Ok I replaced transistors and I tried circuit. When I REPLACE leads of avo-meter in fuse holder it mesures of about 100mA current. Speaker gives verry loud sound and when I remove leads of avo-meter, LED still flashes for few seconds. What can I check? It is this good behavier?
The LED should not flash, it should be continuously lit but it's normal for it to remain lit for a couple of seconds after the power is removed, as the filter capacitor discharges.
Where is the 470R resistor? What do you mean by in front of the speaker? Do you mean in series?
If so, it's not surprising the sound quality is poor due to the poop damping factor. It will be very quiet as the power will be only a few mW.
Where is the 470R resistor? What do you mean by in front of the speaker? Do you mean in series?
If so, it's not surprising the sound quality is poor due to the poop damping factor. It will be very quiet as the power will be only a few mW.
The resistor will cut the power delivered to the speaker and reduce the sound quality, you might as well build a small amplifier with a 0.5W speaker, four AA cells and an LM386 if you want it to sound like a clock radio.
Changing R14 for an incorrect value will not help. There is an error somewhere else, check: the PCB, that all the components are soldered in the right places, the solder joints are all good and that the transistors all all good..
Changing R14 for an incorrect value will not help. There is an error somewhere else, check: the PCB, that all the components are soldered in the right places, the solder joints are all good and that the transistors all all good..
Author said, that I must change value of R10 to bigger one in small steps. When I get right one, it will be ok. I will add potentiometer to R10 and when I messure right current, I will get potentiometer out and messure it's resistance, and then I will know, how much resistance I must give to R10 to be right one. Then I will replace everything with fixed resistor with value of R10+resistance of poteniometer.
I must also check trimmer, becouse I did something, that isn't in original schematic....:
Did I do right thing to connect it like this? I thoud that in original schematic is mistake...
I must also check trimmer, becouse I did something, that isn't in original schematic....:
Did I do right thing to connect it like this? I thoud that in original schematic is mistake...
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You shouldn't need to vary the value of R10.
Do you understand how the biasing of Q5 and Q6 works?
The voltage across Q4 needs to be equal to the VBE of Q5+Q6.
The voltage across Q4 can be calculated using the following formula:
VCE = VBE
Do you understand how the biasing of Q5 and Q6 works?
The voltage across Q4 needs to be equal to the VBE of Q5+Q6.
The voltage across Q4 can be calculated using the following formula:
VCE = VBE
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That could explain differend current, through fuse, but not 10x higher, am I right? So I won't change nothing, even if I put r10 with 10k resistance in...I should check something else..but what? I checked every connection..If one of the transistors would be dead, amplifier wouldn't work at all would it? So, What do you suggest? What if I connected loud speaker wrong? In original, this speaker had a tweeter connected on (I think it was parallel). Could that be the problem? Should I connect tweeter with woofer?
Oh, and you forgot to answer..
Oh, and you forgot to answer..
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audioguru2
- Apr 6, 2004
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You connected your trimpot as a rheostat (variable resistor).
The trimpot in the circuit's schematic is shown differently as a rheostat.
The trimpot in the circuit's schematic is shown differently as a rheostat.
Please expand further audioguru.
A rheostat and variable resistor are the same thing.
He appears to have connected up the resistors correctly.
audioguru2
- Apr 6, 2004
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The rheostat trimpot in the project is drawn a little different than Fikic draws it but it is exactly the same. It is a variable resistance.
I really don't know what to do anymore. I checked everything and it's like in the original schematic. Why it doesn't work like it should? Resistors are right, especially R10 and R11 (I checked them hundreds of time). It is useable to change value of R14? Maybe it will work?
R9 is 2k but 2k2 will do fine.
I've simulated it in LTSpice and R9 is very sensitive, with it set to 1k5 the current is 18.9mA and when it's set to 1k6 the current is 29.6mA.
I haven't tested the cross over distortion yet and I've substituted some of the transistors as I don't have the models for all of them.
I've simulated it in LTSpice and R9 is very sensitive, with it set to 1k5 the current is 18.9mA and when it's set to 1k6 the current is 29.6mA.
I haven't tested the cross over distortion yet and I've substituted some of the transistors as I don't have the models for all of them.
Connecting a 470R resistor in series with the speaker makes very little difference to the the current, it increases slightly to 29.8mA.
I'm beginning to question whether it's actually possible to trim the current to between 25mA to 30mA because R9 is too sensitive: with it set to 1k54 the current drops to 21.9mA.
I would say trim the current to between 15mA and 30mA - you'll probably need a multi-turn trimmer to do that.
Varying R14 is not the answer, it will alter the gain and biasing.
It sounds like one of the components are faulty or there's a poor solder joint somewhere.
My guess is there's a problem with Q4, perhaps its gain is much lower than it should be?
Try replacing it.
Have you checked that R9 reads near 0R when its set to its lowest setting?
Measure the following voltages:
VBE of Q4 (voltage across R10), should be about 0.6V to 0.7V.
VBE of Q3, should be about 0.6V to 0.7V.
VBE of Q5 and Q6, both should be about 1.2V to 1.4V.
I'll post the LTSpice file if you like.
I'm beginning to question whether it's actually possible to trim the current to between 25mA to 30mA because R9 is too sensitive: with it set to 1k54 the current drops to 21.9mA.
I would say trim the current to between 15mA and 30mA - you'll probably need a multi-turn trimmer to do that.
Varying R14 is not the answer, it will alter the gain and biasing.
It sounds like one of the components are faulty or there's a poor solder joint somewhere.
My guess is there's a problem with Q4, perhaps its gain is much lower than it should be?
Try replacing it.
Have you checked that R9 reads near 0R when its set to its lowest setting?
Measure the following voltages:
VBE of Q4 (voltage across R10), should be about 0.6V to 0.7V.
VBE of Q3, should be about 0.6V to 0.7V.
VBE of Q5 and Q6, both should be about 1.2V to 1.4V.
I'll post the LTSpice file if you like.
Ok, I messured voltages somehow...:
VBE of Q4 (voltage accross R10) was 0.6V
VBE of Q3 was 0.6V
VBE of Q5 was 0.3V
VBE of Q6 was 0.2V
Author said, that I must check also voltage across Collector of Q5 and negative ground and across Emitter of Q5 and negative ground. Both voltages should be half of voltage supply. However, voltage across Collector and negative ground was 34V, and across Emitter and negative ground was 15V. Can that explain my problem? What should I do?
VBE of Q4 (voltage accross R10) was 0.6V
VBE of Q3 was 0.6V
VBE of Q5 was 0.3V
VBE of Q6 was 0.2V
Author said, that I must check also voltage across Collector of Q5 and negative ground and across Emitter of Q5 and negative ground. Both voltages should be half of voltage supply. However, voltage across Collector and negative ground was 34V, and across Emitter and negative ground was 15V. Can that explain my problem? What should I do?
