Here's my current bench lash-up DAC based on three hexacaps. Total capacitance of the order of 1.3F.
32fs EIAJ is fed in, balanced, from the left, the central hexacap feeds two TDA1545ATs. Filters are TDK inductor based, six stage balanced with X7R caps (gasp!). Caps in the centre provide the mid-rail power (2.5V) which the I/V resistors are fed from. The two hexacaps on the right power the post amps based on AD815ARBs, gain set roughly to 26dB with a little NOS droop correction. They're the postage-stamp sized boards roughly in the middle. Output is differential and fed to ferrite-cored transformers to convert to SE.
I'm blown away by the dynamics of this, given the right recording. Next up is an amp based on paralleling 8 or 10 AD815s. Each fed from its own hexacap, of course....
Update1 - the DAC supply has now been upgraded to two tiers of the 4700uFs, interconnected by six 1.5mm diameter solid copper wires. Capacitive reactance measures 2.5mohm...
Posted 17th March 2014 at 10:49 AM bygooglyone Updated 17th March 2014 at 10:58 AM bygooglyone(incomplete)
It must be close to 12 months ago that I saw the PIC32MX450. I was convinced that a chip that included two I2S audio interfaces, and ran at 100MHz implementing 32bit arithmetic in pretty snappy time would surely be able to do some fun audio stuff.
I started with getting a board designed, and porting the code that I use to control my Analog Devices audio DSP. Then I generated the code to implement a direct digital synthesiser. These worked fine.
Over the last couple of weeks I have completed the "core code" for a digital crossover all implemented in the PIC32 itself.
The basic implementation is:
- An analogue to digital converter
- The PIC
- A digital to analogue converter
(Oh and an interface PCB done on veroboard to route the MCLK and power to the A/D and D/As. I am kind of tempted to re-spin the PIC32 board to do this for me...)
The PIC drives the I2S interface as master, generating...
I bought a pair of these in about '95 and modified them a bit. I blocked up the ports, stuffed them with fluff, replaced the inductor with an air cored type, and rewired with CAT5. I also removed the top cages and grills which subsequently got lost, and also removed the rubber from the woofer voice coil, and put in a phase plug fashioned from some dowel.
I lived happily with these for about 10 years, ending up with an 18" subwoofer which used an equalised bass guitar speaker. The advantage of this equalisation was a degree of cancellation between the sub and the little woofers, resulting in a very smooth lump free transition in the listening position.
I found them to be an excellent tool in the development of audio stuff, it was quite easy to discern the difference between different types of cables, for me and friends, etc etc.
I bought another pair but I was wondering if the tweeters were fried as they sounded a bit dull compared to my modded...
Here's my latest crazy project, intended to answer the question 'How low can you go?' in power supply impedance. No listening tests yet, I've just finished assembling this stack of hexagon capacitor arrays out of cheap electrolytics. Total capacitance of the top 4 tiers is comfortably above 1F. The top tier is designed for lowest ESR at HF using Nichicon 1500uFs, the next tier is a trade between ESR and uF. The last two are the highest capacitance density I could find (a total of 182 4700uFs). I've yet to connect the bottom two hexagons to the upper tiers but the top two are connected by wires running through the gaps between caps.
The whole is intended to push the limits of low frequency noise in powering a couple of TDA1545s which will sit on a small PCB supported on thick wires from the centre of the top tier. I am aiming to get a capacitive reactance (at 50Hz) at the DAC supply pins below 3mohm and ESR below 1.5mohm.
I have a pair of tired Goodmans Magnum SLs. They are in reasonable condition, but one mid is blown, and they have no front covers. I tried grafting a new mid on to the broken one, expecting I would need to do some crossover work, but it really did not sound good. A very unpleasant sound that I found quite fatiguing and unpleasant to listen to coming from the upper bass lower mid area, quite a relief to turn it off actually. I put bigger and bigger choke on the woofer to try and filter it away, but that didn't work. I took of the dust cap in case that was causing it but no.
I am thinking of re-purposing the boxes now, but first did just check the woofer to make sure it has a problem. Below are some distortion plots, for a good one, the bad one and also for a Wharfedale Dovedale 3 woofer, with fixed rubber (Note the different scale). These plots were done at quite low level, like normal neighbour friendly listening levels.
So you have a small handful of parts and want to build a (simple) discrete voltage regulator instead of using an IC. What to do?
For line-level audio circuits, especially op amp based (IC or discrete) preamps with high PSRR, something like the Z-reg is generally sufficient. Robust, works well, has enough ripple rejection to cut power line noise from the preamp output.
If you add just a couple more parts, however, you can add feedback to the Z-reg circuit, a simple error amplifier in the form of an additional transistor Q2, with the output-sampling voltage divider R1,R2.
The ripple rejection is not vastly superior to the circuit without the feedback unless some additional bypass capacitors are added as shown in the first version of the circuit below. The output impedance, however, improves from a few ohms to a few tenths of an ohm as a result of the feedback. Which could, in principle, be of use.
I'm not going to spend too much time on this one. The idea is to increase the input impedance of the pass transistor by buffering it with a jFET so it will support a high-impedance passive CRCRC filter section that generates a low noise reference voltage. The reference is defined not by a Zener or diode stack, but by a simple voltage divider. There is a LM317 pre-regulator on the front, but it is traditionally configured and works independent of the following circuit so it is omited here together with the additional transistor that speeds up the charging of the reference voltage filter capacitors.
The basic problem is that lowering the noise of the reference cannot lower the output noise indefinitely. After a point the output noise is defined by the performance of the pass transistor instead.
Two versions are presented, one with all the protection diodes and a simplified version with extraneous components removed.
LTSpice simulation shows so-so performance into a light load, with about 70 dB of ripple rejection and a fairly high output impedance, but the drop out voltage is respectably low and we must factor in - coming directly from the Jung Super Regulator - that this is just a two transistor circuit, with no error amplifier to provide feedback.
As a frame of reference, it is quite similar in performance to the Z-reg we looked at back in part III.
The k-multipler is of a class of voltage regulators where the output is referred to the input voltage, rather than to ground. It provides "X volts less than the input", rather than the traditional regulator which provides "X volts above zero"....
The Kmultiplier is as far as I know the best power filter you could possibly squeeze out of just 2 active components. Not only this, but due to its simplicity it's RF PSRR is greater than the majority of regulators. It has output impedance lower than most lytics, doesn't oscillate into film bypass, and won't glitch on fast load signals.