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The Art of Analog Design

Discrete transistor based CFB I/V stage

Posted 25th September 2015 at 01:56 PM by abraxalito
Updated 26th September 2015 at 01:35 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 20th September 2015 at 11:54 PM by abraxalito
Updated 25th September 2015 at 01:20 AM by abraxalito

I've been getting a lot of use out of Simon Bramble's webpage for designing active filters recently - 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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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 -

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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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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Force/sense shunt

Posted 24th April 2013 at 08:35 AM by abraxalito
Updated 24th May 2013 at 03:59 AM by abraxalito (Added pics of prototype fs-shunt. Added noise update.)

I'm a recent convert of the lowest possible impedance of power supply based on my experience of adding caps to my chipamp. So I figure the signal stages can't be harmed by reducing their supply impedance either, particularly at LF.

I note there are a few aftermarket regulators around - I had a look at Paul Hynes and Belleson in the past few days. They're a bit pricey for my tastes, given the cost of the components they're using can't be over single digit $ so I've had a look at "doing it at home, only cheaper".

First off, a simple TL431 is about the best bang for the buck achievable, as the part here is 0.2rmb. But the dynamic impedance is typically 0.2ohms and I was hoping and aiming to go a bit lower than this - perhaps an order of magnitude lower, to around 10mohms. Lower than this and the resistance of the PCB tracks come into play and its also very hard to maintain such a low impedance beyond the audio band as cap ESRs (for the best ones) are of...
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What's in a datasheet?

Posted 7th October 2011 at 05:38 AM by abraxalito
Updated 14th November 2012 at 01:47 AM by abraxalito (Updated with link to new article 14th Nov 2012)

Yesterday I had this very interesting exchange with RocketScientist about his open source design for a headphone amp, the O2.

The nub of the issue raised here is - should designers stick only to what datasheets tell them about parts or to what extent use what's 'common knowledge' about parts to eek out better performance?

I was surprised to learn from RS that offsets within dual opamps are so closely matched in practice - its a really new discovery for me. So why don't semiconductor manufacturers tout this feature? Or perhaps RS just 'got lucky' with the relatively few samples he tested?

My experience of reading opamp datasheets is that the specs for offsets (both the typicals and the max) degrade in going from single to dual devices, where the devices are all on one die. Let's have a look at a relevant opamp from...
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An engineer's engineer passes on...

Posted 14th June 2011 at 02:35 AM by abraxalito
Updated 8th September 2011 at 03:13 AM by abraxalito

Analog guru Jim Williams dies after stroke - 2011-06-13 15:26:32 | EDN
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More transparency in active speakers - part II

Posted 15th November 2010 at 12:59 PM by abraxalito
Updated 5th March 2012 at 02:58 AM by abraxalito

Its not something I claim originality for, this use of Cat6 cable for running to speakers. What though I believe is original is combining the function of speaker cable with the sharing resistors - I've seen that nowhere else. Remember guys, you saw it here first

Not only does this save on having low value fairly high power resistors but it comes with another benefit which suits diyers. That's the fact that the amps can be easily tested separately - they're not going to be in parallel until the drive unit is connected to the output, since the cables join together only at the far end, at the terminals of the driver. I've certainly found this an advantage in development - the individual gains I've checked with a multimeter between the outputs when playing a 400Hz sinewave. This frequency was chosen because I'm too lazy to check the frequency response of my multimeter and I wanted a mid-range test.

The gains of the individual chip amps do need matching very...
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More transparency in active speakers

Posted 13th November 2010 at 03:23 AM by abraxalito
Updated 5th March 2012 at 03:05 AM by abraxalito

Those of you who read my earlier posts about improving my Fayou actives might have realised by now I'm a bit fanatical about grounding and its effect on sound quality. I had an idea - could a system be put together without any ground at all, and might this solve many of the sound quality issues I've found in my setup? I could only find out by trying it. So, the idea was born - 'Goundless Sound'.

First up, a groundless amplifier is just another name for a balanced, or bridged design, at least in respect of the output being balanced. A truly balanced amp would also need a balanced input - that's not really ground-breaking (pardon the pun, could not resist) either. Putting the two together is hardly rocket science. But I decided on a few twists - more about those in just a moment.

Convention is a useful thing. Ponder a world without convention in respect of computer keyboards - Asus and Toshiba having one layout of the keys on their notebooks, Dell and Lenovo...
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