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Which type of large decoupling capacitor for amplifier audio output?

Discussion in 'Audio' started by carebare47, Jul 18, 2014.

  1. carebare47

    carebare47

    66
    1
    Oct 21, 2010
    Hello,
    I am currently trying to build a Theremin. I am using someone elses design, I have parts lists sorted out and am almost ready to try and get a prototype working. Being a bit of a novice still I am unsure on what type of capacitor to use for the output of the audio amplifier stage of the theremin. The circuit diagram for the audio amplifier stage can be found here. The capacitor in question is the 1000uF non-polarised one by the output of the op-amp. I don't know much about capacitors, but are non-electrolytic capacitors made that large? Could I use a non-polar or bipolar electrolytic capacitor? I have ran the circuit in multisim (a simulation program) and there appears to be about 11V DC before the capacitor (and no apparent DC offset before it), so it looks important. Any help would be greatly appreciated.
    Many thanks,
    Tom
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    It's a standard polarised electrolytic capacitor. The polarity is not marked on the diagram, but the side that connects to the TDA2003 is the positive plate, and the speaker side is the negative plate.
     
  3. carebare47

    carebare47

    66
    1
    Oct 21, 2010
    Thank you very much for your help and the quick reply. Do you know if electrolytic capacitors have an effect on audio quality? I'll get one anyway, it's just so I can keep it in mind for future modifications.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    All components "have an effect on audio quality" but the question is whether it's detectable by anyone's ears. In many cases this question would be answered by double blind testing, but folks who claim to be able to hear subtle differences are very resistant to testing themselves in any reliable way.

    In this case, the presence of the capacitor will cause the response to roll off at low frequencies, i.e. the bass response will be reduced, and this should be detectable by ear. Using a higher capacitance value will improve the bass response.

    A better option is a DC-coupled amplifier that doesn't have an output capacitor at all; ICs are available to implement these, but split supply rails are needed, unless you use the bridge-tied load ("BTL") configuration. Google those keywords for more information.
     
  5. carebare47

    carebare47

    66
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    Oct 21, 2010
    Okay, thank you. I don't suppose you know how to calculate where the low frequency roll off will occur? I will use this design for the prototype, but if there is an audible problem with bass response I'll look into different amplifier stages or modifying this one. Thank you for your advice, your responses have been very valuable over the last week or so.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    It's hard to say accurately, because the loudspeaker is a complex load - it consists of resistance, inductance, and motional inductance, due to the movement of the cone, which is affected by the enclosure and environment, and it contains an impedance peak at the natural resonant frequency of the speaker, which is between around 200 Hz for a small speaker down to about 40 Hz for a 15" subwoofer. These factors make it impossible to calculate the rolloff using the standard C-R filter formula which assumes a constant value for R.

    You can calculate a corner frequency using that formula, but the actual -3dB point could be higher or lower than that value by a factor of 2 or more.

    1000 µF is definitely on the low side for an 8Ω load. If you need good bass response, you should look into DC coupled amplifiers - either a single-ended one, which requires split supplies, or a BTL one, as I mentioned. Both of these have no output series capacitor at all, so they can have a very good low-end frequency response and they may also be more compact.

    The BTL has the advantage of higher output power for a given power supply voltage and load impedance, because both ends of the speaker are driven, and it is widely used in car audio. The TDA2009A can be used in BTL configuration, as shown in this schematic from the data sheet. But since both halves are used for a single channel, you would need two TDA2009As for stereo. The TDA2004A looks very similar to the 2009A and should also be able to be used in BTL configuration, but this is not shown in the data sheet.

    TDA2009 BTL.png

    P.S. There are many other amplifiers that can be used in BTL configuration, including many with much higher output power specifications and better availability. Look on digikey.com in the category Integrated Circuits - Linear - Amplifiers - Audio.
     
  7. BobK

    BobK

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    Jan 5, 2010
    1000uF and 8 Ohms will be down 3dB at 19.80Hz. Unless you are using a huge subwoofer, the speaker will cut off way above that. A typical 5 or 6 in speaker is not going below 40Hz or so.

    The speaker and enclosure are much more important to bass response, as Kris said.

    Bob
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yes, you're right Bob. It looks like 1000 µF would actually be plenty. Thanks for the correction.
     
  9. carebare47

    carebare47

    66
    1
    Oct 21, 2010
    Just realized I never follow-up posted. Ended up using a 2200uF cap and it worked wonderfully. Thank you Kris for your useful, detailed explanation, and Bob for your quick practical advice :)
     
    davenn likes this.
  10. Zerotolerance

    Zerotolerance

    12
    0
    Sep 19, 2011
    For audio, I recommend Low Impedance Electrolytic Caps
     
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