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Old 4th January 2015, 06:15 PM   #1
ief is offline ief  Netherlands
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Default AB international precedent 1450 rail voltage?

Does anyone here know (or make an educated guess) what the rail voltage might be of this class AB amp?

Specs:

Two channel amplifier
145w per channel @ 8 ohms
240w per channel @ 4 ohms
200w per channel @ 2 ohms

1 pair of MJ15012 and 1 pair MJ15011 per channel

And how much of a tollerance would there be on a different voltage then original? (give or take)

Tnx in advance.
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Old 4th January 2015, 09:54 PM   #2
mjf is offline mjf  Austria
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I guess ,it is +-50Vdc (minimum).......(amplifier not bridged).
Have a look at the psu, there should be big electrolytics with e.g. 63V voltrating.
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Old 4th January 2015, 10:13 PM   #3
ief is offline ief  Netherlands
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Tnx, caps are rated 80 Volts but that could be for longer life as well (?) Not that it helped, they gone bad just the same.
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Old 5th January 2015, 05:21 PM   #4
ief is offline ief  Netherlands
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Feeding it with 64 Volts DC now and it seems fine on that. Still can't figure out if this thing is working the way it should but it sounds fine as far as I can tell.

Main worry is/ was 5 w watt resistors that are getting pretty hot. Had a light distorted sound before so thought something was wrong and the getting hot of the resistors (feeding the 16 Volt part of the cirquit) was due to that. They get hot on 50 Volts to btw.

Anyway, found a bad diode, swapped that, distortion gone but resistors still get hot just the same. Reading a little more makes me believe it might be just 'normal'

Ceramic Resistor Max Temps?

Mine are straight on the pcb AND next to eachother.

Another thing I'm not sure of is the bias, can get a max of 14 mV over a 0.5 ohm resistor when warmed up and that seems way to low (?)

Perhaps still not there

Last edited by ief; 5th January 2015 at 05:25 PM.
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Old 5th January 2015, 05:57 PM   #5
ief is offline ief  Netherlands
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Perhaps a schematic is of use, from a different one, same pcb, just some different parts here and there.
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Old 5th January 2015, 06:01 PM   #6
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I would say the rail voltage is closer to 65 volts, hence the need for 80 volt caps. Most manufacturers don't use over rated parts if they don't need to because it costs more. The use of 5 watt resistors would indicate they indeed get hot during operation. Again for the same reason as the caps. 14mV across .5 ohms is only 28mA, so there may be an issue there, however if both channels are the same it may be designed that way. It is a PA type amp so heat and reliability are generally a large part of the design.
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Old 5th January 2015, 06:53 PM   #7
ief is offline ief  Netherlands
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I was thinking along the same lines but still scared to use as is (want to power a sub with it.)

Say the original rail voltage was higher, am I then right in presuming R138 and R137 would have an influence on the hight of the bias voltage, i.e. could it be possible those are to high now? 68 ohm in schematics, 35 ohm on mine.

I'll see what happens when lifting the 5 watters of the pcb, can't even touch them for a second as is.

Rest of the amp is cool as a cucumber.
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Old 5th January 2015, 07:37 PM   #8
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R137/8 have to do more with the activation of the anti latch transistors Q110/111. The output bias should be pretty much solely controlled by Q130,R159/160 and TVR102. Might check that the wiper hasn't gone open circuit on TVR102 in which case you will get a minimum bias with no adjustment.
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Old 5th January 2015, 08:07 PM   #9
mjf is offline mjf  Austria
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you can measure the voltage drop across the 5W Res.......
UČ/R gives you the power dissipation of this part.......;

with a bias of 28mA the amp is in a working condition, i would not change it at the moment. let the amp play for some time, test it. you can change this later on if there is a need ......
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Old 5th January 2015, 09:34 PM   #10
ief is offline ief  Netherlands
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Tnx for thinking along guys!

Lemmy see, bias pots seem fine, R159 is 1 K in this amp (R160 same as schematic) I did calculate, but math not being my strong suit.... I came at 1.5 watt give or take (64 volt, 17 volt, 1K5 resistor)

Put the resistors upright, not looking verry tidy but hope they cool down a bit, wil put it together, re adjust ofset and bias (pot full open that is on the bias ) and go from there, tnx again!
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