Posted 4th June 2016 at 04:42 AM bygooglyone Updated 4th June 2016 at 05:18 AM bygooglyone
I finally got around to rolling out the distortion test set and the Amplifier of 100 Transistors to measure its performance.
I would like to say that I don't care - and that the whole thing is an engineering abortion. A complicated joke, and that the measurements don''t matter. The fact that I am making the measurement would however show me to be a liar - as if I didn't care, then why did I do this?
Anyway, with low distortion measurements, getting your head around the baseline of your test gear is key. With all gains / levels being equal, here is the loop-back distortion of the test system:
Which is fine, rolls along at about 0.0003% across the band.
Then I ran a sweep of the Amplifier of 100 Transistors with NO load at 3dB below clipping:
OK, this is saying the amplifier distortion raises it's head above the noise floor at 1KHz and is...
Posted 28th May 2016 at 02:18 AM bygooglyone Updated 28th May 2016 at 02:22 AM bygooglyone
In my previous post I presented an idea spawned from a very bad place, primarily boredom and probably too much alcohol. Plane trips from Australia do that...
The Amplifier of 100 Transistors was the result.
Between that posting and this a few things have happened. I finished the design - adding extra decoupling and 100 Ohm base resistors to all 104 output devices. I did this because I sincerely thought I was building more of an RF oscillator than amplifier.
And I built it.
And I got it working.
This is the beast from the back before I loaded the "output devices":
Note the ludicrous number of emitter and base resistors! Half way through I concluded that I was bonkers, and wasting several days hand building a complete folly.
Then again, even with the thing running, it is still a folly!
Posted 14th May 2016 at 03:48 AM bygooglyone Updated 14th May 2016 at 03:53 AM bygooglyone
As a kid, which depending on my wives mood could be stated to be "right now" right through to "you have never been a kid", I once made the statement that "if it can't be done with a BC549 it is not worth doing".
this statement was made in jest at the time, and probably stolen from a similar a similar assertion about the NE555. (those of you who are < 30 years old probably haven't seen these used in real anger!)
Here I am travelling, and flying from Adelaide (Australia) to the USA. This is a long, boring flight. In a fit of boredom I set myself a challenge.
So what is the challenge? Something cool and completely different for once. Hmm. Make a power amplifier using BS549's. If you have seem my play room, amplifiers are made to scare speakers and annoy the neighbours. So this can't be a lightweight 100mW job. It must be something that actually works, and is able to make real noise.
Posted 5th January 2016 at 10:48 AM bygooglyone Updated 10th January 2016 at 05:45 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 11:47 AM bygooglyone Updated 6th September 2015 at 11:56 AM 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...