what rate of transformer i should use?

[motbuddy]

Do all the resistors have to be a 1 watt or a 1/2 watt resistors?

thanks.

 

[CBS240]

Well, figure it. 28.8V/4.9K=5.9mA, ~170mW. 5.8V on R1, about 35mW. R2 has 135mW. R3?, uW.

R4 & R5 have ~0.7V across them. No big power hogs there.

I would use at least 2 or 5W resistors for R6 & R7.

Also R8 should be at least 1/2W metal film flame proof. I have had amp circuits burst out in RF oscillation and smoked the Zobel resistor to a cinder.😱

S*** like that happens sometimes.......🙄

 

[motbuddy]

i get R3 has 16.4W. Is it reasonable for it? In the tutorial the author mentioned,

The main load resistor R2 of the Q2 common-emitter amplifier stage is bootstrapped by C2 and DC biased by R3. This network should set the quiescent output voltage at about half the power supply value. If it does not, alter the value of R3.

Any idea about this?

thanks.

 

[CBS240]

Well, figure it. 28.8V/4.9K=5.9mA, ~170mW. 5.8V on R1, about 35mW. R2 has 135mW. R3?, uW.

R4 & R5 have ~0.7V across them. No big power hogs there.

I would use at least 2 or 5W resistors for R6 & R7.

Also R8 should be at least 1/2W metal film flame proof. I have had amp circuits burst out in RF oscillation and smoked the Zobel resistor to a cinder.😱

S*** like that happens sometimes.......🙄

My bad, I just noticed it is a single supply amp. so ~15V/4.9K=3mA, ~44mW. 3V on R1, ~9mW. R2 has 35mW.

R3 is 470,000 Ohms, with about half the supply voltage. ~500uW, no?

Also R3 is providing some level of feedback.

 

[motbuddy]

how do u calculate for R3?

thanks,

 

[motbuddy]

??

 

[godfrey]

how do u calculate for R3?

You can't calculate it exactly.
The value you need depends on the current gain of Q2.

Try 470K then:
If voltage at output (collector of Q6) is too high, then R3 must be lower value (maybe 330K).
If voltage at output is too low, then R3 must be higher value (maybe 680K).

Voltage at collector of Q6 should be about half of supply voltage.

If you use a 1 mega-ohm potentiometer for R3, it will be easy to adjust.

 

[AndrewT]

If you use a 1 mega-ohm potentiometer for R3, it will be easy to adjust.

only to adjust and then replace with a fixed value resistor.
The maximum current through a 1M 500mW pot is 0.7mA, if one wants long life and good reliability. Some, me included never run pots at more than 50% power rating, i.e. <0.5mA
The maximum current through the wiper and through any part of the track must not exceed 0.7mA irrespective of what position the wiper is at.

 

[Speedskater]

But 1M and 250mW is 500V is this a vacuum tube circuit?

 

[motbuddy]

hi,


http://www.sentex.ca/~mec1995/tutorial/xtor/xtor6/xtor6.html
Does the potentiometer R9 use to control the volume? When i turn on the power. The volume is too high till the audio sound become very terrible.

thanks.

 

[CBS240]

Yes, R9 is the input pot. Is the circuit bias correctly? You should measure 15V at the collector of Q6, the + side of the output cap. If you do not measure 15V at this node, adjust R3 until you do. If you see any oscillation, increase C3 accordingly.


A couple of small issues, wipers for pots are usually noisy locations. A small cap from the wiper of the input pot to gnd for that and RF filtration wouldn't hurt. Also R10 is a problem. If the wiper becomes disconnected for some reason, the output transistors will over-bias and melt. To prevent this, use a voltage divider, a resistor from collector to base, then place R10 between the base and emitter of Q1 and short the wiper to one end. There should be 2.1V between the bases of Q3 & Q4.

 

[motbuddy]

To prevent this, use a voltage divider, a resistor from collector to base, then place R10 between the base and emitter of Q1 and short the wiper to one end. There should be 2.1V between the bases of Q3 & Q4.

hi thanks for the reply.Isnt it R10 works as a voltage divider?

thanks,

 

[motbuddy]

i just measured the voltage at Q6, i get 12V. So does it means i need to adjust R3?i would like to try a 1M pot just like Godfrey said.

 

[godfrey]

12V is pretty close. It should work fine like that - maximum power will just be a bit lower.

I'd just try increasing R3 a bit, maybe add an extra 100K resistor in series with the 470K.

Don't forget to adjust R10 to set the bias for the output transistors.

 

[motbuddy]

Don't forget to adjust R10 to set the bias for the output transistors.

ok.i left the Q6 to be 12V. Now i'll go for the bias part. How could i set the bias of the transistor. By referring to the waveform of the output transistor?

thanks.

 

[motbuddy]

after i change the bias and then i inputted a 1Khz 0.5Vp-p to the input.the sound is better and when i tune the input POT i can low down the volume.

But when i try to input from a DVD player. The sound become too loud and terrible again even the volume is tune to the lowest. Is it because of i didnt tune the bias correctly?

thanks,

 

[motbuddy]

??

 

[motbuddy]

how do i know whether i set the bias correctly?

thanks.

 

[godfrey]

I think it will be good to put a resistor (maybe 10K) in series with C1. This should make it less loud and not sound terrible.

maybe it is easy to connect the extra resistor between R9 and C1. It does not have to be 10K - You can try anything between about 4.7K and 47K.
-------------------------------
Q5 and Q6 will be damaged if they get too hot. They must not be too hot to hold your finger on.

These transistors should be on heatsinks to help keep them cool.

If more than one is on the same heatsink, then they must be insulated from it so there is no electrical connection between them.
-------------------------------
To set the bias is difficult because it will change when some transistors get warm. To fix this, it is good to put Q3, Q4, Q5 and Q6 all on one big heatsink, and then put Q1 on the same heatsink or glue it on top of Q5 or Q6.

This will help to keep them closer in temperature.
-------------------------------
Did you build it on a circuit board? Or is it loose parts connected together? Do you have a photo?

Regards - Godfrey