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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.
Preamps
Old

Removing hum from Aikido Cathode Follower (ACF-2 9-Pin)

Posted 7th March 2017 at 03:00 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, 25 views)
File Type: pdf acf-2.pdf (51.1 KB, 12 views)
File Type: pdf acf-2 one solution.pdf (51.8 KB, 13 views)
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Old

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, 174 views)
File Type: zip MicroPre_gerbers.zip (53.9 KB, 73 views)
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Old

Grounded Grid Preamp

Posted 7th July 2011 at 12:43 AM by alexcp (My DIY projects)
Updated 16th March 2013 at 02:35 PM by alexcp

Built from the kit from Bruce Rozenblit. Very nice one, but is not being used - it has too much gain for any of my systems. I will probably sell it.
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Old

First Watt B1 clone

Posted 12th August 2010 at 07:19 PM by alexcp (My DIY projects)
Updated 11th February 2017 at 02:16 PM by alexcp (Added photos)

What to say... B1 is a great design by Nelson Pass. Easy to build, absolutely transparent to the sound, compact, PCBs are available. I built one, using Nelson Pass' gerbers for making my own PCB, in an enclosure from diyenclosures.com (which sadly seems to be defunct now). The big blue film caps are 10uF Evox Rifa PHE426.

Update: I added a relay input selector, which reduced crosstalk between inputs. I also built another B1 clone (the last three photos), in an enclosure 1U (40mm) high.
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Attached Files
File Type: pdf B1 Buffer Preamp.pdf (273.8 KB, 181 views)
File Type: zip B1 R0 DIY GERBERS.ZIP (423.6 KB, 62 views)
File Type: zip relay_selector.zip (131.7 KB, 20 views)
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