Audio signal adder for headphones

Thread Starter

raisintoe

Joined Mar 24, 2012
46
I am creating an audio signal adder using this op-amp. I do not need to amplify the audio.

I want to drive my headphones directly from the op-amp, but I do not know how to tell if this op-amp can handle the load of a typical headphone set, (there is no power dissipation rating or anything like that in the op-amp datasheet).

If I should not try a direct drive to my headphones, how can I add a simple transistor circuit to drive my headphones?
 

crutschow

Joined Mar 14, 2008
24,047
If you look on page 6 of the data sheet you referenced you will see that the smallest load it is rated to drive is 600Ω so, if your headphones have a lower impedance than that (and they likely do) then you will need an added amplifier.
The easiest is to just use a IC audio amp such as one of these.
 

Thread Starter

raisintoe

Joined Mar 24, 2012
46
OK, I would rather make an amplifier using an npn transistor, since I have those on hand. I guess what I want is a common emitter amplifier, but I still have trouble understanding how they work.

Here is an example, I have trouble getting this to work properly on LtSpice.
 

ScottWang

Joined Aug 23, 2012
6,859
OK, I would rather make an amplifier using an npn transistor, since I have those on hand. I guess what I want is a common emitter amplifier, but I still have trouble understanding how they work.

Here is an example, I have trouble getting this to work properly on LtSpice.
An easy method you may try it, using a class A amplifier as the first circuit in your linked page, and adding a bjt Totem-Pole to amplifier the signal.
 

Thread Starter

raisintoe

Joined Mar 24, 2012
46
If you don't have any PNP transistors, then an all NPN amplifier will not be very efficient. What is your intended power source?

ak
I will set up a DC power supply under 12 volts. So I can choose anything under 12V DC

Also, I might have a few PNP transistors hanging around, that may be a possibility.
 
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Thread Starter

raisintoe

Joined Mar 24, 2012
46
Here is a schematic I drew up, using the amplifier circuit that Crutschow found.

The 32 ohm resistor at the right is my headphones

Schematic.png
 

AnalogKid

Joined Aug 1, 2013
8,251
The idea is ok, but the implimentation needs a little work...

Even though you have bipolar supplies, the output stage swings only between P and GND. This limits the output power to 1/4 its possible maximum value.

The opamp has a totem pole output stage, meaning it can both source and sink current. For this reason, I don't think you need R8 as a pull down.

R7 can be smaller to increase the output voltage swing.

If you search for opamp booster schematic, you will find many examples of ways to boost the opamp output current while maximizing output voltage swing. Some are so efficient that you would get more output with only one power rail than your circuit gets with two.

ak
 

Thread Starter

raisintoe

Joined Mar 24, 2012
46
This is the exact circuit that I have on my breadboard. It works pretty good, but sounds muddy, It's good for a first try! I think the low quality is mostly due to R14, I gave it a large resistance to avoid pulling too much current through it. What is a good way around this problem?Breadboard circuit.png
 

AnalogKid

Joined Aug 1, 2013
8,251
By the time you have R14 down low enough that the circuit almost has a symmetrical output impedance, Q3 is burning up due to the static current. Congratulations - you've just re-discovered the problem with Class A audio circuits - they suck - - up lots of power.

The only way around the limitations of a single emitter follower circuit is not to use an emitter follower circuit. If you search for things like '1 watt audio amplifier schematic' or 'opamp output booster', you will get hundreds of circuits to choose from. My favorite is the one with one NPN and one PNP in the power leads of the opamp, but most people prefer a complimentary emitter follower on the opamp output.

Plus there a threads on this site that go into this.

ak
 
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Thread Starter

raisintoe

Joined Mar 24, 2012
46
Yes, I did search for opamp booster. I have seen the simple PNP NPN setup, but how do I keep this circuit from clipping between 0.7 and -0.7 volts? The signal from the opamp has to pass the threshold of the transistors before they respond.
 

AnalogKid

Joined Aug 1, 2013
8,251
Depending on the exact configuration, the opamp supplies some of the output current in this voltage range, plus the negative feedback loop shifts the output stage operating point to supply the rest. Remember, when the two emitters are making +1 V (for example) the output of the opamp is at +1.6 V. If you post a schematic, we can discuss it. No schematic, no details.

ak
 

Thread Starter

raisintoe

Joined Mar 24, 2012
46
Depending on the exact configuration, the opamp supplies some of the output current in this voltage range, plus the negative feedback loop shifts the output stage operating point to supply the rest. Remember, when the two emitters are making +1 V (for example) the output of the opamp is at +1.6 V. If you post a schematic, we can discuss it. No schematic, no details.

ak
This is all I am talking about

Opamp booster.png
 

AnalogKid

Joined Aug 1, 2013
8,251
Q2 is upside-down.

Disconnect the right side of R1 from the op37 and connect it to the two emitters. This brings the two transistor base-emitter junctions inside the feedback loop, where the opamp can compensate for them. to reduce crossover distortion, add a resistor from the two bases to the two emitters so the opamp can provide some current to the output directly.

ak
 

Thread Starter

raisintoe

Joined Mar 24, 2012
46
Awesome! I made the changes, and will put this circuit together today. How does the added resistor reduce distortion? Or, how does distortion occur during crossover? Is it simply the inrush of current when the transistor is turned on that causes distortion?
 
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Thread Starter

raisintoe

Joined Mar 24, 2012
46
I've built the circuit, it works really well! I placed a 1K resistor between the emitters and bases, there is still some distortion, I am not sure how much current I can draw through the opamp.
 
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