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abraxalito abraxalito is offline

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About Me

  • About abraxalito
    Biography
    Ex professional- and consumer audio designer. Prior to working in audio I was in embedded systems.
    Location
    Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
    Interests
    Maximizing dynamics and soundstage depth for the least money with multibit DACs
    Occupation
    English teacher, disruptive innovator with ARM Cortex M-series
    Country
    United Kingdom
  • Signature
    A new scientific truth does not triumph by convincing its opponents ... but rather because its opponents eventually die, and a new generation grows up that is familiar with it. - Max Planck

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  • Last Activity: Today 03:12 PM
  • Join Date: 19th September 2007

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Posted 25th May 2015 at 07:00 AM by abraxalito Comments 2
Here's an interesting post by Charles Hansen (of Ayre fame) about his preference for diamond buffers in amp output stages - http://audioasylum.com/forums/amp/me...18/185962.html

He's saying that diamond buffers sound better but that he has no idea why this would be.

Simulating the PSRR of the diamond vs a traditional EF2 reveals a significant difference - about 6dB better PSRR for the diamond. Could this explain the SQ improvement? Charles designs his amps without GNFB so you'd tend to think that his OPS PSRR is really rather critical.

Since making this discovery I've been on the look-out for opamps with diamond buffer OPSs - OP260 was one I found (courtesy of Esperado) but there are a few from Linear Technology which I've ordered up a few of. In particular, LT1886 and LT1723 look very interesting as potential amp/buffer stages in DACs. LT is fairly unique in that they publish a simplified internal schematic in all the DSs I've looked at...

Posted 16th April 2015 at 01:28 AM by abraxalito Comments 6
I asked this question of Yahoo and didn't find anything much - a few people mentioning the PSRR of their C-multipliers but no simulation results and no algebra. So I fired up LTspice for myself to take a look. The transistor models are the usual Gummel-Poon ones LTspice provides (one up from hybrid-pi) which look to be decent enough for this purpose.

I went for two EFs (sometimes called EF2) as that's probably a more practical arrangement in an audio amp. Some designers even prefer EF3 to get much lighter loading on the VAS/TIS. The EF load I made independent of the bias current so I could learn more about biassing. I used AC simulation to have a look at how the PSRR varied over the audio band.

Turns out the PSRR depends on at least three circuit details and one inherent characteristic of the EF transistor used. In no particular order the circuit aspects which matter are the source impedance seen by the base of the EF, secondly the load impedance seen and finally...

Posted 15th April 2015 at 04:19 AM by abraxalito Comments 9
Posted in Power chip amps
My experiments last autumn with putting a transformer between a chipamp and a drive unit left me with one nagging question. Why was the sound so much clearer with the trafo than without, given that the trafo's voltage ratio was relatively low (1.5:1)? Even Frank was surprised at so much change and suggested some other effect was in play (amp instability). I've not ruled out amp instability but I have made some progress on understanding chipamp PSRR.

The first trafo experiment was done with the TDA7265 - I've since tried trafos on two other chipamps - TDA8947 and LM4766 with similar subjective results - a much clearer sound, more depth to the soundstage and so many tiny details on recordings becoming clearer that the experience has become akin to headphone listening. The trafo ratio has increased - 2.5:1 for the TDA8947 (21V rails, bridged) and 5:1 in the case of the LM4766 (again bridged), running on 62V total supply.

My current listening system is LM4766 with...

Posted 7th April 2015 at 07:03 AM by abraxalito Comments 0
Posted in Uncategorized
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...

Posted 31st March 2015 at 02:26 AM by abraxalito Comments 0
Posted in Digital
On the face of it, this article gives the appearance that Atmel's raced into the lead by a substantial margin in the realm of low power ARM offerings - New ARM-powered chip aims for battery life measured in decades | Ars Technica

However a quick peek beneath the surface reveals all is not what it seems at first sight.

First a stand-out from the linked article :

Atmel, the San Jose-based microcontroller maker, today released samples of a new type of ultra-low power, ARM based microcontroller that could radically extend the battery life of small low-power intelligent devices. The new ... (MCUs) consume less than 35 microamps of power per megahertz of processing speed while active,

Radically eh? Less than 35 microamps /MHz is the claim here - wanna see how that stacks up?

Ostensibly the 35uA headline figure is a worthwhile improvement over NXP's latest offering - where NXP is claiming 55uA/MHz (the LPC5410x I posted about...
Recent Comments
I agree Charles' 'explanation'...
Posted 26th May 2015 at 01:56 AM by abraxalito abraxalito is offline
The economy of the diamond...
Posted 26th May 2015 at 12:22 AM by rjm rjm is offline
Some additional measurements...
Posted 15th May 2015 at 07:11 PM by aive aive is offline
Low hanging fruit all...
Posted 11th May 2015 at 06:11 AM by abraxalito abraxalito is offline
Ol' Diminishing Returns...
Posted 11th May 2015 at 03:46 AM by fas42 fas42 is offline
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