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An interesting read from someone from Apogee

Aug 10, 2018

I haven't finished the whole thing yet, as I am reading it from work. Which my work involves riding motorcycle...

The interview come across wide topics from digital audio, MQA, practicality of using 96k...

The thing caught my eyes was the explanation to apogee groove have a higher than normal impedance. Is it true that variable voltage output can overcome the impedance matching problem?


Staff Member
CFO (Chief Fun Officer)
Feb 13, 2016
Seattle Area
No. He is assuming that your headphone has hundreds of ohm impedance so what they have is very good. He doesn't realize there are consumer headphones with very low impedances.


Major Contributor
Mar 1, 2018
Gold Coast, Queensland, Australia
Amir, he's basically saying the amplifier output stage is configured a current source instead of a voltage source (hence the high output impedance you measured), but in reality, your testing showed it clearly is limited by the internal rails wouldn't you say?

NP did some experimentation with full range speaker drivers (like headphone design) and transconductance amps:


"The most precise way to develop that specific current is with a current-source amplifier. Such
an amplifier ignores the impedances in series with the circuit, the resistance and inductance
of the wire and voice coil and the back electromotive force (EMF) produced by the cone
motion. As I said, most speakers are designed around voltage sources but there are few
instances where a current source can be used to advantage. One of the best ones is the
category of full-range high-efficiency drivers.
Why is that? First, such drivers are able to take advantage of acoustic and suspension
resistance to achieve some or all of the damping that they need to prevent excessive
overhang because their moving mass is very light. With their efficient motors, even a high
source impedance is often enough to give critical damping. Second, their impedance curve
tends to reflect their needs – more current both at low frequency resonance and in the treble,
two areas where frequency response has fallen off with increased speaker impedance. If you
want, the current through the voice coil can be made constant regardless of the variations in
the acoustic environment. The voice coil force is invariant whether the cone is loaded into a
horn, sealed box, bass reflex or whatever else you care to mount it in."

The above linked Apogee article has a few issues IMO...

"Roger: Of course. Let’s take a 48 kHz 24 bit file. 24 bit means you should have several hundred thousand different voltage levels. You have 24 ‘levels’ to define a voltage level between 0 dBFS and -144 dBFS if you’re in the digital domain. I think you have around 600,000 individual bit levels for a 24 bit file (recording format), and for a 16 bit file (CD audio) you have about 16,000 bit levels. The total amount of levels are your dynamic range. In the signal path, you are asking a computer or converter chip to look at the analogue signal 48,000 times per second (assuming a 48kHz file), look at the incoming voltage and finally associate that voltage with one of 600,000 different levels. For a 96kHz file, the converter has to do this 96,000 times per second and so on."
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