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Old 17th May 2012, 02:50 PM   #1
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Question Power Amplifier design with multiple speaker drive capability

Hi Guys,
I'm new to this forum and new to amplifier design as well, basically i'm a digital design engineer, but now i need to design an amplifier, there are a lot of things that i don't know when it comes to amplifier design so i'm seeking the help of all the Stalwarts in this forum, let me give you a brief description of my amplifier specifications:

1 - Bandwidth -- 20 Hz to 20 KHz
2 - Nominal power output -- 50W r.m.s
3 - Nominal sound output -- 107db
4 - Output drive -- Driving of 4/8 Ohm loudspeaker at 70V/ 100V AC line
5 - Low Harmonic Distortion -- <1%
6 - Should be able to drive multiple speakers as given by the above output drive.

with the given above specs i have selected MAX9709 to do the amplification job but i am a little perplexed with how to interface the o/ps with the multiple speakers and i came across a concept called audio transformers so do i need to use them in my design.

Kindly Help

Thanks & Regards
Bhavani Singh
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Old 17th May 2012, 04:04 PM   #2
AP2 is offline AP2  Italy
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Hi,
If there are quantities of supply, (i mean to compact module for both, speaker 4R and 100V ac line) please send me pm

Regards
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Old 17th May 2012, 10:11 PM   #3
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What distances are these speakers distributed over? How many? Type? Rating? What is the intended use? When you say multiple speakers, do you mean two or two thousand? What level of fidelity are you looking for...what are the speakers, source material, and associated equipment?

It is common in distributed PA paging work to drive high-voltage 70 volt lines with a power amp that has a lot of voltage capabaility (usually 2 channels bridged for voltage) or using an output transformer. This is a common feature of paging amplifiers. Then at each speaker there is small transformer so the 70 volt line can drive the comparatively low-impedance speaker. This high distribution voltage allows the use of very cheap wire for paging systems via ceiling speakers at a shopping center, or with cheap horns for the announcer at the dog track etc. It is not intended for high-fidelity music. The paralleled high-voltage transformers and each one's speaker start working more as current-following devices than the way conventional speakers follow voltage, and they just add more in parallel until the load aproaches the current capability of the amp. The transformers used at the speakers are usually not high-quality, and neither are the speakers.

For shorter distances, you are better off using larger-guage speaker wire and wiring the speakers into some kind of network where you put speakers in parallel to average their impedance anomalies then put those sets in series to achieve the desired impedance.

For distributed high-fidelity music, you should consider balanced low-impedance lines to power amplifiers at distant locations. Then take the short-distnace approach at each diverse location.

Your sound output spec means nothing without a reference level and distance etc.

Look at chip-amps, encapsulated high-power op amps, and surface-mounted board amps. I know little about them. You may be comfortable with switching-mode power supplies and switching-mode amplifiers with a digital design and RF background.
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Old 18th May 2012, 06:59 AM   #4
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With 100V line systems (70V line systems in the USA), the amplifier has to be able to drive 100V rms of audio into a variable load impedance. The multiple loudspeaker loads each have a tapped coupling transformer with nominal power settings indicated for each tapping point. If (for example) you have a 200W amplifier with a 100V output, then you can drive up to 200W of attached loudspeakers, which might be 100 speakers each tapped at 2W or 20 each tapped at 10W.
The advantages of this system is that the output level of each speaker is selectable at the speaker by altering the tapping point on the built-in transformer in the speaker, and that transmission at a higher voltage means that you can use thinner speaker cable.

The amplifier can either be a conventional one with an attached step-up output transformer, or you can buy amplifiers with a high-voltage direct-drive output, where the amp boards are running off high-voltage dc rails so that they can produce the 100V rms output without requiring the step-up transformers.
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Old 18th May 2012, 10:42 AM   #5
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Default More Technical info

Hi guys thanks for the replies
I would like to answer the questions of @cycleclamper
What distances are these speakers distributed over?
1 meter (requiring 107db sound output)
How many?
10
Type?
unknown
Rating?
50W
What is the intended use?
Broadcast
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)

Thanks & Regards
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Old 18th May 2012, 01:40 PM   #6
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Hi,
Class D requires a current limiter to drive the transformer.
not the on / off type, but which may allow the normal operation during the peaks (due to saturation)

Regards
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Old 18th May 2012, 01:54 PM   #7
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As AP2 says, current limiting can be a problem if you are adding a step-up output transformer to an existing class-D amplifier.

The transformer will have a very low dc primary resistance, so any dc offset at the amp output may cause high currents to flow in the primary which may cause early saturation on one polarity of the output waveform.
The transformer may also have inadequate primary inductance, so low-frequency signals may cause excessive magnetising current to flow in the primary.

Step-up OPTs on the output of a class-D amp can be a nightmare!
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Old 18th May 2012, 02:16 PM   #8
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Quote:
Originally Posted by Ouroboros View Post
As AP2 says, current limiting can be a problem if you are adding a step-up output transformer to an existing class-D amplifier.

The transformer will have a very low dc primary resistance, so any dc offset at the amp output may cause high currents to flow in the primary which may cause early saturation on one polarity of the output waveform.
The transformer may also have inadequate primary inductance, so low-frequency signals may cause excessive magnetising current to flow in the primary.

Step-up OPTs on the output of a class-D amp can be a nightmare!
Yes, it is right. and add...very nice problem eheh!
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