LM383 anti-hiss filter

Hi I am looking of a way to reduce the hiss (heard when no input to the amplifier) on the LM383. Any ideas?

Hi,

You can hear this hiss, thus it is in the audible frequency range.
If you supress this hiss, you need to suppress audible frequencies.
The hiss is in the high frequency range, so you need a low pass filter to reduce the noise.
The lower the cutoff frequency of the filter, the less hiss you will hear.
But you will the miss the high frequencies when listen to music.

Reducing the hiss means to reduce the audio frequency range = to reduce audio quality.

--> if you want good audio quality, you should look for a low noise amplifier.
LM383 is not a good amplifier and I think it is not produced now, for many years...
Why use such an outdated IC?

Klaus

Cheap and very old amplifier ICs are not high fidelity so they produce distortion and hiss.
There are many audio amplifier ICs made today that are high fidelity and produce very low distortion and no hiss.

KlausST said:

Hi,

You can hear this hiss, thus it is in the audible frequency range.
If you supress this hiss, you need to suppress audible frequencies.
The hiss is in the high frequency range, so you need a low pass filter to reduce the noise.
The lower the cutoff frequency of the filter, the less hiss you will hear.
But you will the miss the high frequencies when listen to music.

Reducing the hiss means to reduce the audio frequency range = to reduce audio quality.

--> if you want good audio quality, you should look for a low noise amplifier.
LM383 is not a good amplifier and I think it is not produced now, for many years...
Why use such an outdated IC?

Klaus

Click to expand...

My mistake I did not mention it. It is to be used in an SSB receiver, and the hiss is way above the 3KHz, so no problem to cut this off. But how? In the LM386 they do it by some feedback https://dmohankumar.files.wordpress.com/2015/03/hiss-free-lm-386-amplifier.png but I have no idea if this can be done in the LM383 as well.

I am building the "magical audio filter" and he has used this there. It works fine apart from the hiss. I think it is not the problem of the 741 chips, cause I have replaced them with TL071 and the hiss is the same.
However, when I set the notch pot to notch out high frequencies some of the hiss is reduced. What does these things tell you?

Yeah, SSB uses noisy AM modulation and a very narrow audio bandwidth.
You do not want to use 741 opamps that are 52 years old and produce hiss, and you do not want to make magical Notch or Peaking filters.
Instead you need an active lowpass filter to cut off hiss (and all the important consonant sounds of speech). When you remove all the s, t, f, sh, ch and other consonants then people listening will say, "What did you say, what did you say, over and over.

Here is a fairly simple second-order lowpass filter that produces -12dB per octave:

Audioguru said:

Instead you need an active lowpass filter to cut off hiss (and all the important consonant sounds of speech). When you remove all the s, t, f, sh, ch and other consonants then people listening will say, "What did you say, what did you say, over and over.

Click to expand...

Absolutely not true. In CW you can use CW IF filters (expensive if your TRX does not already have one) to achieve the same effect in the IF instead of the AF. However, problems start when on SSB mode. You need the SSB IF filter of course, but many nasty things can happen. Carriers interfering with the received SSB stations, near-by interference by other stations, noise from over the horizon radar intruders or just noise that can be reduced (along with some intelligence) using LPF/BPF/HPF or variable peak frequency and bandwidth filters.
A simple LPF or static BPF does not solve most of these problems.
The current filter offers some of these features, it is a communications AF filter, that has other specs than the AF filters used on Hi-Fi.

I did not understand. It is still the antique LM383 producing the hiss, not the SSB radio. You cannot filter the LM383 so throw it away and use a better newer audio power amplifier IC.
Your hiss problem might be caused by the speaker that might have a peak in its high frequency response. Try different speaker.
Good Luck removing radio interference from the SSB radio.

Audioguru said:

I did not understand. It is still the antique LM383 producing the hiss, not the SSB radio. You cannot filter the LM383 so throw it away and use a better newer audio power amplifier IC.
Your hiss problem might be caused by the speaker that might have a peak in its high frequency response. Try different speaker.
Good Luck removing radio interference from the SSB radio.

Click to expand...

I disconnected the LM383 from the rest of the circuit. The major hiss is from the rest of the circuit. I would say by ear a 80-90% from the circuit and a 10-20% from the LM383. So it is not this chip problem.

Instead of changing the circuit, can't I just add an RC LPF at the output (speaker side)?

I was also thinking of a stereo pot. one end connected to the input of the circuit, the other to the output (through coupling caps. Because the output would also be attenuated, the hiss would not be noticed at low volume levels. But can a standard stereo pot be used at a few W of output power this amp can provide?

A speaker's resonance sounds like a bongo drum unless it is damped by the extremely low output impedance (about 0.04 ohms or less) of a modern amplifier. A resistor in series with the output of the amplifier and a capacitor to ground makes a poor filter, reduces the output levels and ruins the damping.
A stereo pot is not made for power. Its entire track can survive 0.5W but half its track would be 0.25W and 0.1 of its track can survive only 0.05W. You do not want a low resistance pot at the input of an amplifier and you do not want a high resistance pot at the output.

Instead of changing the circuit, can't I just add an RC LPF at the output (speaker side)?

Click to expand...

Yes you can but consider the values you need and the losses they would introduce. With such low impedance, you would be looking at R being a few Ohms and C being several uF, quite a lot of amplifier output would be sunk straight into the filter

I was also thinking of a stereo pot. one end connected to the input of the circuit, the other to the output (through coupling caps. Because the output would also be attenuated, the hiss would not be noticed at low volume levels. But can a standard stereo pot be used at a few W of output power this amp can provide?

Click to expand...

You are not thinking that through... Lets assume a 5K stereo pot, at 100% volume you have no resistance between the top of the track and the wiper so you get maximum volume. Now back it off a few degrees, you get a slight drop in input signal from the first gang and maybe 100 Ohms in series with the loudspeaker, hence very little volume. Back it off a few degrees more and you get another slight drop in input level but maybe 500 Ohms in series with the speak, and almost no sound at all.

You would need a dual gang pot with say 5K or 10K on one gang and about 100 Ohms with several Watts rating in the other.

Brian.