Toroidal transformer ratings

strider

Is there a way to figure out the ratings of a toroidal transformer using a multimeter? I've got an old Audiosource amp and a LM3875 gainclone kit laying around and think the two might have the makings of a complete amp, but would like to know if the transformer would work first.

Systems

If the Audiosource amp is operational you can check the DC voltages,or you can directly measure the AC voltage of the secondary output windings. The LM 3875 works best with a transformer secondary voltage in the 22-25 volts AC range.The final DC voltage should not exceed about 35 volts DC assuming an 8 ohm load,and it should be lower(max of approx.30VDC) if using a 4 ohm load.

ShinAce

Which Audiosource amp?

strider

ShinAce wrote: »

Which Audiosource amp?

The Amp Three.

Systems

strider wrote: »

The Amp Three.

A quick search shows that to be 150 watt per ch. so I suspect it's secondary voltages would be too high for use with an LM3875.Is it in working order?If so,I don't know about SQ but it would surpass the Gainclone in the power dept.

strider

Still kicking my idea around. I'll measure the transformer before I take it out of the amp chassis, but I've conceded the fact I'll need to buy new transformers for this project if I go through with it.

Right now I've got a Rotel 980BX powering the bass cabinets in my system. The bass cabinets are 2 8" dual voice coil woofers per side, wired to a single pair of binding posts to show a nominal 4 ohm load. The cabinets are still rough finished, no sanding/painting/final woodwork done yet. I was thinking of adding a second set of binding posts to each cabinet when I take them back apart to finish them so it would be easier to fool around with different wiring configurations.

The Audiosource still works, but doesn't necessarily thrill me soundwise. It does have internal heatsinks, 4 pairs of binding posts, RCA jacks, IEC connector, etc etc. It also has room for 2 of these transformers. I'm interested in hearing what the gainclones would sound like driving the woofers. IIRC the LM3875 puts out around 56 watts per channel into 8 ohms. I've got more then enough boards/parts for an amp channel per woofer, which would mean about 110 watts per side. A bit less then the 200 watts per side I've got with the Rotel currently, but I don't think I'm too worried about that. The bass cabinets are crossed over at 200hz, they just need to keep up with the single drivers that are fed 6 watts per channel.

This is the problem I run into when waiting on orders from Digikey/Parts Connexion etc. Once the parts get here I'll be busy for quite some time, it's just the dead time in between planning and building when my mind runs crazy on what to do next.

Systems

That Antek transformer is ideal and a great price.(as long as it is void of mechanical noise)

strider wrote: »

IIRC the LM3875 puts out around 56 watts per channel into 8 ohms.

I did some actual measurements on a 3875 unit I built and would say the 56 watt rating is abit too optimistic,see post #3 for results.http://www.polkaudio.com/forums/showthread.php?t=71781&highlight=gainclone+makeover

You have some options depending on how many modules you have.You could try a bridged/ parallel arangement
(google BPA150,BPA300) or if you want to drive a 4ohm load exclusively then using 2 or 3 modules in a straight parallel configuration can work well.

strider

Got the toroid measured today, about 42v at the secondary, so it's a bit too much power. My father in law mentioned I could unwind the toroid 'til I got down to the rating that I'd need, but at $36 I think I'd rather just buy one with the correct ratings.

The BPA amps look very interesting but a bit out of my league at the moment. I may just build a stereo amp using one of the kits for the single driver speakers and keep the Rotel where it is. The kit is from chipamp.com/BrianGT which seemed to be pretty popular with the DIYAudio.com set.