Audio feedback problem

[audioguru]

The intellegibility of your extremely narrow band AM radio is almost zero. My wife asked, "What is that awful noise?" then she heard some Spanish (she is from Spain).

 

[JimB]

I agree that there is a lot of wideband hiss on the audio.
Consider that this could be coming from the IF amplifier.
I assume that you have a nice narrow xtal filter at the front of the IF amp, but do you have much selectivity inside or at the end of the IF amp?
A high gain IF amplifier can contribute a LOT of wideband hiss if it is not stopped by a narrow filter at the end of the IF amp before the (product) detector.

Before you ask how I know, I have been there, heard the hiss and got the T-shirt, then added the filter - much better.

JimB GM3ZMA

 

[Space Varmint]

The background hiss is worse than the hum. Your audio gain distribution is still whacked. Don't blame it on A to D sampling of your PC, your audio is horrible. Follow advice given in previous threads. Have you done a signal/noise test of the radio? At any rate, building a radio that works is an accomplishment so be proud. Still love me?

No, your right. I toned it down. Those things don't bother you as much at first. Eventually the details come into play. Yeah, your still OK

 

[audioguru]

Don't tell anybody but I actually listen to AM radio news and traffic reports in my car. Its frequency response is from about 60Hz to about 3kHz at -3dB and it doesn't sound TOO bad. I think the AM radio stations have a peak at about 4khz so it sounds a little crisper. There is no noise what so ever except in a lightning storm and when I drive past crackling high voltage transmission lines.

 

[Nigel Goodwin]

What do mean? The damn thing is decoupled. He said decouple the base, so I asked him what he meant.....choke it? I got no answer.

How can you try and design a radio without the simplest knowledge of how to do so? - feed each stage from the supply rail via a resistor with decoupling capacitors to chassis, both an electrolytic and a smaller one in parallel.

Standard circuit design - it's there for a reason.

Anyway it is clean as a whistle, no hum on battery. So yeah, A better power supply could be in order.

You can't get hum on battery - because there's no hum involved - you're feeding the supply hum directly to the bases of the many unrequired transistors as you don't have the required decoupling circuits.

Throw all the unrequired transistors out, and the problem may go away in any case - they aren't doing anything, apart from injecting hum.

 

[Space Varmint]

Don't tell anybody but I actually listen to AM radio news and traffic reports in my car. Its frequency response is from about 60Hz to about 3kHz at -3dB and it doesn't sound TOO bad. I think the AM radio stations have a peak at about 4khz so it sounds a little crisper. There is no noise what so ever except in a lightning storm and when I drive past crackling high voltage transmission lines.

That sounds like a nice receiver. Is it adjustable band pass? What do you listen to? Just locale AM broadcast?

 

[Space Varmint]

Here's what I was after.....communications!

I played with the beat note a little then went down to about 7.140 MHz and tuned in some Morse code.

 

[Space Varmint]

Sorry it was a long file and kicked out recording. Here's one I did 2 minutes ago on 40 Meters. Same thing. Starts at top of band with SSB then down to CW at low end.

 

[audioguru]

I don't call all that noise, "communications".
The voices are extremely narrow-band but the noise is wide-band.

It would sound a lot better if the audio response went to 3kHz or 4kHz and there is no noise.

 

[transistor495]

My cats ran away while playing the space voomit's radio file

 

[Space Varmint]

I don't call all that noise, "communications".
The voices are extremely narrow-band but the noise is wide-band.

It would sound a lot better if the audio response went to 3kHz or 4kHz and there is no noise.

Oh yeah, well, an audio filter would be nice. Actually what's happening with the audio is if I go to straight AM it is little too basey. I don't like that and I'm thinking about using a small coupling cap before that chip to cut it back.

I have a couple of situations. First of all I'm using a big Altec Lansing speaker is a wood or particle board box. So that will bring out the base. I'm mean it sounds great with head phones, just off the '741. That's just what I want. But if I used smaller series resistors going to the chip, THAT'S WERE I RAN INTO THE FEEDBACK PROBLEM.

 

[Space Varmint]

You'll notice allot of noise is atmospheric noise though. But you guys were right about an excessive hiss. I scoped through the audio and found on the very first audio amp I put a .01uf cap directly on the base to filter highs. Well it had a bad adverse reaction. It actually caused the hiss. I looked at the collector output and there was spurious white noise. I removed it and it went back to normal.

I'm glad I have you guys technical ear. Even Nigel though he can be a P_____ at times....lol.

 

[Space Varmint]

80 meters : https://www.mediafire.com/file/mktgnyexgzi/80MetersA.wma

 

[Nigel Goodwin]

I'm glad I have you guys technical ear. Even Nigel though he can be a P_____ at times....lol.

I've never even listened to the files, I know what SSB and AM sound like, and your's are going to be no better (and most probably worse - as you have no proper filtering, and really poor design).

But it's about COMMUNICATION, not QUALITY, anyway - but the limitation should be on the transmission and the path, not in the receiver.

 

[Space Varmint]

I've never even listened to the files, I know what SSB and AM sound like, and your's are going to be no better (and most probably worse - as you have no proper filtering, and really poor design).

But it's about COMMUNICATION, not QUALITY, anyway - but the limitation should be on the transmission and the path, not in the receiver.

Man that is pure baloney! Where do you come up with this crap?

Anyway I filtered the audio some more after I finished the pre-selector. It sounds great but I really need to pick up an inline mike. A friend was telling me it makes a big difference when making these recordings.

Now suppose you tell me how the receiver doesn't matter. This should be good...

 

[kchriste]

I really need to pick up an inline mike. A friend was telling me it makes a big difference when making these recordings.

Steal a 600Ω transformer out of an old modem or something and couple the receiver's audio straight into the "line input" on your sound card. Then there will be no dispute about frequency response.

 

[audioguru]

Hi Mr Gone, aka Space Varmint,
You recorded your radio with a microphone in front of a speaker?
An Altec Lansing speaker is pretty good and if the microphone is an electret type with the proper bias voltage then it should sound OK.
But your "radio" has too much gain, too much attenuation and its high frequencies are cut too much.

A low-pass filter for audio is supposed to have a flat frequency response up to its cutoff frequency then a sharp drop. Your low-pass filter begins cutting important voice frequencies and has a very droopy response.

You say that your AM section has too much bass and you said before that the power amplifier is boosting the bass when it doesn't.
Do you like the very thin squeaky sounds of a cheap kid's AM radio? It doesn't have any bass.

 

[Space Varmint]

Hi Mr Gone, aka Space Varmint,
You recorded your radio with a microphone in front of a speaker?
An Altec Lansing speaker is pretty good and if the microphone is an electret type with the proper bias voltage then it should sound OK.
But your "radio" has too much gain, too much attenuation and its high frequencies are cut too much.

A low-pass filter for audio is supposed to have a flat frequency response up to its cutoff frequency then a sharp drop. Your low-pass filter begins cutting important voice frequencies and has a very droopy response.

You say that your AM section has too much bass and you said before that the power amplifier is boosting the bass when it doesn't.
Do you like the very thin squeaky sounds of a cheap kid's AM radio? It doesn't have any bass.

No, I've been using the built in mike on my laptop. The speaker is a good 3 feet away.

Right now I got one problem left. Something Nigel told me about maybe a week ago. The VOLUME control. It don't work right. Everything else is great. It's done except that damn volume control. I got my AGC just pefect. No clipping at all. It's exactly where I want it. So I need to catch the volume control between the 741 OP Amp and the rest of the audio.
I'm not sure what the problem is but the only variable POT I have left is a wire wound. That might be the problem. It just does nothing at all if I put it in series anywhere at that stage of the game. So I did that little rig where I run it to ground and found that it grossly distorts my audio.

What do I do? The audio is beautiful coming off the OP AMP except it clips because I interrupt the AGC to put the head phones in. Hed to do it like that to keep from adding more shielding (cans....lol).

Is there a way to back down the audio in series? Do I need a carbon type resistor?

 

[Nigel Goodwin]

Man that is pure baloney! Where do you come up with this crap?

Anyway I filtered the audio some more after I finished the pre-selector. It sounds great but I really need to pick up an inline mike. A friend was telling me it makes a big difference when making these recordings.

Now suppose you tell me how the receiver doesn't matter. This should be good...

It's trivial to make a receiver that's greatly higher quality than the transmitter and reception path, in fact it would be difficult NOT to make a receiver of much higher quality (although you do seem to have managed it?).

 

[Space Varmint]

Current Audio:

Pot miss placed....Help!