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Sanwu FLAC and Bluetooth player has a significant flaw

Posted 16th March 2016 at 12:59 AM by abraxalito
Updated 17th March 2016 at 03:18 PM by abraxalito

I was going to write a post praising this player for the superb value for money (its the only cheap single-chip FLAC player I've found) but this morning it produced an alarming series of whistles and pops from what I presume is a corrupt file on my TFcard. So now its only recommended if you're sure you have perfect data on your card - it doesn't seem to mute the audio when an error is found.

Apart from this major howler at just 30rmb its great, providing as it does FLAC, WAV and mp3 support along with Bluetooth running from a USB power source at 5V. The audio performance is decent when run through my modified XuanZu headamp as preamp - the level is rather low otherwise and I suspect it needs a high-impedance buffer for best dynamics.

I put the TFcard which gave the player hiccups into my PC reader and uploaded the 'problem' file to Audacity. No glitches noticed there so looks like I might have to dig a bit deeper to find out what went wrong. I shall try playing...
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Old

An LC filter for supplying a line stage

Posted 19th February 2016 at 12:33 AM by abraxalito
Updated 22nd February 2016 at 12:51 AM by abraxalito

I've designed LC filters for classAB amp power supplies before - for those applications iron powder toroids work fine for the Ls. However for the power supply in my latest DAC design I wanted more supply rejection and this calls for higher value inductors - in the tens of mH. Creating a 10mH inductor on a toroid takes way too long and is hugely fiddly as the wire length needed is substantial and I don't have any specialized winding machine. So toroids are really out of the question at such values.

I have some largeish inductors in the right range wound on bobbin cores but when I checked the DCR it was a little high, 20ohms or so. As I might need up to 100mA, a 2V drop is too great. In any case, in LTspice this resulted in rather an overdamped response - what I really needed is something in the range 1 to 2 ohms. The solution seemed to be use ferrite cores of the kind normally used to make transformers. Which means breaking a kind of informal rule I made for myself about not...
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Old

Highly affordable digital audio CPU board

Posted 8th January 2016 at 03:02 AM by abraxalito

STM's lowest cost ARM Cortex M4 CPU is now available on a very low cost evaluation board here - https://item.taobao.com/item.htm?spm...cket=13#detail

Its quite a good fit for audio purposes because there are a couple of I2S interfaces with a dedicated audio PLL. There's plenty of RAM (64k) and according to the benchmarks, it'll do the equivalent of around 100MIPs. Its also respectably low power when running out of RAM - use the flash memory accelerator though and its not quite so frugal.
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Old

BBftB headphone amp PSU

Posted 14th October 2015 at 02:57 AM by abraxalito

Here's the power supply I've lashed up to feed the balanced SE classA amp.

Its fed from a 5VA EI transformer with a 65VAC secondary. One 390uF cap follows the rectifier, then there's a 30mH choke, two 390uF caps beyond that.

A series regulator is made from a string of 3 TL431s as reference (the max from a single one is 36V - I've gone for a total of 78V) and that's followed by a 2SK213 simply because I had no other high voltage transistor to hand. There's an RC filter feeding the gate of the MOSFET to reduce the output noise from the shunts (68k,200nF).

Output ripple isn't visible on my scope but I plan to feed the output into my AC millivoltmeter and see what its giving out in terms of noise.
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Old

Taobao headphone amp is an FX box

Posted 14th September 2015 at 01:19 PM by abraxalito

I bought this amp because the case attracted me - no pics on the Taobao page were giving away anything about the insides, quite unusual. After receiving it I couldn't resist having a quick listen and it turns out its a hardwired tone control with bass and treble turned up to the max, about +16dB wrt 1.3kHz. What a surprise! - no matter as I was going to strip out the innards anyway to use as a test platform for my amp.
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Old

Can you identify this opamp from its simplified schematic and PSRR plot?

Posted 10th September 2015 at 01:28 AM by abraxalito
Updated 21st September 2015 at 12:23 AM by abraxalito

In my search for opamps with better real-world PSRR behaviour, I came across this beauty. For now I'll just post up its simplified schematic and PSRR plot - if any of you know of it please put your deduction in the comments. I may add more clues later if nobody nails it early on.

The reason I find this part interesting is its cascoded output stage - I believe this is what leads to the 'hump' in the positive rail PSRR. I've never seen that behaviour on any other device.

Well over a hundred views now and not one single stab at the answer. Its the industrial-strength version of the now obsolete LM308, a Bob Widlar special with super-beta input stage. The output stage cascode I take it isn't primarily to improve the PSRR rather its due to the high maximum operating voltage (72V). TI does still have the DS on its website though it doesn't put in an appearance in parametric search as its been obsolete for a while. Even more interesting is its decompensated variant which...
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Old

Headphone amps designed for PSRR

Posted 28th August 2015 at 04:38 AM by abraxalito
Updated 6th September 2015 at 10:57 PM by abraxalito

Since acquiring and modding my Taobao headphone amp I've been enamoured of creating a much more portable headphone solution to deliver aural nirvana but on the move. Whereas transformers are a very practical solution for a desktop amp, steel and copper is not only bulky but also jolly heavy and hence a no-no for anything pocket-sized.

How else to get the dynamics I'm seeking though? For these amps I'm toying with different solutions to getting better PSRR, particularly in the all-important bass region which tends to suffer in commercial portable amps. The OPS (output stage) is what needs most attention in any classAB amp - the signal stages can all be classA but for efficiency (and hence battery life) the output stage can't be conducting all the time.

A fully discrete output stage where the output devices are cascoded looks to be one solution but initially I'm looking for a simpler, more cost-effective solution with lower overhead on the supplies if possible....
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Old

Cost optimized active crossover

Posted 7th April 2015 at 07:03 AM by abraxalito

I've driven the BOM cost of this board down quite hard by choosing some 'disrespected' opamps to build it around - TL084s. They go for 0.5RMB on Taobao (about $0.08, same as a recycled TDA1387). Listening so far is positive but I've got no audiophile opamp reference to check it against. In terms of bang for the buck though, its going to be very hard to beat. All filter caps are NP0 ceramics, resistors are bog-standard thick film 0805s.

What's different about this build for me is its fully balanced and classA operation in order to cancel power supply noise. To get classA operation from TL084s means the feedback/filter networks must be very high impedance so as to draw under 100uA peak currents. Opamps aren't used as output buffers, rather I've gone for emitter followers loaded by two-transistor CCSs.

The picture is my second prototype, using the best (aka lowest ESR) caps I could get hold of - Nichicon HZ and Nippon Chemi KZE. All the copper wires sticking up around...
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Old

Supercapacitor b*ll*cks

Posted 15th February 2015 at 11:50 PM by abraxalito
Updated 6th September 2015 at 05:28 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 (the text at this link has since been updated to remove the 'total rating...whopping' remark).

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....
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Old

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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