All About Circuits

what specs can I expect from this circuit?

chunkmartinez

Thread Starter

chunkmartinez

Joined Jan 6, 2007
180
http://www.allaboutcircuits.com/vol_6/chpt_6/10.html

What power can I expect from this circuit? I assume the op amp will swing about 10v and divide by the 8 ohm speaker load to get the current draw and then calculate power? This would give me 18w peak, don't I divide the p2p value by .707 for rms? And what kind of losses can I expect?

Does the decoupling cap attenuate any low frequencies by a significant amount?
 
thatoneguy

thatoneguy

Joined Feb 19, 2009
6,359
The transistors are only rated for 600mW. So, the output power is limited to that.

For higher power, you would need to add a stage to drive lower gain output transistors (power transistors with heat sink attachment capability, rated for many watts).

You'll get better response/performance/power from an Amplifier IC with the correct components, or, if looking at more than 30W, an off the shelf receiver for the same or lower cost.
 
Audioguru

Audioguru

Joined Dec 20, 2007
11,248
The circuit has a HUGE problem. So it probably will have very low output power.
The output current from the opamp is low but the power transistors have low hFE so they need much more base current than the opamp can supply.

The text says to use fairly high power TIP41 and TIP42 power transistors. Their minimum hFE is about 22 at 100mA to 1A.
The opamp has a maximum output voltage loss of 5V when its output current is only 5mA.
For a base current of 5mA then the output current from the transistors is 110mA peak.
Then the peak output power is only (110mA squared) x 8= 97mW and the real output power is only 68mW.
 
chunkmartinez

Thread Starter

chunkmartinez

Joined Jan 6, 2007
180
Thanks for the responses.

Any ideas you can give me to throw inbetween this circuit to up the output? If those bjt's can't handle much power, any suggestions on replacements? And what would I need to add inbetween the op amp and output transistors(or rplace the op amp with a diff one)? Would using lower hfe transistors work, or mosfets instead? I want to keep this a class B design, or close...for efficiency and expirimentation.
 
Audioguru

Audioguru

Joined Dec 20, 2007
11,248
chunkmartinez said:
If those bjt's can't handle much power, any suggestions on replacements?
Click to expand...
ThatOneGuy was COMPLETELY WRONG! The TIP41 and TIP42 POWER TRANSISTORS are rated for 65W each, not 0.6W.

And what would I need to add inbetween the op amp and output transistors(or rplace the op amp with a diff one)?
Click to expand...
It is a stupid circuit that was not designed properly (then why is it in the tutorial??). A power amplifier IC should be used instead.
A school kid can use darlington output transistors that require as very low input currents.

Would using lower hfe transistors work, or mosfets instead?
Click to expand...
You need higher hfe, not lower hFE.
mosfets are very difficult to bias and need a higher supply voltage for the opamp which will destroy the opamp.

I want to keep this a class B design, or close...for efficiency and expirimentation.
Click to expand...
Class B produces crossover distortion. Audio amplifiers use class AB with no crossover distortion.
A class B or a class AB amplifier is linear so it is NOT efficient. A class D switching amplifier is efficient.
 
chunkmartinez

Thread Starter

chunkmartinez

Joined Jan 6, 2007
180
Audioguru said:
ThatOneGuy was COMPLETELY WRONG! The TIP41 and TIP42 POWER TRANSISTORS are rated for 65W each, not 0.6W.


It is a stupid circuit that was not designed properly (then why is it in the tutorial??). A power amplifier IC should be used instead.
A school kid can use darlington output transistors that require as very low input currents.


You need higher hfe, not lower hFE.
mosfets are very difficult to bias and need a higher supply voltage for the opamp which will destroy the opamp.


Class B produces crossover distortion. Audio amplifiers use class AB with no crossover distortion.
A class B or a class AB amplifier is linear so it is NOT efficient. A class D switching amplifier is efficient.
Click to expand...
Thanks for that info, I understand about the different classes, I wanted to eqpiriment with class B to see if I notice the crossover distortion. I actually wanted to use a small screw POT to adjust the bias so that I can switch between B and AB for fun.

What power amp IC's do you reccomend? And what output can I expect from the tip41 and 42?
 
chunkmartinez

Thread Starter

chunkmartinez

Joined Jan 6, 2007
180
Thankyou. Btw, was the power amp IC suggested in place of the op amp or to replace the whole circuit? Because I am wanting to use the bjt pair as a push pull project and I also have some comming in through mail.
 
Audioguru

Audioguru

Joined Dec 20, 2007
11,248
chunkmartinez said:
Thanks for that info, I understand about the different classes, I wanted to experiment with class B to see if I notice the crossover distortion. I actually wanted to use a small screw POT to adjust the bias so that I can switch between B and AB for fun.
Click to expand...
Crossover distortion causes low level sounds to be fuzzy and buzzy.
NO audio amplifier today has crossover distortion.

What power amp IC's do you recommend? And what output can I expect from the tip41 and 42?
Click to expand...
The LM3775 and LM3886 ICs are excellent. They have very low noise and distortion and are wideband. They protect themselves. Their output is between 50W and 60W into 4 or 8 ohms.

You can use TIP41 and TIP42 output transistors in an audio amp that supplies about 25W into 8 ohms.
 
chunkmartinez

Thread Starter

chunkmartinez

Joined Jan 6, 2007
180
Audioguru said:
Crossover distortion causes low level sounds to be fuzzy and buzzy.
NO audio amplifier today has crossover distortion.


The LM3775 and LM3886 ICs are excellent. They have very low noise and distortion and are wideband. They protect themselves. Their output is between 50W and 60W into 4 or 8 ohms.

You can use TIP41 and TIP42 output transistors in an audio amp that supplies about 25W into 8 ohms.
Click to expand...
Do you have any ideas for a design or have you seen any(for the tip41, 42)? Will the lm833 work to drive the pair? I don't mean to ask all these question but i'm new to datasheets and I figured people know the specs at the top of their head of given components.
 
Last edited:
Audioguru

Audioguru

Joined Dec 20, 2007
11,248
You found the same circuit that you found before.
Look at the datasheet for the LM833 dual audio opamp. It is also spec'd with 10V into 2k ohms which is only 5mA peak. It uses the same power supply and the same output transistors as the other horrible amp. There is no way it can produce 15W into 8 ohms which is 31V peak-to peak. How can the output be 31V when the total supply voltage is only 24V??

The opamp cannot drive the output transistors!

Maybe Circuits Today does not know anything about simple arithmatic and electronics.
 
chunkmartinez

Thread Starter

chunkmartinez

Joined Jan 6, 2007
180
Audioguru said:
You found the same circuit that you found before.
Look at the datasheet for the LM833 dual audio opamp. It is also spec'd with 10V into 2k ohms which is only 5mA peak. It uses the same power supply and the same output transistors as the other horrible amp. There is no way it can produce 15W into 8 ohms which is 31V peak-to peak. How can the output be 31V when the total supply voltage is only 24V??

The opamp cannot drive the output transistors!

Maybe Circuits Today does not know anything about simple arithmatic and electronics.
Click to expand...
Thats why I asked I was just making sure....So....What can i use to drive the pair?
 
Audioguru

Audioguru

Joined Dec 20, 2007
11,248
chunkmartinez said:
Thats why I asked I was just making sure....So....What can i use to drive the pair?
Click to expand...
The TIP41 and TIP42 output transistors need a pair of driver transistors.
Use darlington transistors instead. Each darlington transistor is a driver transistor and an output transistor combined.
 
chunkmartinez

Thread Starter

chunkmartinez

Joined Jan 6, 2007
180
Audioguru said:
The TIP41 and TIP42 output transistors need a pair of driver transistors.
Use darlington transistors instead. Each darlington transistor is a driver transistor and an output transistor combined.
Click to expand...
So you are suggesting darlington transistors for the drivers and output instead of using the tip pair? I was wanting to use the tip pair no matter what since I ordered some already. Or can I use a darlington pair to drive each TIP transistor?
 
Audioguru

Audioguru

Joined Dec 20, 2007
11,248
You can use ordinary little transistors to drive the TIP41 and TIP42 power transistors.
ANY little ordinary transistor can supply 110mA.
 
chunkmartinez

Thread Starter

chunkmartinez

Joined Jan 6, 2007
180
Audioguru said:
You can use ordinary little transistors to drive the TIP41 and TIP42 power transistors.
ANY little ordinary transistor can supply 110mA.
Click to expand...
So, If I drive the TIP bjts with 110ma will that give me a current gain of around 2 amperes?(calculates a little higher but...)

Also, I have 5 pairs comming in the mail of complimentary TIP bjts. I was only going to use a pair originally but could I use a few pairs for more current sink/sourcing and thus more power output? I understand this would require a higher driver current to drive more pairs. And to keep the design more efficient can I design the driver circuit to produce little voltage since bjts bias with current and not voltage?(as long as it is high enough for the forward voltage drop)
 
Last edited:
Audioguru

Audioguru

Joined Dec 20, 2007
11,248
2A into 8 ohms is 16V peak. The opamp will need a supply that is about plus and minus 20V or more which is too high for it.

Parallel transistors will not increase the power output. A higher supply voltage is needed to increase the output power to one speaker but that will blow up the opamps.

Ordinary inexpensive car amplifier ICs use a bridged circuit. Two amplifiers where one amp drives one wire of the speaker and the other amp drives the other wire of the speaker with opposite phase. Then the voltage swing is almost doubled and the current swing is also almost doubled. The resulting power is about 3.5 times more than an ordinary amplifier.
 
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