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Thread: Can I connect balanced output straight to Quad 405?

  1. #21
    Join Date: Mar 2017

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    I'm Dennis.

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    Agree.

  2. #22
    Join Date: Mar 2008

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    I'm inthescottishmafia.

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    Quote Originally Posted by Pharos View Post
    I am not familiar with your feeding device, but;

    In general signal sources IME are not in danger when shorting their O/P because there is sufficient O/P impedance to prevent damage, but this is certainly not the case when power is involved.

    Examples IME, O/P of a preamp, O/P of a tape recorder, O/P from a preamp to a tape recorder.
    That was my understanding, worst that would happen is no output as signal is going to ground.
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  3. #23
    Join Date: Jan 2009

    Location: Essex

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    I'm openingabottleofwine.

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    Taken from https://web.archive.org/web/20081227...m/note151.html :



    Figure 5 shows recommended wiring for all combinations of balanced and unbalanced I/O interconnections when 2-conductor shielded cable is used. Figure 5 also includes the two most common manufacturer shield-grounding schemes; signal-grounding the shield and chassis-grounding the shield. Identifying these schemes for every unit in a system is essential to debug system hum and buzz. This is no simple task since chassis and signal grounds are connected together. The goal is to find out if the manufacturer connected them together is such a way that shield currents do not affect the audio signal. The dashed lines in Figure 5 represents the units' chassis boundary. Connections between dashed lines are functions of the cable. Connections outside these lines are the manufacturer's choosing, whether conscious or unconscious.

    Figure 5 is arranged such that the top and left most figure (5a) is the theoretical "best" way to connect equipment with optimal results. The "best" way being, everything completely balanced with all shields (pin 1s) connected to chassis ground at the point of entry. As one moves down or to the right, degradation in performance is expected. Whether a system operates acceptably or obeys these theoretical predictions is too system-specific to predict accurately. However, one must start somewhere.

    The quality and configuration of the input and output circuits are omitted from Figure 5 and the ensuing discussion, to focus on cable wiring and the internal wiring of the units. The I/O circuitry is assumed ideal.

    Figure 5. Interconnectivity using shielded 2-conductor cable only. Asterisks denote usability with off-the-shelf cable.



    Balanced Output Driving Unbalanced Input

    The third column in Figure 5 is the most troublesome, balanced outputs driving unbalanced inputs. Since the input stage is not balanced, induced noise on the signal conductors is not rejected. If you must use an unbalanced input, use as short an input cable as possible. This reduces the induced noise. There's a reason it's hard to find and buy unbalanced RCA cables longer than 12 feet. Figure 5i shows both ends of the cable shield connected to units with chassis-grounded shields. If the units are far apart, the chance of the shield currents inducing noise on the signal conductors is greater. Keeping this cable very short reduces the shield current and therefore reduces the noise that is not rejected by the unbalanced input stage. Most systems may require disconnecting one end of the shield for the Figure 5i case. Even a small current in the shield may prove too much for an unbalanced input stage. Again, support your favorite one-end-only political position.

    Disconnect the shield at units with signal-grounded shields (Figures 5j & 5k). If both ends have signal-grounded shields, run for your favorite one-end-only political party. (Figure 5l).

    This scheme connects the balanced output's negative output to signal ground, rather than a high impedance input. Many balanced output circuits will attempt to drive this signal ground, causing great distortion and potentially damaging the output stage.

    Other balanced output stages are termed "floating" balanced. (Analog Devices SSM-2142 Balanced Line Driver chip is one example.) Also called a cross-coupled output, these circuits mimic the performance of fully balanced transformer solutions and are designed so the negative output can short to signal ground. If you find or use this scheme, be sure that the balanced output stage can properly handle signal ground on its negative output.

    (My emphasis)
    Barry

  4. #24
    Join Date: May 2020

    Location: NorthWest

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    I'm Philip.

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    I've finally got all the bits together after a long wait for an amp, but the cable I've made, as per diagram 4b is introducing noise. I've double checked the source output, and the amp from a different source, and checked the cable for continuity several times.

    Currently then the cable (2 core shielded) is wired as so;

    Female XLR end;
    hot to pin 2
    shield to pin 1

    RCA end;
    hot to tip
    shield to sleeve.

    I'm following the articles linked to by Barry but my lack of technical know-how is letting me down really. I've no idea if my output stage is cross coupled, for example. I'm going to try the next best looking diagram, no.6 which has the addition of cold connected to pin 3 at the XLR end (short to shield) and to the sleeve at the RCA end, but I'm curious to know whats introducing the noise. The cable is 4m long and as far as I'm concerned it's terminated well. I'm trying to get in touch with Behringer for some proper specs as the only manual I can find is a quick-start guide.
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  5. #25
    Join Date: May 2020

    Location: NorthWest

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    I'm Philip.

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    Well as far as I can tell, the job is done.

    Hot > 2
    Cold > 3
    Shield > 1 (nothing shorted)

    Hot > Tip
    Cold/Shield > Sleeve

    It's clean as a whistle, unless I'm missing something I can't see or hear. Thanks again to everyone for all of your help and all the links provided, i've definitely learnt a few things.
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  6. #26
    Join Date: May 2008

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    I'm Rob.

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    Good to hear you sorted it.
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  7. #27
    Join Date: Jan 2009

    Location: Essex

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    I'm openingabottleofwine.

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    Quote Originally Posted by HarryCrumb View Post
    Well as far as I can tell, the job is done.

    Hot > 2
    Cold > 3
    Shield > 1 (nothing shorted)

    Hot > Tip
    Cold/Shield > Sleeve

    It's clean as a whistle, unless I'm missing something I can't see or hear. Thanks again to everyone for all of your help and all the links provided, i've definitely learnt a few things.
    That arrangement is the one that is most often used in the ready-made leads available on-line. But the 'cold' (or negative-going) output is shorted to the shield (connected to pin 1) at the RCA end. Clearly the balanced output of your pre-amp can cope with this.

    As all the articles cited have shown, the best way to arrange for a balanced output to feed into an un-balanced input, does depend on the specific items, and a certain amount of trial and error is needed.

    I'm delighted it's now all working well, and to add some reassurance, I have fed a (modified) Quad 405-2 amp from a pre-amp having a balanced output using exactly the arrangement you describe (but that was because the manufacturer of the pre-amp advised such arrangement).

    Time now to enjoy some music.
    Barry

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