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abraxalito abraxalito is online now

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About Me

  • About abraxalito
    Biography
    Ex professional- and consumer audio designer. Prior to working in audio I was in embedded systems.
    Location
    Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
    Interests
    Maximizing dynamics and soundstage depth for the least money with multibit DACs
    Occupation
    English teacher, disruptive innovator with ARM Cortex M-series
    Country
    United Kingdom
  • Signature
    No matter if we meanwhile surrender every value for which we stand, we must strive to cajole the majority into imagining itself on our side - Everett Dean Martin

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  • Last Activity: Today 11:32 AM
  • Join Date: 19th September 2007

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Posted 25th August 2014 at 06:09 AM by abraxalito Comments 6
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...

Posted 14th August 2014 at 11:44 PM by abraxalito Comments 1
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)...

Posted 30th July 2014 at 09:06 AM by abraxalito Comments 5
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'....

Posted 22nd July 2014 at 07:22 AM by abraxalito Comments 1
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

Posted 21st July 2014 at 11:59 PM by abraxalito Comments 0
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.
Recent Comments
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Posted 30th August 2014 at 12:40 PM by abraxalito abraxalito is online now
Yes I've noticed a change...
Posted 27th August 2014 at 07:26 AM by abraxalito abraxalito is online now
Heh, heh ... know exactly...
Posted 27th August 2014 at 05:40 AM by fas42 fas42 is online now
No, not yet tried that...
Posted 27th August 2014 at 05:24 AM by abraxalito abraxalito is online now
Have you tried scoping...
Posted 27th August 2014 at 05:03 AM by fas42 fas42 is online now
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