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SAA7220 filter replacement project

Posted 20th September 2016 at 12:24 PM by abraxalito

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.

In order to kick this project off,...
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Taobao shopping for 'digital' amps

Posted 18th September 2016 at 02:45 AM by abraxalito
Updated 18th October 2016 at 07:08 AM by abraxalito

A number of interesting amps have caught my eye on Taobao of late, they (except for the last) are true digital amps in the sense that they're 'power DACs'. The first is using ST's STA333W which is a phenomenally cheap fully-integrated digital amp chip, hence the very low price of this board - ns=1&abbucket=13#detail ns=1&abbucket=13#detail USB input, speaker out but needs an external power brick of some sort, up to 20V.

The second one's a boxed product, albeit with an external power supply. Its using Intersil's D2-audio chipset (D2-41051), the first implementation I've noticed on Taobao. It uses discrete MOSFETs in the output stage. The enclosure and controls look to be in the PS Audio territory, a cut above the usual Taobao cheap stuff -
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Posted 7th July 2016 at 11:58 PM by abraxalito
Updated 8th July 2016 at 12:01 AM by abraxalito

I've not posted for a few months here as I've been involved in moving apartment which was quite a major project given the quantities of parts and assemblies I've accumulated. Even though I've been in my new place for over two weeks now, very little has been unpacked so far, but I have just yesterday rebooted my desktop active XO/amp system which had been quite literally assembled on my desktop with no casework whatsoever Its still without casework and survived the move with only a few wires falling off but now 'installed' in a drawer (pic attached). Its being fed from my 'Domino' balanced TDA1387 DAC and Taobao TFcard player and delivering bags of emotional satisfaction through stand mounted '3Nod' two-ways.

The amps are all LM4766 bridged running from 60V total but with heroic measures to keep supply noise under control, the central PSU has a CLC configuration and ferrite input transformers are used for coupling between AXO and amps. Output transformers (ferrite for...
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Transformers and crossfeed

Posted 15th April 2016 at 04:09 AM by abraxalito
Updated 25th April 2016 at 03:15 AM by abraxalito

Listening to headphones creates, on many recordings, an unnaturally wide soundstage. I decided to play with the idea of 'crossfeed' whereby some deliberate crosstalk is introduced between the two channels. With transformers its jolly easy to do - just create an extra winding with the right number of turns then put this winding in series with the main output of the opposite channel.

In the picture I've done this with some EP17 ferrite cores and added a DPDT changeover switch to A/B between the original and crossfed version. The crossfeed factor I've gone for is 25%. It works in practice in that there's a more natural presentation without any 'extreme' stereo effects (almost a kind of phasiness on some recordings). But sometimes I enjoy the 'all around' effect so switchable is the name of the game

Trafo winding details as follows - primary 500T of 0.1mm, two secondaries first 120T 0.21mm, second 40T 0.21mm.

Made a second one as a colleague liked...
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Mighty mite balanced SE classA headphone amp

Posted 5th April 2016 at 09:39 AM by abraxalito
Updated 25th April 2016 at 11:06 AM by abraxalito

I was going to call this amp 'fleapowered' but after doing some searching online I found fleapowered amps in general were much higher output power than this - single digit Watts typically. Whereas this one is more than a couple of orders of magnitude lower than that (under 10mW) so deserved a different moniker. Seeing as a mite is smaller than a flea, 'mighty mite' seemed to me as good as any descriptor for it.

The mighty mite started life in my imagination after I walked into a local headphones shop and tried out some cans that the shop assistant recommended to me - AT MSR7s. I'd brought my 'Buffalito' headphone buffer with me and some step down transformers for its output - fed from my mobile phone its way too loud for my AKGs and Superluxes. However the MSR7s were too loud even with the 2:1 step down trafo so I told the assistant I'd come back and listen again once I had a more suitable trafo. I figured it needed to be 4:1 step down, so I built a pair of those (PQ32/20)...
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Pocketable headphone transformers

Posted 24th March 2016 at 02:44 AM by abraxalito
Updated 25th March 2016 at 11:54 PM by abraxalito

Seems I got a bit carried away with listening to my heavily modded XuanZu amp in the last-to-one blog post. I even considered it was superior to my current headphones - that would mean shelling out on some new ones. Before I went shopping though I did try it into my DT880s, which are 600ohms (hence I usually only use them for special occasions) - they sounded cleaner, although considerably quieter. This amp doesn't have enough voltage swing available to deliver the SPLs into such a high impedance. So was the cleanliness of these phones due to their being higher quality (they're at least 4* the cost of the others)? Or just because of listening at a lower level?

When my over-enthuasiasm for the amp had subsided a bit I decided to consider a way to answer these questions. If the amp was indeed not producing the artifacts which I was hearing on piano into the low-Z cans, then putting a 2:1 step-down transformer on its output would make no difference at all. The amp has plenty...
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Sanwu FLAC and Bluetooth player has a significant flaw

Posted 16th March 2016 at 12:59 AM by abraxalito
Updated 17th March 2016 at 03:18 PM by abraxalito

I was going to write a post praising this player for the superb value for money (its the only cheap single-chip FLAC player I've found) but this morning it produced an alarming series of whistles and pops from what I presume is a corrupt file on my TFcard. So now its only recommended if you're sure you have perfect data on your card - it doesn't seem to mute the audio when an error is found.

Apart from this major howler at just 30rmb its great, providing as it does FLAC, WAV and mp3 support along with Bluetooth running from a USB power source at 5V. The audio performance is decent when run through my modified XuanZu headamp as preamp - the level is rather low otherwise and I suspect it needs a high-impedance buffer for best dynamics.

I put the TFcard which gave the player hiccups into my PC reader and uploaded the 'problem' file to Audacity. No glitches noticed there so looks like I might have to dig a bit deeper to find out what went wrong. I shall try playing...
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Upgrading the XuanZu portable headamp

Posted 26th February 2016 at 11:22 PM by abraxalito
Updated 14th March 2016 at 06:23 AM by abraxalito

This device is a steal on Taobao, but having had a quick listen last night it could sound clearer. When connected to my smartphone (Meizu MX4 pro) and compared side by side with my 'Buffalito' (not a blind comparison mind) into my SuperLuxes, there were a few notable deficiencies.

First the soundstage air was less apparent. Second there's some sibilance noticeable on voices. And third the background hiss is slightly more apparent and a slight whine comes from the power supply. So I figured - open her up.....

Inside its fairly simple, the more or less standard configuration of a pot, then opamp gain stage then discrete diamond buffer. Which is great because I already have experience with this topology. The power supply is a built in LiIon cell with a boost converter supplying 12V in a single rail and there's a passive rail splitter. The dual opamp is an EL2244, one I've not seen before in such a setup.

First up - the input pot is too low a value at...
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An LC filter for supplying a line stage

Posted 19th February 2016 at 12:33 AM by abraxalito
Updated 22nd February 2016 at 12:51 AM by abraxalito

I've designed LC filters for classAB amp power supplies before - for those applications iron powder toroids work fine for the Ls. However for the power supply in my latest DAC design I wanted more supply rejection and this calls for higher value inductors - in the tens of mH. Creating a 10mH inductor on a toroid takes way too long and is hugely fiddly as the wire length needed is substantial and I don't have any specialized winding machine. So toroids are really out of the question at such values.

I have some largeish inductors in the right range wound on bobbin cores but when I checked the DCR it was a little high, 20ohms or so. As I might need up to 100mA, a 2V drop is too great. In any case, in LTspice this resulted in rather an overdamped response - what I really needed is something in the range 1 to 2 ohms. The solution seemed to be use ferrite cores of the kind normally used to make transformers. Which means breaking a kind of informal rule I made for myself about not...
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Highly affordable digital audio CPU board

Posted 8th January 2016 at 03:02 AM by abraxalito

STM's lowest cost ARM Cortex M4 CPU is now available on a very low cost evaluation board here -

Its quite a good fit for audio purposes because there are a couple of I2S interfaces with a dedicated audio PLL. There's plenty of RAM (64k) and according to the benchmarks, it'll do the equivalent of around 100MIPs. Its also respectably low power when running out of RAM - use the flash memory accelerator though and its not quite so frugal.
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