Hmm.... this is worse than exaggerated ratings. Three pairs of any large plastic (TO264) power transistor will not IME, reliably deliver more than ~ 180/8R, 300/4R. 2R loads would be out of the question with 85V rail voltage. This is not a genuine professional device, so we understand your problem to a degree.
This brings us back to the transformer and the rating guess-timate of 800VA which is now unnecessarily high. I don't trust this rating or estimates unless it is a stereo amp and bridged, which makes it useless for anything under 8R loads. How about measuring that transformer's winding dimensions properly (OD, ID, H) and we get some more facts into the estimates?
It is likely you need local assistance, pro.design and products to give you anything like the power you specify. Building a really high power amplifier to work reliably at a wide range of temperature and abuse is a difficult design and protection matter. Considering the number of used but good amplifiers available cheaply in many countries, the DIY option is not often, economical.
I guess I don't need to say how ridiculously cheap and efficient class D amplifiers of immense power are now. That is why so many class AB amps are also cheap or just scrap.
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I believe we advised on something similar and agreed that it would not be sufficient.
Class AB amplifiers are only around 60% efficient at best with light 8R loads so when you add losses for the low ranges of 4 and 2R it can fall as low as 45% which adds up to 1500VA continuous rating but some would under-rate this for music programme to 1000VA or less, assuming it will never run for long whilst at maxinum power with limiting occurring. As a home, monster power amp it would be fine but if you want it to entertain reliably as a disco amp, it needs to be tough and run cool at full output, all day in 35+ degree heat. The cooling is one problem but parts de-rating is important too.
It would make more sense to restrict the amplifier to 4R loads and at least make use of such an expensive transformer. If the design is beefed up to deliver say, 500W into 4R nominal impedance, you may have a good proposition but let's see what the rating is likely to be first.
Before seriously designing, the end use must be certain as there is no point in advising for the wrong application.
Class AB amplifiers are only around 60% efficient at best with light 8R loads so when you add losses for the low ranges of 4 and 2R it can fall as low as 45% which adds up to 1500VA continuous rating but some would under-rate this for music programme to 1000VA or less, assuming it will never run for long whilst at maxinum power with limiting occurring. As a home, monster power amp it would be fine but if you want it to entertain reliably as a disco amp, it needs to be tough and run cool at full output, all day in 35+ degree heat. The cooling is one problem but parts de-rating is important too.
It would make more sense to restrict the amplifier to 4R loads and at least make use of such an expensive transformer. If the design is beefed up to deliver say, 500W into 4R nominal impedance, you may have a good proposition but let's see what the rating is likely to be first.
Before seriously designing, the end use must be certain as there is no point in advising for the wrong application.
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Thanks Shelah, It seems hard to access
but a quick check of local manufacturers product selection guides suggests
that this is probably only 500VA rating and I guess you understand now what
this means. The amplifier you build from this cannot be rated for continuous
output beyond approximately 250 WRMS at the lowest load impedance likely.
It could be used at higher music power ratings on higher impedance loads like
8 ohms but that is not something I have much experience with. It would seem
that the broken amplidier was about right in its capability, even if the rail
voltage is too high for low impedance use.
If the transformer design was rated at 600VA, thet would be a useful voltage
IMHO but at the mass of only 4 kg, even the iron is not there to do it on a 50
Hz. power supply.
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thanks Ian, so if i am to build the 700w leach amp on the link 700W Leach Amplifier what number of output transistors would you recommend for use?
I have not read the full build article in the link, but it looks like this is a Lo Tim version extended to a 9pair output stage. Or is it an 8pair output stage with the same devices used as drivers?
The 260W into 8r0 should be possible from a 45+45Vac transformer rated at 300 to 600VA.
The 450W into 4r0 should be possible if the transformer is rated at >=600VA.
I have not modeled the output stage, but I doubt it can drive a 2ohm speaker. so in my view this build cannot be described as a 680W into 2ohm amplifier.
I would expect the amplifier to properly drive a 2r0 test resistor, but that is only a minimum test result to show that the amplifier can deliver adequate current to be able to drive a reactive speaker of 4ohms.
This build is at least 500W short of your 1000W target.
Wake up and set yourself sensible targets.
The 260W into 8r0 should be possible from a 45+45Vac transformer rated at 300 to 600VA.
The 450W into 4r0 should be possible if the transformer is rated at >=600VA.
I have not modeled the output stage, but I doubt it can drive a 2ohm speaker. so in my view this build cannot be described as a 680W into 2ohm amplifier.
I would expect the amplifier to properly drive a 2r0 test resistor, but that is only a minimum test result to show that the amplifier can deliver adequate current to be able to drive a reactive speaker of 4ohms.
This build is at least 500W short of your 1000W target.
Wake up and set yourself sensible targets.
Thanks Andrew for that. Just some other clarifications and calculations.
Ian said that my trafo could be 500VA. Am still going ahead with making the amp. How much power into 4r0 can I get from this amp using this 500va trafo And In terms of number of output teansistors, how many can I use to be in the safe region of the estimated power?
Ian said that my trafo could be 500VA. Am still going ahead with making the amp. How much power into 4r0 can I get from this amp using this 500va trafo And In terms of number of output teansistors, how many can I use to be in the safe region of the estimated power?
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