seem to be a good buy for $100.
Last edited by Ken Manson on April 7, 2014 16:44.
