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High Performance Class A Current Feeback Headphone Amplifier
High Performance Class A Current Feeback Headphone Amplifier
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Old 6th November 2014, 11:14 PM   #1
monty78pig is offline monty78pig  England
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High Performance Class A Current Feeback Headphone Amplifier
Lightbulb High Performance Class A Current Feeback Headphone Amplifier

Here's a headphone amplifier designed specifically for Sennheiser HD600 headphones. It will work well with any 150+ ohm headphones.

The design is a somewhat unconventional 2 stage amplifier, it runs off unregulated supplies (12VAC), is current feedback, and uses a singleton input stage.

The input stage is linearised by two constant current sources (Q2,Q3 and Q4, Q5) drawing a current of about 2 mA through Q1. R4 allows the DC offset inherent to the single transistor input stage to be adjusted to zero. The VAS is also the output stage loaded by an SRRP style current source biased to roughly 46mA. I have not yet worked out the value of C7, perhaps it is not even necessary due to the simple nature of the amplifier. R21 and C10 form the Zobel network although I doubt that even this is actually necessary for stability. R13 and C5 prevent any supply ripple from being injected onto the output via the current source. C6 allows the current source to 'float' at AC output voltages (assuming you're not amplifying DC to your headphones).

Gain is about 8.5dB, power consumption is modest for Class A at about 1.6W/channel.
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Old 6th November 2014, 11:23 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The feedback loop should have a capacitor in series with R8, to block the DC!
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Old 6th November 2014, 11:32 PM   #3
monty78pig is offline monty78pig  England
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High Performance Class A Current Feeback Headphone Amplifier
Nope, look a little closer and you'll see the DC is adjusted to 0 by altering the CCS current source injected into the emitter of Q1. Adding a capacitor only affects the amplification of the base to emitter voltage error, which is corrected by the current source Q2, Q3.

There's simply no need for one if you look a little more closely.
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Old 6th November 2014, 11:48 PM   #4
jcx is offline jcx  United States
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most with experience adopt the very practical rule - "don't design circuits that depend on BJT Hfe/Beta being some particular value or being stable"
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Old 6th November 2014, 11:52 PM   #5
monty78pig is offline monty78pig  England
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High Performance Class A Current Feeback Headphone Amplifier
Can you give an example where this circuit relies on that?
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Old 6th November 2014, 11:59 PM   #6
jcx is offline jcx  United States
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sorry, I was trying too hard to make sense of:
Quote:
Nope, look a little closer and you'll see the DC is adjusted to 0 by altering the CCS current source injected into the emitter of Q1.
rather than dismiss it as simply wrong
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Old 7th November 2014, 12:01 AM   #7
monty78pig is offline monty78pig  England
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High Performance Class A Current Feeback Headphone Amplifier
Yeah, sorry. Not a great explanation on my part. Redundant acronym syndrome strikes again
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Old 7th November 2014, 12:05 AM   #8
woody is offline woody
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I am not a headphone person but have a question? Anyone thought of building a low power
Son of Zen amp as a headphone amp?
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Old 7th November 2014, 12:36 AM   #9
monty78pig is offline monty78pig  England
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High Performance Class A Current Feeback Headphone Amplifier
The Son of Zens linearity would be pretty poor compared to this circuit, even if it is balanced. It couldn't be implemented with conventional TRS connectors as the output is BTL.

Also, efficiency ...

Check out Rod Elliots DoZ (Death of Zen headphone amplifier) for a good read if you're into that sort of thing...

Death of Zen Class-A - Use it as a headphone amplifier

Better efficiency, supply rejection etc. Pretty much better than the Zen amps in every way.
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Old 7th November 2014, 01:32 AM   #10
jerluwoo is offline jerluwoo  United States
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Curious if it would be a bit touchy have 2 ccs's trying to control the current through q1?
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