fikic Posted October 27, 2009 Author Report Share Posted October 27, 2009 I checked them all. They are all correct soldered. Only option left, I think is poor solder joint somewhere in the PCB. I am now making new PCB with differend software to make more mechanical compact pads. I will report. Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 27, 2009 Report Share Posted October 27, 2009 Here's the LTSpice file, plus all the required libraries.Remove the .txt extension and make sure they're all in the same directory, alternatively copy the .sub file to LTSpice's lib/sub directory and the .asy to the lib/sym/Misc directory.10W_Guitar_Amp.asc.txtpotentiometer.sub.txtpotentiometer_EU.asy..txt Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 27, 2009 Report Share Posted October 27, 2009 You'll need to upgrade LTSpice's default transistor library.Back up the existing LTSpice/lib/cmp/standard.bjtRemove the .txt extension from the attachment and save it in the LTSpice/lib/cmp directory..I've also included a picture of my LTSpice implementation of the circuit, not that Q5 and Q6 are drawn as two transistors because I couldn't find a Darlington model.standard.bjt.txt Quote Link to comment Share on other sites More sharing options...
fikic Posted October 28, 2009 Author Report Share Posted October 28, 2009 How can I see voltage betwen ground and some element. C13 for example...There should be higher voltage then 24V. And how can I turn trimmer? I don't know this program much. Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 28, 2009 Report Share Posted October 28, 2009 You've probably noticed that there is no C13 or transformer in the schematic. This is because, to make the simulation run faster, I've replaced them both with V1 which is set to 42V as it is the voltage you said is across C13.I assume you're talking about the DC voltage?There are two ways to get the voltage at a certain point.1) Use the transient analysis, should be the default, and set the V2 to 0V and select run from the simulate menu. Now click on the node and a straight line will appear on the graph, if you click on the legend on the graph a cursor will appear, allowing you to read the exact voltage.2) Use the 'calculate DC operating point' tool. Select 'Edit Simulation Command' from the simulation menu, click on the 'DC op pnt' tab, click 'ok', place the cursor anywhere on the schematic and click, you'll see a label '.op' appear, this is the simulation command. Select 'run' from the simulate menu, a list of voltages at all the nodes will appear, the trouble is that the nodes will have obscure names such as V(n006) unless you've names them using the label net tool. To turn a pot, right click on the symbol and edit the 'wiper=' setting, the number should be from 0 to 1 and represents the position of the wiper, for example 0.5 is half way along. LTSpice is not an interactive package so you'll need re-run the simulation every time you've made an adjustment.LTSpice is a very powerful simulation package, but it does have a steep learning curve if you don't have much experience with electronics or other simulation software. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 29, 2009 Author Report Share Posted October 29, 2009 These voltages are just betwen mouse pointer and 0V. But how can I messure VBE of some transistor for example? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 29, 2009 Report Share Posted October 29, 2009 The voltages are all referenced to 0V.To measure the voltage between two points, simply subtract the voltage at one point from another.It's probably easier to use a calculator but LTSpice can do it for you.For example with this circuit:Set the simulation command to transient analysis and run the simulation command. Click Q5d's base to get the base voltage. With the schematic window selected wave the mouse cursor over Q5's emitter, the cursor will change to a red probe, note the net name displayed in the bottom left hand side of the screen 'Click to plot V(N009)' Right click on the graph legend 'V(n005)', a dialogue box will appear. In the text box, append '- V(n009)' (with out the ') to the text, it should look like 'V(n005)-V(n009)' Don't worry about whether it's upper or lower case, SPICE is case insensitive. Click OK. The line on the graph will be equal to the voltage between the base and emitter, again you can click on the graph legend to get the voltage at any point in time.In this example, with the input set to zero Q5(VBE) = 1.24709V, note that because Q5 is really made from Q5 and Q5d, the base emitter voltage has been measured across them both. Quote Link to comment Share on other sites More sharing options...
fikic Posted October 29, 2009 Author Report Share Posted October 29, 2009 I made new differend and better PCB, but circuit is still oscilating...only change, I made from original schematic was n-fet transistors Q1 and Q2. I used BF244AIt is kind of a inposible to make new PCB and connect transistors wrong again to make circuit oscillate...I checked them again and I can't find error. What else can cause oscillating? Quote Link to comment Share on other sites More sharing options...
fikic Posted October 30, 2009 Author Report Share Posted October 30, 2009 I forgot to say, that it's only oscillating, when I power on circuit and have volume potentiometer set to minimum resistance..If I turn it to something bigger than 0Ohms it's cool to turn it back to 0Ohm when I power the circuit on. I am now reading current drawing from fuse of 25mA (I turned R9 to half of full resistance)....But sound is not ok..it is waving. What voltages should I messure and must be the same as in the LTSpice? I am posting some pictures from my messuring procedure. Second picture is speaker I am using now for testing. It's 4Ohms 15W speaker from old TV: Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 30, 2009 Report Share Posted October 30, 2009 Did you use the same transistors or did you replace them all?It sounds like there's a problem with one of the transistors.Your speaker is quite old, Yugoslavia no longer exists. ;D Quote Link to comment Share on other sites More sharing options...
fikic Posted October 30, 2009 Author Report Share Posted October 30, 2009 I used same transistors. Which one should I suspect?Hehe I am quite young..I was born, when yugoslavia died :)..Then we got Slovenia :) Quote Link to comment Share on other sites More sharing options...
Hero999 Posted November 1, 2009 Report Share Posted November 1, 2009 Try replacing Q5 and Q6 first, then Q3 and Q4. Quote Link to comment Share on other sites More sharing options...
fikic Posted November 1, 2009 Author Report Share Posted November 1, 2009 What if I should put this thing in iron case and ground everything and use shielding like this guy Quote Link to comment Share on other sites More sharing options...
Hero999 Posted November 1, 2009 Report Share Posted November 1, 2009 Yes, that might help. Quote Link to comment Share on other sites More sharing options...
fikic Posted November 1, 2009 Author Report Share Posted November 1, 2009 But how do I have to connect things? And for connection like this I would aquire supply circuit by itself, not togheter with preamp and power amp... Quote Link to comment Share on other sites More sharing options...
Hero999 Posted November 1, 2009 Report Share Posted November 1, 2009 Doryy, I don't know wht you're talking about. Quote Link to comment Share on other sites More sharing options...
fikic Posted November 2, 2009 Author Report Share Posted November 2, 2009 I am saying, that I don't know, how to connect everything with the chassis. I also don't know, how to shield inputs. Can you tell me that? Beside that, I have power supply circuit on the same PCB than preamplifier and power amplifier Quote Link to comment Share on other sites More sharing options...
Hero999 Posted November 2, 2009 Report Share Posted November 2, 2009 The 0V supply rail should be connected to the case which should also be connected to safety earth/ground.The inputs should be connected to whatever you're connecting the amplifier to with screened cabe. Quote Link to comment Share on other sites More sharing options...
fikic Posted November 2, 2009 Author Report Share Posted November 2, 2009 i did that. I connected 0V rail to radiator. What is screened cabe? In Slovenian forum, I got advice, to remove C10, becouse this capacitor causes many unwanted effects. It drives signal from power amplifier back to preamp and cause oscilating. Amplification without this capacitor is much lower, but reduces oscilating. What about that? Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 2, 2009 Report Share Posted November 2, 2009 Screened cable is shielded audio cable that is used to connect stereo components together.It stops interference and feedback. The volume control should be connected with it and the input from the guitar should use it.The power amplifier should be disconnected from the input circuit to see if the feedback comes from the power amplifier (it is simply 3 transistors with a 4th one as a variable couple of diodes).Do not remove C10 because it is a "bootstrap" capacitor that increases the positive-going output swing of the amplifier which increases its max power to 10W into 8 ohms.If the power amplifier still oscillates when the volume control is turned down then add a 1k resistor in series with C9.Here is the extremely simple power amplifier parts: Quote Link to comment Share on other sites More sharing options...
fikic Posted November 2, 2009 Author Report Share Posted November 2, 2009 I disconected preamplifier and power amplifier still oscilated. I connected 1k resistor in series with C9, like you said and oscilation stoped. I hear very quiet hum in speaker, with volume potentiometer set to 0. I must try connect preamplifier back and try put some signal on. I will report.EDIT: Now it oscilates when I turn Volume potentiometer to maximum... ??? Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 3, 2009 Report Share Posted November 3, 2009 I disconected preamplifier and power amplifier still oscilated. I connected 1k resistor in series with C9, like you said and oscilation stoped. I hear very quiet hum in speaker, with volume potentiometer set to 0. I must try connect preamplifier back and try put some signal on. I will report.EDIT: Now it oscilates when I turn Volume potentiometer to maximum... ???Then the volume control wiring or the input wire are not shielded and are picking up interference from the power supply wiring or from the output wire.Or maybe you did not connect the grounds as a "star". Quote Link to comment Share on other sites More sharing options...
fikic Posted November 3, 2009 Author Report Share Posted November 3, 2009 Yes I will shield everything when I make case from aluminium and put everything in. I will then make grounding on the case and I will insulate case to prevent it to electricute me. So you thing shielding will make this oscilating go? Quote Link to comment Share on other sites More sharing options...
fikic Posted November 3, 2009 Author Report Share Posted November 3, 2009 Can you tell me something..How can I use star grounding with my PCB. I think, i can't, becouse I used a copper ground...I can't wire every ground to the star connection with different wire. I am confused now. Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 3, 2009 Report Share Posted November 3, 2009 I notice that the power amplifier is inverting so capacitive coupling through the air from the output to unshielded wires at the input cause negative feedback, not positive feedback. oscillation occurs with positive feedback so that is not the cause of the oscillation.Since you have the input circuits disconnected then they cannot cause the oscillation.The supply voltage to the power amplifier will bounce up and down a lot especially if you have a low current fuse. Then the supply voltage can capacitive-couple to the input if the input wiring is not shielded and cause feedback.Star grounding makes everything connect to a single point, usually the main filter capacitor's ground. Quote Link to comment Share on other sites More sharing options...
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