I talked earlier about soundcard measurements, and here is some news: Pioneer Hill Software updated SpectraPLUS 5 so that it can now use ASIO.

ASIO (Audio Stream Input-Output) replaces Windows' own MME (MultiMedia Extensions) or DirectSound. As far as I understand, the beauty of ASIO is that it bypasses Windows Mixer and thus doesn't bring any issues associated with sample rate conversion or level adjustment.

The ASIO drivers that came with my E-MU 0204 crash every time (on Windows 7 and WIndows 8). Thankfully, there is ASIO4ALL, which works well for me.

First picture shows two spectra taken with SpectraPLUS: red is the E-MU 0204 output fed directly into E-MU's input; and blue is the output of the E-MU fed into Pete Millett's interface, output of the interface fed to the input of the interface, and then back to the sound card. THD+N is 0.000781% is the first case and 0.001272% in the second.

Next two screenshots are THD+N vs frequency...

Here's what I've ended up with in my SDcard DAC for eliminating the pull-up current sources within the TDA1543 - a heavily filtered 3.75V reference. For a single TDA1543 just connect the L and R 870R resistors to the output of this filter instead of the normal connection to 0V. Then the input's fed with the TL431 - I'm using 3k and 1k5 output voltage setting resistors. The first resistor and inductor on the left are my attempt to simulate the output impedance of the 431.

The electrolytics are Sancon 16V types with 0.1R ESR - the 440uF being made of two in parallel. The inductors need to have the right ESR (about 0,1ohm also) for which I had to wind my own 68uH - the ready made ones I have are too lossy Varying these parameters might result in a lumpy frequency response.

Attenuation is -70dB by 20kHz but almost flat to 1.6kHz and output impedance remains low down to DC.

Eliminating the current sources means that pin7 of TDA1543 now goes unconnected....

This board is really a fun one to tweak mainly because its so simple and relatively self-contained, so results of mods can be verified very quickly.

In stock form, the sound is fairly typical 1543, nothing unpleasant but an almost total lack of soundstage depth. On recordings with a high ratio of ambient to direct sounds (Nimbus) the acoustic cues are muddied. The flatness of presentation is one of the reasons I was so slow to accept 1543's other virtues during the course of development of my DACs. Soundstage depth is a biggie for me. There's also a lack of 'finesse' on the HF - lack of 'poise' or 'delicacy'.

I started by fixing up the grounding to enforce a star earth on the analog outputs and power supply. Then went on to filter the DAC's supply regulator (7805) with series ferrite beads and additional lytics and ceramics. It later transpired that the beads were being shorted by a parallel track on the other side of the board. I installed filtering prior to...

Its a TDA1543 SDcard player. Not hooked up yet to my amp but looks to be a really simple and quick way to get analog-like digital sound on the cheap.... The digital part is a dsPIC, it would have been better if it were an ARM (much lower power) but this looks to be a good start. Can't argue with a price of 150rmb, considerably cheaper than the QA550 and it does have an I2S output connector For those who prefer to do their own DAC, the vendor also looked to be selling the pre-programmed dsPIC for 40rmb.

Update - I tried powering this up and no sound . There's most likely a problem with the format or type of SDcard I'm using. There's no I2S signal being sent to the DAC which suggests the dsPIC hasn't recognised what's on the card. But there is activity on the card pins so its definitely attempting to read it.

Update 2 - had a chat on QQ with the designer/seller and he mentioned that the .wav files must be placed into directories and I was trying with just a...

My first completed build. I used an Alps in a box for this one before I finished my awesome B1.

It's a tight fit, and I get some noise from the toroid, but it's hardly audible and not at all when the music is playing.

Takman Resistors, Toroidy transformer, DH Labs and Kimber hookup, Connex RCA's. Its not the most powerfull amp, but it drives my PSB Image B6 just fine and then some. The most awesome sound I have ever had in my own house, and I actually don't miss a thing. I would put it in a bigger chassis and perhaps get some copper to further shield the toroid instead of building another amp.

But I have pcb's for a Zen v9, so that's it

Now the digital input section is completed!





Wave I/O for USB input --> SPDIF/AES input board --> FIFO --> Isolator board -->
Dual Clock board with Crystek CCHD-957 clocks (powerd exclusive from a A123 Systems LiFePo4 cell) -->
Ian´s I2S to PCM V2.0 Module --> Red Baron



Because i use two Red Baron´s parallel, i added two additional U-FL connectors for DR and DL.

Aug. 27th, 2013 edit:
Tapping the output transformer as a balanced secondary is totally goofy as was determined by Osvaldo de Banfield and Cnpope (Chris) here:
https://www.diyaudio.com/forums/tubes...madness-2.html

I'll leave it posted here just to show that I'll try just about anything and to show that it's unwise.

Aug. 7, 2013 edit:
I was just looking for the Hawksford and Mills current amp paper on google and I found this very blog post so I thought I would point out a potential drawback to the following feedback technique. Hawksford and Mills claim that using a sense resistor to the ground return from the speaker introduces a distortion similar to "interface distortion" in voltage amplifiers.

There is very little I could find about interface distortion.

As far as I can see it, this feedback scheme may introduce back emf from the speaker and feed it back to the amp...

Continued after some listening and more tinkering from here:
https://www.diyaudio.com/forums/blogs...e-big-amp.html

It seemed lacking in the high end so I added a bit more capacitance to the bypass cap on the cathode resistor on V1.

This picked the high end up nicely. I then hooked the speaker up to the 16 ohm tap and used the 8 ohm tap as the feedback center tap. This resulted in a bit more output and I like the sound just fine.

The volume seems to increase after it's turned on a while. Also, it needs a B+ fuse in the event that V1 doesn't draw then V2 will have no bias and will draw tons of current and then will dump tons of current into it's cathode resistor.

The schematic posted here is updated with voltages and values done to the best of my ability and it sounds very nice. It's not a finished design but great for experimenting.

I will be setting this aside as my curiosity is leading...

I've spent many hours recently poring over webpages and catalogues of inductors, looking for the best bang-for-the-buck in relation to anti-imaging filter inductors.

The gold standard, where performance is uppermost and cost and size are subsidiary, looks to be still the gapped ferrite pot cores I started out using. Their disadvantages are they need to be custom wound, not being available off-the-shelf, and their consequent higher cost. They also produce relatively bulky filters when a dozen or more are called for in a design. I've found nothing to beat them in performance, particularly accuracy and repeatability of their value and in terms of achievable Q (I've seen 500-plus in some cases on my meter).

I do hanker after a more portable and easier to replicate design though hence my investigations into cheaper alternatives.

Right down the bottom of the pile are the bobbin-wound coils, available in a wide variety of values here, at stunningly affordable...

Lots going on, my making things mojo has been busy. I have been reading 6 moons, and was interested by the idea of the directly heated triode preamp which sounds quite well apparently, and also repelled by the thought of using a huge output tube, slightly. So after some thought, I realised that the old battery radios used the directly heated ones, so a few weeks later some 3A5s were winging them selves to me.

Oh, (sigh) I had spent some time tweaking it first (the not passive pre) the most exciting feature was that the copied guru circuit pulled the output cathode to the HT rail when the heater is turned off! (I have big reservoir caps and small bleed). To be fair guru circuit is direct coupled so things are all over the place, but Anyway.

I decided to graft in the 3A5 in to this circuit, which was simple in principle, but ended up with a complete redesign of the power supply several times, as the addition of that one little tube hugely increased the heat generated...