Difference between Push-pull and Balanced

Balanced here is used to refer to each channel (left and right) using its own amplifier circuit, rather than a single stereo amplifier, it is two stereo amps. The cabling is only in reference to them having to be separated (no shared grounds). 

Push pull refers to how each of those amps can work. I think, for instance, an amp with an active ground, could be considered push-pull. But two active grounds (one per channel) would be balanced. 

Quote:

Originally Posted by xnor 


Nope. Have you seen the schematic in the wiki article where the load is driven by the two output devices?
With balanced audio you have two conductors of the same type with equal impedances .. in other words there is no conductor that is connected to the ground / no common return for both the left and right channel.
I guess you have noticed that these are two separate things. Therefore asking about the difference between the two doesn't make much sense. Push-pull is a type of electronic circuit and balanced audio is a way of interconnecting equip.

Well, "push-pull" is really a set of different types of amplifiers that share one particular characteristic - encompassing some class A amplifiers and all class B (and class A/B) amplifiers.

"Balanced" audio is in fact a type of electronic circuit as well.  Unfortunately, it can refer to several different things...  A traditional double-entry headphone cable with four conductors up to the TRS plug is in fact balanced, as are transducers (i.e. speaker/headphone audio drivers) - they both reject common mode noise (although not from before the plug).  The same goes for speaker cables and speakers, of course.  The key is that unwanted noise imparted equally on both sides of the cable results in no change in voltage differential over the transducer - thus the noise is cancelled out.

Differential signaling is what is usually called a "balanced amplifier".  It means that you actually have two sets of amplifiers amplifying a signal and driving the load in that manner, but in opposite polarity of each other.  Interestingly enough, this is exactly the same principle as a push-pull class A amplifier that has a common ground - it's just implemented at a different point in the amplifier topology.

In a push-pull class A amplifier with common ground, an output transformer after two tubes (or rarely, transistors) working in opposite polarity to each other actually adds the opposite polarity signals back together with the same polarity.  That is, only one of the two cables to the transducer is "hot" with the audio signal (the other being the ground) - even though the signal is both "pushed" and "pulled" before hand.  If you remove the output transformer that combines the two signals together with like polarity, you've just created a balanced amplifier which is often still referred to as a push-pull amplifier (I'm not sure if that use of the term is correct or not).  Note that class B amplifiers are different and you can't convert one from common ground to balanced just by changing the output transformer configuration, like with a push-pull class A type.

Of course, you could always have a balanced push-pull class A amplifier that combines the two principles.  That would be two of the push-pull class A amplifiers I just described (each a pair of tubes/transistors with a transformer combining the opposite polarity signals), but working in opposite polarity to each other - one on each side of the transducer.  You're just combining two amplifiers - and in fact, this is exactly what bridging an amplifier is (i.e. a bridged amplifier is a balanced amplifier).

Remember, the transducer only cares about the voltage difference across it - so if in a balanced amplifier the signal is +9 V on one side (from the amplifier working in normal polarity) and -9 V on the other side (from the amplifier working in negative polarity), the transducer sees it the same as a normal non-balanced +18 V on one side and 0 V on the other (ground).  That's the basic idea of a balanced amplifier.

So the exact application of the term "push-pull" can be rather wishy-washy (I didn't even address class B biased amplifiers, which are of course different than push-pull class A amplifiers, balanced or not), and such an amplifier can be both "push-pull" and balanced.  I don't care so much for word games, but I hope I've cleared up some confusion.

The most important distinction to a consumer is whether an amplifier is balanced so there's no common ground, or unbalanced so there is a common ground.  You definitely don't want to short together the black/negative terminals on a balanced amplifier.  This is particularly important in a couple of cases - most notably that Polk SDA speakers cannot use non-common ground amplifiers under any circumstances.

Quote:

Originally Posted by BlackbeardBen 

 

The most important distinction to a consumer is whether an amplifier is balanced so there's no common ground, or unbalanced so there is a common ground.  You definitely don't want to short together the black/negative terminals on a balanced amplifier.  

Does this mean if you have a balanced (no common ground) amplifier but only single ended headphones - you could not simply make a converter cable that combines the (-) pins into a common? 

technically "balanced" headphone amps with 2 amps per channel are "bridged output" - and no you can't use them with TRS - they absolutely require 4 wire connection

Quote:

Originally Posted by BlackbeardBen 

 

Well, "push-pull" is really a set of different types of amplifiers that share one particular characteristic - encompassing some class A amplifiers and all class B (and class A/B) amplifiers.

"Balanced" audio is in fact a type of electronic circuit as well.  Unfortunately, it can refer to several different things...  A traditional double-entry headphone cable with four conductors up to the TRS plug is in fact balanced, as are transducers (i.e. speaker/headphone audio drivers) - they both reject common mode noise (although not from before the plug).  The same goes for speaker cables and speakers, of course.  The key is that unwanted noise imparted equally on both sides of the cable results in no change in voltage differential over the transducer - thus the noise is cancelled out.

 

Differential signaling is what is usually called a "balanced amplifier".  It means that you actually have two sets of amplifiers amplifying a signal and driving the load in that manner, but in opposite polarity of each other.  Interestingly enough, this is exactly the same principle as a push-pull class A amplifier that has a common ground - it's just implemented at a different point in the amplifier topology.

 

In a push-pull class A amplifier with common ground, an output transformer after two tubes (or rarely, transistors) working in opposite polarity to each other actually adds the opposite polarity signals back together with the same polarity.  That is, only one of the two cables to the transducer is "hot" with the audio signal (the other being the ground) - even though the signal is both "pushed" and "pulled" before hand.  If you remove the output transformer that combines the two signals together with like polarity, you've just created a balanced amplifier which is often still referred to as a push-pull amplifier (I'm not sure if that use of the term is correct or not).  Note that class B amplifiers are different and you can't convert one from common ground to balanced just by changing the output transformer configuration, like with a push-pull class A type.

 

Of course, you could always have a balanced push-pull class A amplifier that combines the two principles.  That would be two of the push-pull class A amplifiers I just described (each a pair of tubes/transistors with a transformer combining the opposite polarity signals), but working in opposite polarity to each other - one on each side of the transducer.  You're just combining two amplifiers - and in fact, this is exactly what bridging an amplifier is (i.e. a bridged amplifier is a balanced amplifier).

 

Remember, the transducer only cares about the voltage difference across it - so if in a balanced amplifier the signal is +9 V on one side (from the amplifier working in normal polarity) and -9 V on the other side (from the amplifier working in negative polarity), the transducer sees it the same as a normal non-balanced +18 V on one side and 0 V on the other (ground).  That's the basic idea of a balanced amplifier.

 

So the exact application of the term "push-pull" can be rather wishy-washy (I didn't even address class B biased amplifiers, which are of course different than push-pull class A amplifiers, balanced or not), and such an amplifier can be both "push-pull" and balanced.  I don't care so much for word games, but I hope I've cleared up some confusion.

 

The most important distinction to a consumer is whether an amplifier is balanced so there's no common ground, or unbalanced so there is a common ground.  You definitely don't want to short together the black/negative terminals on a balanced amplifier.  This is particularly important in a couple of cases - most notably that Polk SDA speakers cannot use non-common ground amplifiers under any circumstances.

nice post, I was wondering when someone might get it right and you did, mostly....but for this one detail. there is actually a distinction between bridged and balanced, one that seems to often be missed on this forum as many of the amplifiers promoted as balanced here, are actually just bridged.

in a bridged amplifier, you can achieve 'balanced' output from 2 single ended amps tied together, but each signal phase in the amp is still referenced to ground and may even be made up of all N or P channel devices. Some call this balanced simply because the amp is fed a +/- differential signal and the signal is floated on the output to the headphones and ground ignored. In this example even if the 2 signal wires are antiphase, this is still just a bridged amp.

in a balanced amp the differential signal, made up of in-phase and anti-phase components, is generally fed to a differential pair of N-channel and P-channel devices like jfets, mosfets, or perhaps bipolar NPN and PNP transistor devices. the signal is not referenced to ground, but to the opposite phase, the only connection to ground is through the power supply. there are also balanced in/out IC's, although not all that many, more are designed to be ADSL drivers, or LVDS receivers/transceivers, rather than audio amplifiers

then you have special cases like Circlotrons and cross-coupled Balanced single-ended (X'd) amps like those from Nelson Pass that can use all P or N channel devices due to non-standard feedback mechanisms, but lets not go there

Quote:

Originally Posted by qusp 

 

nice post, I was wondering when someone might get it right and you did, mostly....but for this one detail. there is actually a distinction between bridged and balanced, one that seems to often be missed on this forum as many of the amplifiers promoted as balanced here, are actually just bridged.

 

in a bridged amplifier, you can achieve 'balanced' output from 2 single ended amps tied together, but each signal phase in the amp is still referenced to ground and may even be made up of all N or P channel devices. Some call this balanced simply because the amp is fed a +/- differential signal and the signal is floated on the output to the headphones and ground ignored. In this example even if the 2 signal wires are antiphase, this is still just a bridged amp.

 

in a balanced amp the differential signal, made up of in-phase and anti-phase components, is generally fed to a differential pair of N-channel and P-channel devices like jfets, mosfets, or perhaps bipolar NPN and PNP transistor devices. the signal is not referenced to ground, but to the opposite phase, the only connection to ground is through the power supply. there are also balanced in/out IC's, although not all that many, more are designed to be ADSL drivers, or LVDS receivers/transceivers, rather than audio amplifiers

 

then you have special cases like Circlotrons and cross-coupled Balanced single-ended (X'd) amps like those from Nelson Pass that can use all P or N channel devices due to non-standard feedback mechanisms, but lets not go there

Thanks for the correction.  I knew there was a distinction between bridged and balanced amplifiers but I had no idea what it was!  As far as users are concerned they both are differential output amplifiers - I think that's important to note.