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More Tubes - Interstage Transformer-Coupled Type 45 Headphone Amp

L0rdGwyn

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I have another DIY project to show off! Going to keep this post short and sweet, and light on measurements, not likely they'll impress anyone lol.

This is a type 45 headphone amplifier built for a friend, Zach Mehrbach of ZMF headphones. Here are the design highlights:

-LED biased 5687 / E182CC / 7044 input stage
-Nickel core interstage transformers by Intact Audio
-Fixed bias type 45 output stage
-Electra-Print output transformers with 300ohm, 120ohm, 32ohm taps
-Separate B+ HV regulators for each channel, as well as separate bias shunt regulators for each channel
-DC regulated heater supply for the 5687 / E182CC / 7044 inputs
-Rod Coleman filament regulators
-Vintage Amphenol 9-pin noval and UX4 sockets
-Simpson DC milliammeters
-Chassis designed by me and machined / coated by Landfall Systems, color is Distant Blue, inspired by vintage JBL speaker baffles

Here is the circuit.

PXL_20211205_185816326.jpg


And the finished amplifier. Yeah, it's big.

PXL_20211205_190701995.jpg


Rear panel with power switch. Pretty boring!

PXL_20211205_191353279.jpg


Here it is with a pair of Philco globe 45 and Amperex E182CC. I like this combination best.

PXL_20211205_191126830-2.jpg


PXL_20211205_191239144.jpg


Listening with some ZMF dynamics, front panel toggle switch changes the output tap from 300ohm (up), 120ohm (middle), and 32ohm (down).

PXL_20211205_192526032.NIGHT.jpg


Measurements in brief, THD at 1mW into a 300ohm dummy on the 300ohm tap is 0.07%, all H2.

Here is the frequency response off the 300ohm tap with a 300ohm dummy. Down around 2dB at 20kHz.

ZMF 45 FR 300ohm.png


This amp has some of the best clarity and holographic staging I have heard in a headphone amp. Just shipped it off, will be very eager to see what Zach thinks :)

Thanks for looking!
 
Last edited:

mhardy6647

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Beautiful... and globe 45s! Sweet.
I like your star ground, too! :)

A nice SE 45 stereo amplifier deserves fine, high sensitivity loudspeakers, though. ;)
 

levimax

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Beautiful design and workmanship! At first I thought you were trying to do a "McIntosh Green LED tube thing" but I see they are used to set this bias.
 
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L0rdGwyn

L0rdGwyn

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Beautiful... and globe 45s! Sweet.
I like your star ground, too! :)

A nice SE 45 stereo amplifier deserves fine, high sensitivity loudspeakers, though. ;)

Thank you! I have a 45 speaker amplifier myself ;) Sowter iron, a parafeed output design however. The input and output stages are cap-coupled in that design, I plan to direct-couple it at some point.

Beautiful design and workmanship! At first I thought you were trying to do a "McIntosh Green LED tube thing" but I see they are used to set this bias.

Thanks! Yeah the green lighting is a secondary effect, looks kinda cool though coming out of the ventilation in the dark.
 
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