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abraxalito abraxalito is offline

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  • Join Date: 19th September 2007

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Posted 8th January 2016 at 04:02 AM by abraxalito Comments 0
Posted in Uncategorized
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.

Posted 20th October 2015 at 08:49 AM by abraxalito Comments 0
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...

Posted 14th October 2015 at 03:57 AM by abraxalito Comments 2
Posted in Uncategorized
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.

Posted 10th October 2015 at 02:55 AM by abraxalito Comments 4
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...

Posted 25th September 2015 at 02:56 PM by abraxalito Comments 2
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...
Recent Comments
I tried higher voltage...
Posted 21st January 2016 at 02:26 AM by abraxalito abraxalito is offline
Lol no need to answer...
Posted 20th January 2016 at 06:52 PM by nige2000 nige2000 is offline
Sounds good
have...
Posted 20th January 2016 at 05:55 PM by nige2000 nige2000 is offline
I've heard there are...
Posted 14th January 2016 at 01:02 AM by abraxalito abraxalito is offline
is there a simple way...
Posted 13th January 2016 at 07:39 PM by nige2000 nige2000 is offline
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