Finished my 45 parafeed headphone amp today, nothing innovative, but wanted to share it, I think it turned out well. Been at this hobby since January, so this is my second from scratch design-to-build amplifier, the other being a 6A5G-based SET.
It is a MOSFET cascdoe CCS loaded 6J5 input, MOSFET cascode CCS loaded 45 output parafeed design for high-end headphones. Mains transformer is a custom job by Sowter to my spec, OPT are Sowter 8983. Filament regulators are Rod Coleman's, coupling and parafeed caps are Rike Audio S-Cap 2 (paper, poly, aluminum, oil). Power supply is a single rail CLCRC.
Here is the schematic, although it is not comprehensive. Parafeed capacitance was determined experimentally by taking FR sweeps of various capacitances (courtesy of cheap Solen caps), along with subjective listening impressions. Ultimately decided on 6.6uF made up of two 3.3uF in parallel.
Bias points:
6J5 B+: 380V
6J5 plate to cathode: 200V
6J5 cathode: 6.2V
6J5 plate current: 8.5mA
45 B+: 398V
45 plate to cathode: 206V
45 cathode: 35.5V
45 plate current: 35mA
Here are some shots of the interior. Bottom plate is perforated aluminum sheet for ventilation.
Here is a FFT of the left channel at 350mV into 300ohms.
And the frequency response.
Thanks for checking it out! Going to experiment with changing the 6J5 unbypassed cathode resistor to some stacked HLMP-6000 LEDs later today.
It is a MOSFET cascdoe CCS loaded 6J5 input, MOSFET cascode CCS loaded 45 output parafeed design for high-end headphones. Mains transformer is a custom job by Sowter to my spec, OPT are Sowter 8983. Filament regulators are Rod Coleman's, coupling and parafeed caps are Rike Audio S-Cap 2 (paper, poly, aluminum, oil). Power supply is a single rail CLCRC.
Here is the schematic, although it is not comprehensive. Parafeed capacitance was determined experimentally by taking FR sweeps of various capacitances (courtesy of cheap Solen caps), along with subjective listening impressions. Ultimately decided on 6.6uF made up of two 3.3uF in parallel.
Bias points:
6J5 B+: 380V
6J5 plate to cathode: 200V
6J5 cathode: 6.2V
6J5 plate current: 8.5mA
45 B+: 398V
45 plate to cathode: 206V
45 cathode: 35.5V
45 plate current: 35mA
Here are some shots of the interior. Bottom plate is perforated aluminum sheet for ventilation.
Here is a FFT of the left channel at 350mV into 300ohms.
And the frequency response.
Thanks for checking it out! Going to experiment with changing the 6J5 unbypassed cathode resistor to some stacked HLMP-6000 LEDs later today.
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Hi L0rdGwyn,
Very nice build indeed. Just a quick question: why do you use two CCS’s in series? What’s the advantage when measuring (spectrum/THD) and when listening?
Regards, Gerrit
Very nice build indeed. Just a quick question: why do you use two CCS’s in series? What’s the advantage when measuring (spectrum/THD) and when listening?
Regards, Gerrit
I love seeing what people make, nice! And a 45 to boot, wow. Can I ask where you sourced the spring clips holding down the black electrolytic caps? I need a bag of those.
Apologies if you are already aware of some of this, but the CCS load affords the triode a near ideal load, an "infinite" AC impedance (more on the order of 50Mohm), in parallel with the grid leak resistor of the following stage, this gives the 6J5 driver tube a near flat load line, which lowers second harmonic distortion. In addition, the CCS has exceptional PSRR. So from a listening standpoint, lower noise floor, lower distortion, better imaging, soundstage, and detail, from my listening.
Most parafeed builds use some sort of constant current source on the output tube plate, more common use is a plate choke, which allows the plate voltage to swing beyond the B+. The advantage of using a MOSFET CCS load here is higher PSRR than a plate choke and size. The disadvantage is a higher B+ requirement as this will determine the maximum plate voltage swing when the grid is at its most negative. The MOSFET CCS also has to dissipate a significant amount of heat, around 7W in my amp, which is why there are heat sinks mounted to the back of the chassis.
Thanks very much! And sure thing, they were very helpful in this build, nice to be able to mount a 35mm diameter cap with a single screw. They are pretty expensive though as far as parts go.
TH25 Cornell Dubilier - CDE | Mouser
Just … as an aside, it seems to me that having an old fashioned switch on the output for the 8 Ω resistive load might be warranted: One could easily countenance hooking up an actual real, live, 8 Ω conventional speaker to the amplifier. Nothing lost, really.
And who knows… you would get a couple of watts out, too. Just add a couple of conventional speaker connections, and the DPDT switch either switches to speaker, or to 8 Ω resistive secondary load.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
And who knows… you would get a couple of watts out, too. Just add a couple of conventional speaker connections, and the DPDT switch either switches to speaker, or to 8 Ω resistive secondary load.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
This is actually a single current source with the two devices coupled through the gate resistor.
What’s All This JFET Constant-Current Stuff, Anyhow? | Electronic Design
This was taken into consideration and is exactly the approach taken with my 6A5 amplifier.
As built, this amp can swing 200Vpp into a 5K load, limited by 0V grid current, so ~1W into 8 ohms. But the goal of the build was to make a somewhat small footprint headphone amplifier. Adding speaker binding posts would have cost chassis real estate. Ideally, to make a 2W speaker/headphone amp, I would have used a 275V bias point on the 45, which would have necessitated a >525V B+ to give the active load its necessary headroom. Could have considered a plate choke to drop the B+ requirement, but again, real estate.
So long story short, I considered it but decided to do a dedicated headphone amp that can fit on my desk at bias points where the 45 will last forever. Thanks for the input!
You are very welcome, good sir.
It surely is a choice of 'surplus of riches', I would imagine. Building a lovely steampunk dedicated piece of glowing glass magnificence implies that you very likely also have competent 'ordinary' amplifiers for your speaker system. So, I get it.
As a matter of design esthetic, I would have squeezed a SpeakOn jack back there (from Neutrik), as they are really sweet connectors; I also would have cobbled together a purpose-designed 'adapter cable' with the usual banana binding posts at one end, and the Neutrik mate at the other. And to add to the steampunk routine, I'd have mounted little beryllium-copper spring clamps on the upper back edge, and arranged for the adaptor cable to be exactly just long enough to snugly fit, taught, therein.
As I said… a matter of design esthetic, of course.
I find absolutely no fault or shortcoming at all with your choices!
Best wishes, Lord.
Keep safe the castle's keep!
Yarrr....
⋅-=≡ GoatGuy ✓ ≡=-⋅
I look forward to seeing what else you make a few Januaries from now! Nothing I scratch built in my first five years of this hobby was anywhere close to as nice as your effort here.
Thanks, GoatGuy, and duly noted! I will keep those design ideas in mind for the future. Yes, I am very happy with my speaker amplifier, it is another DIY job like the one above. Those Neutrik jacks are pretty sharp, I'll keep an eye on them...
Well thank you for the very kind compliment audiowize, and thanks for answering my questions around the forum, you have been most helpful. Hopefully I'm in a bigger house five Januaries from now, if I build many more amps in my tiny house, my girlfriend will kill me 😱 I would like to work on an 801A design next, have some ideas cooking but I will need a break.
Thanks! They are from Kevin at K&K Audio actually. Need to take the time to design my own boards here sooner or later.
Appreciate the link, they are kinda pricey. I found stainless steel fluorescent bulb clips in 5 different fluorescent tube diameters for pennies each. They stand higher but wire can pass under that space. These are probably worth a try.
(24-Pack) JESLED T8 U Clips Holder Bracket for 8FT LED Light Bulbs, LED Fluorescent Tube Replacement Mounting Accessories, Stainless Steel Lamp Support, Lampholder, Pipe Clamps to Prevent Sagging - - Amazon.com