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Old

Pocketable headphone transformers

Posted 24th March 2016 at 02:44 AM by abraxalito
Updated 25th March 2016 at 11:54 PM by abraxalito

Seems I got a bit carried away with listening to my heavily modded XuanZu amp in the last-to-one blog post. I even considered it was superior to my current headphones - that would mean shelling out on some new ones. Before I went shopping though I did try it into my DT880s, which are 600ohms (hence I usually only use them for special occasions) - they sounded cleaner, although considerably quieter. This amp doesn't have enough voltage swing available to deliver the SPLs into such a high impedance. So was the cleanliness of these phones due to their being higher quality (they're at least 4* the cost of the others)? Or just because of listening at a lower level?

When my over-enthuasiasm for the amp had subsided a bit I decided to consider a way to answer these questions. If the amp was indeed not producing the artifacts which I was hearing on piano into the low-Z cans, then putting a 2:1 step-down transformer on its output would make no difference at all. The amp has plenty...
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Old

Sanwu FLAC and Bluetooth player has a significant flaw

Posted 16th March 2016 at 12:59 AM by abraxalito
Updated 17th March 2016 at 03:18 PM by abraxalito

I was going to write a post praising this player for the superb value for money (its the only cheap single-chip FLAC player I've found) but this morning it produced an alarming series of whistles and pops from what I presume is a corrupt file on my TFcard. So now its only recommended if you're sure you have perfect data on your card - it doesn't seem to mute the audio when an error is found.

Apart from this major howler at just 30rmb its great, providing as it does FLAC, WAV and mp3 support along with Bluetooth running from a USB power source at 5V. The audio performance is decent when run through my modified XuanZu headamp as preamp - the level is rather low otherwise and I suspect it needs a high-impedance buffer for best dynamics.

I put the TFcard which gave the player hiccups into my PC reader and uploaded the 'problem' file to Audacity. No glitches noticed there so looks like I might have to dig a bit deeper to find out what went wrong. I shall try playing...
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Old

Rant on the SET critics

Posted 8th March 2016 at 05:16 PM by DougL

I am so tired of hearing that anyone who listens to SET amplifiers "Must like distortion".

It must be comforting to be that simple.

The reality of the situation is a single ended tube amplifier is a very specific solution to a very specific problem.

The topology results in an implementation that has vanishingly low distortion at very low power levels.

This is a direct result of three factors,
Low distortion of the audio triodes used, the lack of multiple devices and the avoidance of the non-linear magnetic behavior around zero crossing.

FWIW, this only works in the tube world for very low power on very efficient speakers.

I still have memories of listening to a Klipsch horn with a Phase Linear 400. I could not stay in the room at any power level. It was near class B and arguably the worst amplifier ever made. Still, it took an efficient speaker to highlight its faults.
...
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Old

Matching JFETs

Posted 8th March 2016 at 12:29 PM by rjm (RJM Audio Blog)
Updated 6th May 2016 at 09:27 AM by rjm

with only two resistors, a 9 V battery, and a voltmeter...

The current-voltage relationship for a jfet device is approximately a quadratic expression defined by just two parameters, the saturation current, I_dss, and the pinch-off voltage, which I'll call V_gs0.

I = I_dss (1-V/V_gs0)^2

In principle, therefore, to characterize the device all we need is two data points (I1, V1) and (I2, V2) to solve the expression above for I_dss and V_gs0. We don't need to measure I_dss or V_gs0 directly.

All you need to do is connect the jfet device-under-test (DUT) as shown, and measure the voltages across two different source resistances. That's it. The excel worksheet computes the I_dss and V_gs0 values for you (or you can do it by hand, the formulas are provided.)

The math is a bit messy, but if you can solve a quadratic expression it's easy enough.

*****

Note: I found it was important to include...
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Old

V-FETRON 75 UPDATE #1

Posted 6th March 2016 at 03:05 PM by Michael Rothacher (Audiohobby.com)
Updated 6th March 2016 at 10:34 PM by Michael Rothacher

Achievement Unlocked: 75 Watt Class A Zero Feedback SIT Circlotron

Click the image to open in full size.
Soft clipping at 75W

Because of the high input capacitance of the 2SK180’s and the gain of this circuit, we have to pull out all the stops to get our 75W beast to make audio bandwidth. Here you can see that each SIT is getting its own transformer and buffer.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.
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Old

CrystalFET Phono Stage

Posted 3rd March 2016 at 04:55 AM by rjm (RJM Audio Blog)
Updated 29th April 2016 at 02:10 AM by rjm

Development thread here.

CrystalFET is a J113 jfet-based two-stage phono preamp, with passive equalization and on-board MOSFET-based shunt voltage regulator.

Black boards pictured are the original rev. 1.1a prototype, which I ended up using for mc operation. There was a connectivity error in the schematic used to make the boards, so the layout was redone as rev. 1.2a.

1.2a are for mc operation only, about 55-56 dB. I'm giving these away for $5 for one pair, see here. [only four sets left!] Rev. 1.2a is up and working. No abnormalities, near-perfect agreement with the LTSpice simulation.

Last, rev. 1.3c features switchable 35/56 dB gain for moving magnet and moving coil cartridges, and adds a jumper for the regulator "boost" feature. The basic circuit hasn't changed, just fixes and refinements of the concept. Pictured below.
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Old

Upgrading the XuanZu portable headamp

Posted 26th February 2016 at 11:22 PM by abraxalito
Updated 14th March 2016 at 06:23 AM by abraxalito

This device is a steal on Taobao, but having had a quick listen last night it could sound clearer. When connected to my smartphone (Meizu MX4 pro) and compared side by side with my 'Buffalito' (not a blind comparison mind) into my SuperLuxes, there were a few notable deficiencies.

First the soundstage air was less apparent. Second there's some sibilance noticeable on voices. And third the background hiss is slightly more apparent and a slight whine comes from the power supply. So I figured - open her up.....

Inside its fairly simple, the more or less standard configuration of a pot, then opamp gain stage then discrete diamond buffer. Which is great because I already have experience with this topology. The power supply is a built in LiIon cell with a boost converter supplying 12V in a single rail and there's a passive rail splitter. The dual opamp is an EL2244, one I've not seen before in such a setup.

First up - the input pot is too low a value at...
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Old

Chromecast Audio Output Noise

Posted 26th February 2016 at 11:11 AM by rjm (RJM Audio Blog)
Updated 27th February 2016 at 12:18 AM by rjm

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.
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Old

Voltage Regulators for Line Level Audio. Part 11 : The Crystal M Shunt

Posted 20th February 2016 at 12:49 AM by rjm (RJM Audio Blog)
Updated 22nd February 2016 at 08:35 AM by rjm

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.

It's lifted directly on the Salas shunt design (as reworked by me for my own jFET phono stage), but the circuit can also be considered a distant, DC-coupled relative of the Zen amp.

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...
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Old

An LC filter for supplying a line stage

Posted 19th February 2016 at 12:33 AM by abraxalito
Updated 22nd February 2016 at 12:51 AM by abraxalito

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...
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