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Bi-amping the Piano Black Ref3A MM De Capo Be (92db 8ohms)

Discussion in 'General Audio Discussion' started by KAC, Jan 2, 2018.

  1. KAC

    KAC New Member

    Messages:
    27
    I was driving the DC with a 300B SET monoblock amplifier rated at 8WPC although realistically probably around 5 to 6 WPC. I am using a Marantz based tube preamplifier with 2 outputs. I also have a spare Antique Sound Lab tube preamplifier also with dual outputs. I recently got hold of a Conrad Johnson MF 2100 solid state amplifier (100 WPC) so I decided to try bi-amping the DC. The manufacturer says that the DC has no internal crossover on the midrange woofer-- the driver’s natural upper-frequency rolloff was used to integrate its output with that of the 1” tweeter, which had a simple high-pass filter to roll off its low end.

    The bi-amp was pretty straightforward. The 300b SET was connected to output 1 of the Marantz based preamp and powers the tweeters. The input of the ASL was connected to output 2 of the Marantz based preamp then its output connected to the CJ to control the level and power the woofers.

    So far so good although using the 300B SET alone to power both drivers produces a cleaner and airier sound noticeable during critical listening albeit a tad brighter. I am hesitant to use an external electronic crossover as it will add something to the signal chain.

    Note that the 300B SET can power the DC but adding the CJ enables it to play rock music with heavy hands.

    Just sharing. Let me know if you have other ideas. At some point, I will have level controls installed on the CJ so I don't have to use the ASL preamp.
     
    Last edited: Jan 2, 2018

     

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  2. onplane

    onplane What! No Wake???

    Messages:
    2,310
    Location:
    Rochester, NY
    Well, your bi-amp is hardly straightforward given that you have cascaded pre-amps. Instead, try taking the rec out on the Marantz into the aux in on the ASL.

    This will give you a single pre-amp for each speaker half.

    Next issue is phase. There is no guarantee your amps are in phase/polarity. To test, use each amp to power a single speaker full range with a mono source. Listen carefully to a track with decent bass. Then reverse the leads on just one speaker and listen again. The configuration with the loudest sound has the amps in phase and is the way you should connect in your bi-amp.

    Regards,
    Jerry
     
    unfairlane likes this.
  3. KAC

    KAC New Member

    Messages:
    27
    Thanks for the suggestion Jerry. I have checked phase using a test disc and a mono recording and they appear to be correct but I will check again using your method. The second output of the Marantz based preamp is connected to Line 1 of the ASL, which is then connected to the input of the CJ to control volume. I asked the guy who assembled the Marantz based preamp to provide 2 outputs so I can use it to biamp. I just need to eliminated the ASL to make it simpler by installing a level control on the CJ but it would involve punching a hole on the CJ for the level knob.
     
  4. onplane

    onplane What! No Wake???

    Messages:
    2,310
    Location:
    Rochester, NY
    Just remember whenever you check phase/polarity, you must connect everything just as it would be when bi-amping. That is, if you are going to cascade pre-amps (and this is never a good idea), you need to test cascaded.

    As for putting a level control on the CJ, this might not work. Remember the key is that both amps should be putting out equivalent power. In order for two amps to do that, the gains of both amps would have to be identical. This is rarely the case so the ability to cut back gain on one amp to match the other is necessary. Problem is you must "cut back" on the amp with the highest gain. If your control is on the amp with the lowest gain, no matter what you do with that amp it can never catch up with the other. In your case, how do you know that the CJ has the higher gain?

    I can make a further case for keeping your two pre-amps. With dedicated pre-amps (i.e. one pre-amp for each speaker half), you can set the bass tone control full OFF for the tweeter amp and the treble control full OFF for the woofer amp.

    What this does is limit the frequencies being amplified in the respective amps and any frequency limitations can only reduce IM distortion. It also makes sense for the drivers. That is, why send high frequencies to your woofers and at the same time low frequencies to your tweeters?

    Regards,
    Jerry
     
  5. KAC

    KAC New Member

    Messages:
    27
    Appreciate your inputs Jerry. I tested cascaded. Using the phase test track, the voice came from the middle when in phase and all over the place when out of phase. But I will double check.

    I will talk to the tech who will modify the CJ and mention your inputs.

    Unfortunately, both my preamps do not have tone controls. The woofer of the speaker has a natural roll off for the highs. The tweeter, upon the other hand has a high pass filter.
     

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