Posted 26th February 2016 at 11:11 AM byrjm (RJM Audio Blog)
Updated 27th February 2016 at 12:18 AM byrjm
Measured at 24/96 with my Asus Xonar STX soundcard (~ -147 dB noise floor)
The Chromecast Audio output noise powered with the included USB wall wart supply is -130 dB at 1 kHz, rising gradually at lower frequencies and showing some switching power supply noise peaks at 4763 Hz and higher multiples, never exceeding about -120 dB.
This is respectable performance given its price point.
Posted 20th February 2016 at 12:49 AM byrjm (RJM Audio Blog)
Updated 22nd February 2016 at 08:35 AM byrjm
A while back I did a series of blogs on voltage regulators. Back with a new entry today: The Crystal M, configured here for 40 V DC output and a 25 mA load.
The circuit is based on two p-channel MOSFETs, the top one is a constant current source, the bottom one a constant voltage source. As the load current changes, the voltage source adjusts its current to balance.
I trick, I discovered, to getting it to work nicely - the attached screencap shows it well-behaved while handling a full-swing output current pulse - is the source resistor R10. This resistance dials-down the current gain of the MOSFET, damping out the overshoot.
The ripple rejection is about 70 dB over the audio bandwidth. The output impedance is about 0.05 ohms over the same frequency...
Posted 19th February 2016 at 12:33 AM byabraxalito Updated 22nd February 2016 at 12:51 AM byabraxalito
I've designed LC filters for classAB amp power supplies before - for those applications iron powder toroids work fine for the Ls. However for the power supply in my latest DAC design I wanted more supply rejection and this calls for higher value inductors - in the tens of mH. Creating a 10mH inductor on a toroid takes way too long and is hugely fiddly as the wire length needed is substantial and I don't have any specialized winding machine. So toroids are really out of the question at such values.
I have some largeish inductors in the right range wound on bobbin cores but when I checked the DCR it was a little high, 20ohms or so. As I might need up to 100mA, a 2V drop is too great. In any case, in LTspice this resulted in rather an overdamped response - what I really needed is something in the range 1 to 2 ohms. The solution seemed to be use ferrite cores of the kind normally used to make transformers. Which means breaking a kind of informal rule I made for myself about not...
Posted 18th February 2016 at 11:14 PM byrjm (RJM Audio Blog)
Updated 7th April 2016 at 06:58 AM byrjm
I've never put everything into a single LTSpice worksheet like this before: I find it fascinating. You can really pull apart a circuit to see what makes it tick, before solder ever hits the iron.
Power supply ripple, frequency response, gain, and crosstalk can be established. You can look at turn on and turn off transients, inrush currents, and conductance angle, and check peak currents in the filter capacitors. It's all there if you care to peek in and poke around.
I'm such a huge fan of LTSpice...
The only problem, really, is it is too perfect: all devices are perfectly matched, every part value is exact, and the temperature is always 25 C. Ground loops, wiring inductance, and thermal runaway do not exist. So no, of course there are no guarantees - but as a tool to get you 90% of the way there with the minimum of fuss and bother it is truly indispensable.
Actually I find the more experience you have the more useful LTSpice...
Posted 16th February 2016 at 01:27 PM byrjm (RJM Audio Blog)
Updated 16th February 2016 at 11:51 PM byrjm
There are various tricks, like parallel input devices and active current sources, that I have avoided here in the interests of simplicity. If you want to go down that road, you can get an idea where it leads, here. Instead, the circuit below is basically a JFET version of my old 6DJ8 amp, here. A single JFET was getting me nowhere in terms of output impedance - around 10kohms! - so I moved to a compound stage buffering each amplifier with a source follower.
Noise and distortion figures look okay. The gain is only 30 dB. A bit low. The main trick is the PSRR, which is awful. The two stage circuit actually amplifies the power supply noise onto the output. So considerable effort must be put into the power supply regulation and filtering. I note that this is pretty much par for the course with this circuit topology where resistors are used instead of current sources on the JFET drains.
The circuit below leaves out the usual RC filter inserted between the power supply...
Posted 16th February 2016 at 08:21 AM byrjm (RJM Audio Blog)
Updated 16th February 2016 at 11:47 PM byrjm
This Excel (2013) worksheet will help you fine tune the values of the resistors and capacitors used in the passive RIAA network found in any number of two stage tube, op amp, and FET phono stage circuits.
Excel handles complex numbers well enough now that this job isn't particularly difficult, though for simplicity the DC blocking cap (Cc) is left out of the calculation.
Posted 15th February 2016 at 06:45 AM byrjm (RJM Audio Blog)
Updated 19th February 2016 at 10:59 PM byrjm
There are lots of phono stage circuits floating around based on two jFET amplifier stages and a passive RIAA network. I'm not sure who did what first, but there's the Boozehound, LePacific, and of course Salas versions.
Setting aside concerns about the ripple rejection**, today I'd just like to focus on the distortion and noise of the circuit itself. The passive RIAA stage is a large obstacle. It attenuates the signal substantially at all but treble frequencies, and it generally presents a large series impedance - both of which tends to increase circuit noise.
The jFET themselves meanwhile are a fine balance between low current, low noise operation with high distortion, or running at high current, high noise, with low distortion. Circuit gain must be paid for meanwhile with added distortion since the two stage design struggles to manage 40 dB.
After spending some time in LTSpice with this, I realized it was the proverbial rock and hard place...
The customers original problem with this amp was that it had no sound on one of the two channels, after replacing various components in the amp section and the power supply section it turns out that all it was, was a bad crossover selector switch. The contacts had broken on the inside of the switch. Sourcing this switch is a problem because most companies carry it but you have to buy like 100 or more at $.40/ea. Did find that I can get one from Mouser electronics next month. So in the mean time for testing purposes I hard wired the contacts in the switch together with solder until I can get a new switch.
Posted 2nd February 2016 at 07:11 AM byrjm (RJM Audio Blog)
Updated 24th February 2016 at 01:02 AM byrjm
The discussion thread at the headphone forum is here, but I wanted to throw out the problem to the general blog-reading community here at diyaudio to see if anyone can nail this.
The earthed chassis (light blue) must connect to the circuit common i.e. "ground" (pale green). I do not know where the best place on the circuit ground is to tie that connection.
(COM and GND are completely equivalent pads on the circuit board, while IN- and OUT- also pads on the board but physically further away on the ground plane.)
Answer: as long as it connects at one point only, or the same point of both channels, it doesn't seem to matter at all. I have it connected at the ground tab of the headphone jack and that seems to be as good as anywhere.
The noise was in fact magnetic interference emanating from the transformers. Grounding layout changes / electrostatic...