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  1. #1
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    Touch output caps

    As many of you know by now the SB Touch includes inexpensive electrolytic caps on its outputs. These caps are polarized, which means they are supposed to be run with a bias voltage across the cap, this bias actually makes the cap work properly. But in the case of the touch there is NO bias voltage, I've been trying to find out what this lack of bias voltage does to the sound.

    I finally tracked down some articles on the web where people have been looking into this. It turns out that without bias a polarized capacitor still behaves as a capacitor for small AC voltages, BUT the distortion through the capacitor goes up exponentially as the AC voltage increases. The upshot is that at the full output of the Touch (2V RMS) the unbiased cap has 22 times as much distortion as a film capacitor does. At lower output voltages this distortion goes down significantly, but even at its best it's still about 5-6 times as much as a film cap.

    This has some interesting ramifications for connections to a system. The conventional wisdom says that you shoulkd run the Touch at 100% volume all the time and use an analog domain volume control, thus maximizing the bits used and the signal to noise ratio. But that doesn't take into account this exponential distortion. A number of people have reported that they get much better sound running the Touch directly into a power amp and using the digital volume to bring the level down. This is usually attributed to preamps not being very "transparent", but in this case it might be the significantly lower distortion when you run the Touch at a lower output level.

    Of course all the above is JUST for the analog outs, it has nothing to do with running the digital signal into an external DAC.

    So what to do about it? You can use an external DAC, that has been talked about a LOT. You can open up the Touch and bypass the output caps. Opening the Touch and performing surgery and getiing it back together is not an easy task, so don't attempt this unless you know what you are doing and are willing to risk destroying it. There are threads on this over in the DIY forum.

    You can also build an external bias circuit that biases the capacitors without having to open up the Touch. I'll try and get a thread on this started over in the DIY forum in a day or two. I'm kind of surprised that none of the companies that supply "tweaks" haven't marketed one of these yet. It very simple, a 6V source (AC supply or battery) and 2 resistors. You can put it in a box with connectors, but then you need an extra set of interconnects. You can have pictails sticking out of a box, you can take an interconnect and cut it in half and insert this etc. There are several ways you can do it.

    Its still not as good as getting rid of the capacitors all together, but its much lower distortion than what you get out of the box, and you don't have to do anything to the Touch itself.

    John S.

  2. #2
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    You might find this attachment helpful.

    Edit: Also check this Cyril Bateman article.
    Attached Images Attached Images
    Last edited by andy_c; 2012-02-28 at 15:12.

  3. #3
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    That article is for a properly biased electrolytic, an unbiased one is MUCH worse, which is the situation we find in the Touch.

    John S.

  4. #4
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    No, it is unbiased. See Figure 2.14.

  5. #5

    Smile

    Interesting reading.

    Its a real shame that Wima stopped their MKS2 range with the 5mm lead spacing as these were tiny compared to most. I have a few sets of 22uF 16v that I've used specifically for DC blocking on outputs. I used 1 set on my SB3 before I went transport only. In the Touch, I've removed the said elco's and bridged the pads as I've been playing to see how good the stock DAC can sound with a few tweeks as well as running it through an external DAC.

    The concept of applying DC to bias the caps is interesting. I would assume that you would need 2 sets of caps with the DC applied between the 2 sets? This assumption is based on the fact that you wouldn't want to inject DC to the DAC and the same would apply to the amp inputs. The caps are polarised and so would need be set positive to the supply (assuming a positive voltage is applied).

    What about and DC blocking caps on the amp input. In theory if they are polorised the oposite way to those in the touch, you already have 2 sets of caps?

    Just thinking out loud! Interesting....

    DAC output >.... -][+ >.... +ve >.... +][- >.... Amp
    Last edited by UV101; 2012-02-28 at 16:05.

  6. #6
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    Would installing a couple of non polar's solve the problem?

  7. #7
    Senior Member Mnyb's Avatar
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    Quote Originally Posted by JohnSwenson View Post
    As many of you know by now the SB Touch includes inexpensive electrolytic caps on its outputs. These caps are polarized, which means they are supposed to be run with a bias voltage across the cap, this bias actually makes the cap work properly. But in the case of the touch there is NO bias voltage, I've been trying to find out what this lack of bias voltage does to the sound.

    I finally tracked down some articles on the web where people have been looking into this. It turns out that without bias a polarized capacitor still behaves as a capacitor for small AC voltages, BUT the distortion through the capacitor goes up exponentially as the AC voltage increases. The upshot is that at the full output of the Touch (2V RMS) the unbiased cap has 22 times as much distortion as a film capacitor does. At lower output voltages this distortion goes down significantly, but even at its best it's still about 5-6 times as much as a film cap.

    This has some interesting ramifications for connections to a system. The conventional wisdom says that you shoulkd run the Touch at 100% volume all the time and use an analog domain volume control, thus maximizing the bits used and the signal to noise ratio. But that doesn't take into account this exponential distortion. A number of people have reported that they get much better sound running the Touch directly into a power amp and using the digital volume to bring the level down. This is usually attributed to preamps not being very "transparent", but in this case it might be the significantly lower distortion when you run the Touch at a lower output level.

    Of course all the above is JUST for the analog outs, it has nothing to do with running the digital signal into an external DAC.

    So what to do about it? You can use an external DAC, that has been talked about a LOT. You can open up the Touch and bypass the output caps. Opening the Touch and performing surgery and getiing it back together is not an easy task, so don't attempt this unless you know what you are doing and are willing to risk destroying it. There are threads on this over in the DIY forum.

    You can also build an external bias circuit that biases the capacitors without having to open up the Touch. I'll try and get a thread on this started over in the DIY forum in a day or two. I'm kind of surprised that none of the companies that supply "tweaks" haven't marketed one of these yet. It very simple, a 6V source (AC supply or battery) and 2 resistors. You can put it in a box with connectors, but then you need an extra set of interconnects. You can have pictails sticking out of a box, you can take an interconnect and cut it in half and insert this etc. There are several ways you can do it.

    Its still not as good as getting rid of the capacitors all together, but its much lower distortion than what you get out of the box, and you don't have to do anything to the Touch itself.

    John S.
    22 times ! as measured separately or in this actual application ?

    Touch has 0.0014% thd acording to stereophile so will it be 0.0014%/22 after for example removing the caps ?

    If I used the analog out I would remove them , so I'm fairly confident in your original assessment on how they work (or rather don't work ) plausible enough for me for an almost free mod and btw great initiative to actually have solution that can make people keep the warranty on the unit .
    --------------------------------------------------------------------
    Main hifi: Touch + CIA PS +MeridianG68J MeridianHD621 MeridianG98DH 2 x MeridianDSP5200 MeridianDSP5200HC 2 xMeridianDSP3100 +Rel Stadium 3 sub.
    Bedroom/Office: Boom
    Kitchen: Touch + powered Fostex PM0.4
    Misc use: Radio (with battery)
    iPad1 with iPengHD & SqueezePad
    (in storage SB3, reciever ,controller )
    server HP proliant micro server N36L with ClearOS Linux

    http://people.xiph.org/~xiphmont/demo/neil-young.html

  8. #8
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    Quote Originally Posted by Mnyb View Post
    22 times ! as measured separately or in this actual application ?
    I think he's referring to a Cyril Bateman article in which Bateman measured the THD of an electrolytic cap at about 22x that of a low-distortion film cap. I have the Bateman articles, which he used to provide for free at his web site, but that web site is gone now - it was on geocities. I've provided an alternate link here.

    The thing is, this distortion was measured with 1VRMS across the cap. But this is a coupling application, and if the cap is chosen to be large enough, the AC voltage across it is negligible. That's why the Douglas Self excerpt I posted shows the distortion approaching zero as the cap value is increased.

    Bateman did find that, for polar electrolytic caps, a reduction of distortion could be had by DC biasing. But that technique is wasteful. The absolute level of the second harmonic distortion component is proportional to the square of the amplitude of the AC voltage across the cap, while the absolute value of the third harmonic is proportional to the amplitude cubed of the AC voltage component across the cap. So decreasing that amplitude gives great benefits. I've attached a plot of that relationship from Bateman's articles. That's the relationship that Self is taking advantage of by limiting the AC voltage component across the cap to tens of mV.

    Bateman also found that non-polar electrolytics had lower distortion than polar ones, so they are a good choice.
    Attached Images Attached Images  

  9. #9
    Senior Member Mnyb's Avatar
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    Thumbs up

    Quote Originally Posted by andy_c View Post
    I think he's referring to a Cyril Bateman article in which Bateman measured the THD of an electrolytic cap at about 22x that of a low-distortion film cap. I have the Bateman articles, which he used to provide for free at his web site, but that web site is gone now - it was on geocities. I've provided an alternate link here.

    The thing is, this distortion was measured with 1VRMS across the cap. But this is a coupling application, and if the cap is chosen to be large enough, the AC voltage across it is negligible. That's why the Douglas Self excerpt I posted shows the distortion approaching zero as the cap value is increased.

    Bateman did find that, for polar electrolytic caps, a reduction of distortion could be had by DC biasing. But that technique is wasteful. The absolute level of the second harmonic distortion component is proportional to the square of the amplitude of the AC voltage across the cap, while the absolute value of the third harmonic is proportional to the amplitude cubed of the AC voltage component across the cap. So decreasing that amplitude gives great benefits. I've attached a plot of that relationship from Bateman's articles. That's the relationship that Self is taking advantage of by limiting the AC voltage component across the cap to tens of mV.

    Bateman also found that non-polar electrolytics had lower distortion than polar ones, so they are a good choice.
    Cool I'm not that kind of engineer so I can't really have a well grounded opinion on circuit specifics just thought the number was a bit out of context now you provided some more context great now I understand more.

    Any engineered opinion on how it applies to Touchs output stage , but you may need schematics to have an opinion on that ?
    --------------------------------------------------------------------
    Main hifi: Touch + CIA PS +MeridianG68J MeridianHD621 MeridianG98DH 2 x MeridianDSP5200 MeridianDSP5200HC 2 xMeridianDSP3100 +Rel Stadium 3 sub.
    Bedroom/Office: Boom
    Kitchen: Touch + powered Fostex PM0.4
    Misc use: Radio (with battery)
    iPad1 with iPengHD & SqueezePad
    (in storage SB3, reciever ,controller )
    server HP proliant micro server N36L with ClearOS Linux

    http://people.xiph.org/~xiphmont/demo/neil-young.html

  10. #10
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    Quote Originally Posted by Mnyb View Post
    Any engineered opinion on how it applies to Touchs output stage , but you may need schematics to have an opinion on that ?
    I'm not familiar with the circuit details of the Touch. I have an SB2 going into a Benchmark DAC.

    I must also mention this, about the audiophile double standard regarding harmonic distortion. If a cap has distortion levels in the ppm range, it seems to cause all kinds of hand-wringing. Yet for tube power amps with upwards of 1 percent THD, the distortion is routinely dismissed as benign.

    So, which is it? :-)

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