Upgrade of Music Angel XD8502AIII amplifier

Discussion in 'Power Amps' started by Zampotech, May 19, 2019.

  1. Zampotech

    Zampotech Friend

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    Part one. Entrance

    Moscow firm asked me to upgrade 10 pieces of amplifiers music angel XD8502AIII.

    [​IMG]

    On photo 9 amplifiers. 10 amp is already on my desktop.

    Her customers complained of a dull sound.


    This SE amplifier tubes DCC90+6N9P+300B

    [​IMG]

    [​IMG]

    The appearance of the amplifier is very good.

    The cost of these amplifiers is relatively low. In Moscow, they are sold at a price of $770

    I will not criticize my Chinese colleagues, I will show only the problem areas and the possibility of improvement.

    First of all, I took the amplitude-frequency response.

    [​IMG]

    Here is a graph of the amplitude-frequency response

    [​IMG]

    The graph shows that the reason for the "dullness" of the sound is a limited range in the high frequencies.

    I have an assumption that the high frequency limitation is due to the 6N9P driver tube. This tube has a high output impedance and is probably shunted at a high frequency by a 300B tube capacity.

    The second problem is due to the strong microphone effect of the DCC90 tube. This tube is unknown to me
     
    Richgard, elmoe, crazychile and 4 others like this.
  2. frenchbat

    frenchbat Almost "Made"

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    Is that really a roll off of 25dB between 1kHz and 10kHz ?
     
  3. Zampotech

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    No. I didn't sign the graph. Excuse me

    The vertical scale is the voltage at a load of 4 ohms per dB (0.775V). My voltmeter has 0 dB at 0.775 v
     
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  4. Zampotech

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    Part two.Inside the amplifier

    This is what the amplifier looks like inside.

    [​IMG]


    Good components are installed. Most of the resistors are similar to the Allen Bradley carbon ones. Film capacitors are mostly of WIMA. Installation is neat and tough.

    The technologist of the Chinese factory did a good job.

    My next step will be to re-engineering
     
  5. Zampotech

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    Part three. Drawing up a schematic diagram.

    Reverse engineering has led to such an amplifier circuit.

    [​IMG]

    Forecasts for improving the sound of the amplifier are very favorable.

    Pleasant surprise. Electrical parameters of the tube 300B-98 produced by ELECTRON TUBE Shenguang fully comply with the electrical specifications and parameters of the tube produced by JJ Electronics.

    [​IMG]

    Original 300B unfortunately I have no.
     
    Last edited: May 20, 2019
  6. Zampotech

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    Part four. Modification

    My upgrade plan

    1. Replacement tube DСС90 double triode 6N1P
    This will greatly reduce the microphone effect.


    2. Replacement tube 6N9P (6N9S, 6SL7) to dual triode 6N8S (6SN7)
    This will extend the frequency range on the driver tube - output tube section


    3. A small correction of the scheme and the denominations of the elements.

    The result of the correction:

    [​IMG]


    The modes were tested on the model and it is hoped that the upgrade will effectively.
     
    Richgard and Dzerh like this.

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