Posted 31st October 2016 at 08:42 AM byabraxalito Updated 1st December 2016 at 03:23 AM byabraxalito
The Intersil D2 chipset based amp arrived and while it sucked pretty badly on speakers I had a hunch that like the STA333BW boards I've been playing with recently, it would acquit itself admirably on headphones, given the much lower demands on the supply.
The mods I've done to this are fairly straightforward - changing the output filter to work more optimally with headphones (via a 7:1 step-down transformer) and tweaking up the power supply a bit.
To run with cans I went for an output load impedance of 1100ohms. This was determined primarily by the choice of inductor (MSS1210 8.2mH from Coilcraft) which in turn was chosen so that the carrier frequency would be below the SRF of the coil. This choice is fairly arbitrary in that the inductor would still work fine above its SRF just it's starting to look like a capacitor. However it turned out that with 8.2mH the cap value required was fairly simple, 5nF so I stuck two 10nF NP0s in series.
Posted 28th October 2016 at 03:19 AM byabraxalito Updated 4th November 2016 at 03:03 AM byabraxalito
I've been mighty impressed with the STA333BW board (just 62rmb on Taobao, see an earlier post for the link) driving headphones via a transformer (ferrite cored step down 7:1, I've tried various sizes). So much so that I don't much feel the need to upgrade my headphones beyond my Superluxes (HD668B). The sound I'm getting is immensely satisfying - rich (which I got from my TDA1387-based DACs, but not from my phone with its ES9018K2M, that doesn't qualify as rich in tonality) and dynamic beyond anything I've so far heard. I noticed in a recent thread post by TNT (here - http://www.diyaudio.com/forums/vendors-bazaar/283672-new-audio-op-amp-opa1622-6.html#post4868193) he says he doesn't much like listening on headphones due to the 'in-head' quality of the sound. Yeah I used to get that, transformers pretty much get rid of it giving an out-of-the-head-all-around-holographic kind of experience, along with stupendous dynamics. I like listening to classical piano and that's very demanding...
Posted 20th September 2016 at 12:24 PM byabraxalito
Some years ago I reverse engineered the coefficients of the Philips SAA7220 filter, just for fun. At that time the microcontroller I implemented the filter on (LPC1114) didn't have a fully implemented I2S interface so I needed additional logic to get the data in (an array of 74HC595s) and I used the SSP to output a serial stream, albeit not reliably as I didn't implement a foolproof method of generating WS. Hence the filter wasn't ready for primetime.
Now though STM has introduced a Cortex M0 MCU which does have the dedicated I2S ports to realize a pukka emulation of the SAA7220. At least that's what I'm hoping - I know the code worked on the LPC1114 but I seem to recall I needed a slight overclock - there is a report that the STM32F072 can be overclocked but I shall need to check this out for myself. One major advantage this replacement will have is going to be lower power consumption - something of the order of 100mW or under.
The Schitt Yggy DAC has already created something of a stir over on Head-Fi amongst those who've heard pre-production prototypes. Its of interest not just because of the pre-launch buzz but also because the designer (Mike Moffat) is one of the long-time seasoned guys in the field. He says this is the best practical DAC he knows how to build. And its priced ISTM very reasonably given the amount of tech it embodies ($2300) - the DAC chips come from ADI and are $64 a piece on the manu's website. He's using 4 of them but says he had to address the glitching of the DAC without using a sample-hold which sounds like ***.
The chip is the AD5791BRUZ which ADI designed for industrial/scientific/medical applications rather than for audio. Datasheet attached.
Having looked over the DS what strikes me as interesting is that this is a 20bit DAC (1ppm) yet the 1kHz THD performance (p4) isn't anything to write home about (-97dB) and that figure is given at a very low sample rate...
Thanks to Matthieu (Malefoda) for the heads up on this. It looks distinctly like a DAC inspired by the DAC-AH - note not only the 8 DAC chips but also the PCB layout closely follows that design's. The price on Taobao is about half the AH (368rmb) so I shall be ordering up one or more to have a play. Matthieu found it on another website with prices in USD, here - http://www.cart100.com/Product/42124644877/.
Note there are two options here (blue, black) - the cheaper one is a kit of parts I believe, the more expensive ($84) the finished unit. Going on the translation of the Taobao page, the kit does not include the transformers, mains switch or the case.
Update : here's the summary of the mods so far, for details go to Malefoda's thread linked in the comments.
1) Re-route the output ground so the opamp filtering caps aren't subject to injected CM noise from the destination component (amp or pre).
2) Reduce the supply voltage to the output opamps...
Posted 12th January 2015 at 01:57 AM byabraxalito Updated 12th January 2015 at 02:07 AM byabraxalito
Last night the battery of my I2S source ran out so while it was on charge, I unboxed my last remaining Lite DAC-AH to have a listen to that, fed from my (mains powered) QA550 wav player. I'd bought 3 DAC-AHs a few years back when I was playing with them.
The differences between the stock DAC-AH and the Ozone Pagoda were apparent in the areas of soundstage depth, dynamics (especially at LF) and coarseness at higher levels. So this got me wondering what would be the lowest hanging fruit in terms of mods to it. My procedure before had been to implement everything I could think of before having a listen - this way its not possible to know which mods are the most effective ones and which can be omitted.
This time I'm going to be a bit more disciplined for the purposes of discovering which mods deliver the best bang for the buck. The first one I'm going to try is a passive LC filter between the DACs and the AD847s. Here's the schematic for it - its using the cheap and...
A single TDA1387 feeds a 3 inductor quasi-elliptic filter followed by AD8017s as buffer-amps. The large ferrite cores in the base do the bal-SE conversion. The power supply is 4 * AA NiMH cells which should in theory last for a whole day's music.
The design is really a 'MkII' version of the Ozone portable where the AD815 buffers have been replaced to allow a more compact construction and lower power draw without the constraint of being able to drive IEMs directly. Whereas the portable used a stack of 1387s due to the choice of 7mm TDK inductors, this one's using pot-core chokes giving a much higher working impedance and hence higher output levels from just the one DAC chip. The desktop footprint is about that of a CD.
Update : I found some bargain Jamicon caps on Taobao which give the tower more elegant proportions, as well as improving the supply impedance to the buffer-amps and giving more breathing...
Posted 15th November 2014 at 02:51 AM byabraxalito Updated 19th November 2014 at 05:43 AM byabraxalito
I'll outline here some thinking in choosing the major building blocks (aka ICs) for this card - any comments welcome as this progresses.
First up the DAC chip will be the TDA1387 initially. I don't know for sure that the output from the ARM/Xilinx card is I2S but I'm going to verify that fairly soon. There's nowhere near enough room for the passive shunt I've adopted previously so the bass performance probably is going to have to suffer. I shall pay considerable attention to the power supply arrangements though in an attempt to make up for the LF lack.
After the DAC, passive I/V will follow and then a filter using the TDK 7mm inductors I've used previously. I've slotted them into the gap between the PCB and the case and there's just enough height available. Since space is at a premium I'll experiment with a 3 inductor design - the stop band attenuation will suffer but probably I'll add a secondary LC filter at the output to make up for that somewhat. The secondary...
Here's the next experiment - a higher working impedance anti-imaging filter which allows operation without any active voltage gain stage following it. Its also one you can build with Mouser parts - Fastron make inductors suitable for this - substitute their 27mH for the 30mH for only a modest degradation in the FR. Or add a Panasonic 2.7mH in series with the 27mH if you'd like to go the whole hog.
The frequency response is -3dB at 18.5kHz and about -55dB by 24.1kHz. Passband ripple is <0.3dB.
Update - after winding all the coils I realized that I don't have a system right now to slot a full bandwidth DAC into - mine at present is fully activated. So to test out the buffer design's audible qualities I need to build a limited bandwidth DAC (for my bass/mid, up to 3.5kHz). Hence another version of the bass/mid LPF is called for, with the highest possible working impedance. Turns out I can wind a 125mH coil with wire which doesn't break too easily (0.13mm dia)...