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About bobkatz

  • Birthday 03/12/1949


  • Biography
  • Location
    Orlando, FL
  • Interests
    Audio and Music, of course
  • Occupation
    Mastering Engineer
  • Headphones
    Three Stax, Sennheiser 600 and lots more....
  • Headphone Amps
    Custom built & a Stax SRA amp
  • Sources
    The usual, plus lots of masters and master tapes
  • Other Audio Gear
    Two studios worth...

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  1. Oh, please stay away from Canare Star-Quad. Very high capacitance. The so-called advantages of the twisted starquad are offset by its disadvantages. I suggest Mogami cable designated for AES/EBU, but it's really very low capacitance balanced cable great for analog: http://www.mogamicable.com/category/bulk/dig_interface/aes_ebu/ It's very neutral, no sonic color, very wide bandwidth. The only down side of the Mogami is you have to be very fast on the soldering iron because the insulation melts very quickly. But then, good soldering technique is a badge of being a DIYer.
  2. I just bought an m2tech hiface. Coming from teac by ups ground. I'll let you know how it performs. Really the best thing is to have the crystal oscillator in the dac as spdif means another PLL is involved. But I needed the flexibility to use existing dacs that i have. Asynchronous with the Centrance driver is the way to go. They have their software and hardware drivers down pat. Centrance has licensed their technology to many manufacturers and have just come out with a USB dac themselves http://www.m2tech.biz/
  3. Would that be 60 pf balanced or 15 pf (two caps in series)? I just calculated a circuit using MacSpice: with a 20K source resistance, 10K load resistance and either 30 or 60 pf across the load resistor. That approximates what I have hooked directly to the input of my KGSS (without the equalization cap). With 30 pf load it's down less than .01 dB at 20K. Even if you double the input resistor to 40K you're not going to see an order of magnitude change and even that would be less than 0.1 dB. I expected that or else our typical solid state audio gear would be pretty bad news. And my KGSS measured frequency response is not affected significantly by the position of the 1 dB/step attenuator. You guys are going to have to wait till I have the inclination or a free weekend to take my input circuit apart and just measure the frequency response hooking a low impedance generator directly to the input terminals. Something's very strange here, there's no legitimate reason for the rolloff that I measured in my amp and all theory says it is not due to the input attenuator. My colleague in Belgium will also make a measurement when he gets his KGSS, made exactly like mine except using DACT attenuators. Stand by for the fireworks ).
  4. I've never used the power-matched Krell systems. You're saying a 50 ohm source impedance and a termination resistor at the input to the amplifier? I had a Krell KSA-100 amp for years, class A, excellent sound, but standard high impedance input. "I'm from Missouri, you'll have to show me", so I'll continue with my own experientially-derived reactions to low impedance loads (less than 10K) on all kinds of circuits that I've tried. I didn't come about this conclusion by theory or measurement, but by listening. Once I was using a variable o-pad at the input to my power amplifier to adjust its sensitivity. The driver circuit is the excellent discrete opamp in the Cranesong Avocet, with excellent headroom (it clips at about +37 dBu balanced). This driver circuit is rated to drive most any load down to below 600 ohms. For various reasons, during a certain period of time, the attenuator turned out to be presenting a 1k ohm resistance to the Avocet. I redesigned the gain structure to produce a 10K or higher load and immediately noted a better sound quality to the system, a more open quality. So I tend to stay away from low-impedance loads on my line drivers. All levels in my system are calibrated, so the level to the loudspeaker was matched to 0.1 dB in all these comparisons. I realize there is a contradiction here in that RF circuits and loudspeaker driver circuits themselves, are able to drive very low impedance loads, but "once bitten, twice shy" so I've chosen to stay away from low impedance attenuators.
  5. ONLY if you are not using a passive attenuator at the input to the amp of too high an impedance. (. If you are, I think the safest thing for you is to measure the output frequency response with your attenuator at various positions. Here are my current conclusions after my most recent round of mods: 1/2 dB of loss at 20 kHz with modified O2s (port sealed) sounds just right to my ears. I've listened to a wide variety of material that I'm extremely familiar with and it varies equally on either side of neutral when the 20K is -1/2 (no load). To my ears, the 1.8 dB of loss that I initially measured is probably excessive. Is this loss due to the approximately 10K of source resistance I'm presenting right at the input terminals? I'd have to test, or if Kevin would be kind enough to calculate. Kevin probably nailed the reasons for potential rolloff to be the input capacitance of the FET front end. In that case, it should be made clear to buyers that the KGSS is susceptible to such problems and either an attenuator of such and such impedance be used, or that it be driven by a low impedance source and a buffered volume control should be used. While some engineers I know are fond of using 2K impedance attenuators (volume controls), I've found that it seems to "choke" all the sources I've tried. Even discrete transistor circuits like to breathe, to my ears; they sound better with bridging loads, even if they are spedced to drive low impedance loads. I learned it the hard way, by actual experiment, that 600 ohm load is definitely too low, that 2K load is too low sonically, and that somewhere above that (10K is safe) is the minimum load for all the various driving circuits I've tried. This includes 5532s as well as esoteric discrete opamps with dc servos, such as Forssell and Dave Hill's excellent circuits. This leaves me with an interesting choice. I already designed this calibrated attenuator into my KGSS. Perhaps the simplest thing is for me to add a buffer stage if it is really true that the KGSS's input FET has a fairly high capacitance. Kevin? BK
  6. How significant is the load capacitance to the measured frequency response of the KGSS?
  7. Dear Kevin: Right. Well, there is always some form of RC filter, often due to the input wiring capacitance and any medium-to-high impedance input attenuator. I assumed my input wiring capacitance is negligible being of very short length. Do you think my measurements of (if I recall correctly) 1.8 dB down at 20 kHz can be explained by the input attenuator/potentiometer circuit I also posted? If so, then what you are saying is that automatically ANY mid-to-high-impedance input level control will introduce considerable HF rolloff in the KGSS. If so, then a slight advance HF compensation is not out of order. I just settled on four hand-matched 200 pf caps as sounding correct in my front end circuit with the O2 Mk2/Spritzer port modded! What is the input capacitance? It might be possible to design a 600 ohm (2 kohm if possible) input variable input attenuator circuit that would not be significantly affected by the FET's capacitance. This would be the best way to go about designing an attenuator so not to need an eq circuit. BK
  8. Right you are. I just sealed the ports with Blu Tack today. It tightens the bottom end very nicely. Shame on Stax for having porting the Mk2 and thank you Spritzer for discovering the anomaly. I imagine they had gotten a lot of complaints from unsophisticated users about the "fart". In fact, the seal on my head is so tight that I get quite an annoying snappy fart when placing the cans on my head and/or pushing down even slightly on the sides of the phones. But I'll live with it for the more accurate bass that the sealed port produces. As a result of the tighter bass I'm considering taking down the 20k about 1/2 dB by changing the 270 pf cap to 180 pf. With some sources the high end on the O2's is a bit "high fi" but pleasantly so with 270 pf, which gives an amp boost of 1.3 dB at 20K. I think perhaps an amp boost 1/2 dB less (0.8 dB boost at 20K) would complement these phones very well. I wonder if my SR5-Golds have a port? They really don't fart much at all.
  9. I should really move this question over to the port mod thread. Spritzer, are you saying that the perceived "porty quality" of the phones, which I perceive as extra energy below about 60 Hz (probably lower, more like 40) will be tamed a bit by sealing the port?
  10. Well, if Kevin's prototype measures flat (into an open circuit, that's how I measured mine), we have to wait till someone else measures a current issue KGSS with its current PC board layout to know 100% for sure. Does the headphone load produce a resonant circuit? I have a friend in Belgium who just had a KGSS built by Justin and bought an O2 at my recommendations and and who has the laboratory gear to make accurate measurements (he works at Galaxy Studios) so when he gets the amp he'll measure it for us. I know that Robin is reading this thread, so since he has the "standard" (DACT?) input attenuators, I'll ask him to measure the response (balanced) at the input terminals as well since there is a difference there, should not affect response but we have to be scientific about it. BK
  11. Right, I'll take a further look at it! But I'd first like to hear from others who have the latest iteration of the KGSS and can measure its response to confirm this is not an anomaly. Bob
  12. Let's just say you're right, that's more convenient, but making your own 1 dB/step attenuator out of a 24-step switch is more rewarding. You sing your mantra as you work through the weekend sorting and installing Dale resistors. I generally wouldn't change the gain structure or feedback on the amp, but either attenuating the source (with a control on the DAC) or adding an attenuator at the input terminals is legitimate. ). BK
  13. Thanks Kevin. Your contributions to the headphone and audio community are legendary! Your quote below, my reply above. The reason why I felt the gain was too much was a combination of using a professional level source (which I can adjust, however) and a 1 dB/step attenuator that has to be optimized as it only has 24 steps. This amp has the two 500 K resistors to ground installed on the input side but it is not the HV model. The 500K resistors are included on your schematic at http://gilmore2.chem.northwestern.edu/projects/showfile.php?file=gilmore2_prj.htm. Who knows how I got it to oscillate (it was painful but I had my hands on the pot, so it was momentary) and it was only with an open input and only the unbalanced input. I don't think I have any ground loops, but the unshielded twisted pair input cables are not that far from the outputs. Let's leave it, the additional input resistors have removed the oscillation (or, simply, plugging in any source) and it's not my concern. As for the freq. response, it would be educational to see data. What do you think of my suggestion of removing the feedback and measuring the response at each stage in the currently-made unit (Justin says the only changes have been a couple of transistors)? Thanks again, Bob
  14. I've been running an optimized 1 dB/step monitor gain control in various rooms and studios around the house for about 8-10 years now. By optimized I mean that the monitor gain has been calibrated to produce 83 dB SPL with -20 dBFS RMS pink noise per channel. After optimizing the power amp gain this way, the control typically runs in a range of 4 to 6 dB around a -9 center, for the majority of music that's ever been recorded, and as high as 0 dB in rare cases (certain audiophile recordings) and as low as -16 dB in rare cases (certain egregiously-distorted and overmodulated hypercompressed recordings). The music can be made as loud as the most insensitive ears can take and as soft as anyone would wish. 24 steps are definitely enough if you optimize the gain structure versus SPL. This includes a range including surround reproduction of wide-range theatrical films (which can be made to sound annoyingly "too loud" at the top of the pot), stereo reproduction of audiophile material, stereo reproduction of well-recorded music, and stereo reproduction of hypercompressed material as hot as "Black Eyed Peas", or "Green Day", for example. Once the gain range has been optimized to this SPL, even the most sensitive listener on the planet will play Green Day at perhaps -18, which still leaves 6 dB at the bottom for extreme circumstances. I once ran a TKD 1 dB/step attenuator with as many as 48 steps if I recall correctly, and never went below about -16 and found the rest of the steps to be unnecessary, once the gains were optimized. If they're not optimized, then you find the attenuator just runs in a different average position but still within a range of about 6 dB or less on an average day, or 16 dB on the extremes. If after this optimization, anyone who finds they need more than 24 dB max attenuation for normal music listening, AND can find music that they can tolerate near 0 dB on the same attenuator, then I swear they must be deaf. I also have installed a dim switch on my KGSS to take care of phone calls or in-room conversation if you want the amp to be quiet. That's the only legitimate need for an extra low step below -24 ). You could also make the last step be -40 instead of -24, but a dim switch is much faster, more convenient and more ergonomic. Conclusion: 24 1 dB steps are enough, when the system is optimized. BK
  15. Hmmmm, we may be getting somewhere. Can KG take a measurement of the original circuit for reference (or, I assume, your original specs on the Headamp site referred to the original circuit). But you don't have any compensation caps, do you? We could spice this to the end of days, but if you want to measure first, and find which stage is the source of the rolloff, one way would be to remove the negative feedback and (very carefully) measure frequency response at each stage---with a DC isolated, battery-operated voltmeter with adequate bandwidth. Not being very familiar with the fancy current mode topology, I'm not sure 100% if this is the correct procedure. It may be over my head. BK
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