Posted 22nd April 2017 at 04:30 AM bygooglyone Updated 22nd April 2017 at 04:40 AM bygooglyone
After a few months of playing with loudspeaker drivers and software fixes to code that I wished I had never written in the first place, the lure of the BJT loomed large again...
There I was, sitting at my desk, and pondering where these dark feelings were coming from and how to scratch that evil itch, and my eyes came to rest upon that most ludicrous of things - the amplifier of 100 transistors.
About a year ago, or so it seems to me sitting here, I built this homage to the BC549, or as the engineer in me says, "Laughably stupid thing, which probably shouldn't work. Who would be dumb enough to try and find out?".
It needed a case. It needed a power supply. It deserved each of the above that would allow it to work, and ideally take an equally "left field" approach. it should never have been built, but it had. It should have been put in the back of a very dark, locked cupboard, but it had not.
Years ago I moved from the country back into the city. This was a move that required me to clean out heaps of stuff that I really didn't want to clear out. In our old place we had two houses on the block, one of which had been converted into an enormous studio room with NO walls, and a 14 by 14 metre shed. To say that I amassed a collection of speakers and stuff would be an understatement.
One of the things I got rid of were a pair of Richard Allan CG-10 drivers that I never got around to sticking in a box. Indeed, I never even got around to measuring their parameters.
Recently on EBAY I came across a pair of these...
I was surprised at the condition of these - the cones look new. Needless to say they are not, the date code is 1973.
I have measured and used the CG-12, HD-15 and a bunch of other Richard Allan drivers in the past and found them...
in my electronics workshop I have always had a stereo. Eight years ago I looked on the shelf and decided there was all the makings of an integrated preamp - DSP - Crossover and four power amplifiers.
That is the price of always making an extra board or two when I build a "thing" - you end up with a massive collection of bits.
I duly shoe-horned this lot into a case that is about 3" high. Tuned it up and didn't think a lot more.
Fast forward 8 years to two weeks ago, and there I was listening to the radio while wondering what to build next, and I thought the left tweeter sounded a bit low.
I swapped input to the amp L-R, and yep, it was. Odd - as the DSP treats L and R exactly the same, there is no balance control in there. So I swapped the speakers. Hmm, it wasn't the tweeter. It was inside the "box".
Posted 8th January 2017 at 01:35 AM bygooglyone Updated 8th January 2017 at 01:49 AM bygooglyone
Well here I am at the end of another summer holiday - with yet another thing I have built for which I have absolutely no plans at all!
I have over the years built several (several dozen I would expect!) discrete operational amplifiers. The challenge I set myself here was:
- Actually get some proper measurements of it's performance, now that I have some test gear that is up to the job.
- Make the device in something close to a DIL-8 package. (slight fail here but close enough)
- Use some of the multitude of parts I have collected of late.
- The ability to configure the op amp as a buffer or amplifier with non-inverting gain. This means adding feedback resistors and a DC block cap.
- The ability to include input RF filter (typical few hundred ohms series and 1nF across the input)
The schematic won't surprise anyone - over the years I have designed ridiculously complex and simple amplifiers, and have run...
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.