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Old 13th February 2013, 05:27 PM   #1
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Default Pieter's take on an all DHT headphone amplifier.

Please find attached the pdf.

Personally I find this interesting because vapourware might turn into hardware.
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File Type: pdf DHT Headphone amp.pdf (327.5 KB, 298 views)
 
Old 13th February 2013, 05:31 PM   #2
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I am in no way affiliated with Pieter apart from country of residence and interest in diy audio

Therefore anything I might post is purely my own point of view.
 
Old 13th February 2013, 05:33 PM   #3
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Pieter's schematic for those who just wish to glance at the design.

Click the image to open in full size.

And two excerpts:

Quote:
Apart from the output transformer I do not claim to be original, and please note this design has not been built yet.
Very likely transformer winding ratios and load impedances need some experimentation.
My inspiration comes from the designs of Lynn Olsen and the late Allen Wright.
The input transformer is a 1:1+1 phase splitter; it is wound on a nanocrystalline toroidal core, and it is the same as used in Lynn Olsen's Karna amplifier.
The two 4P1L tubes work in pushpull class A; the cathodes see a common current source (or better "sink"?) like Allen Wright's PP 300B design. For this application I think there is enough headroom without the risk of the current sink running out of "steam" causing hard clipping.
Why pushpull? I want to maintain the high permeability of the output transformer in this design; with a single ended output stage the required airgap would impair permeability, and besides the SE topology would require an additional gain stage. A pushpull transformer will be smaller than an SE one which is good for winding capacitance and DC resistances.
This output transformer is rather special. Primary impedance ratio is 6k anodeanode with a 600
ohm load across the full secondary; at the same time it will function as an inductive volume control.
It will have a nanocrystalline cut ccore with a very minor airgap to withstand minor DC imbalance, at the same time maintaining permeability . This core material is also known as "Finemet".
The idea is this: the output transformer is basically 6k anodeanode (the 6k is not yet fixed), and 600 ohm secondary. Step down ratio therefore is 3,16:1. The secondary is center tapped to keep
everything symmetrical. I like the idea to drive a headphone with a balanced signal.
The secondary winding has a number of taps which, attached to a 24 position switch (Elma or a cheapo Chinese one), functions as volume control and impedance matching at the same time.
At maximum volume we have a 6k 600 ohm ratio; one step down (a minus 3 dB tap) and we have already a 6k 300 ohm ratio; 2 steps down gives 150 ohm, 3 steps down 75 ohm, and 4 steps down (minus 12 dB) gives a proper drive for a 38 ohm load. We can use all our further steps down to control the volume, at the same time having a 6k 38 ohm ratio or better. In other words: in practice the tubes will always see a very high load impedance which is good for linearity.
Quote:
Why not keep it all direct heated with a 5R4 rectifier.
Semi choke input (0.5 uF) followed by two chokes to split the power supply for each channel.
Each channel an additional CLC.
Options for regulation (shunt?); this is just one example of many possible power supplies.
Cathode/filament supply by Rod Coleman.
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Last edited by Bas Horneman; 13th February 2013 at 05:40 PM.
 
Old 13th February 2013, 05:42 PM   #4
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In addition to Rod Coleman's filament regs. Salas' HV Shunt V2 Regulator would be great as the shunt reg for the psu.
 
Old 13th February 2013, 06:02 PM   #5
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Quote:
Originally Posted by Bas Horneman View Post
Please find attached the pdf.

Personally I find this interesting because vapourware might turn into hardware.

I read the article of Pieter and a little correction i like to make: amourphous nano crystalline material? It should be amorphous OR nano crystalline because amorphous means without crystal structure.

A long time ago i made a transformer for a Stax electrostatic headphone based on a amorphous c-core. Mr Treurniet listened to my transformer. I know for sure that he had not such a transformer in the past, maybe he has now but i would be surprised if he really did.......
 
Old 13th February 2013, 06:20 PM   #6
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May I make a little correction to the little correction ?

This particular PDF speaks about "More recent are core materials like amorphous alloy (iron or cobalt based), and amorphous alloys with a nanocrystalline structure."

Isn't the case that this material starts as a amorphous alloy and then that material gets a heat treatment where a nano crystalline structure is created ?
The addition of the 'with a' is already a nuance ...


Quote:
Originally Posted by esltransformer View Post
I read the article of Pieter and a little correction i like to make: amourphous nano crystalline material? It should be amorphous OR nano crystalline because amorphous means without crystal structure.
 
Old 13th February 2013, 07:09 PM   #7
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An alternative could be...if straight from DAC.
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Old 13th February 2013, 07:16 PM   #8
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Quote:
Originally Posted by daanve View Post
May I make a little correction to the little correction ?

This particular PDF speaks about "More recent are core materials like amorphous alloy (iron or cobalt based), and amorphous alloys with a nanocrystalline structure."

Isn't the case that this material starts as a amorphous alloy and then that material gets a heat treatment where a nano crystalline structure is created ?
The addition of the 'with a' is already a nuance ...
Yes that's indeed the case. Nano crystalline materials are heat treated amorphous materials. But after heat treatment it ain't amorph anymore.
 
Old 13th February 2013, 09:29 PM   #9
regal is offline regal  United States
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From personal experience with transformers for headphones I agree highly with the assesment that the low level signals being dealt with need more than just a typical iron core OPT. P-P is a good way to deal with the transformer issue, but I think I would want an amp with more gain than this.
 
Old 14th February 2013, 02:36 AM   #10
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As I found out just recently, for 300 ohm cans and the right OPT, there is enough gain.
Of course staring at the four 4P1L tubes in front of me I thought about P-P. A little step up at the input would not hurt. Could make a nice amp.
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