John Warren

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John Warren last won the day on October 4 2014

John Warren had the most liked content!

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About John Warren

  • Rank
    "So much for the experts on this board"

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  • Interests
    Engineering-Audio, magnetics, materials for electronic and magnetic applications, engineering models and simulation, SPICE, MATLAB, FORTRAN, acoustics, complex algebra, physics of sound, microphones, vintage audio, loudspeaker design, amplifier design, McIntosh amplifiers, discrete semiconductor devices.....and movies including silents, foreign and indies.
  • My System
    12" Utah Tri-axial drivers mounted in LRE bass "reflex" enclosures.

    Sony Superscope FM only

    Lafayette Solid State Stereophonic Integrated Amp

    16 GA Lamp Wire

    Koss Pro 4AA

    Technics SL-QD33

    CD Player:
    NAD 325i (modified)

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  1. No so sure. Film caps like Polypropylene or Polyester (i.e. Mylar) don't drift or otherwise degrade unless abused. The electrolytic capacitor life is limited by time operating at maximum ripple current rating and its frequency. That creates heat and that's what dries out the electrolyte paste in the cap. In filter applications like power supplies this is an issue but in loudspeaker no. There's no downside to changing the electrolytics with higher voltage ratings the biggest risk being damage to the PC board. Lead spacing might be different. Vintage Technics is top tier hardware.
  2. A signal at 300Hz, within the range where the bass horn is operating, has even and odd harmonics that extend well into the into the midrange. Consider impulse type signals from a single source with sound energy over a wide range of frequencies. The low stuff, say below 400Hz, and the high stuff, say above 400Hz, arrive at the listener at oddly different times. It's the stuff you listen too that's within the range where the drivers are operating is where the medicine needs to be applied.
  3. To eliminate the differences due to the path lengths must be DSP. No passive or analog active can do it.
  4. The capacitors C1, C2, C3 and C4 determine the low end, -3dB frequency. I designed the filter such that C1=C2=C3=C4 so changing the -3dB frequency means changing all caps from one value, say 1.0uF, to another, like 1.2uF. There are two channels on the board, a total of 8 caps need to be swapped out so not "end user" friendly but hardly difficult for anyone that has basic solder/de-solder skills. The caps in this configuration are polyester film at about $1 each. I used Panasonic on the first builds and the results were excellent.
  5. The plot below is the small signal bandwidth of the XLi with the filter installed. The low end responses shown are with different R/C combinations on one section of the filter. The 6dB max assumes balanced inputs. The response is the signal being sent to the error op-amp (where output stage feedback is introduced). The -3dB frequencies shown are 47, 36, 28 and 25Hz. The top end -3dB is 460Hz. The 4th order bandpass at 36Hz eliminates the low end stuff where the Klipschorn no longer behaves as a horn, it's filtered out. Need a sub however to handle the low stuff.
  6. This was an earlier version of the board layout. Caps C1-C4 and C11-14 are Panasonic Polyesters. U1 and U2 are AD713s. All resistors are film and the remaining caps are MLCCs.
  7. Been there, done that. Works perfect. It's a project I did some time back.
  8. I pulled power for the board from the +/-50VDC lower rail leads that connect the power supply to the amplifier board. The filter has Zener regulated supplies to provide the +/- 15V rails for the op-amp packages (AD713). If there's one place where this amplifier can be improved is the power supply,
  9. The photo shows how the filter board was installed in the first units I modified. The bracket at the left is screwed into a chassis ground location and was used as the ground for the filter.
  10. Turning the wick up on the input signal from 500 to 800mV shows how duty cycle at switching increases. Note that the top rails are beginning to sag a bit.
  11. Here's how the rails switch in the XLi. Takes about half a second for the amp to turn on. The power supply takes about 150ms due to charging the filter caps. There's a power up relay that keeps the amp silent until its awake. In the plots, green is the high positive rail, red is high negative rail, light blue is the low negative rail and the pink is the low positive rail. The dark blue signal is 500Hz sine wave at the amp output, the input is 500Hz, 500mV balanced input from 500 Ohm source. On both the positive and negative rails, feedback from the output is sensed and used to pulse the gate of a power MOSFET which performs switching between low and high rails. As output voltage at the load increase, the duty cycle of the ON pulse increases. That's shown in the second plot. The switching enables high power without high idle dissipation.
  12. The idea was to develop an active bandpass filter for any bass horn (say a Klipschorn) that's otherwise impractical to realize using passive components. The XLi is low cost, has excellent %THD below 1kHz, is class AB and lends itself to the mod. The actual filter is text book Sallen-Key, very easy to implement in the small signal section of the amplifier. In the schematic, the filter is outlined in red. I've placed it between the input op-amp (U2) and front panel pot comprised of R35, R44 (pot is at full CW). The schematic shows just the small signal processing of the XLi. U1 is the gain amp, U3 is the error amp. The response plots shown are small signal simulations that are possible using cascaded Sallen-Key filters. The PC board for the filter is installed vertically sharing a couple of the mounting screws used to mount the amplifier board to the chassis. By implementing a 4th order bandpass in the small signal domain the amp drives the Klipschorn to work within its horn loaded range. The horn loaded range is where it makes the best sound.
  13. That's great to hear Mark! I own a couple myself, love them. The purist have issues with the basic design but with the right speakers the amp can make a lot of people smile.
  14. It's a low cost class AB amp with rail switching and decent distortion numbers. It's straightforward to install an active, steep slope filter inside the unit and convert each channel into bass horn amplifier. Implementing a 4th order band-pass for, say an Klipschorn or LaScala, takes a PC board the size of a credit card.
  15. The plot shows %THD v. power sweep at 5 frequencies, the bottom is 100Hz, green is 1k, gold is 5k, light blue is 10k and dark blue 20kHz. Distortion is function of power and frequency. Suppliers, if they even publish a result plot 1kHz only. Distortion tends to increase considerably in the higher end of the spectrum (case in point). It's the amplifier board and includes both small and large signal stages. All on one board. So yes, I'm serious.