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Highly affordable digital audio CPU board

Posted 8th January 2016 at 04:02 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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Discrete headphone buffer - 'Buffalito'

Posted 20th October 2015 at 08:49 AM by abraxalito
Updated 26th October 2015 at 02:42 PM by abraxalito

Nowadays with discrete transistors as affordable as they are, the most cost-effective solution for a particular audio application may well be a discrete one when SQ (rather than numbers) is uppermost. Audiophile faddishness about discretes aside.

Here's a case in point - my pic shows a headphone buffer where the design aims were lowest cost, smallest size and lowest battery drain, while maintaining acceptable SQ. There are 28 transistors which go for 0.04rmb each on Taobao. That's 1.12rmb. OK so you can also buy 2 NJM4556s for that, but how do they sound? In my experience of building an O2-alike, not so great. They're also going to take 15mA at 7V whereas this design takes 6.5mA at 3.6V input. So an integrated design will be more than 4X as power hungry. With a 2600mAh single cell LiIon this could run for 400hrs - over two weeks continuous if played at low level.

The power supply is created by an LM2662 which inverts the 3.6V positive input for a -3.6V rail. It...
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BBftB headphone amp PSU

Posted 14th October 2015 at 03:57 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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Designing the best bang-for-the-buck headphone amp

Posted 10th October 2015 at 02:55 AM by abraxalito
Updated 16th October 2015 at 06:36 AM by abraxalito

Headphone amps aren't any different from speaker amps in principle - what you hear (apart from a bigger version of the input signal) is the power supply's noise coupled through the inadequate PSRR of the electronics.

SE classA operation is a way to minimize the generation of power supply noise by arranging the current flow to be constant to a first order so that any remaining ripple on the supply is the result of the finite output impedance of the follower's loading current source and those of the driving stages. But how significant are these 2nd order effects? This design is an attempt to find out - by reducing them as far as practicable.

The idea is to run SE classA at a much higher voltage than is needed to drive the 'phones (balanced, with 80V supplies giving 144V peak-peak) then step down the output voltage with a custom-wound output transformer. This has the effect of increasing the PSRR of the amp's output stage. I'm not worried overmuch about the PSRR...
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Old

Discrete transistor based CFB I/V stage

Posted 25th September 2015 at 02:56 PM by abraxalito
Updated 26th September 2015 at 02:35 AM by abraxalito

Since I figured out the reason for needing all those caps in my earlier DAC designs was all brought on by using passive I/V, I'm now a total convert of active I/V in order to do away with the sheer bulk.

Having tried single transistor I/V and loved it, I found there was still some improvement to be gained by biassing the common-base transistor with additional current sources to reduce its input impedance. Since getting down to the region of 1ohm would require some 25mA of bias which isn't well suited to portable applications I decided to have a go at using feedback to obtain the impedance I'm seeking.

I'm not using an off-the-peg CFB amp because they still turn out to be fairly power supply quality susceptible (subjectively speaking) so here's a design I hope that greatly reduces the supply impedance requirements so that it can be used in a portable player.

The picture shows the second prototype I/V stage, coupled to a 6th order Chebyshev anti-imaging...
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Designing high order active filters

Posted 21st September 2015 at 12:54 AM by abraxalito
Updated 25th September 2015 at 02:20 AM by abraxalito

I've been getting a lot of use out of Simon Bramble's webpage for designing active filters recently - http://www.simonbramble.co.uk/techar...ter_design.htm. Its a great resource.

Right down at the bottom of the page the last filter he shows the schematic of is a 9th order Chebyshev, 1dB ripple, with a corner frequency of 1kHz. A textbook frequency response plot is obtained using LTC6241s. I latched on to this and tried changing the corner frequency to 18kHz, wondering if I could use such a design for an anti-imaging filter for my DACs. So I divided all the capacitor values by 18 and ran the sim. Disaster! The frequency response I obtained is below - a 7dB spike at 17kHz.

The problem seems to be inadequate Q - high order filters are composed of sections which increase in Q (more positive feedback) and the chosen opamps aren't fast enough (18MHz GBW). I went to a faster opamp for the highest Q stage which brought about some improvement...
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Old

Taobao headphone amp is an FX box

Posted 14th September 2015 at 02:19 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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Can you identify this opamp from its simplified schematic and PSRR plot?

Posted 10th September 2015 at 02:28 AM by abraxalito
Updated 21st September 2015 at 01:23 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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Headphone amps designed for PSRR

Posted 28th August 2015 at 05:38 AM by abraxalito
Updated 6th September 2015 at 11:57 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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Another incredibly cheap Taobao toy

Posted 19th July 2015 at 11:00 AM by abraxalito
Updated 10th August 2015 at 11:44 AM by abraxalito (Listening report added; chip pinout added)

When browsing Taobao I can't resist the temptation to try the very cheap stuff. This little TFcard player really takes the biscuit for the lowest price of a digital audio source - 9.9rmb. So I ordered up three, I might just order more soon.

First powering up I tried some FLAC files, these don't work but wav and mp3 work fine and it even finds files below the root directory. Power supply is nominally 12V but its using an EF fed by a zener diode, not an IC reg so almost anything from 6V will probably work. I haven't managed to find the technical blurb for the chip so far so a little reverse engineering was needed. It incorporates a 3.3V regulator to power the IR receiver and the TFcard - I have fitted an additional regulator for the first mod because I wondered if the low-level noise I was hearing was due to interference from the flash card's power draw. Turns out no as when playing .wav files this noise disappears.

Incidentally the 9.9rmb cost includes the remote...
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