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...
Posted 31st January 2016 at 01:06 PM byFrex Updated 2nd March 2016 at 04:34 PM byFrex
High performance 160W triple outputs Lab power supply
PCB GROUP BUY
Frex, the 31 Junary, 2016.
I opened here a new blog page to replace the dedicated wiki page previously
made for this group buy list.
The DIYaudio wiki is unavailable for weeks now and that will take some time
to be re-opened. So for this reason i move the group buy here on this new blog.
I will probably also take advantage of this new blog to share many others things.
Until now, 12 pre-order are done on 25 required. We will wait end of February,
and then if we stay less than 25, i will make a new price estimation for a lower batch.
The buyers list you will find below can only be...
I'm not sure whether its possible to build a passive, permanent device that dissipates/neutralizes electrical charge. But the scientist part of me finds the claims that you can interesting.
Static electricity will eventually dissipate by attracting counter ions from the air. This happens more quickly if the humidity is high.
So seriously, if you just stuck a wire into a bucket of dirt, how much "earthing" would that actually provide? Is there any way to amplify that effect by using special materials or even passive electrical components?
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!