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Just for fun - an ultra-low impedance AIF

Posted 18th June 2014 at 11:40 PM by abraxalito

DIYing high enough Q inductors for anti-imaging filters is a bit of a challenge so here's another way to approach this. I found its possible to build very high Q inductors by simply slipping ferrite beads over a fairly thick copper wire. The limitation though is that the beads I have offer about 1.2uH per bead. So making a ferrite bead version of a 660uH inductor will call for over 500 beads - rather impractical.

To make such a ferrite bead inductor filter a practical possibility its necessary to work at a much lower impedance. I've taken a 1ohm working impedance inititally to develop this filter. Now the largest inductor just needs 10 beads. Given this very low impedance the post-amplifier's noise performance becomes rather a challenge - but a transformer will probably do the trick nicely. I plan to try this with a 1:7 step up trafo and see how it goes.

This kind of filter is probably very suitable for DACs with higher output current - the communications type...
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Old

Tweeter line level passive XO

Posted 13th June 2014 at 04:47 AM by abraxalito
Updated 16th June 2014 at 04:51 AM by abraxalito

I'm really impressed with the bang for the buck with my 228rmb speakers, but have a theory they're rather being limited by their crossover. Especially the first order tweeter crossover which hasn't enough LF rejection.

In search of a steeper XO for the tweeter I've decided to go the whole hog and design a kick-*** filter that'll allow me to fully activate these puppies and see just how much they're limited by their electronics. kinku just pointed me to a filter design program called AADE which I've just finished using to design my first LC bandpass filter.

Its bandpass because I want to stick it straight after the DAC and it needs the anti-imaging function, meaning a steep low pass around 18kHz. The tweeter doesn't need this as they're fairly low on IMD, but the tweeter amp sure benefits from having no ultrasonics. Hence here is my first attempt at a tweeter bandpass filter, to go between the DAC and a dedicated tweeter amp.

For now I'll use the TDA1521...
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Old

Linsley Hood amp on Taobao

Posted 4th June 2014 at 05:27 AM by abraxalito

Looks like a bargain, around $120. Sorely tempted to get one to play with - what do you think ?
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Cheapo passive speakers breaking in

Posted 1st June 2014 at 06:02 AM by abraxalito
Updated 2nd June 2014 at 03:54 AM by abraxalito

A pair of these were delivered yesterday. At first I thought their bass sucked and the top end was too 'phasy' - it was only after a while I realized I'd connected them out of phase

Now they're wired up correctly on my TDA1521 amps, they're starting to come on-stream. I rate them as a bargain at the asking price of 228rmb (around $35).

After they've fully broken in I plan to explore what the lowest hanging fruit is by way of tweaks to the crossover....

Update1 - here's the XO, very simple 1st order. 250uH inductor on the bass (0.1ohm DCR, 6ohm bass/mid) and 1.5uF cap for the tweeter (8ohm). The NP cap for the tweeter has a rather high ESR (3ohms at 1kHz) so there might be some mileage in substituting this for a foil cap.

Plotting this arrangement on LTSpice gives a crossover frequency (if -6dB) around 7kHz which seems a little high. If we were to assume a -3dB crossover (because they're in quadrature for a 1st order) then it would...
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Old

TDA8932 class D amp

Posted 27th May 2014 at 12:39 AM by abraxalito
Updated 16th June 2014 at 01:09 AM by abraxalito

One channel is completed - barring the off-board hexacap shunt - second channel under construction. Then for a comparison with the TDA1521s

Update1 - both channels running now, 160,000uF on each. Bass is tuneful but LF colourations noticeable on piano (Ashkenazy Diabelli Variations). Solution to this is simple - more caps

Update2 - an extra ring of caps around the original hexacaps now, so we're around 250,000uF. Bass is so much better than from the TPA3116 but the HF is more ragged, less transparency overall than the TPA3116. So I'll next try an outrageously decoupled implementation just to check I'm not limiting the chip with poor PSU hygiene....

Update3 - I have over 30 1206 10uF caps per side now as local decoupling - HF problems are slight. Makes me wonder if I use it only on the bass/mid and a TDA1521 for the tweeter duty we might reach audio nirvana.... Looking at the DS its easy to see why the HF's a bit of a mess - the PSRR is about...
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Old

TDA1521 chip amps rock!

Posted 23rd May 2014 at 03:13 AM by abraxalito

Here's my prototype TDA1521 dual mono arrangement - two chips per channel in bridged/parallel, sat atop 400,000uF hexacaps. The neutrality has taken a while to get used to but I'm lovin' em....
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Old

Ziggurat passive shunt for chipamp

Posted 5th May 2014 at 04:34 AM by abraxalito
Updated 23rd May 2014 at 12:34 AM by abraxalito

Two tiers in order to achieve the lowest ESR - top is using 4700uF and bottom, 15,000uF. Total around 0.8F, 25V.

I plan to try this out first with my TDA8566 but that's limited to 18V so if that sounds promising I'll look for a new chip which can handle the full 25V supply capability.

Update1 : Wired the top tier only underneath my new TPA3116 class D amp board from Taobao. Seems here the dynamics aren't limited by the capacitance on the output stage supply, rather on the analog supply (pin17 of the TPA3116) which goes to an internal LDO regulator. No amount of capacitance on this pin (I went up to 100,000uF) gives the chip the LF dynamics of my class AB amp so I've been off in search of a class D chip which doesn't have an internal reg so might be amenable to a hexacap on its supply...

Update2 - NXP's TDA8932 is on order now - this is a chip which can be DIYed because its in a manageably small SO32 package, so no need to order up an eval board. Over...
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Stack-a-DAC filter module taking shape

Posted 28th April 2014 at 01:27 PM by abraxalito
Updated 29th April 2014 at 12:36 AM by abraxalito

I'm prototyping this baby which will probably be the first Ozone board to go to PCB layout. It implements the 10th order Chebyshev filter shown earlier and is intended to be mounted atop a 'hexacap' passive shunt. The DAC stack in the centre is made up of six TDA1387s. Input will be I2S and output unbalanced audio 40mV RMS.

Update - over on a thread about AD815 Eldam asked about the inductors - how many I need to buy to get the values in the schematic - so I'll talk about that a little here.

I have bought a big reel of 1k pieces of these 680uH inductors. The price was around $100 but I think I probably should have paid a little more as although the reel is marked up as TDK, they might well be fakes. I suspect that because their appearance isn't as professional as some 1mH parts I bought earlier from another seller. Added to that is the spread of values - having measured at least 100, only one has shown up as 680uH (within 10uH). The rest have varied from 800uH...
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Old

First transportable Ozone incarnation

Posted 27th April 2014 at 02:48 PM by abraxalito
Updated 28th April 2014 at 05:33 AM by abraxalito

I took this to the park this morning, it survived the journey in my backpack but it exposed a dodgy RCA connector. The case I used (temporarily) for it is a cylindrical tea carton - the red 'tower' behind. Battery power comes from 3 NiMH AA cells giving around 10hrs of listening.

I've just done a mental estimate of the BOM cost of this DAC (labour cost is way dominant, but this is DIY right?) and it turns out to be dominated by the cost of the caps in the top three tiers (the small black ones). These are real Japanese Rubycons, chosen for their diminutive height (16mm) vs the Chinese ones at 25mm height. So if you didn't mind adding 3cm to the overall neight you could build this with all Chinese caps and save a few dollars. As it is the total parts cost is under $30 including batteries. If you wanted S/PDIF input (currently its I2S) you'd add a couple more $.
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Old

Ozone Chebyshev filter

Posted 18th April 2014 at 02:39 AM by abraxalito

I've munged the earlier 8 inductor filters down to only 5 - this one is using off-the-shelf TDK inductors, SLF7045-681, albeit they need to be hand-selected to get the correct values as the production tolerance is 20%. X7R caps sound just fine, surprisingly

FR is -3dB at 17,8kHz with around 0.25dB of passband ripple.
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