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
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.
Hi MikeWeed thanks for the reply
I would like to make things more clear
What distances are these speakers distributed over?
1 meter (requiring 107db sound output)
What is the intended use?
what level of fidelity?
High Fidelity (THD<1%)
And the topology of my design is of a Constant voltage system
i.e., 70V/100V system as
Amplifier --> Step up transformer-->Distribution N/W-->Step down transformer--> Speaker
Now i hope the system is clear, so i would like to ask some more questions
1 - I'm stuck up at choosing Step up transformer, what should be the criteria for choosing the step up transformer, what are its specifications like insertion loss etc that i should take care of? if please suggest me any such parts and any reference designs for the above given topology.
2 - I'm using MAX9709 for the amplification job so now the system would be like this ; MAX9709-->Step up Transformer-->Cables--> Speakers(in built step down transformers) (Note: Max9709 gives 4/8 ohms outputs with 50W rating), my question does the MAX9709's output drive current capability affect the system? (Note: 70/100V Line)
I have also noted the fact that i can't use MAX9709 for my design from your reply, but i would be using some similar component so keeping that in mind can you please answer the above questions.
Thanks & Regards
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.
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.
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
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
"Never argue with an idiot. People might not be able to tell the difference."
"Against stupidity the gods themselves contend in vain."
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.
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."
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.
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?
That was one heck of a discussion, the best info that i got was that, MikeWeed is probably 70+ years aged, "Boss accept my venerations".
as for nestoribio you have really understood my specs clearly, so everything said, let me answer MikeWeed he has some serious questions.
1 -- Why do you need 10 speakers and not some other number?
I am supposed to design this amplifier which is a constant voltage 70/100V system with ten speakers with 5W rating each.
important point is the system is fixed so my system should be a replacement of the previous amplifier that the people were using in the past
2 -- Will the speakers be separated by some distance, or clustered together in one location?
roughly the gap between the speakers will be equal to the gap between two bogeys of a train(i have no knowledge of this metric) they are not clustered.
Nothing with the orientation as well, i'm much worried about the system till the step up transformer part as the distance of speakers and their orientation and stuff like that are required for me to decide the wire guage, which i'm not interested in right now.
3 -- By 50 watts of power, did you mean 50 watts per speaker or 50 watts for the entire system?
I suppose i answered it in the first question itself but let me answer it again, 10 speakers, each 5W, amplifer power 50W since(5x10=50W).
Now guys please go through my questions again and see if you can come up with any answer.
One thing i want to make clear is the Distribution system is fixed which is 70V/100V system so i'm bound to using a step up transformer.
one question that i would like to add is, how to interface my step up transformer and the distribution system to the amplifier, i don't think i can directly do that, any clues?
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.
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
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.
Some EDCOR suitable transformers:
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...
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.
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.
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