Pseudo Balanced rb-971

Discussion in 'Hi-Fi and General Audio' started by johnandchris, Apr 12, 2012.

  1. johnandchris

    johnandchris

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    I have recently aquired a Michi RHC-10 which has replaced my RC-995. I am very pleased with it, it gives a lovely sound.
    I am currently running 2 RB-971 amps in bridged mode and using each as a monoblock per speaker (single wired).
    I have read on the internet that an even better was of connecting them is in "PSEUDO BALANCED" mode as described here.

    but there's an even better way of doing it if you can live with a little less on the effect side compared to the bridged mode, namely "pseudo-balanced" mode.
    You'll of cause need two RB 981', and then you short-fuse one channel on each of the 981's by the help of a shortfused jack. Then connect each channel of the cable connecting the pre-amplifier to the remaining channel on each of the 981's - and the power-anplifiers are in pseudo-balanced mode.
    The result is an astonizing amount of control because the transistors in one side of this stereo-amplifier now is dedicated to "dampen" the signal when needed. The noise-floor comes down to virtually nothing and the dynamics gets astonising - even copared to the bridged mode.

    My question is, what is meant by a sort fuse on the channel. Is it simply connecting positive to negative?
    I would like to try it if possible. Any help welcomed.

    Thanks John
     
    johnandchris, Apr 12, 2012
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  2. johnandchris

    johnandchris

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    Bump. Anyone?
     
    johnandchris, Apr 15, 2012
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  3. johnandchris

    RobHolt Moderator

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    Hi John,

    I've not come across this term.
    He seems to be saying that each half of the stereo power amplifier handles half of the waveform. Sounds very like what we'd understand to be normal bridging to me, where signal phase is inverted at input on one channel and output is taken from the two positive terminals. If the comments have appeared on a forum or article, perhaps give use the link ?

    What worries me about the comment are terms such as 'astonishing' because again we end up back asking the question, is what the Rotel doing currently exceeding the requirements for it to be transparent.
    In standard form is has very low noise and distortion, and very high damping factor. I'm at a loss as to to how taking these things further can do anything audibly astonishing.

    I would need to see more details of what is being suggested.
     
    RobHolt, Apr 15, 2012
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  4. johnandchris

    Werner

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    I think he grounds one channel's input, and then connects its 'hot' speaker output to the speaker's ground terminal.

    Sort of an active ground then.



    How much is 'astonishing' in dBs?
     
    Werner, Apr 16, 2012
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  5. johnandchris

    Tenson Moderator

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    Is this a ground compensated output? I guess it might be taking the back EMF on the speaker negative terminal and feeding it back into the amp input via an attenuator. The amp output then carries the same signal on the posative terminal and the EMF becomes common-mode voltage across the speaker coil?
     
    Tenson, Apr 16, 2012
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  6. johnandchris

    johnandchris

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    Interesting. The information i published above was a direct copy of what he published.
    If i am going to try it, what exactly would i need to do (ie what to connect where).
    Thanks
    John
     
    johnandchris, Apr 16, 2012
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  7. johnandchris

    I-S Good Evening.... Infidel

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    John PMed me on this subject because it has sort of lead-on from a previous suggestion made regarding bridging a power amplifier through stereo inputs from an XLR balanced output from a preamp.

    As tenson suggests, this is effectively an active ground to the speaker.

    Its benefit, compared to a single-ended setup, is much greater Common-mode rejection ratio, and any PSU noise in the amplifier will become common mode (so Power supply rejection ratio will increase).

    However, it has all the disadvantages of standard bridging/balanced operation (namely higher output impedance and potentially higher output noise) without the benefits of increased voltage swing and dynamic range.

    Overall, I would stick with genuine, full-balanced operation.
     
    I-S, Jul 27, 2012
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  8. johnandchris

    Tenson Moderator

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    Thanks for the report Isaac.

    As you say it seems like a balanced input driving a bridged amp would be a preferable method.
     
    Tenson, Jul 27, 2012
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  9. johnandchris

    Sergeauckland

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    If the pre-amp has a balanced output, then bridging tow power amps is trivially easy.

    Connect XLR pin 2 output to Pin 2 of Power amp 1
    Connect XLR pin 3 output to Pin 2 of Power amp 2
    Connect XLR Pin 1 output to Pin 1 of both power amps.

    Connect XLR pin 3 of both power amps to pin 1 (i.e. signal ground)

    The effect of this will be to drive both power amps but Amp 2 will be inverted. Taking the loudspeaker output between the + terminal of each power amp will double the voltage output.

    S.
     
    Sergeauckland, Jul 27, 2012
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  10. johnandchris

    Tenson Moderator

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    Or perhaps drive each input with both phases, but reverse pin 2 and 3 on one amp?
     
    Tenson, Jul 27, 2012
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  11. johnandchris

    Sergeauckland

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    That's equally good. I have a horror of cables with phase invertion, work of the Devil they are, so I try and avoid having any in my system.....one day that cable will get used elsewhere......

    S.
     
    Sergeauckland, Jul 27, 2012
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  12. johnandchris

    I-S Good Evening.... Infidel

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    If you are describing using 4 mono power amps this way, then that's actually not advisable.

    Bridging like this works best when the bridged amplifier channels share a power supply - in other words, turn a stereo amp into a mono one.

    The reason for this is that if the two amplifier channels have separate power supplies, be it in one box or in two, then the power supply noise on each one will be different. When the power supply noise is the same on both channels (due to sharing of the power supply) it cancels out. If it's different, it isn't cancelled.

    The problem is that the hifi industry has done a good job of selling everyone on "more separate power supplies is better" - not always the case. It IS, however, good to separate the two channels from one another, by bridging separate stereo amps into monoblocks.
     
    I-S, Jul 27, 2012
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  13. johnandchris

    Sergeauckland

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    I agree that bridging two mono amplifiers won't reduce power supply noise (although it might have some effect on hum). However, the purpose of bridging is to increase the available volts rather than reduce any power supply noise, which should be thoroughly inaudible anyway. This is particularly so if the bridging is done to an already high power amplifier, as it can be assumed (not always a safe thing to do) that the loudspeakers are pretty inefficient, and so shouldn't be showing up any power supply noise.

    S.
     
    Sergeauckland, Jul 27, 2012
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  14. johnandchris

    I-S Good Evening.... Infidel

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    Not necessarily - there are plenty of amps, both with and without feedback, that have significant power supply noise. This is often hidden in specs by the use of an a-weighting filter for SNR measurements. The use of bridging can massively reduce such noise.

    For most domestic installations the additional power is not a necessity.
     
    I-S, Jul 27, 2012
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  15. johnandchris

    Sergeauckland

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    I agree about the use of A weighting for noise...hides a multitude of sins...I would be suspicious of any company that quoted noise A weighted without a corresponding unweighted figure, and quote the bandwidth measured over.

    I wonder what amplifiers you're referring to, as I haven't come across any such amplifiers, but then I've led a somewhat sheltered life.

    My own amplifiers had some power supply noise but increasing the size of the main reservoirs cured that issue. Unfortunately, they're double mono, so bridging them wouldn't have helped all that much.

    S.
     
    Sergeauckland, Jul 27, 2012
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  16. johnandchris

    I-S Good Evening.... Infidel

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    Contexually appropriate for this thread, the Rotel 971 has a fair amount of PSU noise.

    In the 971 it leads to an interesting problem - although THD figures are very good, there is a significant amount of intermodulation distortion between any signal and the power supply noise - eg putting a 1kHz tone through it results in a significant number of sidebands at 50Hz intervals.

    AES paper 6551 contains details...
     
    I-S, Jul 27, 2012
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  17. johnandchris

    MadGus

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    Hi . I am new to this forum. Does anybody have a photo of this psuedo shot fuse jack connection... ?
    I have 2 x 971 amps and would like to try this idea ?
     
    MadGus, Aug 28, 2020
    #17
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