Just a little thing about this amplifier...

[-_nando-_]

Hi guys ! Take a look:

http://www.ampslab.com/c300.htm



The PSU transformer should be 50+50, so after retification deliver 70 + 70, or the PSU should be 36 + 36 to deliver 50+50 after retification?

An other thing...



What's the BIAS VR in this schematic? R22 or R10... R22 right?

Can I adjust bias without a scope?



Thankssss !!

[lineup]

R10 is used to set output to 0.0V
R22 for bias, yes

to see how much current output bias
you measure voltage across R33

current will be V/R33 resistance
and total current will be this x4, as there are 4 transistors in output

[-_nando-_]

V / R33

V = The +Volt from Supply ?

I didn't understand anything...

Please, tell me more detailed


And asking again, PSU Rails = 50 + 50 mean that this is the voltage AFTER or BEFORE retification?


Thanks !!!

[jacco vermeulen]

200 watts in 8 ohms is 40 volts continuous.
Peak voltage is 1.41 higher, 56.4 volts.
Rail voltage needs to be higher than that.
V rails is after rectification, so it is the 70 volts.

[-_nando-_]

70 + 70v

Now I believe that this amp can deliver 300W RMS

[AndrewT]

Hi Nando,
vr10 adjusts the output offset.
Set your dmm to DC volts, start with a low sensitivity try 200Vdc max scale to start with.
Measure the voltage across the speaker terminals with NO load on the output. Adjust the DMM down to maximum sensitivity without over loading it. Now adjust vr10 until you have 0mVdc at the terminals. Anything less than 10mVdc is OK. It is worth checking this cold and hot allow upto an hour for the hot reading.

vr22 adjusts the output stage bias (Iq).
Find r33 (emitter resistor) and all it's partners. Measure the voltage across each resistor, again start with the meter set high.
Now calculate the current in each emitter resistor using that formula, I=V/R. Keep all your units in Amperes and Volts and Ohms to avoid losing decimals places. Now calculate the voltage you need across the emitter resistor when it has the correct bias current. Same formula, just turn it around, V=I*R. Adjust vr22 until the emitter resistor voltage is as you have just calculated. Check each of the other 7 resistors to be sure they are reasonably balanced +-10% should be OK.

What speaker impedance are you planning to use?
If 8ohms then the 400VA transformer per channel is OK. If 4ohm then increase the transformer to 600VA per channel.
Similarly the main smoothing caps need to be +-14mF per channel for 8ohm and +-28mF per channel for 4ohm.

With an input impedance of 10k (r2) the input capacitor (c1) needs to be a big plastic job. 4u7F to 10uF to keep the bass from being cut off early and avoid some phase change at low frequencies. You could use a 22uF 16Vac bipolar electrolytic but this will have poor performance although it is cheaper.

[-_nando-_]

Thank you Andrew, appreciated your answer !



Well, everything is fine to start building this amp, BUT I can't find that MJE 350/340... Which transistor can replace them?

Thanks !!

[-_nando-_]

Oh ! Another thing !

This amp runs at + - 70v, and the transistors are 140v... Too much in the limits don't you think?

Maybe using MJ15024/5 (16A 200v) is better... The original transistors (15003/4) are cheaper, 20A 140v...

I do NOT wanna see transistors blowing like pop corn...

Thanks !!

[lineup]

Originally posted by -_nando-_
... BUT I can't find that MJE 350/340... Which transistor can replace them?

Not many good replacements. Have to take at least 150 Vce.
I suggest you look for to find MJE350/340.
They are good standard and used in many, many amplifiers.
Should not be impossible to find
if you look in another shop or can buy from internet.

Maybe you can try in forum 'Trading Posts' if anyone can sell you some.
I am sure there are 100 of members here, that have MJE350/340 at home.

[lineup]

Originally posted by -_nando-_
This amp runs at + - 70v, and the transistors are 140v...
Maybe using MJ15024/5 (16A 200v) is better... The original transistors (15003/4) are cheaper, 20A 140v...

As you say, I should use MJ15024/5 here.
They very much like MJ15003/4 but can take more voltage.
Even if they cost a bit more, this nothing if you think what box, trafo, heatsink and other things cost.

Transistors are never the expensive stuff in an amplifier.