bobkatz Posted January 24, 2011 Report Share Posted January 24, 2011 (edited) Schematics and photos attached! I got my belated Christmas present from Justin the other day, a lovely KGSS amp. Finally I got to hear my new Stax 007 Mk2 (Omega 2) in all their glory. Knowing that I was going to install my own Goldpoint 1 dB/step attenuators, and a Left to mono switch (to check headphone balance), I asked Justin to agree to construct a KGSS without wiring the input connections to the back panel or installing a volume control (yes, he did test and burn in the board). Leaving me with some fun to do on my own! The first thing I discovered is that the KGSS is extremely sensitive. I judged it has at least 15 dB too much extra gain, even with consumer (nominal -10 dBu) sources. Without an additional attenuator, I even found it easy to incite HF oscillation by opening my Goldpoints fully, with an open unbalanced input. This is not a good idea! So I designed an additional 15 dB attenuator circuit, installed it at the KGSS input. Then I proceeded to listen to a wide variety of reference material through my Benchmark DAC-1. This includes much of my own material which I have mastered and which I know intimately monitored through the Revel loudspeakers in my mastering room. To be honest, I was disappointed with the high end, it was just not open enough nor did the "air" frequencies come close to that of my Revels. Others have reported the Omega 2's to be a little "dark" or closed in and I was prepared for that. This is partly due to the extended bass response of these wonderful headphones, which may be as much as 1 dB too much below about 60 Hz. So, I decided to design a simple passive equalizer to compensate for the Stax response. This took advantage of the 15 dB pad I had already installed. I figured I might need between 1/2 and 1 dB rise from 10K to 20K, and in Spice I designed the filter. Then I went to the bench, and proceeded to make a baseline frequency response measurement before installing the EQ (which consists of two added capacitors per channel, one on each phase of the balanced line). Much to my dismay, I discovered that this KGSS is rolled off. Flat at 1 kHz, it was -.13 dB down at 5k, -.48 dB down at 10k, -1.1 dB down at 16K, and -1.61 dB down at 20kHz!!! This was the explanation for the loss of air and high end, not the Stax phones at all. I measured this rolloff at the headphone outputs, the same rolloff whether balanced or single ended (relative to ground). I made measurements at various gains and pot positions, and nothing significant changed. Left and right channel measured identically (the same rolloff) within 0.1 dB. I conclude that some internal capacitive losses are occurring, despite Kevin's current-mode circuit. Or some unknown. Clearly, whatever is causing the rolloff is fully symmetrical as the plus and minus outputs of both channels measure identically. It sure sounds like the rollof is within the amp itself. I think that Spritzer has been working on a new version of the KGSS, and I'd like to ask him to perform frequency response measurements on a stock unit and a new or modified unit and report here. I leave the possibility of capacitive losses in my input wiring, but it's doubtful, considering the cable lengths and positions of the resistors. I should have measured the response at the input terminals before I installed the amp in the mastering room, but that is not possible without too much effort on my part. I will do that measurement if someone else reports flat response with his KGSS. There was full agreement in response between my high-end Fluke meter, my Audio Toolbox, and a classic Ballantine TVM, so it's not my measurement equipment, and I DID measure the test oscillator in the Audio Toolbox with the same tools, and I compensated for the generator's own slight irregularity (it has to be turned down +0.5 dB at 20kHz, but not at 16 kHz and below, strangely). I am pleased to report that after the installation of the EQ mod, the amp is flat to less than 0.1 dB from 20 through 20 kHz, and it sounds marvelous now! So, what gives? Bottom line: I highly recommend this mod; the one-pole curve of the eq exactly mirrors the losses in the amp, and I no longer sense a loss of air or presence. I'm a happy camper now! I can give anyone who wishes a spreadsheet or fixed calculations for the 1 dB/step attenuator if you wish. Note that the Goldpoint wafer is normally designed for a ladder attenuator, but I didn't like the idea of sending the signal through all those resistors in series, so I designed it as a fixed series resistor with a switched single variable resistor. It's a fully-balanced variable U pad. KGSS mod input circuit.pdf Edited January 24, 2011 by bobkatz Quote Link to comment Share on other sites More sharing options...
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