Is this amplifier circuit connected correctly?

[jackson6612]

Hi

Please remember that I'm not a physics or science student. Just trying to learn things. So, please be simple with your replies. Thanks.

I found this amplifier circuit on this webpage:
http://www.circuit-lab.com/2009/12/30-watt-audio-power-amplifier-schematic.html

Now I have connected the terminals and have a few questions about it, please have a look on this linked image: http://img341.imageshack.us/img341/6779/amplifierx.jpg

1: Are terminals connected correctly?

2: I;m sure you have notice that "A" there. The ground symbol - three horizontal bars over one another - as I'm told, is used to denote negative terminal. But I don't see any need of this in the circuit because the circuit already has designated +ive and -ive terminals. So, what's the reason for this? Please remember I'm not a science or physics student.

3: Now coming to "B". Is that "To Input Switches" terminal connect to a sound source such as tape recorder etc.?

4: What was the reason for having two separate circuits as "Power Amplifier Circuit" and "Pre-Amplifier Circuit"? It could have been one single circuit, I think.

Please help me with this. Thanks a lot for your time.

 

[vk6kro]

Are terminals connected correctly?

Yes

I;m sure you have notice that "A" there. The ground symbol - three horizontal bars over one another - as I'm told, is used to denote negative terminal. But I don't see any need of this in the circuit because the circuit already has designated +ive and -ive terminals. So, what's the reason for this? Please remember I'm not a science or physics student.

This just means you can join these points together and that they are OK for humans to touch and you could connect any shielded wires to these points.

Now coming to "B". Is that "To Input Switches" terminal connect to a sound source such as tape recorder etc.?

Yes.

What was the reason for having two separate circuits as "Power Amplifier Circuit" and "Pre-Amplifier Circuit"? It could have been one single circuit, I think.

It is usual to label the parts of the circuit to explain what they are used for. This does not affect the function of the circuit. Different parts of a circuit can be physically located near each other, or not.

There is a problem with the regulator in this circuit. It has more than 40 volts input, but the LM317 has a maximum input of 40 volts.

 

[jackson6612]

Are terminals connected correctly?

Yes

I;m sure you have notice that "A" there. The ground symbol - three horizontal bars over one another - as I'm told, is used to denote negative terminal. But I don't see any need of this in the circuit because the circuit already has designated +ive and -ive terminals. So, what's the reason for this? Please remember I'm not a science or physics student.

This just means you can join these points together and that they are OK for humans to touch and you could connect any shielded wires to these points.

Now coming to "B". Is that "To Input Switches" terminal connect to a sound source such as tape recorder etc.?

Yes.

What was the reason for having two separate circuits as "Power Amplifier Circuit" and "Pre-Amplifier Circuit"? It could have been one single circuit, I think.

It is usual to label the parts of the circuit to explain what they are used for. This does not affect the function of the circuit. Different parts of a circuit can be physically located near each other, or not.

There is a problem with the regulator in this circuit. It has more than 40 volts input, but the LM317 has a maximum input of 40 volts.

Thank you very much, VK. You have really helped me. I have some questions, though. BTW, please keep in mind that I'm not a physics or science student.

Which point could be connected together? The ones with ground symbols? The 'actual' current flows from -ive terminal so would it be dangerous to touch it. The electrons would start flowing through a living body.

Aren't "Power Amplifier Circuit" and "Pre-Amplifier Circuit" do the same job to increase volume? By the way, why didn't the author simply wrote "Amplifier Circuit", what's the reason to write "Power"?

And about that LM317 you better contact the original author because I don't know what the heck it is. :-)

Best wishes
Jackson

 

[vk6kro]

Which point could be connected together? The ones with ground symbols? The 'actual' current flows from -ive terminal so would it be dangerous to touch it. The electrons would start flowing through a living body.


Yes, you would join the wires with the ground symbols together. These could also be connected to any metal case and to the mains ground.
No, just touching the grounded side of this circuit would not harm you, although if you also touched the +40 volts you would probably feel it as a small electric shock. 40 volts is about the point where you do feel these voltages.

Generally, don't touch anything while the circuit is operating.
Especially, make sure the mains input wiring is completely covered so nobody can touch it. You should not be able to see any metal with mains voltage on it.
You can do this with heat shrink tubing and with suitable terminals.
Mains wiring is something you need to get checked before you connect it. Make sure the mains cord is properly anchored so that if it is pulled, it will not pull other parts of the circuit.


Aren't "Power Amplifier Circuit" and "Pre-Amplifier Circuit" do the same job to increase volume? By the way, why didn't the author simply wrote "Amplifier Circuit", what's the reason to write "Power"?

They are both amplifiers, but there is a difference between power amplifiers and voltage amplifiers.

A power amplifier can drive a speaker while a voltage amplifier (or preamp) probably couldn't, even though they may produce the same voltages without the speaker connected.

The difference is that the power amplifier can still deliver a voltage while a current is being supplied, while the voltage amplifier can't deliver a current and still produce as much voltage output.


And about that LM317 you better contact the original author because I don't know what the heck it is. :-)

The LM317K is a voltage regulator (and a very good one) but it has maximum input voltage ratings which appear to be ignored in this circuit.
So, if you want to build this circuit, you may have to modify the power supply.
This may just mean getting a 25 volt transformer instead of a 35 volt one, although the resistors around the 317 will also need to be changed.

 

[yungman]

Hi

Please remember that I'm not a physics or science student. Just trying to learn things. So, please be simple with your replies. Thanks.

I found this amplifier circuit on this webpage:
http://www.circuit-lab.com/2009/12/30-watt-audio-power-amplifier-schematic.html

Now I have connected the terminals and have a few questions about it, please have a look on this linked image: http://img341.imageshack.us/img341/6779/amplifierx.jpg

1: Are terminals connected correctly?
First pass, I don't see anything particularly wrong.
2: I;m sure you have notice that "A" there. The ground symbol - three horizontal bars over one another - as I'm told, is used to denote negative terminal. But I don't see any need of this in the circuit because the circuit already has designated +ive and -ive terminals. So, what's the reason for this? Please remember I'm not a science or physics student.

3: Now coming to "B". Is that "To Input Switches" terminal connect to a sound source such as tape recorder etc.?

4: What was the reason for having two separate circuits as "Power Amplifier Circuit" and "Pre-Amplifier Circuit"? It could have been one single circuit, I think.
Just for easier to understand.
Please help me with this. Thanks a lot for your time.

Are you trying to build this circuit?

I really don't like the circuit.

1) R4 is to adjust the bias of the two MOSFET and the voltage drop across R4 depend of the instanteneous voltage. Meaning that when the collector voltage of Q1 is higher ( less collector current ), the voltage across R4 is less, then the two MOSFETs (Q2 and Q3) can be into deep class B or even stepping into class C ( off).

2) Bias of Q1 depend on the voltage at the junction of C5, Q2 and Q3. I would like to see a better bias.

3) Q1 is run in the way that is pretty much like a open collector, meaning the rise time is governed by the R4 and some other passive components, fall time is active pull down and is going to be faster. Uneven slew time almost guarantee even harmonics and is usually not good for any audio application.

3) It is really in an OP-Amp configuration where R2 is the feedback resistor and R1 is the input resistor. The closed loop gain idealy is R2/R1. BUT C3 and R6 form a lag lead network and feedback to R4. AND the reference of R6 is +40V instead to ground. It is not something I would design for an amplifier because any ripple on the +40V will complicate the stability calculation.

When you design an amplifier, don't try to make it complicate. C3 and R6 connecting to R4 make it that much harder to predict the pole and zeros of the amplifier. I would like to see a more balanced circuit than this. I would like to see the two power MOSFET biased in a more predictable way, either a class A, AB.

 

[Bob S]

The LM317K is a voltage regulator (and a very good one) but it has maximum input voltage ratings which appear to be ignored in this circuit.
So, if you want to build this circuit, you may have to modify the power supply.
This may just mean getting a 25 volt transformer instead of a 35 volt one, although the resistors around the 317 will also need to be changed.

The maximum voltage rating on the LM317 regulator is V_in - V_out < 40 volts. In this circuit in normal operation, this voltage difference is about 10 or 12 volts (less when the LM317 is pulling down the Q1 base). Also V_out - V_adj = 1.25 volts (regulated value). The very large input filter capacitor C1 (4700uF) limits fast transients on the unfiltered input voltage during turn-on. So the LM317 is operating in a safe region, unless the power supply output is shorted to ground.

It is not obvious why the specs call for a center-tapped secondary on power transformer T1.

Bob S

 

[vk6kro]

No, the LM317 is rated at a maximum output voltage of 37 volts. This is set up for 40 volts.

Also notice what will happen when the capacitor from ADJ to ground (C3) is charging.
The output will be 1.25 volts and the input will be about 51 volts. So the input minus output differential would approach 50 volts.

One problem with Internet circuits is that anyone can publish anything.
If you are lucky, they might have built it and got good results, but they may have been lucky with some overspec parts.
I have seen circuits where the author must have "designed" it on paper and didn't even build it.

 

[yungman]

I did not look that deep into the spec of LM317. If the 317 is spec 37V from into out. It is ok to design a 40V supply using a regulator that is spec for less, but the designer has to provide protection...Like putting a zener from into out so if a short at the output that pull the output to ground, the zener will turn on and pull the input of the regular down. The design got none, that is bad. If anything cause the output to short to ground, before the over current protection can even kick in, the it would be over 40V across the into out and the chip will pop. No margin in the design.

I have seen plenty of working power amp design, this is not one of them.