Below are some cars that I owned over the years and regret selling…
This is a follow on to my earlier post on OPT characterization. Instead of the differential amplifier, I tried using an 1:1 transformer on the output of the signal generator, so I could have floating outputs to drive the reference resistor and the transformer-under-test (“TUT”). This is the same method used by Ayumi Nakabayashi to characterize his transformers. The result was much better than the old method using the differential amplifier, since both the resistor and the TUT could be grounded on one end, making the measurement much more accurate, without noise and line interferences. Below is the resultant plot of a generic 5k SE OPT:
I recently got one for cheap, the moment I plugged it in, I noticed that it had excessive noise – a non-stop rustling sound, especially when the “Warmth” was turned all the way up. Google search turned up some threads and confirmed my suspicion that the PSU was not well filtered, and the PCB traces were not properly routed. In addition, running the 12AX7’s at 48V did not help its performance either.
Instead of doing some of the mods found at groupdiy, which involved replacing all the IC’s, and adding more local bypassing caps to the PCB. I decided it would be much easier to just scrap the stock PS PCB and replace it with a SMPS. At the same time, the tubes will run at HV with a DC-DC converter that I already have. And finally, the I/O PCB will be replaced with I/O transformers. The total budget for the above modifications should be less than $100, as I already have some of the parts on hand.
I got an idler turntable this weekend, it’s the ELAC Miracord 50H-II. It’s in pretty good condition except for the left channel RCA plug which was cut off, the arylic cover had some bad scratches on it, but otherwise, the mechanical condition was quite good for such old deck. The cartrige was junk, so I tried to put on a Shure M97 I had laying around, for some odd reason, I had to reverse the left hot and cold wiring, otherwise, it shorted out when the right channel was plugged in, weird! With the wires reversed, I was expecting some sort of L-R cancellation, but it played just fine, what was that about?!
The sound, even without any adjustment at all, no stylus alignment, VTA, etc., already sounded much better than the Technics SL-1200MK5G, so there’s something to the idler magic after all. I am mulling over whether to build a custom plinth for it or not. Since I like the way it sounds, I may just get some corner supports to replace the springs… we’ll see.
Today, I build a simple dufferential instrumentation amplifier consisting of TL072 and INA128. With it, I can now measure the impedance response of the OPT, choke, etc. The task is made simple by using the Syscomp CGR201’s built-in network analyzer function, which is set to sweep from 10Hz to 1MHz, and the voltages across the reference resistor (3.3k) and the primary winding the OPT are fed to Ch 1 and Ch 2 respectively. After exporting the resulting data to Excel, something like the following impedance vs. frequency plot can be plotted. From which, all the parameters of the OPT can be estimated.
I found some good MIDI tracks for New Order’s Bizzare Love Triangle, which could be great for remixing, so I used Bandstand and Audacity to create the individual tracks. The trick was to record the output from Bandstand with Audacity, after some fiddling around, this is the setup that I came up with, notice the routing via the Virtual Audio Cable in the Audacity settings below: