I've been meaning to get around to updating this by folding in the improvements to the diamond buffer stage made during development of the Sapphire 3 headphone amplifier. Here is the first look of the bboard v2 under LTSpice.
I've gone back to simple emitter resistors on the input, running under much lower current to keep the input impedance high. The output is simplified to a basic Sziklai compound transistor pair with the bulk of the bias current running in the second transistor.
In terms of distortion, for line level output level, CCS loaded input has no advantage. I'll have to double-check PSRR and a few other things before signing off on this version though.
I admit I did not give ASUS the benefit of the doubt and seriously consider their Essence STX soundcard as a replacement for my Onkyo SE200-PCI. ASUS make nice motherboards, but unlike Onkyo have no previous expertise in high end audio.
I am happy to report - a bit late in the game, the card came out in 2009 - that they've done a really good job with it and the drivers for Windows 10, technically still in beta, work just fine.
Asus updated the design recently to the STX II. The PCB has been redone, but the only visible change is the PCM1792A DAC has been moved towards the top of the card closer to the IV conversion op amps. An second LDO regulator IC, U34, empty on the STX, is now populated. A "TXCO" clock source is added next to the ASUS audio controller IC. The four film caps next to the output IC are replaced with WIMA brand. It's basically identical, so it...
Current version is 5.60C, last update was 2012 to be compatible with Windows 8. Driver package 5.60C installs without issues on Windows 10 64 bit. The AudioDeck utility installs as well.
However, all is not well:
On installation, Immezio 3D effects are enabled. This locks the sample rate at 48 kHz. 44, 96, and 192 kHz cannot be selected. Deselecting the Immezio 3D effects prompts a reboot, but the Immezio 3D effects remain enabled after rebooting. The card is stuck at 48 kHz. 3D effects (which enables the DSP processing such as Qsounds, EAX, A3D) cannot be shut off.
I see three possible workarounds:
1. Find a command line switch, or edit the installation batch file to disable Immezio 3D on installation...
Posted 2nd October 2015 at 06:07 AM byrjm Updated 9th November 2015 at 02:44 AM byrjm
This is a headphone amplifier with digital inputs, not a DAC with a headphone jack. Though technically given equal board space, the headphone amp, with hot-running single-ended class-A output stage, is surely the centerpiece of the design. (The Asahi Kasei DAC, with MUSES01 for the I-V, is no slouch mind you.)
First impressions. It is large, solid, and very nicely made, but - after seeing the inside - rather simple, spartan even. From the DAC output to the headphone jack is just two op amps and two transistors, the op amps being shared between channels. A third op amp most likely just buffers the analog line output. Apart from the headliner MUSES01 op amp none of the parts are especially expensive, though many were clearly carefully chosen for sound quality - the 2SC5196 for example. The TE7022 USB receiver is a disappointment, as is, to be honest, the single set of power rails and the use of dual op amps shared between channels.
Posted 26th September 2015 at 12:49 PM byrjm Updated 9th October 2015 at 05:24 AM byrjm
My Onkyo soundcard drivers stopped working when I upgraded to Windows 10. Onkyo says they have no plans to release a patch, so I'm left with no high quality audio solution for my computer. Since I already have a good headphone amplifier, what I'm mainly looking for is a high quality line level analog output.
One options is another soundcard, the ASUS Xonar STX being the obvious choice. I dunno, it doesn't grab me.
I was thinking with going with an external box this time, connected via USB. As this opens up about a zillion options, I'm going to limit things to,
Respected audio brands with a solid reputation for digital audio.
Small enough to be placed on top of my computer case.
Posted 26th June 2015 at 03:59 AM byrjm Updated 1st October 2015 at 08:04 AM byrjm
I'm often asked "which op amp sounds better".
The reply is usually a scowl and muttered "does it look like I care!?" Which is something of a lie... I do care about getting op amps to sound good. It's the phrasing of the question I dislike.
Op amps do not come in "good, better, best" flavors. All it is - and this is pretty obvious I would have thought but apparently not - all this is about is matching an op amp to the job it's going to do; the circuit it's going to be sitting in.
The op amp you'd choose to use as a DAC IV converter is different from the one you'd choose to back a 100k volume potentiometer in a preamp is different from the one you'd choose for an MC phono preamp input stage...
Why do you think there are like a thousands of different op amps to choose from in the first place? It's because there are thousands of combinations of op amp characteristics and properties ... not because companies...
Work stuff. I was writing Labview vis for an hp 4192A LF impedance analyzer and needed something to measure to check the data acquisition program. So I stuck some of my audio capacitors I happened to have into the 16047A test fixture "just to see".
I have no idea what these measurements are telling me other than yes, the 0.47 uF capacitors are indeed 0.47 uF ... up to about 0.5 MHz anyway. Maybe someone can do some technical analysis. I was struck though by just how quickly the inductance of these big film caps kicks in. As audio coupling caps they are fine, but if you are silly enough to use them as power supply bypass for example...
There are some reproducibility issues I'm still coming to grips with, but the differences shown in the plots is definitely from the capacitors themselves and not the leads or random variations. I've measured them several times over with similiar result.
Posted 21st May 2015 at 02:32 PM byrjm Updated 27th May 2015 at 02:54 AM byrjm
This post, about a push-pull MOSFET output stage for a headphone amp, got me thinking again about the Audio Technica AT-HA5000, which is something of a benchmark in its class. The "basic" signal circuit (not a complete schematic, it's clearly missing some ancillary details) is attached below. Probably out of MJ originally.
I think with any circuit like this, the differences are less about the MOSFETs and the operating points and more about the front end and what tricks you do with the power supply. That, and how you make sure it doesn't go up in a puff of vaporized silicon taking your headphones with it.
The Audio Technica schematic has nice old-school Zener regulators, a discrete JFET front end, a long tailed pair + current mirror for voltage gain and "proper" BJT Vbe multiplier and driver stage. Q7 is presumably in thermal contact with Q10,11 providing overtemp protection, and the output has a protection relay (not shown in detail) for...
Posted 20th May 2015 at 07:00 AM byrjm Updated 19th June 2015 at 12:26 AM byrjm(added schematic of original version)
The circuit was originally hosted on Headwize, but the site seems to have gone offline.
It was a single stage resistively-loaded MOSFET follower, a unity gain current buffer used to drive headphones.
Some updated versions provided below. As noted in the comments the "Reverso" version with the CCS on the V+ and a p-channel mosfet has better PSRR performance, especially with voltage divider network R6,R7,C4 on the collector of Q2.
So good in fact that I switched around the n-channel version to use a negative voltage rail to obtain the same result!