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Supercapacitor b*ll*cks

Posted 16th February 2015 at 12:50 AM by abraxalito
Updated 16th February 2015 at 02:34 AM by abraxalito (Red Wine it isn't, just the same designer)

Here's something very misleading in the 6moons preview of Vinni Rossi's (of Red Wine Audio fame) latest supercapacitor powered kit. They have 18 * 350F supercaps in the box and they say '...the total rating becomes a whopping 6300F'. However each capacitor is only 2.7V so presumably to get any audio signal at all out, they need to operate these caps in series. Caps in series don't multiply, they divide so the total capacitance drops by a factor of 18. Hence only 19.4F. Not a small difference from 6300F or did I miss something?

http://www.6moons.com/audioreviews2/vinnierossi/3.html

I think I may have missed something - they aren't using all the supercaps all the time, rather there are 9 discharging and 9 charging. So the total capacitance in the audio circuit at any time is 350/9 = 39F. Still an impressive amount of capacitance but the problem is that the ESRs add too. Here's the datasheet for the Maxwell 350F ultracap they appear to be using - http://www.tecategroup.com/capacitor.../BC_series.pdf...
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Old

TDA1387 octal DAC looks like a good modding base

Posted 16th January 2015 at 12:55 AM by abraxalito
Updated 13th February 2015 at 04:23 AM by abraxalito

Thanks to Matthieu (Malefoda) for the heads up on this. It looks distinctly like a DAC inspired by the DAC-AH - note not only the 8 DAC chips but also the PCB layout closely follows that design's. The price on Taobao is about half the AH (368rmb) so I shall be ordering up one or more to have a play. Matthieu found it on another website with prices in USD, here - http://www.cart100.com/Product/42124644877/.
Note there are two options here (blue, black) - the cheaper one is a kit of parts I believe, the more expensive ($84) the finished unit. Going on the translation of the Taobao page, the kit does not include the transformers, mains switch or the case.

Update : here's the summary of the mods so far, for details go to Malefoda's thread linked in the comments.

1) Re-route the output ground so the opamp filtering caps aren't subject to injected CM noise from the destination component (amp or pre).

2) Reduce the supply voltage to the output opamps...
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Old

Revisiting mods to the Lite DAC-AH

Posted 12th January 2015 at 02:57 AM by abraxalito
Updated 12th January 2015 at 03:07 AM by abraxalito

Last night the battery of my I2S source ran out so while it was on charge, I unboxed my last remaining Lite DAC-AH to have a listen to that, fed from my (mains powered) QA550 wav player. I'd bought 3 DAC-AHs a few years back when I was playing with them.

The differences between the stock DAC-AH and the Ozone Pagoda were apparent in the areas of soundstage depth, dynamics (especially at LF) and coarseness at higher levels. So this got me wondering what would be the lowest hanging fruit in terms of mods to it. My procedure before had been to implement everything I could think of before having a listen - this way its not possible to know which mods are the most effective ones and which can be omitted.

This time I'm going to be a bit more disciplined for the purposes of discovering which mods deliver the best bang for the buck. The first one I'm going to try is a passive LC filter between the DACs and the AD847s. Here's the schematic for it - its using the cheap and...
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Old

New year, new DAC

Posted 31st December 2014 at 08:36 AM by abraxalito
Updated 9th January 2015 at 01:27 AM by abraxalito

Here's the 'Ozone desktop pagoda' DAC for 2015.

A single TDA1387 feeds a 3 inductor quasi-elliptic filter followed by AD8017s as buffer-amps. The large ferrite cores in the base do the bal-SE conversion. The power supply is 4 * AA NiMH cells which should in theory last for a whole day's music.

The design is really a 'MkII' version of the Ozone portable where the AD815 buffers have been replaced to allow a more compact construction and lower power draw without the constraint of being able to drive IEMs directly. Whereas the portable used a stack of 1387s due to the choice of 7mm TDK inductors, this one's using pot-core chokes giving a much higher working impedance and hence higher output levels from just the one DAC chip. The desktop footprint is about that of a CD.

Update : I found some bargain Jamicon caps on Taobao which give the tower more elegant proportions, as well as improving the supply impedance to the buffer-amps and giving more breathing...
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Old

Design ideas for Aune M1 DAC card

Posted 15th November 2014 at 03:51 AM by abraxalito
Updated 19th November 2014 at 06:43 AM by abraxalito

I'll outline here some thinking in choosing the major building blocks (aka ICs) for this card - any comments welcome as this progresses.

First up the DAC chip will be the TDA1387 initially. I don't know for sure that the output from the ARM/Xilinx card is I2S but I'm going to verify that fairly soon. There's nowhere near enough room for the passive shunt I've adopted previously so the bass performance probably is going to have to suffer. I shall pay considerable attention to the power supply arrangements though in an attempt to make up for the LF lack.

After the DAC, passive I/V will follow and then a filter using the TDK 7mm inductors I've used previously. I've slotted them into the gap between the PCB and the case and there's just enough height available. Since space is at a premium I'll experiment with a 3 inductor design - the stop band attenuation will suffer but probably I'll add a secondary LC filter at the output to make up for that somewhat. The secondary...
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Old

New .wav player from Aune looks very interesting

Posted 11th November 2014 at 02:26 AM by abraxalito
Updated 15th November 2014 at 10:27 AM by abraxalito

Thanks to jambul for alerting me to this one - 赵宇为作品 - Aune M1 便携式播放器测评报告 [Soomal·数码多] (link is in Chinese but its mainly for the pics).

Notice that although its using the PCM1793 chip the D/A and analog circuits are all on a daughter board. This gives rise to the possibility of engineering a daughter board with a much better DAC chip (think TDA1387) and improved head-amp...

Street price here is around 800rmb (80, $130) so I shall be ordering one to have a play.

Update - looks like I'm rather slow to catch on, Taobao already has somebody's alternative DAC-AMP card, which appears to be selling fairly well, here : http://item.taobao.com/item.htm?spm=...cket=13#detail

I've now placed the order for the M1, hopefully it'll arrive tomorrow and then I'll follow Dave (EEVBlog)'s advice : 'don't turn it on, take it apart'. The main question I want an answer to is how...
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Old

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

Posted 8th November 2014 at 02:46 AM by abraxalito
Updated 8th November 2014 at 03:00 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 03:42 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 09:11 AM by abraxalito
Updated 6th November 2014 at 11:51 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 03:33 AM by abraxalito
Updated 15th January 2015 at 02:19 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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