Troubleshooting Amplifier Design and PCB Layout

[Enzy]

I want to practice some etching, Does anyone know where I can get some pcb layouts of amplifier circuits like 600 watts and above?

 

[Enzy]

need help figuring out the issue with a circuit I built.

I etched a board for the first time and made an amplifier I saw online.

I prototype the board first and I wasn't getting any sound out so I rebuilt it a couple times ensuring I didn't make any mistakes but still I got no sound out, but in a forum the forum admin said he got the PCB made and it worked no problem, the version I build is a slightly updated version but he said it shouldn't give any issues at all.

After etching the circuit and building circuit just now I still didn't get any sound from it so I am wondering if I can get any assistance, could someone view the schematic and see if there is an issue.

original circuit that the forum admin made:


Circuit I made:

 

[chuckey]

have you got the centre tap of the power supplies connected to earth? What is the voltage at the centre loudspeaker terminals - should be earth +- .1V. The voltage amplifier transistors are in two pairs, compare the voltages on each collector with its twin, that should point you to the problem.
Frank

 

[Enzy]

yes the center tap of the power supply is connected to earth of the circuit which is where the speaker negative terminal is, Its not connected to the ground at the input. I tested between speaker terminals + and - for ac voltage and I got 0.498v.
When testing the Fets you said to test collectors so I tested the Drains of the Fets, I placed one tester lead to the center tap of the power supply and the other to the Drain of each Fet and the readings were 9.67v , 9.68v dc. On my circuit the fets I am using are Ifrz44n and Irf4905.

 

[betwixt]

?? 9.67V ?? The supply is supposed to be +65V and -65V (130V in total).

Brian.

 

[Enzy]

I am using 40v(20v +/-) to power it but I assume it should still be more than that. What could be the issue I am using new FETS, Its not the same as the Fets In the sechmatic but I didnt think that would be an issue.

I just realized that the FETS have the wrong pin outs so I took them out and ran wires from them to the board in the correct manner but the regulators on the power supply got hot instead just like what happend when I tested this circuit on a breadboard, so I am assuming the FETS just need to be the correct type then.

 

[Audioguru]

The Mosfets are not biased correctly so the amplifier will produce plenty of crossover distortion.
Most audio power amplifiers do not use voltage regulators.

 

[chuckey]

It is imperative that the input ground is connected to the power supply centre voltage, else the input transistors have no bias and also no input signal flows.
Frank

 

[c_mitra]

I would love to see your power supply schematics also because problems may be there too.Why the speaker negative terminal is showing some voltage when it should be close to ground?

- - - Updated - - -

I see that you have a centre tapped transformer. The center tap will go to the ground in the power supply board. I also see that you have symmetrical output power supply. Measure all voltages with respect to the centre terminal and write on the schematic diagram.

In post #4 you say it is not connected to the ground at the input- please explain in detail.

 

[Enzy]

All I was saying is that the middle terminal or the power supply isnt directly connected to the section of the circuit labled P2 which is the ground terminal for the input signal.

I would test and show voltages but I dont know how to lable the pic I cant seem to find any softwares to easilyy lable things within a picture, and If i was to lable would I need to label every single thing meaning voltages passing through resistors and caps?

secondly since I was using the wrong Type of FET and I removed them and ran wires to the board in the correct manner and it still dont produce any output I was wondering If I need to use the correct FETs or the Irfz44n and Irf49055 Fets can work for testing purposes since I already changed the connections.

 

[chuckey]

The whole amplifier up tp the loudspeaker capacitor is DC coupled. So generate a small DC voltage , say +10 mV ( a pair of resistors across +Vcc to earth) and feed it into the input socket, the DC level should shift through out the amplifier, you can find out with your DVM, where its going wrong.
Frank

 

[c_mitra]

You are right, P2 is not to be grounded at the amplifier board because you are using a differential stage for input. Most important points for the voltage measurements are the legs of the transistors. You should see that the power supply ground is connected to the amplifier ground using a DVM.

 

[Enzy]

when you say amplifier ground are you refering to P4 or P2?

P2 is labelled ground but its not connected directly to power supply ground (center terminal or power supply)

P4 says zero volt its actually connected directly to the center terminal of the power supply

 

[c_mitra]

Right. Now you can short the input (P1 and P2) and report the voltage on the power FETs.

 

[Enzy]

before I do that I went and bought the correct FETS and also I found another mistake I made the 3.3k resistors that should connect to q9. and q10 I actuually had a 3.3 ohm resistor there. After replacing resistors and FETs I got sound out but I had to max the volume on my laptop to hear it slightly, what I realized is that when I unplug the power supply the audio shortly comes up load then die out. 1 side of the regulator on the power supply gets hot and 1 Fets on the same side gets hot on the amp, I am wondering If I should just build a new power supply without regulators to test it.

Also I am supplying 20v+/- to the amp, testing between power supply 0v and FETs drains I am getting 20v on 1 and 18v on the other. The one that gets hot has a lower drain and gate voltage, I would have to double check to give exact readings.

 

[c_mitra]

1. This is a power amplifier and it will need significant input before you will get building-shaking output. Most likely the headphone output of the laptop will not be enough. You will need line level input.

2. It is time to run a quick check of all the resistors with a multimeter. These mistakes are most common and most difficult to find.

3. If you have shorted the input and getting 20V and 18V on the drains of two FETS (there are 4; which pair you are talking about?) there there are other mistakes.

4. For the time being, leave the power supply with regulators. Identify the FET that gets hot on the diagram. Also make sure that the voltages are +/-20V (after the drop)

 

[chuckey]

I have a big problem with Q11 base current , where does it come from?
Frank

 

[Audioguru]

I have a big problem with Q11 base current , where does it come from?
Frank

I do not know why all the grounds are not connected together.

 

[Enzy]

so your saying I need a preamp stage to actually hear anything out of the amp? (Line level input) what do you mean by that.

I didnt see any other mistake and I only bought a single pair of FEts so I am using 2 FETs.

I dont have the amp where I am now but before I changed the resistors a FET was getting hot along with 1 regulator and after changing the resistors the opposite FET and opposite regulator is getting hot.
But what I will do when im back home is to check carefully if there is a fault with a wrong component or if a component was inserted incorrectly because its confusing at times when placing parts because you know the print is in the inverse

 

[c_mitra]

so your saying I need a preamp stage to actually hear anything out of the amp? (Line level input) what do you mean by that.

It is entirely possible that your laptop music output is meant for headphones and is not at line level. Line level outputs can drive a power amplifier directly...