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More Is Better? 6336 / 6528 OTL Headphone Amplifier

L0rdGwyn

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Showing another amplifier I just finished, decided to adapt my 6AS7G headphone amplifier circuit for 6336 / 6528.

Similar to the 6AS7G circuit, this is a dual-mono power supply design, each channel has its transformer/rectifier/reservoir and Maida style HV regulator. I use a 40 second time delay to allow the tube heaters to warm before soft start B+ is applied. I also included a 5V 3-pin fan header on the power supply PCB, there are two 92mm Noctua fans mounted internally directly below the power tubes for some active cooling. The 6J5 input stage is gyrator loaded and LED biased, the two stages are direct-coupled, and the 6336 or 6528 cathode followers are actively loaded. Output capacitance is 600uF per channel, ClarityCap metallized polypropylene. You might be able to spot them if you look closely.

The two sections of the 6336 or 6528 are run in parallel, each tube biases at 275mA with the components cold. Given the high current, both HV regulators and the power tube loads are mounted on external heat sinks. This amplifier is an engineer's dream in efficiency.

Here are some photos before some basic measurements.

DSCF6752.jpg


DSCF6756.jpg


DSCF6767.jpg


DSCF6762.jpg


Output Z:
6336 - 24ohm
6528 - 15ohm

THD w/ 6528:
1mW into 300ohm 0.007%
1mW into 120ohm 0.013%
1mW into 80ohm 0.029%
1mW into 32ohm 0.11%

All distortion above is pure H2. Distortion measurements were largely dependent on the quality of the 6J5, I chose one that measured well.

Frequency Response:

300ohm

6336 OTL FR 300ohm.png


32ohm

6336 OTL FR 32ohm.png


Some more pics with a new headphone release from ZMF, the Atrium.

DSCF6772.jpg


DSCF6817 (1).jpg


All lit up at night.

PXL_20220322_235318698.NIGHT-2 (1).jpg


The amp sounds great, it's the best OTL headphone amp I have heard, very happy with the result. Thanks for checking it out :)
 

Dogen

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That’s beautiful work, both in design and construction! I love OTL headphone amps, with full knowledge they often don’t measure well, but yours does very well. That much iron and glass driving a little pair of headphones - it holds an irrational appeal. Thanks for sharing your project.
 
OP
L0rdGwyn

L0rdGwyn

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That’s beautiful work, both in design and construction! I love OTL headphone amps, with full knowledge they often don’t measure well, but yours does very well. That much iron and glass driving a little pair of headphones - it holds an irrational appeal. Thanks for sharing your project.

Thanks! A lot of time and planning goes into it, happy it turned out well. Despite what people say, these amplifiers can sound excellent, I think many would be surprised.

One thing that bothered me about the build was I felt the knob was too large, it's a vintage variac knob from General Radio. I was able to find another one, one size smaller, General Radio KNSP-8, much better size for this chassis.

PXL_20220325_234912726.NIGHT.jpg


PXL_20220325_234614840.NIGHT-3.jpg
 

_theLaughingman

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Thanks! A lot of time and planning goes into it, happy it turned out well. Despite what people say, these amplifiers can sound excellent, I think many would be surprised.

One thing that bothered me about the build was I felt the knob was too large, it's a vintage variac knob from General Radio. I was able to find another one, one size smaller, General Radio KNSP-8, much better size for this chassis.

View attachment 195398

View attachment 195399
She's a beauty!! Sexy beast.
 

_theLaughingman

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I do want to get into tube amps for headphones, but I have mostly planars and one high impedance headphones, so hesitating a bit to invest in something like a bottlehead amp.
 
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L0rdGwyn

L0rdGwyn

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I do want to get into tube amps for headphones, but I have mostly planars and one high impedance headphones, so hesitating a bit to invest in something like a bottlehead amp.

If you're running planar magnetics, you'd want to go with a transformer-coupled design, Bottlehead's Mainline amplifier would do the trick!
 
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