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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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Old

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 21st April 2015 at 11:50 PM 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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Old

Supercapacitor b*ll*cks

Posted 15th February 2015 at 11:50 PM by abraxalito
Updated 16th March 2015 at 03:25 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

TDA1387 octal DAC looks like a good modding base

Posted 15th January 2015 at 11:55 PM by abraxalito
Updated 13th February 2015 at 03:23 AM by abraxalito

Thanks to Matthieu (Malefoda) for the heads up on this. It looks distinctly like a DAC inspired by the DAC-AH - note not only the 8 DAC chips but also the PCB layout closely follows that design's. The price on Taobao is about half the AH (368rmb) so I shall be ordering up one or more to have a play. Matthieu found it on another website with prices in USD, here - http://www.cart100.com/Product/42124644877/.
Note there are two options here (blue, black) - the cheaper one is a kit of parts I believe, the more expensive ($84) the finished unit. Going on the translation of the Taobao page, the kit does not include the transformers, mains switch or the case.

Update : here's the summary of the mods so far, for details go to Malefoda's thread linked in the comments.

1) Re-route the output ground so the opamp filtering caps aren't subject to injected CM noise from the destination component (amp or pre).

2) Reduce the supply voltage to the output opamps...
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Old

Revisiting mods to the Lite DAC-AH

Posted 12th January 2015 at 01:57 AM by abraxalito
Updated 12th January 2015 at 02:07 AM by abraxalito

Last night the battery of my I2S source ran out so while it was on charge, I unboxed my last remaining Lite DAC-AH to have a listen to that, fed from my (mains powered) QA550 wav player. I'd bought 3 DAC-AHs a few years back when I was playing with them.

The differences between the stock DAC-AH and the Ozone Pagoda were apparent in the areas of soundstage depth, dynamics (especially at LF) and coarseness at higher levels. So this got me wondering what would be the lowest hanging fruit in terms of mods to it. My procedure before had been to implement everything I could think of before having a listen - this way its not possible to know which mods are the most effective ones and which can be omitted.

This time I'm going to be a bit more disciplined for the purposes of discovering which mods deliver the best bang for the buck. The first one I'm going to try is a passive LC filter between the DACs and the AD847s. Here's the schematic for it - its using the cheap and...
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Old

New year, new DAC

Posted 31st December 2014 at 07:36 AM by abraxalito
Updated 9th January 2015 at 12:27 AM by abraxalito

Here's the 'Ozone desktop pagoda' DAC for 2015.

A single TDA1387 feeds a 3 inductor quasi-elliptic filter followed by AD8017s as buffer-amps. The large ferrite cores in the base do the bal-SE conversion. The power supply is 4 * AA NiMH cells which should in theory last for a whole day's music.

The design is really a 'MkII' version of the Ozone portable where the AD815 buffers have been replaced to allow a more compact construction and lower power draw without the constraint of being able to drive IEMs directly. Whereas the portable used a stack of 1387s due to the choice of 7mm TDK inductors, this one's using pot-core chokes giving a much higher working impedance and hence higher output levels from just the one DAC chip. The desktop footprint is about that of a CD.

Update : I found some bargain Jamicon caps on Taobao which give the tower more elegant proportions, as well as improving the supply impedance to the buffer-amps and giving more breathing...
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