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I returned to DIY electronics in 2009 after a 20 year pause by building a few kits to get in shape. This blog is for me to keep track of my progress.
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Aikido headphone amplifier

Posted 11th June 2017 at 04:16 AM by alexcp (My DIY projects)

I built a headphone amplifier using John Broskie's Aikido All-in-One PCB with Tube Rectifier. The PCB is well made and easy to build, and I am satisfied with how it sounds.

One comment is that R9, R10, and R13 (see the attached schematic) affect the distortion introduced by the circuit. By changing R13, I was able to cut the distortion by as much as 90%, from 0.2% @1kHz 1Vrms output down to 0.02%. My optimum was R13=90ohm with B+ at 310V (using the recommended Hammond 270EX power transformer) and all other values as in the attached schematic.
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Removing hum from Aikido Cathode Follower (ACF-2 9-Pin)

Posted 7th March 2017 at 04:00 PM by alexcp (My DIY projects)
Updated 7th June 2017 at 09:41 PM by alexcp (Edited for brevity and, hopefully, clarity)

I purchased from Glass-Ware.com an Aikido Cathode Follower 2 (ACF-2) All-in-One 9-Pin PCB, designed by John Broskie, the editor of Tube Cad Journal (tubecad.com).

The PCB holds a pair of cathode followers, each loaded by a triode current source, and a respective power supply. In my build, a JJ E88CC with 220ohm cathode resistors shows 0.003% THD @1kHz 1Vrms with pure 2nd harmonic distortion.

As I tested my ACF, I noticed audible buzz at its outputs @0.15% THD+N - that's 1.5mVrms(!), well above the THD, and it was not a grounding problem.

A careful look at the schematic (attached), confirmed by some Spice simulation (an LTSpice model is also attached) revealed that the "Aikido" in the ACF-2 relies on the exact match of the voltages across the capacitors C18 and C19, including the hum component. These capacitors are in series for the rectifier (e.g. hum) current and, together with R4/R7, are in parallel for the signal current. In the ideal...
Attached Files
File Type: asc acf-2.asc (8.0 KB, 50 views)
File Type: pdf acf-2.pdf (51.1 KB, 14 views)
File Type: pdf acf-2 one solution.pdf (51.8 KB, 17 views)
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Restored and modded Cambridge Audio A3i

Posted 9th April 2016 at 01:51 PM by alexcp (My DIY projects)
Updated 13th April 2016 at 03:17 PM by alexcp

The designer of Cambridge Audio A3i had talked about the amp in this forum and suggested some changes to the components that bring the performance of the amp to a new level.

When I saw a cheap A3i on eBay, I decided to try it out. One channel was DOA, but it did not matter as I was about to mod it anyway.

Here is what I've done:
  1. Replaced all rectifiers (D201-D206 on the schematic) with soft recovery diodes and removed the capacitors in paralell with the diodes;
  2. Changed the grounding scheme so that the protection circuit and the relay driver are connected directly to the star ground;
  3. Added a 30ohm resistor in series with the rectifiers of the protection circuit;
  4. Replaced the electrolytic capacitors in the amplifier (but not the protection circuit) with Nichicon KA and replaced the two large power supply capacitors with Nichicon LLS;
  5. Replaced all film capacitors with Kemet PHE426 or, where PHE426 wouldn't fit, with Kemet SMR;
  6. Replaced the rectifiers
...
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File Type: pdf A3I schematic.pdf (1,011.2 KB, 286 views)
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Krell KSA-5 clone

Posted 9th April 2016 at 01:27 PM by alexcp (My DIY projects)
Updated 11th February 2017 at 03:20 PM by alexcp

When I saw the discussions of KSA-5 headphone amplifier on head-case.org and on this forum, I thought it may be a nice use for the bag of 1000uF capacitors I had at the time. This was the main motivation for putting this Krell KSA-5 clone together.

The main challenge was to drill the front panel. The 10mm aluminum panel that came with the enclosure is easy to work with, but I wanted it to be pretty and asked the people at Modushop to CNC it for me.

Although it works ok, I think it is not a very good power amplifier for loudspeakers. As a headphone amplifier, it is no match for my Musical Fidelity X-CANv8.
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File Type: pdf ksa5.pdf (129.1 KB, 866 views)
File Type: pdf ksa5psschem.pdf (54.7 KB, 788 views)
File Type: zip ksa5ampfinal.zip (2.80 MB, 82 views)
File Type: zip ksa5psg.zip (631.6 KB, 53 views)
File Type: zip ksa5psorig.zip (791.3 KB, 61 views)
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NCore based power amplifier

Posted 9th April 2016 at 01:20 PM by alexcp (My DIY projects)
Updated 11th February 2017 at 03:25 PM by alexcp

Ever since I heard about the Hypex NCore technology, I have wanted to build a power amplifier based on Hypex NC400 modules. Here is what I got. It works very well with the preamplifier from the previous post.

I used a spare Arduino to control the SMPSes. For now, it simply allows to use a momentary push button on the front panel. I may add remote control (12V trigger or similar) later.
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File Type: pdf NC400_04xx.pdf (2.97 MB, 136 views)
File Type: pdf SMPS600_DIY_R5.pdf (1.42 MB, 58 views)
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Bruno Putzeys' MicroPre

Posted 9th April 2016 at 01:14 PM by alexcp (My DIY projects)
Updated 11th April 2016 at 08:21 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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Attached Files
File Type: pdf The G Word.pdf (731.5 KB, 177 views)
File Type: zip MicroPre_gerbers.zip (53.9 KB, 75 views)
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From Tube CAD Journal by JRB

Posted 25th June 2015 at 08:05 PM by alexcp (My DIY projects)

For every hobbyist-built amplifier in existence there are probably one thousand amplifiers which are planned on being built and maybe one hundred amplifiers that have all their parts collected in a box waiting to be built and ten amplifiers that are partially built and collecting dust in a closet.

(see full post here)
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Soundcard measurements with ASIO

Posted 7th September 2013 at 09:51 AM by alexcp (My DIY projects)
Updated 17th February 2014 at 06:08 AM by alexcp

I talked earlier about soundcard measurements, and here is some news: Pioneer Hill Software updated SpectraPLUS 5 so that it can now use ASIO.

ASIO (Audio Stream Input-Output) replaces Windows' own MME (MultiMedia Extensions) or DirectSound. As far as I understand, the beauty of ASIO is that it bypasses Windows Mixer and thus doesn't bring any issues associated with sample rate conversion or level adjustment.

The ASIO drivers that came with my E-MU 0204 crash every time (on Windows 7 and WIndows 8). Thankfully, there is ASIO4ALL, which works well for me.

First picture shows two spectra taken with SpectraPLUS: red is the E-MU 0204 output fed directly into E-MU's input; and blue is the output of the E-MU fed into Pete Millett's interface, output of the interface fed to the input of the interface, and then back to the sound card. THD+N is 0.000781% is the first case and 0.001272% in the second.

Next two screenshots are THD+N vs frequency...
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Old

Better than a box of chocolates

Posted 25th June 2013 at 06:17 PM by alexcp (My DIY projects)
Updated 25th June 2013 at 08:27 PM by alexcp

Got this today on the cheap - a box of 6N1P-VI, made in 1988, all new in authentic factory packaging. It is likely to keep me entertained for quite some time

Note: the PDF attachment is a scan of the factory three page insert from the box. It is in Russian and contains the pinout and electrical and limiting parameters of the tube but NO CURVES.
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File Type: pdf 6N1P.pdf (6.03 MB, 217 views)
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Measurements with a sound card

Posted 13th May 2013 at 09:08 PM by alexcp (My DIY projects)
Updated 7th September 2013 at 09:36 AM by alexcp (Updated for ASIO4ALL)

I am using a USB sound card for measurements. It's a pain, but I have nothing better yet.

The sound card is the E-MU 0204. I've chosen it for the specs:
  • Dynamic Range (A-weighted, 1kHz, min gain): 113dB
  • Signal-to-Noise Ratio (A-weighted, min gain): 113dB
  • THD+N (1kHz at - 1dBFS, min gain): -101.9dB (.0008%)

The sound card is intefaced to the real world by Pete Millett's sound card interface.

The software is SpectraPLUS 5.0 by Pioneer Hill Software (PHS). (No link because my antivirus says their web site is infected.) The software runs on an old MacBook running Windows 7 in Boot Camp.

The results do not seem to be affected by nearby computers, flurescent lights, switching power supplies, etc.

The results are affected by:
  • Windows Mixer settings (levels and sampling rates)
  • Software settings (levels and sample rate)
  • Grounding of the sound card, the interface, and the device under test

The greatest source of pain is...
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