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Current Drive for Headphones & The Super Linear Transconductance Amplifier
Current Drive for Headphones & The Super Linear Transconductance Amplifier
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Old 8th October 2018, 05:03 PM   #1
EUVL is offline EUVL  Europe
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Default Current Drive for Headphones & The Super Linear Transconductance Amplifier

As probably the last circuit we shall build for headphone amplifiers, we want to find out how they sound under current drive.
This is not only known for loudspeakers, but also for headphones, especially for planar magnetics.

The circuit is an evolution from our earlier Pioneer Super Linear Headphone Amplifier.
The Pioneer Super Linear Circuit
Only now it has been modified to output current instead of voltage.
Many of the aspects of the SL Voltage such as device matching, etc. still applies.

You can read about the article here :
http://xen-audio.com/documents/cdha/...c%20181006.pdf


Happy reading,
Patrick

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File Type: jpg 180715 SL Trans Top.jpg (111.8 KB, 2744 views)
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Old 8th October 2018, 05:14 PM   #3
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The circuit was also successfully built by a Beta tester, who used the version with DC servo.
In any case, it is still recommended to use a protection circuit, especially when this is non-invasive.

All listening tested were using planar magnetic headphones, HiFi-Man HE500 & HE-400 respectively.
They have constant impedance over the audio band.


Patrick
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Old 9th October 2018, 06:31 PM   #4
Algar_emi is offline Algar_emi  Canada
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Current Drive for Headphones & The Super Linear Transconductance Amplifier
Hi. I was the Beta tester and here my experience with this other great design from Patrick (EUVL). This design was interesting for me because at the time I just got a nice pair of magnaplanar HiFiMan HE-400 headphone and was using them with the Xen Audio Pioneer Ultra linear headphone amp, with excellent results. Patrick asked me if I like the HE-400 with the his amp, and I answered, Yes very much! He told me that even if the Pioneer Super Linear was a great amp, the magnaplanar HP could be driven with even better result with a current amplifier, not a voltage one, as the Pioneer, and guest what! he had a design that he wanted me to try. I was in!

Last edited by Algar_emi; 9th October 2018 at 06:47 PM.
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Old 9th October 2018, 06:35 PM   #5
Algar_emi is offline Algar_emi  Canada
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Current Drive for Headphones & The Super Linear Transconductance Amplifier
To make a long story short, my time as a beta tester was a short one, because out of the box Patrick design worked like a charm. I must confess though that I missed the time to fully tried all function, such as the voltage mode adapter, in part because I already had the nice Pionner Super Linear (voltage mode), and I was pretty busy at the time with other projects. Also I used my extra parts inventory from the Pioneer project to build this amp. The small smd transistor were not as perfectly matched as the previous project, and I was not sure if the amp would be as well balanced, so I used the servo from the start. Other can certainly comments on the kit others features if they have more time than me. Here the final result
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Old 9th October 2018, 06:44 PM   #6
Algar_emi is offline Algar_emi  Canada
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Current Drive for Headphones & The Super Linear Transconductance Amplifier
As some others members can see I'm using my own motherboard design for this built, see the original GB for detail: GB for Pioneer Super Linear Motherboard

If there is enough interest with this new project, and if Patrick doesn't offer his own motherbaord option, I may run an other batch, we'll see.

So this project is a breeze to mount. For better results you'll need Xen Audio great discrete regulators, and the little protection pcb is a must, because this amp has a large DC swing at start-up.

Also take note that the amp, in current mode can be use only with a magnaplanar headphones. For other headphone type, simply use the voltage mode small pcb adapter, to convert the amp to voltage mode. The protection circuit can protect with the both mode, see the excellent documentation for details.

So in my view it may be the perfect headphone amp, can probably used with all the headphone out there (naturally not the electro static ones, as Stax). and sound just great, here my listening impressions:

I replied to Patrick with this simple comment: Well this amp may well be my last headphone amp [not with tube ] that I ever build, it is so good...

With my HifiMan the sound is gourgeous, huge bass, mids to die for, crystal clear highs, and the speed, the speed! As they say in the UK, Lovely...

This latest Patrick's design is very recommended. Thanks Patrick for sharing.

Last edited by Algar_emi; 9th October 2018 at 06:47 PM.
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Old 9th October 2018, 07:18 PM   #7
EUVL is offline EUVL  Europe
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I am glad that you like it. And thank you for being brave to try.

The reasons for using our regulator instead of others are that :
1) it has built-in current limit, which is essential when using voltage mode together with the current sensing shunt protection circuit;
2) it has been tested to be stable with both the current and voltage mode.

That is not to say that other regulators would not do. But you'll have to try.

I personally like the "original" Pioneer SL (voltage) HPA better, marginally.
But that is a matter of personal taste. On top of that I do not own a planar magnetic.
As Algar mentioned you can plug in the voltage-mode for non-planar-magnetic phones.
But that converts the SL Trans to a CFA amp with NFB (negative feedback). In doing so, its character also changes; more like the F5-HA.
As it is known, I personally prefer the ZGF (Zero Global Feedback) sound.

My own prototype is due for independent audition in the coming weeks.
Then you will hear another opinion. We'll see what he says.
He has better ears than I do.

For those guys into current drive loudspeakers, you can easily scale this up to drive 8 ohm chassis in Class A.
So all in all a very interesting experience to say the least.


Patrick
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Old 9th October 2018, 08:42 PM   #8
Algar_emi is offline Algar_emi  Canada
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Current Drive for Headphones & The Super Linear Transconductance Amplifier
I still use my Pioneer Ultra Linear with my others (standard headphones), but with the HE-400, the Transconductance amp is the best...
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Old 9th October 2018, 09:06 PM   #9
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Note that it only has NFB in voltage mode (with stack-on adaptor).
In current mode, it is also purely ZGF.


Patrick
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Old 9th October 2018, 09:39 PM   #10
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For those who want to play with Spice, here the power amp version.
Q9,10 has to be TO92, and preferrably with additional heat sinking.


Patrick
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File Type: asc SL Trans DM Current 8R Tos_Sanken V0 THD.asc (13.1 KB, 122 views)
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