Concept image:

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Chassis form-factor & orientation is designed by Jeremy for his desktop at work, sitting between 2 monitor screens.
To drive Massdrop SENNHEISER HD-6XX phones.
The open-back phones work well in a team work environment.

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Some of the parts are here:

The all-important Power Transformer - not many one-off transformer-winders still exist locally, in particular with the expertise for valve/tube-based equipment.
This PTX was done by Brian Smith in Foxton (designer / builder of the Transmit 8 amplifier).
(Took a bit of a beating from NZ Post ParcelPost on the way up )

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More please

What design did you decide upon to have a transformer made for it?
TSL in Auckland make custom transformers. I used to use TWS in Chch but they don’t seem interested anymore after their valve transformer guru engineer retired.

AMR-iFi R&D

Hi colinf - this will be a 6080 OTL amp, inspired by the Bottlehead Crack, but we will settle on our own operating points for the tubes - perhaps a bit more current if we can.

Personally, I am not very keen on such philosophies as LED bias or SS CCS tube loading, so I'll get him to listen & decide what suits Jeremy - his ears should be better than mine

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The Sennheisers have a 300 ohm impedance so the current needed isn’t very high, around 20mA peak current. Do you know the current the Bottlehead Crack is set at?
Are you making a phase-true input stage?

AMR-iFi R&D

The wood chassis taking shape...
(Jeremy's build.)

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Looking good.

CNC'd top plate?

wood... mmmmmmm

That top plate above is a 'test' template only, to check layout, underneath, which is a bit tight, compact!

The wood is walnut that came from a Tr Me seller - oiled.

Jeremy fancied trying some corner dovetailing.

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Just thinking about the phase issue...a 4 pin XLR is what some balanced headphones use and one could swap the + and - in the connector to preserve absolute phase. Other connectors could be the new 4.4mm TRRS balanced standard, that’s on the new iFi Hip Dac.

AMR-iFi R&D

The tubes are here - 6080 purchased, 12AU7 from my own stash:

6080 is a double triode valve from the 1950s/60s to the 1980s, designed to be used originally as a PSU regulator.
Introduced by Mullard-Philips?
It is rugged & you can see by the size of the plates that it can handle a lot of current.
This NOS sample is a '6080 / CV2984' (Commercial Valve / industrial) version.

The 12AU7 (ECC82) is a 1940s mini double triode, with lowish gain - GE (USA) in this sample.
Some folks here may be using these.

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Thanks colinf for pointing out that this circuit inverts phase.
Each valve stage inverts phase, but the 2nd output stage is 'cathode-follower' connected, for low output impedance (OTL).

I was thinking that we'd simply reverse phase at the outputs - as the headphones are ungrounded.

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Good idea Owen.
Ungrounded....you mean the left and right driver negatives are separated by use of a different plug to a normal TRS?

AMR-iFi R&D

Mar 19, 2020 21:29:40 GMT 12 colinf said:

Good idea Owen.
Ungrounded....you mean the left and right driver negatives are separated by use of a different plug to a normal TRS?

Probably poor technical language by me...
I meant that the headphone amp is driving a 'floating' device - ie unlike another amp stage or active cct.

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Jeremy decided to shoe-horn in a DAC into this chassis - so that he can input either analogue (RCA) or digital (USB) sources.

He accepts that there will now be a switch in the 'signal path'.

This is the Khadas DAC board which has arrived from China (fairly good reviews it seems) - using the ESS 9038 DAC.
Around US$100.

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The Khadas dac looks interesting, such high performance for little outlay.
A TRS connector has the negatives connected together at the sleeve. So you’d have to unsolder the headphone wires in their plug and swap them + to - to invert the phase.
Looking good!

AMR-iFi R&D

Assembly begins:

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Looks lovely.

Keep it coming

The chassis box is a little tight on hindsight (esp now after J decided to add the DAC board).
The PTX is 'rubber mounted'.
The top plate (actually a sub-plate, as a 'cosmetic' top plate will go on top externally) is rather thick, ~4mm - but engineer types like to tap & thread everything
Tube sockets have been rotated for hopefully efficient, short signal paths, point-to-point components, with minimal hook-up wiring.
Vol pot is the tried-and-true ALPS RK27 'blue' - purchased from RS, hopefully genuine, not counterfeit stock. This is a nice balanced sounding plastic film type pot, IIRC.

I roughed up a wiring layout - J is building this, I'm advising on certain bits

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Rather than use capacitor clamps, J decided to make a wood pc to clamp the PSU filter caps to the side.
Caps nothing special, Panasonic TSUP 220uf/450v (a bit over spec'd voltage but it's what I happened to have).



Also a phenolic plate was also cut to support the bridge rectifier board under the PTX.
A bit close proximity to the PTX than I would have preferred myself, but space is tight.

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Rear plate.

Getting close to the fun part - soldering

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jeremy gets credit for the electrical design too ?

i like the dovetail woodwork, reminds me of my old plinus jarrah

Very nice Owen, How was the lettering on the rear plate done, it looks like professional silk screen printing?

Does the power transformer you had made have the same specs as the PT-3, ie 2 x 150v plus 6.3v?
I would like to have one made, do you mind sharing the contact details of the person that did yours thanks.

Mike

when do we get to see the schematic?

Mar 27, 2020 13:31:27 GMT 12 michaelw said:

jeremy gets credit for the electrical design too ?

J uses his 'design' moniker for all his projects.

See - www.instagram.com/jeremyyoungdesign/?hl=en

As said, it's inspired (based on) the Bottlehead Crack circuit design, with our variations & using parts we can obtain.

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Mar 27, 2020 19:15:07 GMT 12 mijoh said:

Very nice Owen, How was the lettering on the rear plate done, it looks like professional silk screen printing?

Does the power transformer you had made have the same specs as the PT-3, ie 2 x 150v plus 6.3v?
I would like to have one made, do you mind sharing the contact details of the person that did yours thanks.

Mike

Hi mijoh - J did the engraving on his CNC cutter (after black spraying the alu plate).

The PTX - yes pretty much the same I think, 0-150v for B+ & 0-6.3 for the heaters (although I'm not exactly sure of the PT-3 spec's).

Brian Smith in Foxton is the only TX winder that I know of - see darklantern.proboards.com/thread/981/brian-smith-transformer-repairs-winding

Mar 28, 2020 9:23:02 GMT 12 sadface said:

when do we get to see the schematic?

sadface - our cct starts with the Bottlehead cct using a 12AU7 direct-coipled to a 6090, with cathode-follower, cap-coupled, OTL output.
The Bottlehead schematic is of course copyright, so we can't show it here. However, anyone interested can see it in their Crack assembly manual. We change a few things, to suit our transformer & will experiment with tube current / operating point. We will also experiment with 12AU7 biasing - LED, traditional bypassed R, etc. The PSU warrants some experimentation too, I think.

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Nice! You’ll probably notice quite a change in sound with adjusting the current through the 6080. It changes the degree of harmonic interaction from the 12au7.

AMR-iFi R&D

Mar 28, 2020 19:09:27 GMT 12 Owen Y said:

Hi mijoh - J did the engraving on his CNC cutter (after black spraying the alu plate).

The PTX - yes pretty much the same I think, 0-150v for B+ & 0-6.3 for the heaters (although I'm not exactly sure of the PT-3 spec's).

Brian Smith in Foxton is the only TX winder that I know of - see darklantern.proboards.com/thread/981/brian-smith-transformer-repairs-winding

Thanks for the info Owen, a CNC cutter is certainly a versatile tool!