I recently was motivated to figure out numerically what might be happening when you get hum out of your power amplifier only when connecting other cables to it. And why a fix I sometimes use works so well.
Before you go blowing your budget on expensive audio isolation transformers, please take a look!
Its quite a good fit for audio purposes because there are a couple of I2S interfaces with a dedicated audio PLL. There's plenty of RAM (64k) and according to the benchmarks, it'll do the equivalent of around 100MIPs. Its also respectably low power when running out of RAM - use the flash memory accelerator though and its not quite so frugal.
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.
WARNING:
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.
While after about 6 months of working on my Fender PD-250 PA System, I finally got it back together tonight and it works great. I don't think that I am going to mess with it any more.
Thanks to everyone on the forums for the help and suggestions. This should be ready to rock the parade season this coming year. Right now I can vibrate the floors of my office/work area with about quarter volume.
Stay tuned to this as I work on the Hiphonics zxi1008 amp that I have on the bench now.
Posted 18th December 2015 at 04:36 AM by2 picoDumbs Updated 4th February 2016 at 11:58 PM by2 picoDumbs
Contained here are all my observations with this amp.
1) The schematic the DIYAUDIO Store PCB is based on does not allow 2nd harmonic adjustment through changing source resistor values.
2) The PCB circuit can be changed to the original degenerating circuit by moving cap leads to other side of resistor lead. This will restore 2nd harmonic adjustment through source resistance modification.
3) Output impedance also appears to be significantly affected (only tested in simulation).
4) As an alternative to the PCB mod, 2nd Harmonic adjustment can be altered on PCB circuit by selecting devices with different transconductance curves if one does not want to modify the PCB.
5) As an alternative to performing transconductance measurements and selecting non-matched devices for 2nd harmonic addition, installing one IRFP250 and one IRFP240 per channel will also give a dominant 2nd harmonic.
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
...
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.
