ToTo Man
the band not the dog
@Paul Burke's recent posting of this article by Scott Hinson on effects of an amplifier's output impedance on the frequency and time response of a loudspeaker has raised a few questions that I’m interested in discussing:
1a) In reality, how common is it to find amplifiers whose Zout varies significantly with frequency?
1b) What is typically the extent of the variation?
1c) Is it possible to generalise and say at which frequencies an amplifier’s Zout is likely to be higher than its published Zout and at which frequencies it’s likely to be lower?
1d) Does the amplifier's Zout curve depend on general circuit topology or can it vary between amplifiers that ostensibly share a largely similar design?
1e) Is there a feasible way for the home listener to measure the Zout curve of their amplifier?
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Next, in this article by Jan Meier and Tyll Hertsens, Meier suggests that too low a Zout can result in overdamping and that a higher Zout may be required to achieve 'critical damping', i.e. optimal impulse response from a given headphone. The article focuses on headphones and headphone amplifiers, which usually have higher load and output impedances than loudspeakers and loudspeaker amplifiers, so I'm not sure how much of this is transferrable, but I'll ask anyway:
2a) At which frequencies is Zout most significant when it comes to affecting the optimal impulse response from a loudspeaker or headphone?
2b) Is there a feasible way for the home listener to determine what Zout yields the optimal impulse response from their particular loudspeaker or headphone?
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An exclusively solid-state amplifier user for the last 20 years with damping factors ranging from 30 to 300, I'm unlikely to have experienced the effects of a high Zout on the performance of my loudspeakers, and I’ve never been intrigued enough to experiment by inserting resistors between my amp and speakers to simulate its effect.
I have however had ample opportunity to experiment with different Zouts on various headphones. This was made especially convenient by auditioning a headphone amp with four Zout taps (0.1Ω, 33Ω, 82Ω and 120Ω). I'll therefore share my subjective experiences of this, for what they're worth (I suspect @Julf and Co. will be “tuning out” at this point ).
My experience is that Zout can have a significant effect on the tuning and/or dynamics of a headphone, especially if the headphone has a low nominal impedance and an impedance curve that varies with frequency.
The most common effect is perhaps heard with open-backed headphones that have an impedance peak in the bass. A higher Zout not only causes an elevated frequency response around the impedance peak, but also makes the bass sound undesirably sluggish / slow to respond, especially if Zout equals or exceeds the nominal impedance of the headphone.
I remember trying the 80Ω Focal Utopia from the amp's 82Ω and 120Ω taps, yielding damping factors of 1 and 0.67 respectively, and it was so slow, boomy and uncontrolled it was unlistenable. The 300Ω Sennheiser HD600 on the other hand sounded fine from all of the Zout taps, even the 120Ω tap (damping factor of 2.5), gaining just a gentle frequency response lift at its Fs without much loss of perceived speed/control. The 600Ω Beyerdynamic DT880 was the least responsive to Zout of all the headphones I tried, its presentation remained very similar whether driven from the 0.1Ω tap (damping factor of 6000) or the 120Ω tap (damping factor of 5).
I noticed that with several headphones, including the HD600 and Utopia, driving them with a higher Zout resulted in a less dry and sweeter treble response, with high frequencies appearing to shimmer/linger for longer giving the illusion of a wider/larger soundstage. As both of these headphones have virtually flat impedance curves through the midrange and treble frequencies, a higher Zout should have no effect on their amplitude response at these frequencies. Switching to the lowest Zout and applying a bass boost with EQ to mimic the effect of the higher Zout did not affect the treble presentation, so I don’t think the increased distortion (IMD or THD?) from boosted bass is the cause of the change in treble presentation. This surely leaves only the increased time response from the higher Zout as the reason for the "wetter" sounding treble, unless it is purely placebo?!
Driving a low impedance (32Ω) closed-back headphone with a ruler-flat impedance curve, there was, as expected, no change whatsoever in the headphone's tonal balance regardless of what Zout tap I used. I did however notice that as I increased Zout, the performance of this headphone steadily worsened and became increasingly "lifeless". At the 120Ω setting (damping factor of 0.27), all of the dynamics from this headphone were effectively crushed so the headphone was clearly being starved of power even at modest SPLs, 0.1Ω was clearly the preferred choice here.
1a) In reality, how common is it to find amplifiers whose Zout varies significantly with frequency?
1b) What is typically the extent of the variation?
1c) Is it possible to generalise and say at which frequencies an amplifier’s Zout is likely to be higher than its published Zout and at which frequencies it’s likely to be lower?
1d) Does the amplifier's Zout curve depend on general circuit topology or can it vary between amplifiers that ostensibly share a largely similar design?
1e) Is there a feasible way for the home listener to measure the Zout curve of their amplifier?
----------
Next, in this article by Jan Meier and Tyll Hertsens, Meier suggests that too low a Zout can result in overdamping and that a higher Zout may be required to achieve 'critical damping', i.e. optimal impulse response from a given headphone. The article focuses on headphones and headphone amplifiers, which usually have higher load and output impedances than loudspeakers and loudspeaker amplifiers, so I'm not sure how much of this is transferrable, but I'll ask anyway:
2a) At which frequencies is Zout most significant when it comes to affecting the optimal impulse response from a loudspeaker or headphone?
2b) Is there a feasible way for the home listener to determine what Zout yields the optimal impulse response from their particular loudspeaker or headphone?
----------
An exclusively solid-state amplifier user for the last 20 years with damping factors ranging from 30 to 300, I'm unlikely to have experienced the effects of a high Zout on the performance of my loudspeakers, and I’ve never been intrigued enough to experiment by inserting resistors between my amp and speakers to simulate its effect.
I have however had ample opportunity to experiment with different Zouts on various headphones. This was made especially convenient by auditioning a headphone amp with four Zout taps (0.1Ω, 33Ω, 82Ω and 120Ω). I'll therefore share my subjective experiences of this, for what they're worth (I suspect @Julf and Co. will be “tuning out” at this point ).
My experience is that Zout can have a significant effect on the tuning and/or dynamics of a headphone, especially if the headphone has a low nominal impedance and an impedance curve that varies with frequency.
The most common effect is perhaps heard with open-backed headphones that have an impedance peak in the bass. A higher Zout not only causes an elevated frequency response around the impedance peak, but also makes the bass sound undesirably sluggish / slow to respond, especially if Zout equals or exceeds the nominal impedance of the headphone.
I remember trying the 80Ω Focal Utopia from the amp's 82Ω and 120Ω taps, yielding damping factors of 1 and 0.67 respectively, and it was so slow, boomy and uncontrolled it was unlistenable. The 300Ω Sennheiser HD600 on the other hand sounded fine from all of the Zout taps, even the 120Ω tap (damping factor of 2.5), gaining just a gentle frequency response lift at its Fs without much loss of perceived speed/control. The 600Ω Beyerdynamic DT880 was the least responsive to Zout of all the headphones I tried, its presentation remained very similar whether driven from the 0.1Ω tap (damping factor of 6000) or the 120Ω tap (damping factor of 5).
I noticed that with several headphones, including the HD600 and Utopia, driving them with a higher Zout resulted in a less dry and sweeter treble response, with high frequencies appearing to shimmer/linger for longer giving the illusion of a wider/larger soundstage. As both of these headphones have virtually flat impedance curves through the midrange and treble frequencies, a higher Zout should have no effect on their amplitude response at these frequencies. Switching to the lowest Zout and applying a bass boost with EQ to mimic the effect of the higher Zout did not affect the treble presentation, so I don’t think the increased distortion (IMD or THD?) from boosted bass is the cause of the change in treble presentation. This surely leaves only the increased time response from the higher Zout as the reason for the "wetter" sounding treble, unless it is purely placebo?!
Driving a low impedance (32Ω) closed-back headphone with a ruler-flat impedance curve, there was, as expected, no change whatsoever in the headphone's tonal balance regardless of what Zout tap I used. I did however notice that as I increased Zout, the performance of this headphone steadily worsened and became increasingly "lifeless". At the 120Ω setting (damping factor of 0.27), all of the dynamics from this headphone were effectively crushed so the headphone was clearly being starved of power even at modest SPLs, 0.1Ω was clearly the preferred choice here.
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