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D1080 MkII 08 actives - under the hood

Posted 14th December 2010 at 11:46 AM by abraxalito
Updated 5th March 2012 at 02:55 AM by abraxalito

I've been spending my free time these past couple of days reverse engineering these speakers, as a prelude to devising the mods. The first stage in doing that is to photograph the PCBs both sides and have both images up on the screen at once, the underside is flipped horizontally so its as though I'm looking through the board. I then annotate the components by hand with the mouse drawing tool. Sometimes I'll write the values in this way too, other times just paste text for the value next to the component. Most of the time though I'm too lazy to add any annotation.

Here's the result for the volume/tone control PCB - one of two PCBs in the design. There are two quad opamps, TL084s - they're marked up in magenta. Interestingly there's no tone control circuitry as such, the bass and treble controls just act as volume to the woofer and tweeter respectively! I've never seen tone controls implemented in this way before...
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How low can you go in active speakers?

Posted 11th December 2010 at 11:54 PM by abraxalito
Updated 5th March 2012 at 02:57 AM by abraxalito

On one of my regular pilgrimages to the computer plaza a couple of days ago, I noticed these little puppies. The price apparently was a shade over $80 the pair so I was indeed very curious. A quick peek around the back showed they were not using a standard speaker cable, rather a custom 4-way umbilical between the two boxes. Whenever I see this, my heart misses a beat because what floats my boat is genuine active speakers, not just powered ones which still sport passive crossovers.

Click the image to open in full size.

Not wishing to ask to disassemble the powered unit on the spot (and doubtless be turned down) I went back home to do some online research. I was pleased to discover these were indeed true actives, and what's more they're powered by TDA8947 chip amps. I'd not come across this part before but its cool at least on paper because its 4 channels in one package - they're using two...
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Old

New scope - update

Posted 25th November 2010 at 01:36 AM by abraxalito

The Tekway is still working well and survived the 35oC temperatures of Hangzhou's summer despite having no internal fan. That is, apart from its CMOS store of the date/time, which for some reason now thinks the year is 2011.

Someone on the EEVBlog just posted this - I haven't read it thoroughly yet but I intend to find out if I can upgrade to 200MHz. Looks promising

Upgrading DST1102B to 200MHz bandwidth
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More transparency in active speakers - part II

Posted 15th November 2010 at 12:59 PM by abraxalito
Updated 5th March 2012 at 02:58 AM by abraxalito

Its not something I claim originality for, this use of Cat6 cable for running to speakers. What though I believe is original is combining the function of speaker cable with the sharing resistors - I've seen that nowhere else. Remember guys, you saw it here first

Not only does this save on having low value fairly high power resistors but it comes with another benefit which suits diyers. That's the fact that the amps can be easily tested separately - they're not going to be in parallel until the drive unit is connected to the output, since the cables join together only at the far end, at the terminals of the driver. I've certainly found this an advantage in development - the individual gains I've checked with a multimeter between the outputs when playing a 400Hz sinewave. This frequency was chosen because I'm too lazy to check the frequency response of my multimeter and I wanted a mid-range test.

The gains of the individual chip amps do need matching very...
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More transparency in active speakers

Posted 13th November 2010 at 03:23 AM by abraxalito
Updated 5th March 2012 at 03:05 AM by abraxalito

Those of you who read my earlier posts about improving my Fayou actives might have realised by now I'm a bit fanatical about grounding and its effect on sound quality. I had an idea - could a system be put together without any ground at all, and might this solve many of the sound quality issues I've found in my setup? I could only find out by trying it. So, the idea was born - 'Goundless Sound'.

First up, a groundless amplifier is just another name for a balanced, or bridged design, at least in respect of the output being balanced. A truly balanced amp would also need a balanced input - that's not really ground-breaking (pardon the pun, could not resist) either. Putting the two together is hardly rocket science. But I decided on a few twists - more about those in just a moment.

Convention is a useful thing. Ponder a world without convention in respect of computer keyboards - Asus and Toshiba having one layout of the keys on their notebooks, Dell and Lenovo...
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Old

How low can you go? - part II

Posted 31st August 2010 at 03:34 AM by abraxalito
Updated 1st September 2010 at 04:23 AM by abraxalito

Here are the schematics - the original output stage and the modified one.

On the first player I bought, I did all those mods in one hit. The third pic shows what they looked like - I took out some of the redundant connectors (guessing the same board is used in players which have more features like component video outputs) to make space for the electrolytics. Having replaced the board (just two screws, one on the rear and another in the base), then I listened. The improvement in bass performance brought about by increasing the value of the coupling caps was noticeable in listening - the cello became more full-bodied - and also on a frequency response plot I made using Audacity. I felt (but can't quite describe) that the soundstaging has improved - music feels more interesting to listen to somehow. I week or so later, I went out and bought a second one (at 99RMB, who's counting?) to remind myself of the original sound. On listening for the first time to a CD I'd just bought...
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How low can you go?

Posted 26th July 2010 at 06:46 AM by abraxalito
Updated 31st August 2010 at 04:28 AM by abraxalito

The prices of DVD players just keep on going down here. Yesterday I bought my very first 99RMB [that's under $15] model in Carrefour. I couldn't wait to get home to see how it sounded.

First up I listened to one of my favourite BIS recordings - Mozart 40 in a version for piano, flute and strings. The sound really wasn't as bad as I'd expected - the cello sounded really 'reedy' rather than full-bodied and the flute was akin to a sopranino recorder in its higher register with very little bloom. But there was a fair approximation to a soundstage and really quite decent dynamics, not at all 'bitstream'.

Rebecca Pidgeon's 'Spanish Harlem' is the test disc I use to check for sibilance (it has lots of 's' sounds), and sure enough there was some but definitely not terrible considering the price.

This undoubtedly had potential for the price - I wondered if it could be improved. Inside almost all the functions are integrated on the MediaTek chip (although it...
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HEC part II

Posted 17th June 2010 at 05:07 AM by abraxalito
Updated 8th September 2011 at 03:04 AM by abraxalito

Here's a pic of the underside of the DAC before I started hacking tracks. Bottom left area is where the digital circuits are, with ground plane. Top left are the analogue output filters. Right is the caps, rectifiers and regs of the power supplies - digital at the bottom and analogue at the top.

The AD1955 sits about the centre of this pic, and if you look closely you'll notice two small holes I drilled in the groundplane either side of it where I will later install the 10uF decouplers.

The second pic shows the component side, prior to any hacks - the 'virgin board' if you like, although sullied by the two small holes near the DAC chip. The analog supply is pin15, that's at top left.

Third, here's the meat of the digital circuitry and I've already added one decoupler to the AVDD (analogue positive supply). To the right of the 24.576 XTAL (not required) is the DIR9001 SPDIF receiver. The DIL chip is a multiplexer allowing both USB and SPDIF input...
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Towards a better DAC - introducing the HEC

Posted 14th June 2010 at 04:16 AM by abraxalito
Updated 8th September 2011 at 03:05 AM by abraxalito

Doug Self's comprehensive work on power amplifiers has been an inspiration to me over a number of years. In it, he coined the notion of the 'blameless amp' as one which was beyond reproach, measurements-wise. So I pondered over running with the notion of 'blameless DAC' along the same lines. Initially this sounded attractive, even though not entirely original. Google found a reference to it already here on diyaudio but could not show me the actual posting.

I decided on reflection that the 'blameless DAC' would be not such a good idea. That's partly because for me, measurements are not the goal of audio design. Enjoyable sound is what does it for me, not vanishingly low THD figures. So a 'blameless DAC' could only be blameless if measurements are God, so to speak. For someone like me for whom auditory experience is all, I decided I'd need another term for my DAC project.

I've settled instead on the term 'highly evolved converter' or HEC for short. This recognises...
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Old

New scope...

Posted 2nd June 2010 at 04:59 AM by abraxalito
Updated 5th March 2012 at 02:52 AM by abraxalito

Click the image to open in full size.

Bought this (well not quite this one, mine has Chinese text on the panel) little puppy yesterday as my old UNI-T was just getting too frustrating to use. In particular its triggering ability is almost useless, giving way too much jitter. That added to the trig level encoder being majorly unreliable (its just over a year old) and lots of apparently self-induced noise on the trace meant I decided to bite the bullet. And boy I'm glad I did - its like getting a new pair of HD800s to replace my aged Sony CD3000s, everything is much clearer.... (and no, I haven't made that particular upgrade yet).

About six months ago I first noticed the younger sibling of this scope in a shop window in the local electronics market. I was blown away - a widescreen scope? That screen was drop dead gorgeous! This pic really doesn't do it justice so hopefully I'll post up one of my own pics soon.

Having been immediately...
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