# I turned one of my amp circuits balanced but I can't understand the point.

#### Aras

##### New Member
I've been designing and building my own amplifiers for years as a hobby. They all sound great. I've been meaning to go balanced since last year, finally I had some time to make a circuit but I don't understand the point at all.

Please ignore the power section. In real life I'm using a center tapped transformer and obviously there are more decoupling and filtering caps on the real thing.

Power draw and THD(0.0002% to 0.0004%) doubled and I am not able to see how this is better than the non-balanced version. Am I doing something wrong or is my thinking wrong?

#### sergeauckland

##### Major Contributor
Forum Donor
Your amplifier isn't balanced, it's bridged. The input is still unbalanced, but the output consists of two amplifiers, one of which (U1B) is inverted. The point of this configuration is to provide 4x the power of a single opamp, albeit with less load tolerance, as each half of the bridge already sees half the load impedance.
Looking at the values, I'm assuming this is to drive headphones, so in my view pointless. I agree that it's not better than a single output. Unfortunately, so-called balanced output for headphones has become something of a marketing buzz-word, even though there's no technical reason it should be better.
For driving loudspeakers there's a good case to be made for bridged outputs, to do with increased power output, but for headphones, single outputs are quite enough, so fairly pointless.

It'll work OK, nothing wrong with it, just unnecessary complexity.
S.

OP
A

#### Aras

##### New Member
Your amplifier isn't balanced, it's bridged. The input is still unbalanced, but the output consists of two amplifiers, one of which (U1B) is inverted. The point of this configuration is to provide 4x the power of a single opamp, albeit with less load tolerance, as each half of the bridge already sees half the load impedance.
Looking at the values, I'm assuming this is to drive headphones, so in my view pointless. I agree that it's not better than a single output. Unfortunately, so-called balanced output for headphones has become something of a marketing buzz-word, even though there's no technical reason it should be better.
For driving loudspeakers there's a good case to be made for bridged outputs, to do with increased power output, but for headphones, single outputs are quite enough, so fairly pointless.

It'll work OK, nothing wrong with it, just unnecessary complexity.
S.

So they are different things. I thought the bridge is some kind of balance since U1B is getting its "+" from a ground but apparently it's not. Thank you for making this clear. I would build the thing for real. You saved me a lot of time.

I really don't understand why balanced should be better either but I wanted to design one with an AB configuration to listen and decide. I should make more research.

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#### DVDdoug

##### Major Contributor
With line-level or microphone-level amplifiers the main advantage is noise-interference rejection which also can reduce ground-loop hum & noise since the signal isn't ground-referenced. Since only your output is balanced, that only applies to the output.

If you're not having problems with an unbalanced design then obviously switching to balance isn't going to improve the sound.

Of you are using pro equipment, most pro equipment is balanced and you'd want to be compatible and stick with the standards.

If this a power amplifier or headphone amplifier*...
In real life I'm using a center tapped transformer and obviously there are more decoupling and filtering caps on the real thing.
With a balanced AKA "bridged") output you can use a single-ended power supply without an output capacitor (both outputs rest at half the power supply voltage so no DC current flows). This is handy in automotive applications.**

Power draw and THD(0.0002% to 0.0004%) doubled and I am not able to see how this is better than the non-balanced version.
You're getting double the voltage to the load and double the voltage means 4 times the power to your speakers (assuming enough current). Again, that's handy in automotive applications where you have limited power supply voltage. Otherwise, it's usually more economical just to use a higher-voltage instead of doubling the components in the output stage.

* With a headphone amplifier, of course you need a special cable-connector because there is no ground and the left & right can't share a common ground connection (or any common connection). IMO - Not worth the trouble.

** High power car stereo amplifiers have a built-in voltage booster so it's not much harder to make one with + & - power supplies.

#### solderdude

##### Grand Contributor
Bridged headphone outputs can be advantageous when the amplifier section has a low voltage rail (think portable equipment) and one wants to drive high impedance headphones as well.
For high imp. headphones there is a 6dB gain in output voltage.

However, this requires 4 wires instead of the usual 3 (2 signal and 1 common return wire).
That in itself can have audible benefits when: The used cable is long or has a relatively high (common return wire) resistance AND has a low impedance.
It really doesn't matter if such a headphone (with 4 wire cable) is connected to a bridged amp or a normal amp where the 2 return wires are connected in the plug.

When you want to keep the entire chain truly balanced (cancelling ground loops and lower even harmonics at the expense of higher odd harmonics) then it could make sense to make an entire chain fully balanced. Not that it will sound any better though.

#### AnalogSteph

##### Major Contributor
As mentioned, mic and line level interconnects are where balanced connections are useful. Headphones just need to have separate ground returns for both drivers, as cable resistance (~1 ohm for 3 m worth of Sennheiser cable) tends to dwarf jack/plug contact resistance (think ~40 mOhms). As a rule of thumb, shared ground return resistance should be <1% of nominal driver impedance.

For a notable example, the Topping L30 and L50 both deliver exemplary performance using single-ended circuitry only. Not much of an issue as long as you've got control of supply voltages. A headphone amplifier can also generally be run on a smallish wall wart just fine, so using unbalanced audio connections is not much of an issue for them either.

Things get more interesting with USB audio interfaces. The most basic ones will run single-ended on +5 V only (line output ~0..+4 dBu tops), past about \$100 you tend to get an inverter for -5 V as well (output ~+10 dBu), and eventually the balanced outputs get BTL drivers (output +15.5-16 dBu). It takes a lower midrange interface for +/-8..12 V supplies to show up; you can get +21 dBu on +/-9 V with BTL output. +/-15 V generally requires either the increased power delivery of Type C connectors or an external power supply.

OP
A

#### Aras

##### New Member
With a balanced AKA "bridged") output you can use a single-ended power supply without an output capacitor (both outputs rest at half the power supply voltage so no DC current flows). This is handy in automotive applications.**
This is actually very interesting. I will test this. Thank you.

As mentioned, mic and line level interconnects are where balanced connections are useful. Headphones just need to have separate ground returns for both drivers, as cable resistance (~1 ohm for 3 m worth of Sennheiser cable) tends to dwarf jack/plug contact resistance (think ~40 mOhms). As a rule of thumb, shared ground return resistance should be <1% of nominal driver impedance.

For a notable example, the Topping L30 and L50 both deliver exemplary performance using single-ended circuitry only. Not much of an issue as long as you've got control of supply voltages. A headphone amplifier can also generally be run on a smallish wall wart just fine, so using unbalanced audio connections is not much of an issue for them either.

Things get more interesting with USB audio interfaces. The most basic ones will run single-ended on +5 V only (line output ~0..+4 dBu tops), past about \$100 you tend to get an inverter for -5 V as well (output ~+10 dBu), and eventually the balanced outputs get BTL drivers (output +15.5-16 dBu). It takes a lower midrange interface for +/-8..12 V supplies to show up; you can get +21 dBu on +/-9 V with BTL output. +/-15 V generally requires either the increased power delivery of Type C connectors or an external power supply.

When I first started making amplifiers, my ground noise was terrible. Because I was using SMPS and even LDOs, RLC filters couldn't fix the hum completely. Then I switched to center tapped transformers and the background was pure silence. Never had a problem again. So, balanced is good only when I'm not able to do that. Got it. Thank you.

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