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I2S transcoder

Posted 7th July 2013 at 12:57 PM by abraxalito
Updated 16th September 2013 at 01:32 AM by abraxalito (Schematics added, pic of 2nd build added)

Here's a circuit that's been a long time in gestation - some logic that converts I2S from 64fs (32bits per sample) down to 32fs (16bits).

Almost all the S/PDIF receiver chips nowadays output a bit clock at 64fs (2.8MHz for RBCD) because the format has the potential to support up to 24bits. When being driven from a CD player though, there's no chance of any useful information occupying those spare bits. As my interest is to run all signals as slow as possible to keep noise to the absolute minimum, 64fs to me is profligate generation of RF when 32fs will do the job. But only the WM8805 supports this format and then only when software programmed.

The other reason for wanting the slower bit clock is that my LAID design relies on shift registers and the 32fs clock gives me twice as good utilization of the serial storage - no bits are being wasted on zero padding.

This circuit is designed to do the job with the fewest standard logic chips I could manage...
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Old

Better sounding anti-imaging filter

Posted 5th June 2013 at 01:46 AM by abraxalito
Updated 5th June 2013 at 02:00 AM by abraxalito

Still in the vein of AIF designs which utilize off-the-shelf inductors, I'm currently listening to this one and its thoroughly enjoyable. I built it with some very cheap bobbin-wound (unscreened) 10mH chokes I found on Taobao.

I don't recommend using unscreened inductors for this but if that's all you have the results are still aural candy. They do pick up a little low level mains hum, not noticeable except when close to the speaker. Also they interact - when I was checking the prototype filter for continuity I hooked up the LCR meter to the 4 inductors in series and found different answers - none measured 40mH. The result was from 32mH to 45mH (from memory - my filters are balanced so I have four sets of series Ls). I have them spaced apart by almost one coil diameter (about 6mm) so if you're going to build this I suggest greater spacing.

You'll note that this one's industrial strength in terms of its stop-band rejection - around 60dB. Whether this is obtained...
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Old

Simple anti-imaging filter

Posted 28th May 2013 at 04:57 AM by abraxalito
Updated 4th June 2013 at 07:34 AM by abraxalito

If you're curious to see how an anti-imaging filter affects the SQ of your NOS DAC but have been hitherto put off by all the coil winding involved, then this circuit might be just the ticket.

I was curious to see how well just two inductors could do and it turns out not at all badly - there's about 23dB stop-band rejection here and the 10mH inductors are off-the-shelf Fastrons. The capacitors can be all 1nF 0805 C0Gs - just buy a strip of 40 and use parallel and one series combination.

I intended to build one and listen to how it fares against my more complex efforts...

Update - a quick listen and its clear on switching back to the original filter that this new filter's noise floor isn't as low. That's not directly perceived as a rise in hiss mind - this is the subconsciously perceived noise floor which translates to more attention given to the music. So I'll put the two inductor filter aside and work on slightly more complex filters next.
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Old

Back to passive filters

Posted 8th May 2013 at 02:11 AM by abraxalito

I didn't much care for the sound of my active elliptic filter - great dynamics in the bass for sure but the upper-end colourations were a bit unnatural sounding. So I've shelved tthat one for now and instead I'm playing with a simplified (by which I mean fewer inductors) passive elliptic.

There are two topologies for building elliptics where the zeroes are realized either by shunt series-LC networks or series paralleled-LC networks. The series created zeroes means fewer inductors are called for. In its most basic, unbalanced form there would be just three inductors for a 7th order filter. This filter though is balanced and designed to feed my Nitro desktop amp directly, without any I/V amplifier stage.
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Old

Force/sense shunt

Posted 24th April 2013 at 08:35 AM by abraxalito
Updated 24th May 2013 at 03:59 AM by abraxalito (Added pics of prototype fs-shunt. Added noise update.)

I'm a recent convert of the lowest possible impedance of power supply based on my experience of adding caps to my chipamp. So I figure the signal stages can't be harmed by reducing their supply impedance either, particularly at LF.

I note there are a few aftermarket regulators around - I had a look at Paul Hynes and Belleson in the past few days. They're a bit pricey for my tastes, given the cost of the components they're using can't be over single digit $ so I've had a look at "doing it at home, only cheaper".

First off, a simple TL431 is about the best bang for the buck achievable, as the part here is 0.2rmb. But the dynamic impedance is typically 0.2ohms and I was hoping and aiming to go a bit lower than this - perhaps an order of magnitude lower, to around 10mohms. Lower than this and the resistance of the PCB tracks come into play and its also very hard to maintain such a low impedance beyond the audio band as cap ESRs (for the best ones) are of...
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Old

Active elliptic - practical implementation

Posted 20th April 2013 at 09:33 AM by abraxalito

Here's the first channel under construction on 2mm pitch double-sided proto board. The opamps in this instance are AD8014 in SOT23-5 packages, hence the choice of the 2mm pitch board. Power supplies will go on another board to be attached to the reverse side.
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Old

Active elliptic anti-imaging filter

Posted 20th March 2013 at 03:18 AM by abraxalito
Updated 20th March 2013 at 03:30 AM by abraxalito (Added FR)

Passive filters rock for SQ, no doubt about it but I'm still curious how good sounding an active DAC I/V post filter might be. So I've figured out an almost equivalent FR active version of my 7th order LC elliptic filter. This active elliptic has been designed using LTSpice's FilterCad program giving the pole/zero positions, then the Williams handbook of filter design helped me translate those numbers into a working circuit. Its using what Williams calls the VCVS 2nd order section based on a twin-T network to realize the zeroes.

My first attempt at an active elliptic filter was using gyrators but that proved very hard (practically impossible) to get stable with CFB opamps due to their HF gain peaking. VFB opamps I ruled out at the start for inadequate SQ - its not hard to make gyrators stable with them. Hence this approach which promises to work with CFBs though I'd guess I'll probably need to add series Rs between the stages in practice. Nothing built yet but thought I'd...
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Old

What is dynamics?

Posted 5th March 2013 at 02:02 AM by abraxalito
Updated 11th March 2013 at 06:35 AM by abraxalito

Fascinating discussion going on about the nature of 'dynamics' over on this thread : http://www.diyaudio.com/forums/multi...nger-what.html

Pano writes (post 1174):

There is a consistency across several decades and cultures that high efficiency speakers sound more dynamic than low efficiency. That's a common subjective assessment, so we might ask "why?"

I'm not going to chip in on that thread because I have bigger fish to fry, but ISTM they're all barking up the wrong tree. That's because the question itself isn't quite posed correctly even though its a great start. Nobody listens to speakers alone, they're always powered by amplifiers. The answer to dynamics lies in amplifiers, not speakers. Put another way 'Its the electronics, stupid!'.

Higher efficiency speakers tax amplifiers (here meaning their power supplies in the main) much less - so there's no way to do an apples-apples comparison...
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Old

Line input RF reject filter

Posted 19th February 2013 at 07:24 AM by abraxalito
Updated 26th February 2013 at 03:09 AM by abraxalito (Added zoomed passband plot)

I've only simmed this so far, no listening tests. The aim here is to have a passive filter which works into a line level impedance (i.e. 10k or greater) and which gives a decent amount of RF rejection without compromising SQ. I plan to incorporate it behind the RCA input jacks of my chipamp.

Design-wise, it started life as a simple 2nd order filter with single series L and shunt C. But when it came to wind the inductor I didn't have a wire diameter small enough to make the required value (44mH). So I split up the inductor into 2 * 22mH and then couldn't resist hanging a cap off the centre tap. The result has turned more into an over-damped elliptic with a higher corner frequency than the original, but with an impeccable phase response to 20kHz. The elliptic-type plot comes about because of the SRF of the two inductors (around 350kHz).

Since fas42 remarked on the phase performance, I've added the group delay plot for the passband. Almost pure time delay of...
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Old

High-end chipamp build project

Posted 24th January 2013 at 07:21 AM by abraxalito
Updated 15th February 2013 at 05:12 AM by abraxalito

Go here : Possibly the most frugal high-end sounding amp?

If you have comments, feedback, critique or improvements and don't want to put it on that thread, feel free to leave it in the comments here.

Here are the first measurements - the Nitro amp being fed by the Ozone DAC with a six-tone multiple, peak digital level -20dB. Peak output power around 100mW in an 8R speaker (connected for this test). It looks from this that the datasheet is a little pessimistic on THD, though here I'm measuring the TDA8566, not the 8561. No measurement graphs are presented in the 8566 DS and the 8561 only has plots for 2R and 4R loads, not 8R.

Schematic now added. BOM here : https://docs.google.com/spreadsheet/...Hc&usp=sharing

Additional schematic showing how to use TDA8566Q in place of TDA8561Q as the latter isn't available from Mouser. Mouser only has fewer than 300 TDA8566Q in stock so best...
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