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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

Gemini Amp - and an idea for yet another headphone amp output stage

Posted 25th April 2014 at 10:13 PM by rjm
Updated 26th April 2014 at 07:19 AM by rjm

As a companion post to the GeminiPS I thought I'd throw the amplifier circuit out there too...

It's not something you'd have any reason to built today I think, but some of the ideas are worth revisiting.

The output stage is what is normally referred to as a complimentary Sziklai pair. The LTSpice circuit below uses the same output, but with the diamond buffer type bias, with it all scaled down to headphone-amplifier voltages and loads. It would be interesting to compare it against i.e. the conventional diamond buffer used in the Sapphire headphone amp. Maybe I'll get around to it. The simulation shows a bit more transient peaking than the straight diamond buffer, ideally there could be some way of adding compensation / reducing the bandwidth to more reasonable levels.
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Voltage regulators for (line level?) audio: The GeminiPS

Posted 22nd April 2014 at 01:40 AM by rjm
Updated 23rd April 2014 at 11:40 PM by rjm

Part of a series.

The GeminiPS is another discrete series voltage regulator, with a Zener reference and bipolar pass transistor. It's an old circuit, published in Practical Electronics in 1970-71, and written by D.S. Gibbs and I.M. Shaw. I happen to have a reprint, but there's a nice overview here.

For reference it might be worth checking back to the two transistor regulator. The GeminiPS circuit is related in the sense that it is a more sophisticated take on the same basic principle. With just a handful of components we have a stabilized, 30 W output with soft turn on and short circuit protection. The circuit can be scaled up and down relatively easily, and the complimentary (negative output) version is an easy modification.

The pass transistor (TR2/3, Q2/3) is between the circuit common and the rectifier anodes. This may seem odd, but it was relatively common back in the day when high voltage transistors were both expensive and rare. The...
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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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Old

How good can the sound get?

Posted 12th April 2014 at 02:40 AM by fas42

This will be a collection of ideas developed and experiences encountered in my personal foray into the world of audio. In part, this is to counter much of the negativity and beliefs out there about the limitations of bog-standard, vin ordinaire, reproduced sound from conventional recordings, heard via a normal pair of speakers.

First of all, the sound can get very, very, very good. Far beyond normal hifi, better than "live" much of the time ... so how can that be so? Because, typically for the recording session the positioning of the microphones, and acoustics, are optimised and tweaked by the sound engineers to pick up a "quality" well beyond what you as an ordinary audience member, concert goer, would experience.

So, how good? Well, for a start you can get "invisible" speakers, meaning that they can't be perceived as being the source of the sound; even if you go up to them and stare at them very closely, and move from side to side...
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Old

Voltage Regulators for Line Level Audio. Part 10 : Simple Shunts

Posted 11th April 2014 at 06:59 AM by rjm
Updated 15th April 2014 at 11:52 PM by rjm

Part of a series.

I've been meaning to take up shunt regulators for some time. I've never got around to building one myself to try, so I'll have to make do by playing in simulation.

Today's circuit is the shunt analog of the Z-reg series regulator: no feedback, Zener reference, single transistor regulation. The output impedance and ripple rejection-characteristics are similar too, with about 40 dB of RR and an output impedance of just a few ohms. It can be built equivalently from either an pnp or pnp transistor. (See attached LTSpice .asc files.)

The difference between shunt and series regulation can best be explained by considering the upstream power supply: In a series regulator an increase in current demand by the load causes the regulator to increase the current to compensate. In a shunt regulator an increase in current demand by the load causes the regulator to decrease the shunt current to balance, so there is no net change in current flowing...
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Old

One Farad capacitor array

Posted 3rd April 2014 at 03:08 AM by abraxalito
Updated 4th May 2014 at 04:53 AM by abraxalito

This array of 127 10,000uF caps measures -j2.9mohms at 50Hz which I reckon makes it comfortably over 1F in capacitance. ESR is 1mohm @ 50Hz.

The individual caps were measuring on average 8,500uF. Although they're marked 10V (the brand is ChengX a slightly shady Chinese one) they're not good for anywhere near this voltage. I reckon they're in reality 6.3V and they seem to have decently low leakage at this working voltage. I shall be using them at 5V or below.

Update1 - seems I wasn't using enough copper in the wiring. I've now built a second one with 2.5mm2 wiring around the loops and 1.5mm diameter cross links and ESR is down to 0.5mohm @ 50Hz now, decreasing at higher freqs. I have enough caps left for a third one...

Update2 - now the first array has a basement level giving 2.1F total with ESR below 0.4mohms @ 50Hz. There are six paralleled TDA1387s atop the combined hexagons giving unprecedented LF dynamics. Amp long, long overdue for an upgrade.......
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Old

The case of the crazy Sapphire amp.

Posted 3rd April 2014 at 01:24 AM by rjm
Updated 3rd April 2014 at 11:07 AM by rjm

Case report:

A set of Sapphire boards gave the proper V+, V- voltages out of the Z-reg, providing about 10.5 and -10.5 to op amp power pins. The output offsets were unusually high however, apparently at about 2 V in one board, and somewhat less in the other. Typically the offsets are in the order of +/-15 mV.

Changing out transistors and op amps did not help, and to all inspection the passive components were installed correctly and working properly. The offset voltages were extremely temperature sensitive. Measurements for the various circuit voltages were just screwy enough to be inconclusive.

I could ask for no more tests, so requested the boards be sent back to me. I found the circuit basically worked as expected, but the offsets were indeed high on both boards, though I measured 0.6 V max rather than 2 V.

***** stop here and make a guess *****

Blowing on the board through a soda straw, the offset shot up when I blew on...
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