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Bruno Putzeys' MicroPre

Posted 9th April 2016 at 02:14 PM by alexcp (My DIY projects)
Updated 11th April 2016 at 09:21 PM by alexcp

In April 2013, Linear Audio published in Vol.5 Bruno Putzeys' article "The G Word, or How to Get Your Audio off the Ground". Linear Audio included a free PCB for the demo project from the article: a balanced volume controller, or rather a minimalist balanced preamp with two pairs of balanced inputs.

I have been looking at balanced interconnects for some time and decided to use the free PCB and build the preamp. Here is what I built.

The enclosure used is Galaxy GX247 with front and back panels custom made by Front Panel Express. It was difficult to place a C14 power inlet on the back panel, as the space is quite limited due to the central placement of the six XLR connectors. An easy solution would be to fix the power cord permanently to the preamp, but I found a compact snap-in connector that fits the 3mm back panel of the enclosure.

I understand the article is available for download at the Hypex web site, and the PCB gerbers and a...
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File Type: pdf The G Word.pdf (731.5 KB, 101 views)
File Type: zip MicroPre_gerbers.zip (53.9 KB, 54 views)
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Old

My Dream System (As of Today)

Posted 9th April 2016 at 08:08 AM by museguy

Fostex T40RP mk3 with mods, including MadDog Pads $140usd

Sennheiser HD600 $377usd

Headphone Amp
Krell KSA-5 Clone

DAC
?
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Old

Print A3-sized schematics from A4 printer (Windows/OSX/Linux)

Posted 8th April 2016 at 03:10 PM by autopoiesis

The situation

1. You have a schematic in PDF format, but it's hard to read when printed on A4
2. Only A4 printer available
3. Printer drivers can't solve the problem
4. A3 size wanted (ie two sheets A4, with some overlap for taping together)
5. Following (free/OSS) programs are available:
GIMP
PosteRazor

To illustrate point 3: my printer will print multiple pages per sheet ("2 in 1" - ie reduce), or print an A4 page on (n x n) sheets (A2 - four times the area), but there's no option to double the size to A3. A4 too small; A2 too big

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About GIMP - https://www.gimp.org/
Open source image manipulation program - sort of free Photoshop, plenty of info around.

About PosteRazor - PosteRazor - Make your own poster!
PosteRazor is designed to split image files into [n by m] individual pages in a single PDF file, on whatever page size...
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Old

Mighty mite balanced SE classA headphone amp

Posted 5th April 2016 at 10:39 AM by abraxalito
Updated 25th April 2016 at 12:06 PM by abraxalito

I was going to call this amp 'fleapowered' but after doing some searching online I found fleapowered amps in general were much higher output power than this - single digit Watts typically. Whereas this one is more than a couple of orders of magnitude lower than that (under 10mW) so deserved a different moniker. Seeing as a mite is smaller than a flea, 'mighty mite' seemed to me as good as any descriptor for it.

The mighty mite started life in my imagination after I walked into a local headphones shop and tried out some cans that the shop assistant recommended to me - AT MSR7s. I'd brought my 'Buffalito' headphone buffer with me and some step down transformers for its output - fed from my mobile phone its way too loud for my AKGs and Superluxes. However the MSR7s were too loud even with the 2:1 step down trafo so I told the assistant I'd come back and listen again once I had a more suitable trafo. I figured it needed to be 4:1 step down, so I built a pair of those (PQ32/20)...
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Old

Supermatched JFETs

Posted 29th March 2016 at 06:10 AM by rjm (RJM Audio Blog)
Updated 19th April 2016 at 12:52 AM by rjm

Truth be told, for a self-biased jfet audio circuit like the CrystalFET the main reason we need to used matched jfets is to ensure that the signal gain is the same in both channels. The operating point of the amplifier stage (the voltages and currents) can be allowed to vary a little so long as the transconductance, g_m is the same, as this is directly proportional to the open loop voltage gain, A, as

A = g_m R_l (transconductance x load resistance)

Now, yes, ideally you would find two jfets with identical saturation current and pinch off voltages, ensuring not just the same gain but also the same operating point. In practice though you are usually binning parts that are close to each other based on some reference parameter like the pinch off voltage (V_gs0) that you hope closely correlates with the signal gain. This is not quite as good though as the calculating the actual transconductance of the particular device in the circuit it is to be used in. And since...
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Old

Pocketable headphone transformers

Posted 24th March 2016 at 03:44 AM by abraxalito
Updated 26th March 2016 at 12:54 AM 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 01:59 AM by abraxalito
Updated 17th March 2016 at 04:18 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 06:16 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 01:29 PM by rjm (RJM Audio Blog)
Updated 6th May 2016 at 10:27 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 04:05 PM by Michael Rothacher (Audiohobby.com)
Updated 6th March 2016 at 11:34 PM by Michael Rothacher

Achievement Unlocked: 75 Watt Class A Zero Feedback SIT Circlotron

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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.

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