fikic Posted October 22, 2009 Author Report Share Posted October 22, 2009 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... Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 22, 2009 Report Share Posted October 22, 2009 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 Quote Link to comment Share on other sites More sharing options...
fikic Posted October 22, 2009 Author Report Share Posted October 22, 2009 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..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... Quote Link to comment Share on other sites More sharing options...
audioguru Posted October 22, 2009 Report Share Posted October 22, 2009 You connected your trimpot as a rheostat (variable resistor).The trimpot in the circuit's schematic is shown differently as a rheostat. Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 22, 2009 Report Share Posted October 22, 2009 You connected your trimpot as a rheostat (variable resistor).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. Quote Link to comment Share on other sites More sharing options...
audioguru Posted October 22, 2009 Report Share Posted October 22, 2009 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. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 22, 2009 Author Report Share Posted October 22, 2009 In the original schematic it's drawn like ordinary resistor with fixed 2k2 resistance, right? Quote Link to comment Share on other sites More sharing options...
fikic Posted October 22, 2009 Author Report Share Posted October 22, 2009 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? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 22, 2009 Report Share Posted October 22, 2009 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. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 22, 2009 Author Report Share Posted October 22, 2009 hm...very strange. I added 10kR potentiometer in series with R14 and turned it to maximum resistance. It doensn't make any difference..but why 470 ohms resistor in series with speaker makes difference?? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 23, 2009 Report Share Posted October 23, 2009 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. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 24, 2009 Author Report Share Posted October 24, 2009 How can I messure voltages, if when I put fuse in socket, speaker screams...? ??? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 24, 2009 Report Share Posted October 24, 2009 It's oscillating.Does it sound like a high pitched whistle or a low frequency buzz or hum?Have you turned the volume down to the minimum setting? Quote Link to comment Share on other sites More sharing options...
fikic Posted October 24, 2009 Author Report Share Posted October 24, 2009 Ok, I messured voltages somehow...:VBE of Q4 (voltage accross R10) was 0.6VVBE of Q3 was 0.6VVBE of Q5 was 0.3VVBE of Q6 was 0.2VAuthor 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? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 24, 2009 Report Share Posted October 24, 2009 Ok, I messured voltages somehow...:VBE of Q4 (voltage accross R10) was 0.6VVBE of Q3 was 0.6VVBE of Q5 was 0.3VVBE of Q6 was 0.2VLooks good up until Q5 and Q6.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?That sounds right to me.Did you manage to stop it oscillating?If not then it's pointless trying to measure DC voltages because the AC could interfere with your meter causing it to generate inaccurate readings.Look at the schematic, rather than just assuming what you've been told is true.The voltage between Q5's collector and 0V should be equal to the DC supply voltage.The voltage between Q5's emitter and 0V should be approximately equal to half the supply voltage.I simulated it with a DC voltage of 30V and the voltage on Q5's emitter is 13.6V which is within 10% of the estimated value.I don't know what is wrong with your circuit, your VBE measurements of Q5 and Q6 suggest that they should both be off, yet the high quiescent current indicates they're both conducting too much and the emitter voltage of Q5 also suggests they're both conducting. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 24, 2009 Author Report Share Posted October 24, 2009 Did you manage to stop it oscillating?I swiched circuit on and loudspeaker screamed, then I quick turned it of and on again and it shuted up so I could make messurements. Should I still account your advice, to replace Q4? I don't understand your explanation for VBE of Q5 and Q6. Why is that voltage so low? Can you explain in newbe language please? Quote Link to comment Share on other sites More sharing options...
fikic Posted October 24, 2009 Author Report Share Posted October 24, 2009 What if DC supply voltage is too high? The secundary of my transformer is 24V, but when I messure voltage in fuse socket (without fuse connected), I read 31V. This is 7V higher. Is this becouse of the filter capacitor, or is supply voltage too high? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 24, 2009 Report Share Posted October 24, 2009 It's pretty normal for a transformer to output a much higher voltage when lightly loaded and the mains is probably also on the high side. The circuit should be designed to account for this but this doesn't seem to be the case as the filter capacitor is specified for 35V, it should be 50V.The DC voltage across the filter capacitor should be about 42V..I'll rerun the simulation with the DC voltage set to 42V and see what happens.EDIT:I've simulated it and it seems to work, R9 needed readjusting but it works.Of course this doesn't mean it'll work in real life but I don't see why not. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 25, 2009 Author Report Share Posted October 25, 2009 Any new suggestions? It seems that somewhere in circuit one component conduct too much current. Somewhere is mistake. But where? Why is current so high in fuse socket? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 25, 2009 Report Share Posted October 25, 2009 Simulate it in LTSpice.That will give you the approximate voltages and currents you'll expect in different parts of the circuit.I'll post the files if you like. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 26, 2009 Author Report Share Posted October 26, 2009 Yes, please, post it. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 26, 2009 Author Report Share Posted October 26, 2009 I tryed circuit without speaker. Instead of a speaker, I connected 12V, 21W lamp. It was unable to read current drawing, becouse one moment was 13mA and in the other was 30mA..it was changing every half second or faster. Lamp was little flashing. What could explain that? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 26, 2009 Report Share Posted October 26, 2009 It's oscillating.An incandescent lamp is a non-linear load, when the filament is cold, its resistance will be very low, when the filament is hot, its resistance will be much higher.The amplifier is only designed to power a 4Ω to 8Ω speaker, connecting a lower impedance load, such as an incandescent lamp with a cold filament, could damage Q5 and Q6.If you want to test it without the speaker, use a 3.9Ω to 8.2Ω resistor as a dummy load, like I told you before. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 26, 2009 Author Report Share Posted October 26, 2009 But why is oscilating? It is also oscilating with speaker. How can I stop that? Quote Link to comment Share on other sites More sharing options...
audioguru Posted October 26, 2009 Report Share Posted October 26, 2009 A transistor performs like an oscillator if its pins are connected backwards and there is a capacitor on it.Check the datasheet for the photo of the pins on every transistor in your circuit. Quote Link to comment Share on other sites More sharing options...
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