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54779 Views 19 Replies Latest reply: May 29, 2012 4:02 PM by nestoribio RSS
Currently Being Moderated

May 17, 2012 7:59 AM

Audio Transformers -- Need a technical Insight.

Hi Folks,

I'm a Digital Design Engineer and would like to get some information from some of you guys. As i said i'm into Digital Domain so I know really less about analog and very less about audio transformers let me give you a brief intro of my design

I need to design an amplifier with 50W o/p & output drive of 4/8 Ohm loudspeaker at 70V/ 100V AC line and capability of driving multiple speakers, I have chosen MAX9709 to do the amplification job, with some R&D i have learnt i might need an audio transformer.


I have some brief questions please try and answer them.


1 - When do we opt for an Audio Transformer in an audio design?

2 - What are the critical metrics associated with an audio transformer?

3 - How to interface the electronic circuitry to the speakers (this is the biggest question for me)?


Please feel free to give info related to this in any way, irrespective of the questions asked.


Thanks & Regards

Schneider

  • MikeWeed Novice 62 posts since
    Oct 21, 2011
    Currently Being Moderated
    May 17, 2012 10:52 AM (in response to Schneider)
    Audio Transformers -- Need a technical Insight.

    You don't need an audio output transformer with the MAX9709, and in

    fact the chip will not work with an audio output transformer. Connect

    the speakers directly to the chip as shown in the data sheet. This

    is a class D amplifier, meaning that the audio output signal is 200kHz

    (in this case) modulated by the signal in the audible range up to 20kHz.

    The speaker inductance averages out the 200kHz to produce audible sound.

    Audio output transformers were originally used with tube type amplifiers

    to convert the high voltage (and high impedance) signal from the plate

    of the tube to a lower voltage for the lower 4 or 8 ohm impedance speaker.

      • MikeWeed Novice 62 posts since
        Oct 21, 2011
        Currently Being Moderated
        May 18, 2012 10:50 AM (in response to Schneider)
        Audio Transformers -- Need a technical Insight.

        Commonly available audio power amplifier chips are not made to drive step-up

        transformers. You should not use a step-up, step-down scheme, but a separate

        power amplifier for each speaker, and feed the input of each amp from your

        signal source. Perhaps the most economical way to drive 10 50W speakers

        (8 ohms each) is to use 5 of the LM4780TA devices. These are class AB amps,

        so will dissipate considerable power, and a good heat sink for each will be

        needed, and plenty of heat-conducting grease! A cooling fan for each may also

        be necessary. The simplest circuit is shown in Figure 1 of the LM4780 data sheet.

        This requires a +Vcc supply of +35V and a -Vee supply of -35V, and for the

        amount of power you need, each supply should be able to provide 12 amps or more.

        The speakers must be mounted as close as possible to the amps to avoid

        problems with the lead inductance. You will get a voltage gain of about 100

        from this circuit, so the peak-to-peak input signal will need to be no larger

        than 1 volt. Also, be sure to connect all the speakers in the same relative

        phase so the output sound waves will add instead of cancel.

  • nestoribio Novice 8 posts since
    May 21, 2012
    Currently Being Moderated
    May 21, 2012 11:42 AM (in response to Schneider)
    Audio Transformers -- Need a technical Insight.

    IMHO, as you need a distribution system it is OK to use transformers. Also as you have choosen a Class D amplifier, wich is good in terms of efficiency, you must preceed the transformer with a low pass filter. Those line transformers are not for vintage tube amplifiers (as wrongly stated), but to feed several speakers on a quite wide area using suitable thin wiring.

     

    Besides it will be more cumbersome to route the signal to several distant places and power those independent amplifiers too. Simply too many wires will make the proyect too expensive and unreliable.

     

    You are on good track: Transformers are OK and they can be found elsewhere. Just look for line transformers. The step-up should be able to handle the full 50W (plus some reserve). Check low frecuency response to avoid core saturation, you may use a high pass filter at the input. High frecuencies are not a problem for transformers itself, but you may need a snubber network to protect your amplifier. Each speaker must have an step-down transformer but just capable of 50/4=12.5 W (I maybe choose 15W ones)

     

    If using class D remember to use a low pass filter, whose design is not simple, but anyway less costly than other solutions given to you.

     

    Datasheets are just a guide, a good one, but you do not have to follow them as a rigid statement.

    • MikeWeed Novice 62 posts since
      Oct 21, 2011
      Currently Being Moderated
      May 22, 2012 9:26 AM (in response to nestoribio)
      Audio Transformers -- Need a technical Insight.

      It takes 500 watts of audio power to drive ten 50W speakers simultaneously.

      Transformers do not exhibit power gain. I suggested using five LM4780TA

      modules, each capable of driving two 50W speakers, for a total amplifier

      cost of $57.65. You could use a single 500W or 600W class D amplifier at

      a lower cost, but these are only available in packages that are difficult for

      hobbyists to use. The advantage of several amplifiers is that, if one fails

      for some reason, the others will continue to produce sound. If the amplifiers

      are near the speakers you need only 3 power leads (+V,-V and ground) and

      a single 1-conductor (plus shield) signal line that runs past all the speakers

      and is tapped to provide an input to each amplifier. The power leads can be

      14 gauge copper or 12 gauge aluminum and do not need to be shielded, as

      there will be bypass capacitors at each amplifier. The signal line is low current;

      24 or 22 gauge should be adequate. This is not complicated nor is it a lot of wires.

      What would be complicated is trying to match the impedance of ten speakers

      of 4 or 8 ohm impedance each to a single amplifier of 2, 4 or 8 ohms output

      impedance. The impedances must match to achieve full power output.

      It has been suggested that you use a step-up, step-down transformer scheme.

      A 500W audio output transformer would be extremely expensive, even if you

      could find one, and add to that the expense of ten 50W audio trnasformers.

      Additionally, long wires between the step-up and step down transformers

      would have to be shielded to prevent noise pickup, and the noise problem

      is aggravated by the higher impedance level of this signal line.

      I wrote "Audio output transformers were originally used with tube type

      amplifiers to convert the high voltage (and high impedance)  signal from the

      plate to the lower voltage for the lower 4 or 8 ohm speaker." Anyone who

      thinks this is an incorrect statement is either ignorant or can't read.

      Chip manufacturers publish well-tested designs in the Application section

      of their data sheets. Is using a simple, economical, proven design being

      too "rigid"?

      I offer advice freely in the hope that someone will benefit from my electronic

      design experience of more than half a century. You can heed my advice, or

      you can try a harebrained scheme that will probably fail and will certainly cost

      much more, even if you could find the parts. It does not seem like a difficult

      decision.

      "Never argue with an idiot. People might not be able to tell the difference."

      "Against stupidity the gods themselves contend in vain."

      Enough said.

      • nestoribio Novice 8 posts since
        May 21, 2012
        Currently Being Moderated
        May 22, 2012 9:06 PM (in response to MikeWeed)
        Audio Transformers -- Need a technical Insight.

        Mike I will quote you and the original post, plus a few comments;

         

        I need to design an amplifier with 50W o/p & output drive of 4/8 Ohm loudspeakers

        It takes 500 watts of audio power to drive ten 50W speakers simultaneously. ¿!? Who is asking that huge amount o f power? Total power 50W, speakers 4 each 8 ohms

        Anyone who thinks this is an incorrect statement is either ignorant or can't read. Maybe you have not read the original post!

        The power leads can be 14 gauge copper or 12 gauge aluminum and do not need to be shielded, but the will be a lot of voltage drop! as there will be bypass capacitors at each amplifier. The signal line is low current; 24 or 22 gauge should be adequate... and it need to be shielded!.

         

        I wonder if in your long experience years if you have seen, (not installed or designed) a distribution system for a building.

        For the original post using 70V or 100V system 24 AWG is more than adecuate. No sheild needed; just a pair or a twisted pair.

        Gainless humble tranformers are not just for valve amplifiers...

         

        "Never argue with an ignorant. Anyone can tell the difference."

        "Against stupidity the gods themselves run away."

         

        Not enough said; you should learn to be humble and polite, but maybe it is dificult for someone who is 60+

        Even if I were completely wrong your statements can be and they are inacurate.

        I asume that this blog is for sharing and not for shouting.

        • MikeWeed Novice 62 posts since
          Oct 21, 2011
          Currently Being Moderated
          May 23, 2012 9:19 AM (in response to nestoribio)
          Audio Transformers -- Need a technical Insight.

          To Nestoribio,

          When it comes to being polite and humble, take a good look at yourself.

          Schneider specified 10 speakers in his second post, and a rating of 50W. He

          did not say whether he meant 50W per speaker or 50W total, but he did specify

          107dB. According to axiomaudio.com/power.html, you need 100W to produce

          106dB with a very sensitive speaker, and most speakers are not that sensitive.

          Schneider could only have meant 50W per speaker for a total of 500W.

          As for your other comments, it is not surprising that someone with your

          emotional problems would mock the great Friedrich Schiller who wrote

          "Mit der Dummheit kampfen Goetter selbst vergebens."

          • nestoribio Novice 8 posts since
            May 21, 2012
            Currently Being Moderated
            May 23, 2012 4:43 PM (in response to MikeWeed)
            Audio Transformers -- Need a technical Insight.

            Mike you seem to smoke some weed! The MAX9709 is just capable of 50W. Just read the data sheet. You just repeat the same over and over in English and in German, puedo seguir en español si te gusta armar una torre de Babel. Pero con los ancianos listos y sabihondos no hay quien les vaya a la contraria.

            Tu ganas, Miguel Yerbita!

            Finalmente Schneider no ha dicho nada y como parece que él mismo no está interesado en ésta inútil polémica, si es por mí considerate ganador y disfurta tu victoria Pírrica, pero no me respondas mas. En fin me encanta eso de 10 amplificadores clase B conectados a una sola fuente de 700 o mas watt. Igual pudiera poner los

            10 MAX9709. Asi se ahorra los transformadores y la señal sin blindaje debe ir de maravillas.

            Y si Schneider va a dar un concierto en un estadium no es ningún hobbista.

            Again you win! 12AWG aluminium wire should be lighter, but keep me out of this non sense.

            • MikeWeed Novice 62 posts since
              Oct 21, 2011
              Currently Being Moderated
              May 24, 2012 8:31 AM (in response to nestoribio)
              Audio Transformers -- Need a technical Insight.

              The optimum solution to this problem cannot be found without more information.

              Questions for Schneider: Why do you need 10 speakers and not some other number?

              Will the speakers be separated by some distance, or clustered together in one

              location? If separated, by what distances, and does their orientation matter?

              If in a cluster, what is their relative orientation? That is, do they all point in the

              same direction in an array, or are they in a semicircle pointing outward, or some

              other arrangement? How did you arrive at the 107 dB figure? How did you arrive

              at the 50 watt figure? By 50 watts of power, did you mean 50 watts per speaker

              or  50 watts for the entire system?

                • MikeWeed Novice 62 posts since
                  Oct 21, 2011
                  Currently Being Moderated
                  May 24, 2012 11:34 AM (in response to Schneider)
                  Audio Transformers -- Need a technical Insight.

                  OK, this is a different problem from what I surmised from your earlier posts. A 50W

                  amplifier will be easy to select, with a little more information, but you will not get

                  107dB from this system. The Web says a train bogey is the carriage that carries

                  a set of wheels, probably around 10 feet wide and perhaps 20 feet long, with a

                  distance of several tens of feet between bogeys. So we have an estimate of the

                  length of wires in the distribution system.

                  It sounds like you are replacing the amplifier and maybe the step-up transformer.

                  Does this mean that the specifications for the distribution wiring, step-down transformers

                  and speakers are set in stone?  I assume the high sides of the step-down transformers

                  will be connected in parallel, but you need to know the impedance of these windings

                  (the low side winding impedance matches the speaker impedance). I never said that

                  step-up transformers are used exclusively for tube type amplifiers. We can find a

                  suitable one once the  other specifications are known.

  • MikeWeed Novice 62 posts since
    Oct 21, 2011
    Currently Being Moderated
    May 24, 2012 1:52 PM (in response to Schneider)
    Audio Transformers -- Need a technical Insight.

    Results after more web research:

    Apparently the step down transformer hi side impedance is unimportant in these 70v/100v

    distribution systems. Good article at ticcorp.com/25v_70v_100v_systems.htm.

    A simple solution might be to use a standard amplifier made for an 8 ohm output,

    followed by an OSD Audio MTR250 amplifier/transformer (amazon.com for $119.95)

    that does the constant voltage step-up. This provides way more power than you need,

    but would be hard to beat at the price.

    I forgot to ask, what is the power supply for all this? Do you have 115vac available,

    or is it for a portable system running on a generator/battery, and if so, what battery voltage is

    available?

  • nestoribio Novice 8 posts since
    May 21, 2012
    Currently Being Moderated
    May 24, 2012 5:57 PM (in response to Schneider)
    Re: Audio Transformers -- Need a technical Insight.

    Hi Schneider and Mike:

     

    I wonder if step-down transformers are not already in place as well as speakers. Maybe the step-up transformer was part of the amplifier you are gone to replace (please correct me if I am wrong). I my searches for transformers the best price I have found is at Edcor (I am supposing you are in USA or Canada, if not there are other choices in Europe, again let me know if I err.), then Hammond a bit more expensive and the very best the Swedish Lundahl, I am ignoring the Chinese ones on purpose. Any of them sell distribution transformers.

     

    As long as know secudary impedance does matter. All primaries are connected in parallel in a distribution system in a way your amplifier "sees" 4 ohms (or its rated impedance) but many times no impedance match is done on the speaker side, you just select the right tap to get the right amount of power. Most distribution line transformers have several taps on primary and secundary sides, but again I am assuming the distribution system is already installed.

     

    As I told you before if you select a class D amplifier you also must provide a low pass filter between the amplifier output and the step-up transformer. I you want to avoid this risky step and efficiency is not an issue (as I suppose it should not be) it may be simpler to choose any class B amplifier that can be direct connected to the steep-up transformer. A snubber network will help the protect the class B solid state amplifier. I think that any of the ones suggested by Mike will do the job nicely.

     

    I am not a young boy. I started my first electronic proyects back in the 60's (I was then a young boy! LOL).

    Anyway I fully apreciate that we are now more calm down and on track.

    .

    • nestoribio Novice 8 posts since
      May 21, 2012
      Currently Being Moderated
      May 24, 2012 5:57 PM (in response to nestoribio)
      Re: Audio Transformers -- Need a technical Insight.

      Some EDCOR suitable transformers:

       

      http://www.edcorusa.com/p/287/em3060

      http://www.edcorusa.com/p/304/wa60-4-70

      http://www.edcorusa.com/p/305/wa60-4-100

      http://www.edcorusa.com/p/309/wa60-8-140

      http://www.edcorusa.com/p/279/ea60

       

      All escept the last one are sold for $28.00 . The last one is priced $31

       

      You are free to choose the one wich better fits your design...

      • nestoribio Novice 8 posts since
        May 21, 2012
        Currently Being Moderated
        May 24, 2012 6:40 PM (in response to nestoribio)
        Re: Audio Transformers -- Need a technical Insight.

        By the way I was reading the specs of the MAX9709 and its distortion is quite high for my taste. I am pretty sure that are better ones out there.

         

        The LM4780TA is a nice amplifier. Used on mono it is capable of 120W, wich gives you ample power reserve and quite low noise and distortion figures.

         

        If you choose it I will suggest the bridge connection (Fig 2 page 5). Just change the speaker for the low side of the distribution transformer. Read carefully the datasheet!

        I wonder if it should not be good to apply the suggestions on page 19 on reactive loading for capacitive loads.

         

        National Semiconductors gives you full details for a good implementation. Datasheet is ample of good technical info.

        For Mike's joy, here you can, and better yet you must follow NS application notes, even for the printed circuit design.

        • MikeWeed Novice 62 posts since
          Oct 21, 2011
          Currently Being Moderated
          May 25, 2012 10:04 AM (in response to nestoribio)
          Audio Transformers -- Need a technical Insight.

          I assume you plan to use the existing distribution network, step-down transformers and speakers, so

          we are only concerned with the step-up transformer and amplifier.

          Either a bare step-up audio transformer of sufficient wattage or a commercial box like the one I mentioned

          above could be used.  The box costs more but has safety features the transformer alone does not.

          My experience is, it is better to spend a little more now and not be faced with an unhappy customer

          later, but this is really a matter of choice.

          The reason I asked about available power is that the box (probably) runs on 115vac, and power for

          the amplifier will most likely also need 115vac.

          I agree with nestoribio (gasp!) that a class D amplifier is risky, especially if you use the commercial box

          step-up amplifier/transformer, because the box specs only say that the input impedance is 8 ohms. Some

          class D amplifiers need output filters and some can drive speakers directly, but may  work well only

          if the load has an inductance like a speaker.

          You may want to consider a commercial box that is made to drive an 8 ohm speaker, or build your

          own amplifier. Many good class B or class AB amplifier modules are available. I like the TDA7295

          because it is very low cost, comes in a package that is easy to use, has low THD, has a closed-loop

          voltage gain of 30dB (1 volt per millivolt), and at 80W gives you some power to spare. Be sure to mount

          this module on a sufficiently large heatsink and use heat conducting grease. If you build your own amplifier, you will need a pair of DC power supplies. Digikey 285-1829-ND should be  adequate.

        • nestoribio Novice 8 posts since
          May 21, 2012
          Currently Being Moderated
          May 29, 2012 4:02 PM (in response to Schneider)
          Audio Transformers -- Need a technical Insight.

          It was a pleasure to help.

          Also Mike helped a lot; without him this topic may be too technical and boring. And besides he helped us to focus on what is really important.

          I agree with Mike on the safety and reliability of the pre-made boxes but also wonder if your boss want you to design (and maybe build) an audio proyect, and then you have to show that you can do it...

          Good luck and best regards

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