[triod750]

Most music takes place below the top actually, so the quality there is of utmost importance.

 

[Zachik]

Most music takes place below the top actually, so the quality there is of utmost importance.

I only care about 250 Hz and below
(just kidding! or am I...)

 

[L0rdGwyn]

I had a listen to the DAC in my main system with the 6E6P-DR in place, I think it sounds pretty darn good, really a step up in terms of clarity and imaging. I would be surprised if I went back to the 12B4A, but you never know! It does have a nice euphonic tonality I think.

Gotta work in the AM, so DIY back on hold, to be continued...

 

[Monsterzero]

From my experience, often I preferred a worse-measuring piece of gear compared to a better measuring alternative.
I mean isn't that what tubes is all about?? Solid state measures often better, but is not as much fun or as musical.

When distortion is reduced to near nil 2nd order harmonics are also removed, giving excellent measuring gear a sterile sound.
2nd order harmonics is what tubes inject into our music that we all love and adore.

I picked up a new solid state amp that measures 0.1 distortion, which on paper hasnt been seen in a solid state amp since the 1970s. The damn thing sounds quite tube like, even though on paper it should sound like garbage.

Probably why I enjoy vintage receivers so much as the 2nd order harmonics havent been stripped away like modern gear has.

 

[Tjj226 Angel]

When distortion is reduced to near nil 2nd order harmonics are also removed, giving excellent measuring gear a sterile sound.
2nd order harmonics is what tubes inject into our music that we all love and adore.

I picked up a new solid state amp that measures 0.1 distortion, which on paper hasnt been seen in a solid state amp since the 1970s. The damn thing sounds quite tube like, even though on paper it should sound like garbage.

Probably why I enjoy vintage receivers so much as the 2nd order harmonics havent been stripped away like modern gear has.

Maybe yes, maybe no. It all depends.

The issue here is that ALL forms of distortion is actually bad. 2nd order just so happens to be the least bad sounding distortion.

The catch is, when you try to fix second order distortion, you generally end up creating more higher order distortion, phase angle distortion, IM distortion, or a whole host of other problems.

There are ways of trying to reduce 2nd order harmonic distortion in a more natural way. Sakuma style amplifiers are one such example.

 

[L0rdGwyn]

Here is an FFT of the right channel of the PCM56 DAC with the 6E6P-DR in place, -6dB undithered 1kHz sine wave, THD 0.014%.



Noise floor is generally below -100dB. The 60Hz and 120Hz noise should improve in the final build with proper grounding and DC regulator heating the output tubes, right now they are on AC. Given the PSRR of both the DAC and B+ supplies, the 120Hz is not ripple, more likely coupling from 120Hz current pulses through transformer-rectifier-reservoir cap loop. To say the layout on the protoboard is not ideal would be a huge understatement. It is also very easy to pick up 60Hz noise and its harmonics from your own home at these levels. The "quietest" room in my house is upstairs, I didn't feel like dragging the prototype up there just for these measurements.

The degree of THD varies in the four 6E6P-DR I have on hand, namely the degree of H2, as low as 0.0095% in the best one.

Getting a feel for how this DAC is going to be laid out - I think I was very optimistic in how compact I thought I could make it. It is more likely going to be a full sized component, probably around 4.25" x 11" x 16". Things start to get very crowded when you put everything on the inside.

 

[Galapac]

Hey @L0rdGwyn, how did that old radio restore turn out?
I remember you getting one awhile back but not sure if you ever posted any pics of it.

 

[L0rdGwyn]

Hey @L0rdGwyn, how did that old radio restore turn out?
I remember you getting one awhile back but not sure if you ever posted any pics of it.

It is on the shelf! Literally. I finished re-capping / re-resistoring it and it is functional, I also finished cleaning up the chassis.



The last bit of work is the alignment - when I was adjusting one of the RF transformers, the adjustment screw crumbled into pieces. So now I have to transplant the same transformer from the spare radio I bought for parts, then work on the alignment. I hit that road bump around the time of my 801A build I just haven't gotten back to it.

I don't have the ideal equipment to do a proper IF / RF alignment, haven't decided what approach I am going to take just yet. There is a guy in Cleveland who restores vintage audio gear, including radios, so I am considering having him do the alignment after I replace the transformer to save me the time and trouble.

 

[L0rdGwyn]

After two recovery days, had the energy to work on some DIY.

I changed the configuration of the output of the DAC - rather than taking the output from the plate of the 6E6P-DR, it was taken from the source of the lower CCS device with the aim of lowering the output impedance. That was accomplished, along with other performance benefits.

Output impedance was reduced from 1150ohm to 275ohm.

Below is an FFT of the right channel again, 1kHz -6dB undithered sine wave.



THD is the same with slightly higher gain, now with a higher H2 and no H3. Higher order harmonics are now visible since the noise floor is reduced from -110dB to -125dB, all are below -100dB.

I took another sweep of the left channel for comparison, the high frequency rolloff seen before is gone, -0.3dB at 20kHz relative to midband.



Now a flat response from this NOS DAC, not bad. I wondered if the HF rolloff was a measurement artifact as a result of the higher output impedance prior to the change, but taking the same measurement with the Linear Audio Autoranger (which is essentially a buffer for taking soundcard measurements) resulted in the same HF attenuation.

I believe the sound has benefitted from the change as well, it sounds great!

Pretty far along into the layout now, heater regulator PCB for the 6E6P-DR is designed, getting close to being able to order the chassis.

 

[Tjj226 Angel]

Those higher order harmonics are interesting. Theoretically everything beyond your 3rd order harmonics should be virtually unmeasurable. Getting rid of 7th order harmonics in particular will make a massive difference in sound quality / listener fatigue.

 

[L0rdGwyn]

Here's a peek at the completed DC heater regulator PCB. Getting a well-regulated 6.3VDC from a 6.3VAC winding is a challenge, as you can see it uses both 6.3VAC windings off the mains transformer. It will supply the power LED as well. The mains transformer for the tube output stage is rated for 115VAC which helps. This regulator is essentially ripple free, overkill really, could heat a DHT off of it! It will maintain regulation down to about 106VAC out of the wall, so no issues with dropout.

Here is where I am headed with the chassis, subject to change of course. About as minimalist as it gets - all of the components will be mounted on the bottom panel, so no hardware will be visible besides the screws for the top plate. Dimensions are 4.25" x 11" x 17", it will be in brushed, silver, anodized aluminum with silver feet. Just a silver power button and low luminosity white power LED on the front panel. IEC inlet, USB in, and RCA out on the back panel. That's it!



There are some parts I have on order that I would like to inspect for dimensions in person, so that is going to delay things somewhat, but better safe than sorry.

 

[triod750]

low luminosity white power LED

Loverly - I hate blue LED.

 

[L0rdGwyn]

Loverly - I hate blue LED.

I'm with you there! Don't want it to stand out much, more for function.

Side note, I got my 30-pack of TDA1387T, teeny tiny little DACs, so darn cute.



No shortage of these to experiment with. I think I am going to change my design though - still a small form factor DAC board with SMD parts, but I think I am going to abandon the resistor I/V stage and do something more creative, going to use tube active I/V. I have a functional schematic using two TDA1387T in parallel per channel with one tube per channel - line level output with a 500ohm output impedance. Will be larger needing a tube mains transformer, but still quite compact I think. No timeline on this one, just brainstorming. Gotta wrap up plans for the PCM56 DAC then build an amplifier.

 

[triod750]

'Guess how many decisions I can make in a second'. You remind me of Al Di Meola, Paco de Lucia and who was the third guy; guess how many notes I can play in a second.
They were also creative.

 

[CJG888]

John McLaughlin