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New active speaker on order from Taobao

Posted 26th September 2014 at 02:49 AM by abraxalito
Updated 29th September 2014 at 02:48 AM by abraxalito

This one has dual TDA7265 chipamps and a nice 60VA toroidal trafo which should give better regulation than the normal EI type. Four electrolytics for the main PSU is encouraging. The opamps are socketed so opamp rolling is on the cards...

Update - received the speakers now. A quick listen showed the typical lack of dynamics opamp sound. After all they're only NJM4558s in there. So I shall re-jig the XO for TL082s by scaling up all the impedances and biassing the opamps into classA. Already all the through-hole caps have come out ready to be replaced by SMT types. The topology tends to suggest this may well be a clone of the D1010 which I'm already familiar with.

Update2 - modded the XO board but so far left the amp board 'as stock'. Mods are 11k resistors to VEE for classA bias, opamps swapped to TL082s, TL431 shunts installed to give +/-5V (from the regulated +/- 12V), plenty of 3,300uF caps across the supplies, 220uH inductors filtering from the shunts, impedances...
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Old

Revisiting active speakers - HiVi D1010

Posted 21st September 2014 at 12:07 AM by abraxalito
Updated 24th September 2014 at 02:51 AM by abraxalito

Back in 2010 I spent many hours tweaking the D1080s with very satisfying results. However now with my Ozone variant DACs I have a far more transparent source than I did then and would like some demo kit which does the DAC justice whilst being fairly compact, portable and not too complex to mod. In 2011 a new smaller and even cheaper model arrived - the D1010 which has undergone some very minor modifications and is now in its mark IV incarnation. Like its older and bigger brother, its also a true active - it uses LM1875 clone ICs for the 100mm bass/mid and a tiny IC amp with a clip-on heatsink powered from a separately regulated supply for the 20mm tweeter. A fairly decent foundation for some extensive hot rodding.

On first connecting a pair of these up to the original portable Ozone (that's the one in the tea canister) I was impressed by the LF soundstage bloom that was reproduced but less than overwhelmed by what happened to the HF. Ragged would be a fair approximation, rather...
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Old

I found some more suitable EDLCs

Posted 25th August 2014 at 06:09 AM by abraxalito
Updated 26th August 2014 at 07:52 AM by abraxalito

Many hours can be whiled away trawling through stuff on Taobao - most recently I've been browsing the selection of electrical double layer caps (aka EDLCs, supercaps).

I looked at supercaps many years ago when they first showed up in the Farnell catalogue, initially they were designed for back-up memory purposes and always had very high ESRs so weren't much good for power supplies where any kind of current is needed. That seems to have changed and now the caps are finding their way into all kinds of energy storage applications where high pulse currents are required. ESRs have dropped to where they're attractive for audio applications which call for a really stiff power supply down to low frequencies.

The current crop (in the pic) are made by Samwha (a Korean firm) and are really nice and affordable here - 4.5rmb a piece (about $0.70). That's for a 2.7V 100F size which comes in a 22mm diameter can, 45mm tall. A 7cap hexacap packs about the same energy and roughly...
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Old

DAC filter for discrete buffer

Posted 14th August 2014 at 11:44 PM by abraxalito
Updated 22nd August 2014 at 03:56 AM by abraxalito

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)...
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Old

Hexasupercap

Posted 30th July 2014 at 09:06 AM by abraxalito
Updated 9th August 2014 at 12:30 AM by abraxalito

Here's a hexacap built with 10F caps, so the total capacitance is 610F. The individual caps have ESRs varying from around 30mohms to 60mhoms - I weeded out those above 60mohms with the aim of getting the ESR (and hence total impedance) below 1mohm for all audio frequencies.

The caps are rated at 2.7V which is plenty enough for powering a stack of TDA1387s. Just they take a while to charge up - with around 500mA being fed in the voltage takes almost an hour to reach 2.7V from cold. I've been forming them up and the leakage current seems to be stabilizing now, around 30mA, or 0.5mA per cap.

Update - I've installed this beneath a stack of 6 TDA1387s and been listening for a couple of days. The long and the short of it is there's now no going back to 'conventional' caps, supercaps are here to stay in my DAC designs. The improvement in SQ is rather hard to describe in words - its lower colouration to the LF which might be described as improved 'weight' or 'authority'....
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Old

Cheapest 32bit CPU to date...

Posted 22nd July 2014 at 07:22 AM by abraxalito
Updated 22nd August 2014 at 04:17 AM by abraxalito

These Cortex M0 modules are 14rmb on Taobao, just over $2 a pop, with more number crunching speed than my first ever PC (40DMIPs).

The CPU is the STM32F030, a 48MHz devuce with limited I/O (I2C, SPI) in a 20 pin TSSOP. The going rate for this chip is 2.4rmb ($0.40).

I have an idea to build a DAC with a handful of these little beauties carrying out some of the filtering.

Update - a price drop just happened on Taobao to a similar board, now down to 9.9rmb - http://item.taobao.com/item.htm?spm=...32&ns=1#detail

Pic attached of this mind-numbingly affordable board.

Aliexpress has the boards I bought now, priced at $6.20 but this includes 'free' shipping - http://www.aliexpress.com/item/stm32...701304725.html
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Old

First prototype PCB-based hexacap

Posted 21st July 2014 at 11:59 PM by abraxalito

Seeing as building hexacaps with wire is very time consuming, I'm investigating ways to get the production cost down. Here's my first attempt at a PCB hexacap, using 2oz copper - I did the layout on EasyEDA - Web-Based EDA, schematic capture, spice circuit simulation and PCB layout Online

Once I've ironed out the minor errors on the groundfill I'll make the PCB public so anyone can order up their own. There are 127 caps on this board giving a 50Hz capacitive reactance just under 8mohms.
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Old

The real story about closed vs open loop class D amps

Posted 11th July 2014 at 12:11 AM by abraxalito
Updated 23rd July 2014 at 09:51 AM by abraxalito

Very interesting paper which I'd not come across before which poses about as many questions as it answers. Thanks to Bob Cordell mentioning it in his book, here's the link :

http://www.eetasia.com/STATIC/PDF/20...URCES=DOWNLOAD

Compare the FFT in fig3 (closed loop) with fig4 (open loop) in the text - which do you think will sound better?

Here's my take on what the paper is claiming, FWIW. That PSRR measurements when the amp has a BTL (balanced) output aren't appropriate because there is output stage cancelling of them. My first head-scratch moment came because this is true of other amp topologies too, not just classD as BTL outputs are very much alive and well in NXP's range of chipamps for automotive use. So why restrict this to just classD?

Secondly what's the argument that PSRR in BTL amps isn't appropriate? They seem to be saying (though they don't express this very clearly) that whilst output stage...
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Old

Amplifier-centric two-way XO (and system) design

Posted 3rd July 2014 at 04:30 AM by abraxalito
Updated 5th July 2014 at 02:55 PM by abraxalito

The traditional view is that the speakers are the weakest link of the audio system and hence they're not really designed with amplifiers in mind. Why make amp design easier when speakers are plenty hard enough to get right?

In my experience speakers don't contribute very much to the listening satisfaction. They're more a matter of taste, but a much better speaker can't undo the faults of an amplifier (or DAC for that matter). That's not to say some speakers don't sound better than others - I mostly prefer my Paiyons to the much cheaper Denons but their 'betterness' is orthogonal to the way to the manner DACs and amps sound better. Electronics has had the biggest impact on my listening satisfaction so far so here is an XO designed to make the electronics' job as easy as possible.

Given that the critical parameters of an amp designed for listener satisfaction aren't THD+N and FR, rather multitone IMD (aka MTPR) and PSRR it makes sense that an amp is given the narrowest...
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Old

Tweeter DAC

Posted 22nd June 2014 at 04:24 AM by abraxalito
Updated 25th June 2014 at 05:49 AM by abraxalito

Hexacaps have their limitations - they provide a great low impedance for lower freqs but according to my LCR meter, turn inductive for frequencies in the mid-single digit kHz. Which is about the lower edge of the passband for a tweeter. So what to do to get a low impedance power supply to a DAC chip from 4kHz and up? Here's my first attempt - there are 7 TDA1387s each sandwiched between two low ESR 4700uF lytics. Sandwiching them together is an attempt to get the inductance as low as possible - the loop area can't practically get any smaller than this. Getting the wires in has tested my patience and that's even before powering the beast up....

Update - well I could have been knocked over with a feather - this worked first time I plugged it in. So now I'm thinking about how to build a markII version, considering if I can get more decoupling close in....
Oh I'm now not sure that my bandpass tweeter filter is really delivering the goods as per its simulation. Connecting...
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