More Tubes - Interstage Transformer-Coupled Type 45 Headphone Amp

[L0rdGwyn]

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.

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

Rear panel with power switch. Pretty boring!

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

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

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.

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!

 

[mhardy6647]

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

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

 

[levimax]

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.

 

[L0rdGwyn]

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.

 

[L0rdGwyn]

If anyone is going to CanJam Chicago this weekend, this amplifier along with one of my 6AS7G OTLs will be at the ZMF table. Not for sale, just amps I built for Zach Mehrbach and his friend. The 45 amp in particular is an interesting listen.