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Old 3rd November 2010, 10:24 PM   #1
neazoi is offline neazoi  Greece
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Default 5KHz simple low pass audio filter needed

hello I design a low power (500mW) audio amplifier using small transistors.
The amplifier is going to be used in a SSB modulator so I need to cut frequencies that exceed 3-4KHz
Is there any simple filter schematic I can use (RC maybe?) with low passband loss in order to cut frequencies higher than 5KHz?
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Old 4th November 2010, 02:06 AM   #2
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Look up Sallen and Key filters.
Also Butterworth filters.
The articles will give you formula to calculate the components required.

1/(2*pi*r*c)
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Old 4th November 2010, 02:23 AM   #3
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Simple can mean a lot of different things. There are several ways to make a low pass filter:
1. first order filter: a series capacitor. This is by far the easiest to do, but attenuation is poor.
2. second order filter:
A. using op amps and sallen key or multiple feedback design
B. time continuous filter ICs like the UAF42 from Burr Brown
C. switched capacitor filters like Linear Devices LTC1062 and related products. Maxim is another company who makes these I think.

There are many different switched capacitor filter chips available. I mention the LTC1062 because it is DC accurate, needs only two or three external components, and implements a 5th order Butterworth filter function. But there are many, many different switched cap filters. One common problem is that they are not as low noise as analog circuits. Most are low pass, but a few are general purpose and/or have HP functions as well.

Which implementation you choose depends on your budget, attenuation needs, available DC power supply voltages (most need +5V DC only), etc. Choose carefully.

-Charlie
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Old 4th November 2010, 08:25 AM   #4
neazoi is offline neazoi  Greece
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Thank you all for your replies.
I usually prefer simpler circuits and avoid using opamps where possible, although I would not expect the performance of an opamp using single transistors.
I have found this http://freecircuitdiagram.com/wp-con...-Equalizer.gif which is a passive graphic equilizer which could be used to let only the voice pass through. Of course this is passive, meaning lossy and I have used an input and output transistor buffer to overcome the loss.
On the other hand I have found this http://www.rlocman.ru/shem/i/users.o...cuits-EQ_3.gif which is an active equalizer but it uses more transistors, I wonder if I could omit a few bands without problem. The problem is that it uses symetric psu +-24v
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Old 4th November 2010, 01:48 PM   #5
DF96 is offline DF96  England
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What type of SSB modulator needs a 500mW audio amp? Most SSB systems modulate at low power, then use linear amplifiers. The few exceptions, such as EER, are rarely used in DIY circuits and require wide bandwidth modulators. What are you trying to achieve?
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Old 4th November 2010, 02:03 PM   #6
Elvee is offline Elvee  Belgium
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These circuits are basically tone controls, they arent very suitable for speech control.
Here are a number of examples (you can modify the capacitors to adapt the frequencies):
Circuit - 3KHz LOW PASS FILTER PLUS AUDIO AMP - Circuits designed by David A. Johnson, P.E.
SSB AF FILTER by SM0VPO
How to build Speech Filter - circuit diagram
http://www.analogzone.com/avt_0806.pdf

Where applicable, you can replace opamps wired in unity-gain by common collector transistor stages
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Old 4th November 2010, 02:32 PM   #7
neazoi is offline neazoi  Greece
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The 500mW is the speaker output power. Part of the amplifier chain will be used for the phasing modulation combined with a passive audio polyphase network.

The thing is that this polyphase network keeps relatively accurate phases at about 300Hz-3KHz. Out of this range the phases loose their accuracy and the unwanted sideband starts to appear. Also, the greatest audio bandwidth is, the greatest the RF bandwidth is, because the phasing method of ssb generation does not have any RF bandpass filtering mechanism. So I need to limit the audio or at least cut off the higher audio frequencies. Unless my considerations are not right...

A simple audio compressor will allow for almost the same RF level for all audio variations (keeping the SSB signal power at maximum), but for bandwidth limiting I need an audio filter.

I am just wondering if a simple filter can be made using a few transistors. The buffered passive filter works ok but I just wonder if a better simple solution can be done.
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Old 4th November 2010, 02:59 PM   #8
DF96 is offline DF96  England
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I am still not clear whether you are building a receiver or a transmitter. In either case you will need good audio filtering for polyphase SSB. Cascaded Sallen-Key is how I would do it, using 2 or 3 stages (i.e. 4 or 6 poles). If you find out about filter design you can learn how to combine stages to get the response you want: Butterworth or Chebyshev. This is a non-trivial exercise, so I suggest you either copy an existing design or start reading about active filters.
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Old 4th November 2010, 04:23 PM   #9
neazoi is offline neazoi  Greece
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So DF96, my consideration about audio bandwidth limit is right indeed? (as far as concern phasing ssb modulator)
The less tha audio bandwidth the less the RF bandwidth?
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Old 4th November 2010, 08:42 PM   #10
DF96 is offline DF96  England
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Yes, provided your modulators are perfect. Any non-linearity causes bandwidth spreading on transmit and poorer selectivity on receive.

You still haven't told us what you are trying to do.
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