Posted 5th January 2016 at 11:48 AM bygooglyone Updated 10th January 2016 at 06:45 AM bygooglyone(Update documents)
I have been asked for the CAD files for the distortion meter I recently built.
NOTE 10 Jan 2016 : I have changed the clock driving and distribution PCB as it really needed to be more versatile to me to run a mix of CS and AKM ADC and DACs. The change allows selection of MCLK at different multiples on the ADC and DAC via jumpers on the board. Again this is prpobably a bit more "hard wired" than a generic consumer device would be, but allows stable operation for fixed sample rate systems.
This project is not a super straight forward "chuck it together and it will all be fine" sort of build. I am providing what is essentially a collection of USB interface (MiniDSP), power supply (mine - open source), backplane, clocking and galvanic isolation (mine - open source), A/D and D/A (mine - open source) and a differential interface and attenuator (Silicon Chip magazine), though I am strongly tempted to do my own.
I have more or less completed the audio analyser based on CS4398 and CS5381.
In an earlier post I suggested I had reached the limits of these IC's. I was wrong. What I had reached the limit of was getting the grounding "OK" for a single (unbalanced) input measurement system.
I have since built a balanced front end - in fact I simply built a Silicon Chip PCB as it was pretty well what I would do - and integrated this to the ADC and DAC.
It looks a bit like this:
What you see is:
- On the left are two independent power supplies
- In the back middle is a MiniDSP USB Streamer card.
- In the middle back is an interface card that
- Does optical isolation of the MiniDsp USB Streamer
- Does more regulation for the ADC and DACs
- Generates local clocks for the DAC and ADC
- Feeds these back to the MiniDSP Streamer
On thinking over the sensitivity of the CS5381 to the drive and input filtering capacitor I decided to explore the capacitors further.
I was also interested to note that a number of manufacturers seem to recommend a range of different capacitor values here.
Purely because it was handy - i.e. right in front of me on the desk - I threw an extra 2nF capacitor across the differential input of the CS5381. Boom - the distortion dropped 6dB straight off.
I muttered a few choice profanities, which made me feel an awful lot better, then arbitrarily threw a 470pf NPO ceramic cap across that lot, just to be sure. Well given I was off doing such arbitrary things - why not?
On analysis, 5nF capacitance at 20kHz is about 1.5K Ohms (reactive) which is within the capability of the op amp to drive.
Which I found rather pleasing, as using the "default" 2n7,...
Posted 6th September 2015 at 12:47 PM bygooglyone Updated 6th September 2015 at 12:56 PM bygooglyone
I have had the time to play with the MiniDSP Streamer and my ADC and DACs now.
Initial results were disappointing - and led to me looking very closely at the ADC drive, and particularly the single ended to differential part of the circuit.
Given I pinched this circuit from an application note (and embarrassingly did not question it adequately) gives me little solace that I built, and used this!
Interestingly, Creative Labs did exactly the same thing on the sound blaster that I had so much trouble improving - the single ended conversion is just wrong. This solves the "Puzzle" that I noted in a blog a few months ago - now I know why I just couldn't get better performance out of that CS5381 drop in to the Sound blaster box.
In the process of improving the ADC drive, and to allow experimentation with the input op amps, I built two versions of the ADC drive, one using non inverting buffers and a second that uses inverting...
I recently set to work the guts of the audio analyser:
- MiniDSP USB Streamer
- Interface card
The ADC and DAC were from an old project. the tests I had been able to do on these were done using simple test gear, I knew the distortion was fairly low - but had been unable to really bottom out the level.
I did know the DAC was down in the 0.001-3% level or below.
Imagine my chargin when I fired the system up, got all the bits running and measured something closer to 0.01% distortion at -1dBc!
I was initially convinced that there was something wrong. After cranking the level down to -10dB, the distortion dropped to 0.001%. Further tesing (I ran the DAC at -1dBc and attenuated the output) proved that it was the ADC dominating the measurements.
Much fiddling led to me concluding that the ADC buffers were the culprit. I would like to blame...
Over the years I have played with measuring - well - just about everything in audio.
distortion measurement has been one of these preoccupations, not the least because it is hard to do, and using commercial gear, expensive.
I have built analogue hear to do this, used mixes of analogue and digital and of late played with using modified commercial sound cards. My efforts on the Sound Blaster x-Fi were interesting but ultimately not rewarding enough for me to leave the mods in there.
I have also been playing with using the A/D and D/A converters from my DSP crossover for this purpose. These are modular and use a common I/O plug with power MLCK, SCLK, LRCLK and Data (alone with I2S for the CS4398 and sometimes a digital volume control).
--- notwithstanding the fact that in previous tests I have shown the digital volume control is the MAJOR source of distortion - I will plug ahead with this, and likely leave that out of the test...
Posted 29th March 2015 at 01:51 PM bygooglyone Updated 7th September 2015 at 01:06 PM bygooglyone
I have been playing with measuring distortion of signal sources and amplifiers a bit over the last year or two.
I am serious enough to spend some time and thought on this, but not serious enough to sink significant cash into application specific test equipment.
I have a couple of USB Sound Blaster X-fi Music (SB1240) sound cards I use for measurements. The original intent of these was for speaker test and general music. I am pressing these into use for more serious measurement.
The ADC in these is the Cirrus Logic CS5361, which is a pretty good ADC. The DAC is the AKM4396, which is a decent DAC.
When making measurements I hit a problem. The raw performance of the CS5361 is specced to be -99dBc guaranteed an -105dBc typical for THD+N. I couldn't get this.
A few things quickly went onto my list of things to try:
-1- Switching - the sound card has a lot of this - how was it implemented?
-2- Op amps...
Posted 27th January 2015 at 03:07 AM bygooglyone Updated 26th January 2015 at 09:02 AM bygooglyone
I recently picked up a pair of, I guess, 1970's Richard Allan three way speakers. The line-up are RA drivers with which I am not familiar. Bass, LP10B, midrange LP5B and tweeters - long dead and replaced with mismatched dome tweeters.
Richard Allan was reasonably popular in Australia in the 70's and 80's, and did some pretty good gear. I was interested to see how these went, but did need to do something about the tweeters.
On pulling the drivers out, I noted a few things:
- The LP10B is a 10 inch bextrene cone woofer, using a 1.5" voicecoil rather than the HP10B's 2" coil. It is also very much an "acoustic suspension" driver - read on.
- The LP5B is very much like a 5" version of the KEF B110 - as used in the LS3A. I read somewhere that Richard Allan made a version of the LS3A under license, but that might be an "internet fact".
- The crossover was made by KRIX, a local speaker manufacturer...