Having disassembled the amp board from its backplate once and got heatsink compound in various undesirable places, I was hoping that the first round of mods would also be the last. No such luck

However, first listening was extremely promising, so I was definitely up for a second round of extreme messiness to gild the lily. I did notice that my grounding changes had introduced a slight hum (50Hz fundamental only it seems) which was only audible when no music was playing and was independent of volume setting. The main gripe I had was that the stereo image was shifted over to one side - this I decided was because the volume pot was being used towards the lower end of its range where the matching is poorest. The chip amps have rather high gain (32dB in bridged) and this can't be reduced. So the solution had to be modding the resistive dividers between the XO and the amp chips - in effect reducing the power amp sensitivity. I went for about a 9dB reduction, determined by the...

Here's the schematic showing the component value tweaks I made to the tone control board. I'll list the changes in detail - only the first three are depicted in the schematic. The remainder aren't as they don't show up on the sim plots

1) The original input stage has attenuation followed by gain. I have removed both, its now a unity gain stage and also no longer rolls off the HF response.

2) The low-end high-pass filter originally had a rather too high Q for my taste. It's been tamed.

3) The bass-mid low-pass filter was rather too low Q, so I've brought this up, making the response flatter across the band - now the hand over to the tweeter is tighter. The tweeter high-pass is left unchanged.

4) The decoupling regime has been changed so as not to contaminate the local ground. Thus the original 100nF ceramics to ground from either supply rail have been moved to decouple between the positive and negative rails. Ground is thereby...

When considering how to hot rod a particular piece of electronics, my first attention always goes to power supplies and layout rather than the somewhat more popular approach of swapping out components for boutique variants. This is because so far I'm not sure that I notice the differences between audiophile grade caps (for example) and the bog standard ones. But I am sure of the differences brought about by improved grounding - to my ears these aren't subtle changes at all. So if I turn out to be dissatisfied with the sound of my layout mods, then I'll turn to tweaks on individual parts.

On examining the layouts of the two PCBs (XO layout already shown in a prior post) it turns out the amp PCB is the one with the relevant power supply components. In more detail, it has two independent supplies coming from separate windings on a standard EI-core transformer. The higher current one is unipolar, unregulated and feeding only the TDA8947s, the small-signal supply is bipolar,...

Here's a somewhat simplified schematic I entered to model the response curves in LTSpice. The volume control isn't shown (as LTSpice doesn't have the symbol) but happens before the first opamp. I've fudged up symbols for the bass and treble controls - they're simply variable resistance in the HF and LF feeds to the power amp board. LT1057 isn't actually used in this unit, its just the nearest library part to the TL084

<edit> I've added the response curves now - look and weep

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...

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.



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...

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

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...

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...

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...