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Discussion in 'd.i.y.' started by a.palfreyman, Dec 17, 2018.

  1. a.palfreyman

    a.palfreyman pfm Member

    Having read these comments in the Qudos thread I decided to try the variation 2) and put 2k7 across the 22k resistor that feeds TR2 of the LTP of my NCC200s (giving as near as dammit 2k4). First hour sounded very bright (but have experienced this before so knew what to expect) and it settled down over the next few hours. There seems less "projection" in a 3D sense, but this is more natural and there was definitely a change in the tonal balance. Following day, played for several hours and found several benefits, pretty much as described above. I had been having problems with a lack of focus in the centre of the image (particularly with trumpet left and sax right which could tend to 'merge' in the middle of the image) and this was now much better. Instruments sound more natural and the soundstaging (particularly depth) seems more natural: piano notes on the right-hand could appear to 'jump-out' at you but not after this mod. I found though that some frequencies seem reduced in output so I can agree with the description of a "tuning" effect with the 22k in place. Will be giving this some time to see if my ears settle with this because to me the improvements far outweigh the negatives.
    Made some rudimentary measurements:
    Before mod: 1k (TR1 feed) has 0.54Vdc across it and 22k (TR2 feed) has 10.1Vdc across it indicating a standing current of 0.54mA and 0.46mA respectively.
    After mod: 1k (TR1 feed) has 0.52Vdc across it and 2k4 (TR2 feed) has 1.15Vdc across it indicating a standing current of 0.52mA and 0.48mA respectively.
    Could this "rebalancing" of the collector currents have an effect on the sound? I don't know enough about electronics to answer that. The only other thing I can think of is the ratio of 22k on TR2 and 27k feed-back resistor. These have an almost exact ration of 9:11 so could this be causing a tuning effect where notes of a similar ratio played simultaneously "affect" each other?
    Forgot to mention this had no vast effect on the bias current or the DC offset on the output
    337alant and S-Man like this.
  2. S-Man

    S-Man Kinkless Tetrode Admirer

    And it marginally improves the phase margin (if anyone was worried about stability).
  3. MJS

    MJS Technical Tinkerer

    I suspect that the fact that these have remained unbalanced over the years is part of the appeal of the sound of this amp circuit. If it ain't broke..etc.

    I've never played with these resistors, or a current mirror here. It's on my (very long) todo list.
  4. martin clark

    martin clark pinko bodger

    +1 @MJS

    By the time you start sticking a current mirror in there, I think you are into regular 'Lin 3-stage' amp (re)design...
  5. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    FWIW a current mirror increases OLG by 12dB and reduces THD from 0.06 to 0.02% @ 50W 1KHz according to a sim (already had the schematic in my simulator). Changing the LTP resistor from 22K to 1K appears to do sweet FA according to my simulator but I only tested at 1KHz into 8R and nothing else...
    Shadders likes this.
  6. a.palfreyman

    a.palfreyman pfm Member

    You have to try it. Played for another 4 hours last night. The sound is opening out, more detail and some "confusion", especially in the centre now pretty much gone. I might have to play around slightly with my mid-tweet cross-over as some higher Freq sax notes can be a little forward, but have yet to try some rock (been listening to Blue Note jazz) as rock always seems a bit darker in terms of mix. There is some delightful cymbal work on The Sidewinder and Takin' Off that really shine through in a natural way without being obvious. I won't be reverting back to 22k.
    I know little about electronics but been speaking to an Instrumentation Engineer at work. In the new year we may try and get some measurements done at 22k and 22k//2k7 (2k4) to see if there are any measurable differences. As he said it is easy to test with single tones but difficult to test with music which is much more complex and single tones may not show up any possible interactions between harmonics of lower Freq notes with higher Freq notes. In the meantime I might build a simple diff amp and connect across the l+r speaker terminals and scope the results. Only problem is it is sounding so good I don't want to mess too much with it as IME newly soldered components (even a single resistor) 'seem' to take at least 10hrs to settle in.

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