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alexcp alexcp is offline

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About Me

  • About alexcp
    United States


Total Posts
Blog - My DIY projects
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General Information
  • Last Activity: Yesterday 03:19 PM
  • Join Date: 9th June 2009


View alexcp's BlogRecent Entries
Latest Blog Entry

Posted 11th June 2017 at 04:16 AM by alexcp Comments 0
Posted in Uncategorized
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.

Posted 7th March 2017 at 03:00 PM by alexcp Comments 0
Posted in Preamps
I purchased from an Aikido Cathode Follower 2 (ACF-2) All-in-One 9-Pin PCB, designed by John Broskie, the editor of Tube Cad Journal (

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...

Posted 9th April 2016 at 01:51 PM by alexcp Comments 0
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

Posted 9th April 2016 at 01:27 PM by alexcp Comments 2
When I saw the discussions of KSA-5 headphone amplifier on 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.

Posted 9th April 2016 at 01:20 PM by alexcp Comments 0
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.
Recent Comments
Thank you for the comment....
Posted 10th May 2017 at 02:29 AM by alexcp alexcp is offline
It looks like a solid...
Posted 25th March 2017 at 12:37 AM by bels bels is offline
KSA-5 is a power amplifier...
Posted 26th February 2017 at 07:48 AM by alexcp alexcp is offline
I wonder how it will...
Posted 24th February 2017 at 01:35 PM by idir idir is offline
Marc, apologies again...
Posted 11th February 2017 at 02:14 PM by alexcp alexcp is offline
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