Basic amplifier question
post-15438382
Thread Starter
Post #1 of 5

Sonic Defender

Headphoneus Supremus
Joined
May 9, 2011
Messages
11,535
Reaction score
2,904
Location
Ottawa, Ontario Canada
Joined
May 9, 2011
Location
Ottawa, Ontario Canada
Posts
11,535
Likes
2,904
Actually, it isn't basic, but the level of understanding I need is basic. So years back when I was getting ready to begin using speaker tap cables to directly drive headphones from the speaker terminals of my amp someone, maybe StanD looked at my scenario and helped. One of the points that I remember that person making, and it was backed up by additional sources now lost to me, was that actually a speaker amplifier will up to a point be more stable as the load that it sees increases. Essentially for an audio amplifier the lower the load it was asked to drive the lower the stability. I don't; however, understand why this is and I would like to relearn this or have my information corrected if I am wrong. Thanks in advance.
 
     Share This Post       
post-15438583
Post #2 of 5

castleofargh

Sound Science Forum Moderator
Joined
Jul 2, 2011
Messages
8,735
Reaction score
4,109
Joined
Jul 2, 2011
Posts
8,735
Likes
4,109
It really depends on the type of amp and context. Basic example, if we consider that you're going to drive 2 different loads at the same voltage, then it's easy. More current will flow into the low impedance load, the total amount of power required is higher, and you could have higher crosstalk, higher noise from heat, etc. And at some point if the load is really low impedance, the amp may simply become unable to provide what you're asking for. Or in a few cases, the heat may become bad, making the load look more like a short circuit than a proper load for the amp.
Trying to keep a signal as clean as possible without any concern about driving a transducer, leads to how DACs do it. The input for the amplifier receiving the signal from the DAC(so the DAC's load) will be several kohm to oppose the flow of current as much as possible. That's the extreme example and headphone or speaker amps do have to drive a transducer, well if possible. So we need as much current as the transducer itself is willing to let through for nominal electrical damping. Otherwise we would just add resistors to match whatever ideal impedance the amps needs to excel.

In practice, all that will often become irrelevant or at least lower in our list of priorities because 2 different headphones probably won't need the same voltage to reach the same loudness, and you might very well end up with less current into the circuit with a very high sensitivity IEM compared to a fullsize headphone. So beside measuring each situations, a vague rule of thumb like "higher impedance load tends to measure better at the amp", probably won't be enough even if it's often true in practice.
Another issue about generalizing, is how some amps simply do better within a specific range of load by design.
 
     Share This Post       
post-15438767
Post #3 of 5

Sonic Defender

Headphoneus Supremus
Joined
May 9, 2011
Messages
11,535
Reaction score
2,904
Location
Ottawa, Ontario Canada
Joined
May 9, 2011
Location
Ottawa, Ontario Canada
Posts
11,535
Likes
2,904
It really depends on the type of amp and context. Basic example, if we consider that you're going to drive 2 different loads at the same voltage, then it's easy. More current will flow into the low impedance load, the total amount of power required is higher, and you could have higher crosstalk, higher noise from heat, etc. And at some point if the load is really low impedance, the amp may simply become unable to provide what you're asking for. Or in a few cases, the heat may become bad, making the load look more like a short circuit than a proper load for the amp.
Trying to keep a signal as clean as possible without any concern about driving a transducer, leads to how DACs do it. The input for the amplifier receiving the signal from the DAC(so the DAC's load) will be several kohm to oppose the flow of current as much as possible. That's the extreme example and headphone or speaker amps do have to drive a transducer, well if possible. So we need as much current as the transducer itself is willing to let through for nominal electrical damping. Otherwise we would just add resistors to match whatever ideal impedance the amps needs to excel.

In practice, all that will often become irrelevant or at least lower in our list of priorities because 2 different headphones probably won't need the same voltage to reach the same loudness, and you might very well end up with less current into the circuit with a very high sensitivity IEM compared to a fullsize headphone. So beside measuring each situations, a vague rule of thumb like "higher impedance load tends to measure better at the amp", probably won't be enough even if it's often true in practice.
Another issue about generalizing, is how some amps simply do better within a specific range of load by design.
Thank you very much, that is extremely helpful and appreciated.
 
     Share This Post       
post-15440088
Post #4 of 5

pinnahertz

Headphoneus Supremus
Joined
Mar 11, 2016
Messages
2,018
Reaction score
676
Joined
Mar 11, 2016
Posts
2,018
Likes
676
Essentially for an audio amplifier the lower the load it was asked to drive the lower the stability. I don't; however, understand why this is...
The statement is far to general to be true in any significant number of real-world cases.

And this is why:
Another issue about generalizing, is how some amps simply do better within a specific range of load by design.
"Stability" means the amplifier doesn't become unstable, breaking into oscillation or creating some objectionable degree of distortion.

If the load is within an amplifier's design parameters, the amplifier should be stable under all possible conditions. All serious amplifier designers have considered the load range very early, because it directly affects the cost of manufacture. If not, the amplifier is poorly designed, or the load is inappropriately applied outside of design parameters. Generally, headphones are not a difficult load to drive as compared with speakers because though impedances may dip, the applied power is microscopic compared with the demands of a speaker. Most headphone/amp mismatches relate to available voltage, with higher impedance headphones requiring more voltage to achieve the required power, and certain types, like IEMS, require very little voltage. Power = V squared over R, so holding total efficiency the same, if you raise R (resistance, or simplified impedance) you must also raise V (voltage) to get the same power and resulting volume. Required power relates to headphone type efficiency, which doesn't change an awful lot within general headphone type categories. Different categories, like low Z IEMs vs high Z full size open, do have notably different requirements.

Because different types of headphones have vastly different requirements, we end up with some combinations that work better or worse, and amps with switches to compensate for different headphone categories. But amplifier stability should not be an issue unless you have a horrible load combination on a really badly designed amp.
 
     Share This Post       
post-15440235
Post #5 of 5

Sonic Defender

Headphoneus Supremus
Joined
May 9, 2011
Messages
11,535
Reaction score
2,904
Location
Ottawa, Ontario Canada
Joined
May 9, 2011
Location
Ottawa, Ontario Canada
Posts
11,535
Likes
2,904
The statement is far to general to be true in any significant number of real-world cases.

And this is why:

"Stability" means the amplifier doesn't become unstable, breaking into oscillation or creating some objectionable degree of distortion.

If the load is within an amplifier's design parameters, the amplifier should be stable under all possible conditions. All serious amplifier designers have considered the load range very early, because it directly affects the cost of manufacture. If not, the amplifier is poorly designed, or the load is inappropriately applied outside of design parameters. Generally, headphones are not a difficult load to drive as compared with speakers because though impedances may dip, the applied power is microscopic compared with the demands of a speaker. Most headphone/amp mismatches relate to available voltage, with higher impedance headphones requiring more voltage to achieve the required power, and certain types, like IEMS, require very little voltage. Power = V squared over R, so holding total efficiency the same, if you raise R (resistance, or simplified impedance) you must also raise V (voltage) to get the same power and resulting volume. Required power relates to headphone type efficiency, which doesn't change an awful lot within general headphone type categories. Different categories, like low Z IEMs vs high Z full size open, do have notably different requirements.

Because different types of headphones have vastly different requirements, we end up with some combinations that work better or worse, and amps with switches to compensate for different headphone categories. But amplifier stability should not be an issue unless you have a horrible load combination on a really badly designed amp.
Good points for sure, thank you for the reply.
 
     Share This Post       

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top