Post by Owen Y on Mar 29, 2020 15:31:43 GMT 12
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. Thanks colinf - would you elaborate a little more about 'harmonic interaction' here.
|
Post by colinf on Mar 29, 2020 20:22:38 GMT 12
Sure Owen, your Sunday digest. Here goes... Two cascaded triode stages partly cancel 2nd harmonic distortion. As the grid-cathode voltage is lowered towards zero the gain of the tube increases slightly, determined by the construction of the individual tube. The control grid construction is most significant. It results in the characteristic gain curve of the tube. It results in 2nd harmonic distortion. 12au7s generate quite a bit of 2nd harmonic, along with 3rd and depending on the brand, upper harmonics as well. It’s about -35 to -40db at 1 volt input for typical setups. Other tubes like 6sn7-6fq7-6cg7-6j5 family have about -50db. 6dj8, about -55db but more upper harmonics. So as the 12au7 receives a positive-going input signal it’s gain increases slightly. As it’s anode is direct coupled to the 6080, the 6080 grid-cathode voltage is increasing which reduces its gain slightly. The amount of current running through a triode changes which area of its characteristic curve it’s running in. Changing the 6080 current while keeping the 12au7 current the same changes the degree of distortion cancellation. It also changes the amount of 3rd and upper harmonics generated. You’d think cancelling the distortion is good but as the ear isn’t very sensitive to 2nd harmonic, what you’re hearing is the change in upper harmonics. The sound you hear from the headphone is the result of all 3 harmonic distortion mechanisms interacting together- the 12au7, 6080 and the headphone itself. The headphone’s harmonic distortion changes with frequency, lessening with increasing frequency but also dependent on the damping and distortion characteristics of the diaphragm material. So there are lots of variables actually. Changing the 6080 current would change one of them. Changing the 12au7 current (while keeping the anode voltage the same) would also change it. Adjust it to sonic taste! Changing the resistors in the circuit to constant current sources allows the tube to operate over a smaller section of its curve and in so doing reducing the distortion.
AMR-iFi R&D
|
Post by Owen Y on Mar 30, 2020 21:15:42 GMT 12
Thanks again colinf - I will need to read that a few times, to see if I can grasp it a bit more. Absolute phase - unfortunately, I realise now that the TRS headph socket does not allow us to simply swap polarities, as the TRS Gnd is common at the socket. I can't immediately think of a way to invert the output - aside from a DAC input source with phase/polarity switching. After the mandatory triple checks, the thing was fired up without tubes, voltage checks done & then the final switch-on with all tubes in place. (The extra loose wires up top are for the DAC board, to be installed later. The vol knob will be painted a gunmetal grey, with matching TX cover.) (J at the work PC at home, using the MassDrop Senn HD 6XX phones - top plate & TX cover won't be done until after lockdown, back at work.) The Khadas DAC, hooked-up outboard - sounds quite good to me (mind you, only listening to Spotify 320kbps).
|
Post by colinf on Mar 30, 2020 22:20:27 GMT 12
Nice work! So satisfying seeing something you’ve made work. Re phase, that’s what I was trying to say. You could use a 4 pin balanced XLR connector on the output with the + and - swapped but you’d also need to change the headphone plug to a matching 4 pin XLR. Otherwise, if not phase inverted digitally, you could add an opamp set to invert the phase before the input of the headphone amp. But that might sound worse than just listening with the phase inverted. It would be down to opamp quality. But is J worried about all that?
AMR-iFi R&D
|
Post by Owen Y on Mar 31, 2020 10:05:43 GMT 12
Ah yes, XLR connections you've mentioned earlier, thanks.
|
Post by Owen Y on Mar 31, 2020 15:23:51 GMT 12
'Gunmetal' volume knob variation - I think I like this, rather stealth (with matching transformer cover to come)...
|
Pundit
Post by beeman on Mar 31, 2020 15:34:07 GMT 12
Very nice - I would pay for this. How does it sound?
|
Post by michaelw on Mar 31, 2020 17:08:13 GMT 12
that knob reminds me of my old rotel avr
|
Post by Owen Y on Apr 1, 2020 10:58:45 GMT 12
Very nice - I would pay for this. How does it sound? Early days yet, beeman. The OTL tube amp is suited mainly to high impedance headphones, notably the Sennheisers (& some AKGs, Beyer Dynamic too I expect). Impression so far is the Senns (MassDrop HD 6XX) benefit from the added warmth, ambient 'envelope' which adds to the already nice openness of the open-back Senns & tube sweetness, lack of harshness at the top (which the previously connected solid state JDS 02 headph amp had to a degree). Vocals & detailing, resolution are very very good. So, jury still out. I begin to hear the 'less-strong points' of the Senn 650/6XX - dynamics on the polite-ish side & some lack of 'bite' when you need it (electric guitars). Further fine tuning (on-board DAC installation, tubes choice, output caps, maybe some resistor-rolling) will aim to mitigate these. Personally, I'm still bothered by the phase inversion of this amp, but I need to familiarise with the 'soundstaging' abilities of these headphones (& headphones in general), because I know that when using loudspeakers, 'stereo-imaging' simply 'collapses' when absolute phase is not maintained . BTW it's obvious that this US$100 Khadas DAC is pretty competent.
|
Post by Owen Y on Apr 1, 2020 11:09:08 GMT 12
Small crisis last night.... While messing around Grounding the heater supply to eliminate a slight hum, the amp suddenly died! Heaters still lighting up, but no high voltage, no sound - silence. Checked input mains volts, the began to suspect the worst - the secondary HT winding had failed. But winding DCR seemed normal (no shorts, no open cct)... Eventually we found a broken solder connection between the rectifier bridge & the first filter cap. Phew, no need to dismantle & dig out the PTX to send back down to Foxton.
|
Pundit
Post by beeman on Apr 1, 2020 11:12:12 GMT 12
fair comment on the Senns - I tend to use Telefunken ECC83 or RCA 7025 to sharpen up the image a bit in my amp. Could be the GE is adding to the smoothness
|
Post by Owen Y on Apr 4, 2020 19:39:02 GMT 12
The Khadas DAC board now installed -in-chassis. A significant improvement in SQ - bass, warmth "bigger soundstage, more dynamic, better overall". I think a few things are at play here: - shorter signal path, and... - solid core 0.7mm dia, (unshielded) twin-ax wiring (vs previously 1.5m of co-ax shielded IC) - Don't laugh but I recommend those cheap, nickel-plated 'naked' RCA plugs (cheap from Jaycar etc), have always sound pretty good to me - ultra short signal path with hollow centre pin, minimal metal mass...is the only way that I can explain it. (Won't impress your friends however ) (USB-in, unused SPDIF-in, RCA audio-out.) (Alum support bracket also helps heat-shield DAC from the (green) 6080 cathode Rs which get pretty hot, dissipate 3.3 watts)
|
Post by colinf on Apr 4, 2020 21:33:50 GMT 12
Would it be even better to remove or bypass the rca sockets and solder the wires straight to the dac board?
AMR-iFi R&D
|
Post by Owen Y on Apr 5, 2020 14:06:25 GMT 12
|
Post by Owen Y on Apr 9, 2020 16:24:06 GMT 12
Transformer cover test fitting - what do you think, silver? Or gunmetal, like the volume knob? (The cosmetic top plate is not in place yet, in black, or all-gunmetal?)
|
Post by colinf on Apr 9, 2020 23:15:11 GMT 12
Black or gunmetal. Everyone has black, gunmetal would look unique!
AMR-iFi R&D
|
Post by cartridgeguyonline on Apr 10, 2020 17:23:47 GMT 12
Am probably off in my own little world again but not a fan of transformer covers. Yes they look tidier but have often found them to resonate and hum. In my old ARC D70 amp it was a real iterative process to sort out the transformer covers and getting the right balance where they wouldnt resonate. My old Leak TL12's never had transformer covers at all, neither do my current amps.
|
Post by Owen Y on Apr 10, 2020 17:44:15 GMT 12
OK, I see... thanks chaps. Jeremy's keen to dress up the PTX with something stylish & as he has access to a 5-axis CNC m/c, this is what he's come up with. I'm glad it's something different from the usual 'box'. This one won't resonate, it's cut from solid aluminium (There's some slots to help vent heat from the underside.)
|
Post by Citroen on Apr 10, 2020 19:17:03 GMT 12
Sort of Rowlandnesque in appearance...
|
Post by Owen Y on Apr 10, 2020 19:21:30 GMT 12
Yes indeed, the top of the TX cover has been lightly 'face-milled'. (eg. like a Jeff Rowland amplifier below - which actually has no 'fluting', it's just had a 'face-milling' cutter run over its flat face, J reckons.). Unfortunately, when painted, this will disappear. But I think 'gunmetal' should look good with the warmth of the wood (walnut).
|
Post by michaelw on Apr 10, 2020 19:37:51 GMT 12
i reviewed a few rowland pieces for audioenz and can confirm the faceplates are indeed flat with clever milling to create the optical illusion of fluting. ircc the finish is quite delicate too and more visually striking in silver than black.
|
Pundit
Post by beeman on Apr 10, 2020 23:34:36 GMT 12
Nah - gunmetal stealth look would work tone down the shiny
|
Post by Owen Y on Apr 11, 2020 10:50:46 GMT 12
Transformer cover received its first spray coat of 'gunmetal' (matching top plate in the background).
|
Pundit
Post by neilsan on Apr 11, 2020 11:00:33 GMT 12
Very nice. If I can butt in Owen, have you tried the Sennheiser HD 660S at all? And if so how does it compare with the 650? Thanks.
|
Post by Owen Y on Apr 11, 2020 12:03:23 GMT 12
Hi neilsan - I am not familiar with the 660S. In fact, I am a newbie to the modern Sennheisers, with only these HD 6XX (650) here to hand. However, Citroen put me on to metal571 headphone guy -
|
Post by Citroen on Apr 11, 2020 17:49:40 GMT 12
The gun metal finish is definitely the way to go.
|
Pundit
Post by neilsan on Apr 11, 2020 20:16:17 GMT 12
Thanks Owen.
|
Post by cooksferry on Apr 13, 2020 18:43:35 GMT 12
Another vote for gunmetal. Black is boring and the silver was a bit too bling.
|
Post by Owen Y on Apr 13, 2020 21:19:16 GMT 12
|
Member
Post by snewt on Apr 13, 2020 21:30:35 GMT 12
That looks the business!
|