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Old 6th September 2005, 09:58 PM   #1
leander is offline leander  Malta
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Default 300w rms amp

I1m currently building this amp--- http://www.ampslab.com/c300.htm
Could anyone help me out in how to set R10, R22 properly.
Thanks

regards


Leander

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Old 6th September 2005, 10:11 PM   #2
CBS240 is offline CBS240  United States
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Looks pretty straight forward to me. R10 is DC offset, R22 controls the amount of current biased through the outputs. Measure Vout DC to ground and adjust R10 until it is as close to 0V as possible. Measure the voltage across the emitter resistors on the output transistors and adjust for the current bias you want....V/R=I.

Might want to adjust output bias first...
This is of course if it is constructed properly.
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Old 6th September 2005, 10:13 PM   #3
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R10 is for setting 0V offset at output.
R22 is for adjusting idle current (bias) of the output transistors, best to measure voltage drop on one of R33-R40.
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Old 6th September 2005, 10:14 PM   #4
jaycee is offline jaycee  United Kingdom
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Adjust R22 to maximum resistance. Attatch a millivolt meter to measure over one of the emitter resistors (R33-R40). Adjust R22 until the millivolt meter reads 20mV (for 55mA idle current in each transistor)

Attach millivolt meter to speaker output. Adjust R10 until output offset is as close to 0mV as you can get it.
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Old 7th September 2005, 10:03 AM   #5
leander is offline leander  Malta
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Thanks for your replys people .

The specs says 300w rms with +/-70 volt rails

Do you think the amp will hold up if I add other 2 power transistors and give +/-80 volt rails ?

do I need to change something else in order to do this modification or I just add two power tansistors and resistors in parallel with the others. Do I need to change the resistors value for example?


Thanks

Leander

Malta
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Old 7th September 2005, 10:03 AM   #6
leander is offline leander  Malta
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I checked out the mj15003/4 specs and there maximum voltage Vceo are 140v therefore I guess that higher voltage than +/-70 will destroy them.

May I replace the 0.39ohm 5 watt resistors, with say 0.22ohm resistors for an increase of power output ???


Note these transistors have a very high current capability
Ic = 20A
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Old 7th September 2005, 10:19 AM   #7
quasi is offline quasi  Australia
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Quote:
Originally posted by leander
I checked out the mj15003/4 specs and there maximum voltage Vceo are 140v therefore I guess that higher voltage than +/-70 will destroy them.

May I replace the 0.39ohm 5 watt resistors, with say 0.22ohm resistors for an increase of power output ???


Note these transistors have a very high current capability
Ic = 20A

Replacing the 0.39 ohm resistors with 0.22 ohm will give you around 0.5 (1/2) extra volts to your speaker. This is something that you will not hear. This substitution though will affect the current sharing amongst the output transistors by 40% and this is less safe.

I would concentrate on the power supply. Improving the power supply will have the biggest affect on the power cabability (and sound quality) of your amp.

Cheers
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Old 7th September 2005, 11:11 AM   #8
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if you have an access to some pair of MJ21194's and MJ21193's
plus/minus 80vdc will do....

never use those 15003's and pairs higher than specified...


enjoy diy-ing

rgds

hienrich
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Old 7th September 2005, 02:40 PM   #9
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Or you can get 350V transistors.

MJL4281/MJL4302 350V, 15A, 230W

Amp I'm building with 5 pairs of those transistors I'm going to run on rails that peak at 70V with a 50-0-50 transformer.

If you parallel enough devices, you could easily run rails over +/- 80V. Also MJL21195/21196 are 250V 16A 200W and would work too.
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Old 8th September 2005, 07:40 PM   #10
leander is offline leander  Malta
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If I use MJ15024/MJ15025 Vceo=250v , May I give +80/-80v without damaging the other circuitry? or is the circuitry just designed to hold +70/-70v and no more ?
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