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At last, NXP has a low-end, low-power Cortex M4

Posted 8th November 2014 at 01:46 AM by abraxalito
Updated 8th November 2014 at 02:00 AM by abraxalito

NXP's ARM offerings I find to be the most power efficient and I've searched for a long time for a current-sipping M4 which is available in a lowish pin count package. Up until now the best offering in that realm has been STM's F411 with 13mA @ 100MHz. NXP's latest offering the LPC54100 beats that by a healthy margin, turning in a sub-10mA draw at the same clock rate. There's also a dual-core variant with a 100MHz M0+ with around half the current requirement. Respect - the fly in the ointment though is only 2 SPI peripherals which don't support TI mode. Pricing looks great at $2/10k.

http://www.nxp.com/news/press-releas...ng-market.html
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Old

Aiming for maximum isolation

Posted 2nd November 2014 at 02:42 AM by abraxalito

Getting isolation from the mains supply is an on-going challenge for me. Here's my latest stab at getting as close to a battery as I can currently manage - a ferrite line-output trafo soon to be pressed into service as a low-capacitance isolation transformer.

The back story is that I'd heard that split bobbin trafos were the way to go for the best isolation from the mains. Having just acquired some for chipamp-drive unit interfacing I was curious to check out their credentials. I measured around 20pF interwinding capacitance which is about the lowest of any mains trafo I've checked. Of that it seems that the majority of the capacitance is directly between the windings, a smaller amount is due to coupling via the steel core (which of course is conductive). So then I figured that even if the windings could be moved infinitely far apart the steel core would be the limiting factor in reducing the capacitance.

Air core trafos are a possibility but then they're hopeless...
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Old

My latest infatuation - transformers

Posted 25th October 2014 at 08:11 AM by abraxalito
Updated 6th November 2014 at 10:51 PM by abraxalito

I've found that some of the el-cheapo trafos (18rmb each) at a shop at the local electronics market are of split bobbin construction. This makes bodging up an audio OPT from two mains trafos a fairly straightforward matter.

I bought some with 9-0-9V and others with 0-12V secondaries. Then I disassembled them (fortunately they're not varnish dipped) and swapped out the 220V primary bobbin for the secondary of the other one. This gives me a trafo with 18V on the primary and 12V on the secondary, a step down of 1.5:1, impedance ratio of 2.25:1. So it makes a 4R drive unit appear as 9ohms to the chipamp.

And when I applied this to the output of the bass/mid of my chipamp (residing in the Phenix active speakers, its a TDA7265), apart from it sounding quieter I suddenly realized how much power supply noise I was still listening to. Incredible

So if you want to know if your chipamp PSU decoupling is really up to snuff, see how much difference a 1.5:1...
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Old

Taobao has a TDA1387 DAC (apparently)

Posted 23rd October 2014 at 02:33 AM by abraxalito
Updated 15th January 2015 at 01:19 AM by abraxalito

Its rather difficult to work out whether there's a line-level output here or if the phono sockets are inputs to the LM1875 amps. The TDA1387 is a plug-in module for the USB-input DAC section which might feed only the amps. Anyone who's good at reading Chinese, please contribute in the comments

http://item.taobao.com/item.htm?spm=...8-3f2abc0f8bd9

Update - seems one particular vendor has quite a range of TDA1387 DACs now, all very affordable. The cheapest is this one (48rmb kit, 88rmb built in my limited understanding) - http://item.taobao.com/item.htm?spm=2013.1.20141002.7.yoDoCx&scm=1007.100 09.2083.i41769520252&id=41937738376&pvid=402c7a10-8221-4bca-82c0-99c1c511877f.
This one has 8 chips, I guess its a tribute DAC to the DAC-AH - http://item.taobao.com/item.htm?spm=...cket=17#detail...
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Old

New active speaker on order from Taobao

Posted 26th September 2014 at 02:49 AM by abraxalito
Updated 6th October 2014 at 02:44 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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