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Headphone amps designed for PSRR

Posted Today at 04:38 AM by abraxalito
Updated Today at 04:42 AM 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

Another incredibly cheap Taobao toy

Posted 19th July 2015 at 10:00 AM by abraxalito
Updated 10th August 2015 at 10:44 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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Old

Bargain beaut - headphone amp

Posted 2nd June 2015 at 05:44 AM by abraxalito
Updated 9th July 2015 at 09:43 AM by abraxalito

When I found this on Taobao a few days ago I could not believe the price wasn't a mistake, or that it was just for the case with nothing inside. But it turned out to be real, so I ordered one - it arrived just now so I'm taking it apart before having a listen. The case oozes quality and the volume control feels silky smooth.

Here - 莱曼(lehmann) 耳放 莱曼构架的好声耳放,全铝机箱,一体耳放-淘宝网

When I've had a listen (gotta search for my 1/4" adapter) I'll get into modding.... Incidentally for those on 110V, it does have a mains voltage selector switch. Amazing.

I traced out the circuit (still haven't listened) and found its pretty much as shown on the Taobao page. 78/79 15V regulators feed BD139/140s and a TL072 sits between the volume pot and the discrete OPS. This stage resembles a diamond, but instead of the more familiar current sources to the rails, it has 1500ohm resistors. LTSpicing this arrangement (schematic shown) reveals its...
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Old

Why might a diamond buffer sound better?

Posted 25th May 2015 at 07:00 AM by abraxalito

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...
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Old

What's the PSRR of an emitter follower?

Posted 16th April 2015 at 01:28 AM by abraxalito

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...
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Old

Are PSRR plots all they're cracked up to be?

Posted 15th April 2015 at 04:19 AM by abraxalito

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...
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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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Atmel plays fast and loose with its new chip's low power credentials

Posted 31st March 2015 at 02:26 AM by abraxalito
Updated 2nd April 2015 at 03:04 AM by abraxalito

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...
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Old

TDA8946AJ headphone amplifier

Posted 20th March 2015 at 04:13 AM by abraxalito
Updated 23rd March 2015 at 06:40 AM by abraxalito

I built this up to see how the TDA8946AJ DC-controlled volume element sounds. According to the datasheet and comparing the figures with those for the same amp minus the volume control (TDA8946J) there's some additional THD so I was curious to hear how transparent it is.

There are transformers both on the input and the output of this. On the input to match levels (4:1 step down) and to do unbal-bal conversion. On the output again for the same reasons, except bal-unbal to drive my phones which I've not modified for 4 wire operation. The trafos are ferrite (=very cheap) RM10 for the input (under the board) and RM12 for output (behind the board). In order to be able to match a variety of phones, I've wound 4 identical secondaries so they can be combined in series or parallel.

From the pic you'll see the customary array of caps - I need to order up some lowESR (preferably NCC) 1800uF caps to do better justice to the rails. Last night I experimented with some ceramics...
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Old

Schitt's flagship DAC uses a not-for-audio DAC chip.

Posted 7th March 2015 at 07:43 AM by abraxalito
Updated 17th August 2015 at 10:38 AM by abraxalito

The Schitt Yggy DAC has already created something of a stir over on Head-Fi amongst those who've heard pre-production prototypes. Its of interest not just because of the pre-launch buzz but also because the designer (Mike Moffat) is one of the long-time seasoned guys in the field. He says this is the best practical DAC he knows how to build. And its priced ISTM very reasonably given the amount of tech it embodies ($2300) - the DAC chips come from ADI and are $64 a piece on the manu's website. He's using 4 of them but says he had to address the glitching of the DAC without using a sample-hold which sounds like ***.

The chip is the AD5791BRUZ which ADI designed for industrial/scientific/medical applications rather than for audio. Datasheet attached.

Having looked over the DS what strikes me as interesting is that this is a 20bit DAC (1ppm) yet the 1kHz THD performance (p4) isn't anything to write home about (-97dB) and that figure is given at a very low sample rate...
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