# Class AB power amp output

I'm in the process of designing a headphone amp for my car (long story). It's been a long long time since I touched on electronics so I've been doing a lot of reading on the topic of amps etc.

The following design works when tested on a breadboard:

This design is an amalgamation of a load of other suggestions I've found online. Whilst it works well I find it starts distorting (particularly on bass) beyond 3/5 volume from my laptop. From my limited knowledge bringing down the gain on the op-amp and bumping up the power stage would solve this... I think?

My understanding is that my 4.5V split rail setup will only go so far without going to a bridged setup (which I can't really do due to other practical reasons). What dictates the power throughput of the power section? I've been reading up on transistors and if I understand correctly the hFE could be what determines amplification?

Would a different set of transistors give me a slight bump in output power? I'm not looking for a huge amount, just enough to give the op-amp a bit more headroom by not running at such a high gain.

Or I could also have misunderstood how the power stage works entirely, in which case any feedback on the circuit is most welcome!

Thanks!

• What does 3/5 volume mean in terms of voltage? Are you overloading it with too high of an input? Nov 10, 2020 at 1:13
• Carlos, one of the main reasons it doesn't work well as you increase the volume is that you are using resistors R4 and R5. Those are there to supply base current but they also stop doing that exactly when you need them the most. When the drive voltage pushes to the top rail, for example, you need the maximum current into the speaker load from Q1. But with only a small voltage across R4 (the drive voltage is close to the rail by definition) there is only a tiny current in R4. But Q1 needs a lot of base current now. So... well, bad stuff.
– jonk
Nov 10, 2020 at 1:37
• Carlos, you either need to study these designs more (probably a lot more) if you want something to work well. Spend a LOT of time talking about your power supply. You mention a car (which is usually 12 V) but show 9 V on the schematic. Are you going to create a split supply rail? If so, how will you generate the high-current ground in between? Are you aware of load-dumps? Do you care? And what's the reason you cannot consider bridging? Why aren't you using an IC like a TDA8551 (5 V rail offered as an example, not a solution)? Look here.
– jonk
Nov 10, 2020 at 1:40

• I would enter via the non-inverting input rather than the inverting input.
• R6 with C5 gives you a low cut frequency of 8.13KHz!, you need to lower that capacitor ten times. Or better yet, lower the feedback resistors.
• Raise the voltage rails to at least the 12V of your car battery.
• Add small degeneration resistors at the emmiters.
• Use beefier/better transistors
• Use a better opamp, for example the OPA2212 which is rail to rail and much better than the OP07.
• Lower R4 and R5
• Add a small isolation resistor at the output of your amp so it drops a couple of V's with low impedance headphones.
• I was also going to say that C5 makes it sound muffled almost as bad an an AM radio. I was also going to say that the OP07 is very very old. Nov 10, 2020 at 17:19
• What resistor would you suggest on the output and on the emitters? I'm not familiar with the concept of a degeneration resistor, is this something to do with thermal runaways? Nov 11, 2020 at 7:46
• A couple of ohms should work, it also depends on the quiescent current you want for the output stage.
– S.s.
Nov 11, 2020 at 13:36

Your opamp U1 does nothing and can be replaced by a piece of wire.

If the circuit is perfect and the battery is brand new then the output voltage swing will be 9V peak-to-peak. Then the undistorted power in an 8 ohm headphone will be 1.27W and into a 32 ohm headphone will be 0.32W which will be extremely loud. But the opamp cannot produce an output voltage swing without a voltage loss.

The opamp U2 does not turn on the output transistors, instead it turns them off. R4 and R5 turn on the output transistors.

The OP07 opamp has a typical maximum output swing of only 2.4V peak when it has a 9V supply. Then its peak output voltages are 2.1V and 6.9V. The output to the headphone will try to be 2.4V peak which is an output current of 300mA into an 8 ohms headphone. With a collector current of 300mA a BD135 or BD136 transistor has a minimum hFE of 30 so the base current must be 10mA.

Each diode adds 0.7V to the peak voltages at the bases of the transistors to 1.4V and 7.6V then resistors R4 and R5 have a current of only 1.4V/2.7k= 5.2mA which is not enough if the transistors have minimum hFE.

Assuming that the opamp and transistors have typical spec's then the 2.4V peak in the 8 ohm headphone produces a power of 0.36W. Turning up the volume will result in severe clipping distortion. If the opamp and transistors have minimum spec's then the maximum undistorted output power will be much less.

Using transistors with higher hFE will not increase the maximum undistorted output power.

• Thank you very much. This answer had really helped understand how this works much better, it now sounds order's of magnitude better. I'm actually using the NE5532 op amp, the image is just a rough sketch with what Ltspice provides. The first op amp comes from a 9V voltage regulator and acts as a buffer (I think) Nov 11, 2020 at 7:43
• The first opamp does nothing and can be replaced with a piece of wire. The "buffer" opamp has the same extremely high input impedance as the second opamp. An NE5532 opamp datasheet shows that its maximum output swing is a little more than the OP07 so the output power of the amplifier is a little more. Maybe your OP07 was a fake one from ebay? Nov 12, 2020 at 17:43