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EUVL 26th May 2019 05:10 PM

The XEN XELF Headphone & Speaker Buffer
2 Attachment(s)
After finsihing our DIP-8 discrete opamps, we want to have a platform to test its sonic performance.
One Last Attempt at Discrete Opamp in DIP8 Footprint

Morde has built a few of our projects and we know he has a WHAMMY.
He was kind enough to test one of the opamps for us against the OPA627, using the WHAMMY as a platform :
(post #1375)

But we thought we should have our own platform to test as well.
So we asked ourselves the question -- how would our own "WHAMMY" look like ?
The result is the XELF, or XEN Lateral FET Buffer.


EUVL 26th May 2019 05:19 PM

Listening Impressions
The brave man also acted as our Alpha tester.
This was deliberately to test out whether it is also suitable for those not-so-experienced DIYer's.
And of course, since he has a WHAMMY, we have a benchmark to go against.

To quote his listening impressions (with his consent) :

"I had some time to spend comparing XELF with the PASS DIY Whammy.
I had installed OPA627 opamps to both.
I did the comparison without Xfeed since it was easier that way and I was able to run them right after the other.
I used my trusty LCD-2f phones for testing and the volume levels were matched with 1kHz sine wave and DMM.

The XELF was powered with regular AA sized batteries and the Whammy obviously via itís own supply.
Bias was ~190mA with XELF and battery voltages ~+-14V.

The listening impressions -- I have to say that XELF has the same kind of easiness and open sound as the UTHAiM.
Very effortless. Some magic with the Exicon lateral FETs?

Bass was deeper and control better with XELF. Evident with samples like:
Children Of Bodom - Blooddrunk, The Mars Volta - Wax Simulacra

Vocals had more presence too with XELF and Norah Jones seemed to be singing in my living room.
Whammy was very close in this regard but I noticed the difference.

Then I plugged in the Xen discrete opamp and oh boy, something just clicked into right position.
Itís like a filter would have been removed between my ears and headphones.
Soundstage is wider and sound is crystal clear. Definitely a bigger change than changing opamps with Whammy.
I guess the XELF buffer has a lot of potential so the opamp makes a bigger effect."

Seems he is keeping the XELF and planning to build the discrete opamps as well.



EUVL 26th May 2019 05:25 PM

Links for the References :

1. One Last Attempt at Discrete Opamp in DIP8 Footprint
3. Auto-Biasing Circuits for Complementary Followers
4. US Patent 4752745 Opto-isolated bias circuit for operating push-pull amplifiers in Class A and Class AB modes
5. JFETs: The New Frontier, Part 2 | audioXpress
6. Taylor Source Follower
7. The Pioneer Super Linear Circuit
8. XCEN Balanced to Single-Ended Converter Revisited


UltimateX86 1st June 2019 06:18 PM

Hi, nice board

This could be uses for power amplifier

EUVL 1st June 2019 06:23 PM

Yes indeed, you can easily wind up the bias with suitable heatsinking.
And with an opamp in a NFB loop also low enough output impedance.


EUVL 5th July 2019 04:30 PM

Driving Speakers
5 Attachment(s)
I received a set of Exicons from a generous donor and built my own prototype today.
Quite straight forward, without any events.
Even without trimming, imbalance between the top and bottom current sources were less than 4ĶA.

As I wanted to test it both for driving headphones and speakers, I reduced the CCS bias a bit to 1mA.
The biasing resistors for the lateral MOSFETs were then set to give 200mA.
If I now then add another 0.5mA in parallel to the CCS, I shall increase the output bias to 550mA.
You can eassily increase further to 1A bias by adding 1mA to the CCS instead.
As expected, with the opamp in place, changing bias does not affect DC offset at all.
And I used a MPC74 0.1R as source resistor, as to be 100% safe when driving speakers.

The XELF alone has a -3dB bandwidth of 2MHz with 33R load, and even higher without load (-1.5dB at 2MHz).
With the discrete opamp in place, bandwidth is limited by the opamp, as per design.
Similar to the UTHAiM, and as mentioned by Morde, it has huge surplus in driving capabilities.
Even with 200mA bias, driving a small 8R speaker to a high loudness was no issue at all.
And you can increase bias easily by a flip of a switch.




Morde 5th July 2019 05:51 PM

Sweet! Power amp and headamp in one go.
Do you have any sonic impressions when feeding speakers? I have a pair of 4ohm/87dB s floorstanders in need of a quality amplification...

EUVL 5th July 2019 05:59 PM

I only tried functionality using lab supplies and a small heatsink.
To test properly you will have to have proper supplies, caps, heatsinks for higher bias, etc.

But for the quick and dirty test, it was good sound.
No reason to believe otherwise, as the LM4702 uses essentially the same output stage :

Easy enough for you to try yourself ? ;)


blackdod 5th July 2019 07:39 PM

Patrick, have you used ECW20N20 & ECW20P20?

EUVL 5th July 2019 07:59 PM


For headphone or small speaker use no need for double die.
Also doubles the capacitances.


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