Current-mode versus voltage-mode amplification

[Dialectic]

I'm not sure I'm an audio beginner, but I have a question for the more expert members of the forum: what is the different between current-mode and voltage-mode amplification, and what are the advantages of each for driving transducers (particularly headphones) and line-level signals?

I've read the Wikipedia article and some marketing literature on the subject, and none of it has advanced my understanding very far.

Current-mode amplification seems to be used only in certain line-level devices (for instance, the Krell CAST technology and the similar Audio-gd technology) and headphone amplifiers (for instance, the Questyle headphone amps). I understand the desirability of low output impedance in amplification devices, and this is one of the reasons I use a Devialet amp, which has very low output impedance, in my second system to drive passive speakers.

I'm interested in this question in the context of headphones, as I have a pair of HifiMan HE-560s, for which the Mytek 192-DSD seems not to have enough juice.

Thank you to ASR's technical experts in advance.

 

[amirm]

I will get us started. Unfortunately this is a complicated topic so my explanation itself may be too difficult to understand.

Let's start with our sources being "voltage sources." This means the signal goes up and down in voltage. Classic amplifier design takes this signal and amplifies its voltage (adds "gain"). Amplification devices are not "linear" meaning they add distortion in different parts of the waveform. That deforms the waveform. To reduce this a sample of the output voltage is taken and compared to the input. That input stage then attempts to minimize this difference. This is called "negative feedback."

One issue here is that amplifiers run out of gain at some bandwidth, i.e. can't amplify too high a frequency. There, feedback stops working as a result and as a result, we need to limit the bandwidth of our amplifier to a point below that. Such an amplifier can also have lower "slew rate" meaning it can't respond to too sharp of a transient. That is because we limited the bandwidth so we cannot handle sharp transitions.

Current mode amplifier works by converting the input voltage to current (so called "V/I" stage). We then amplify that current and sample current to generate our negative feedback. At the output stage we can convert this back to voltage to drive the speaker (or not).

The current amplifier can have much more current to work with than voltage controlled. This as a result allows the bandwidth to be much higher. The resulting amplifier therefore has much better slew rate than voltage control. This can reduce such measurements as Transient Intermodulation (TIM).

The drawback of current amplification is usually higher noise levels, lack of accuracy at DC level ("dc offset") and lower gain.

Because we don't need much bandwidth in audio, they are not a popular choice, the examples you give notwithstanding.

 

[Dialectic]

Thank you very much. I understand the explanation, and it's extremely helpful to me. I do wonder about audible differences between headphone amplifiers using each technology. Perhaps a member has a Questyle headphone amp or Audio-gd DAC with current-mode output that can be submitted for measurement.

 

[amirm]

I am interested too. I am hoping to see the Questyle on Massdrop again to purchase.

 

[RayDunzl]

I do wonder about audible differences between headphone amplifiers using each technology.

I notice no audible difference on-room using CAST or XLR between my preamp and monoblocks.

It does put a little red LED in the center of a triangle of blue LEDs, which is nice, so I've got that going for me.

So, I use the CAST method to free up an XLR output at the preamp.

I don't think I've consciously measured it in-room, but generally speaking, in-room measurements aren't very good at revealing small differences as ambient noise or mic noise randomizes and/or otherwise obscures the tiny stuff.

 

[Wombat]

Rod Elliot's take on this topic:

http://sound.whsites.net/z-effects.htm


Other articles by Rod Elliot:

http://sound.whsites.net/articles.htm

 

[CedarX]

Enleum is launching a new battery-powered HPA with both (separate) voltage and current outputs: the HPA-23RM, see

HPA-23RM - Enleum

The HPA-23RM is Enleum's reference dedicated headphone amplifier, following in the footsteps of the AMP-23R and our legacy products, Bakoon HPA-21 and HPA-01M.

enleum.com

I’m unfortunately not ready to spend $3K to have it drop shipped to Amir, but I’ll be definitely interested to see a comparison of these two type of outputs with some common and “hard to drive” headphones.

 

[raif71]