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Old 1st November 2012, 11:29 PM   #11
TonyTecson is offline TonyTecson  Philippines
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i looked at the alphacore and it seems you might actually have a 1kva torroid core...
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Old 1st November 2012, 11:34 PM   #12
sreten is offline sreten  United Kingdom
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Quote:
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i looked at the alphacore and it seems you might actually have a 1kva torroid core...
Hi, no way is it 1KVA in any reality, rgds, sreten.
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Old 1st November 2012, 11:54 PM   #13
TonyTecson is offline TonyTecson  Philippines
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i said might....i did not say surely a 1kva...
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Old 2nd November 2012, 11:03 AM   #14
Shelah is offline Shelah  Zambia
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primary resistance reloaded: the actual resistance measured is 2.7 ohms.

thanks Ian for your advice. i have taken into consideration that the power will be less. but i want still to make it regardless of the power reduction.

want to ask further if there is going to be other important values like the bias voltage will change if i make the HV23.
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Old 2nd November 2012, 03:37 PM   #15
Ian Finch is offline Ian Finch  Australia
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Originally Posted by Shelah View Post
.... i have taken into consideration that the power will be less. but i want still to make it regardless of the power reduction......the bias voltage will change if i make the HV23.
It's good to have considered the lower power but the issue is that you won't be able to properly drive low impedances like 4R even. The amplifier will need a good, well adjusted protection system specifically for low impedance loads or the transformer will overheat unless it is rated to supply the full current demand appropriate to the rail voltages. This also means knowing how to set it up with current limiting for your transformer, not just protecting the output stage Apexaudio designed it for.

Bias requirements are current, usually measured as a voltage across the source resistors. It is not a critical value in a Mosfet amplifier and doesn't need to change from ~ 50-100mA per each output pair, regardless of power. It does have to be well controlled though, to compensate for varying loads and temperature to prevent thermal runaway.

I'm not saying you can't do what you want, nor that your plan won't work - almost any combination will work, even without limiters for home use, but it will be too bad when you find out the fundamental problems of a weak design when you can ill-afford it, in a hardworking PA amplifier.

I suggest you ask the designer of the amplifier what he recommends in your situation - assuming your transformer is only in the 400VA ball-park and you do want to use it as a PA, disco amp etc.
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Old 2nd November 2012, 07:32 PM   #16
AndrewT is offline AndrewT  Scotland
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for domestic listening a 400VA transformer is good for a total of 200W to 400W of maximum amplifier power.
For PA work I suggest 400VA for 200W of amp power. That probably restricts you to an 8ohms speaker.
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Old 3rd November 2012, 03:20 PM   #17
Shelah is offline Shelah  Zambia
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Thanks Sirs for your advice. Now another thing I wanted to ask. I have an 8channel powered mixer. I occasionally use it and the last time I used, it gave a humming sound through the speakers. I suspected the resister that links the groumd to the earth wire was defective but even after replacin it the hum remained. I changed the. PSU filter caps but still the same. Is there any other culprit that I should look for durin further troubleshooting?
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Old 3rd November 2012, 08:27 PM   #18
Ian Finch is offline Ian Finch  Australia
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A correct schematic and clear pics of the board, wiring and chassis would be needed to make suggestions for an unknown device. Even though the problem is very common, there are so many possibilities that it would be guesswork otherwise. Is it a stereo mixer? Is it working normally apart from the hum?

Perhaps you could start by measuring some basics, like the (~20mV) DC voltage across the amplifier output terminals when neither inputs or speakers are connected to it. Unless this is a Mosfet design, you could carefully (don't slip!) measure the base-emitter DC voltage (~0.6V) of all the transistors to get an indication they are functional. Post anything you think is wrong.

It would be better to start a new thread for a this new topic.
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Old 4th November 2012, 07:14 PM   #19
Shelah is offline Shelah  Zambia
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Its a stereo mixer. Apart from the hum, all is well. I bought the mixer with a messed up mixer so I replaced it with a modified version of the MX600 by the chinese maker MAX. I will update you when it do more troubleshooting.
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Old 5th November 2012, 11:26 AM   #20
Shelah is offline Shelah  Zambia
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Previous post reloaded.
i did a few checks including what Ian told me. they are as follows:
output NPN transistors: Vbe= 0.292v
output PNP transistors: Vbe= 0.105v
output terminals: Left channel: 24.76mv
Right Channel: 20.35mv
i also tested for defects in all transistors of both amplifiers, non of them were defective.
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