RadioBanter - SX-190 buzz in audio

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Mark Rehorst wrote:
On Sat, 01 Nov 2003 16:31:45 -0400, --exray-- wrote:

Could you elaborate a bit more on what it sounds like? Is it an AC buzz
or an AC hum? Is it 60 cycles?
Is the power supply a plain-jane bridge or is it a switching type?

-Bill

It is a buzz. Connecting a scope in parallel with the speaker reveals
a very complex waveform that is in sync with the 120 Hz power supply
ripple.

A mod article in 73 magazine suggested raising the feedback
capacitance to flatten the audio response and reduce hum and noise. I
tried it and it had no audible effect on the buzz.

I also tried putting capacitance in parallel with the rectifiers.
Again, no joy.

I tried replacing the rectifiers with Schottky types. No change.

I am going to try adding a 3 terminal regulator to supply the audio
stage from a regulated source instead of the unregulated power it now
uses. If that works, maybe I'll replace the audio board with a
regulated power supply and IC power amp...

MR

Are you getting a lot of harmonics of the 120Hz?. Maybe a small value
cap like .05 across the filter caps would be worth a try.
If you redo the audio stage what about the 'modulation' of other stages?

-Bill

Are you getting a lot of harmonics of the 120Hz?. Maybe a small value
cap like .05 across the filter caps would be worth a try.
If you redo the audio stage what about the 'modulation' of other stages?

-Bill

There is only one filter cap- 2200 uF. I'll try bypassing it.

The audio stage is the only one that gets unregulated output from the
supply. The rest of the radio is powered via a regulator.

MR

I tried adding caps across the rectifiers- no good.
I tried bypassing the main filter cap with a 20nf ceramic cap. No
good.
I tried swapping out the rectifiers. No good.
I added a 3 terminal voltage regulator, reducing the audio amp supply
voltage to about 10V. Buzz all but gone. I have to press my ear to
the speaker to hear any buzz now.

The distortion in the audio seems to have gone up a bit, so maybe an
amp that can operate from a lower voltage would be better. I'll look
into audio amp chips and see if I can find a suitable device.

Thanks for all input.

MR

Mark Rehorst wrote:

I added a 3 terminal voltage regulator, reducing the audio amp supply
voltage to about 10V. Buzz all but gone. I have to press my ear to
the speaker to hear any buzz now.

The distortion in the audio seems to have gone up a bit, so maybe an
amp that can operate from a lower voltage would be better.

Killing the audio amp is a very effective way of eliminating all sorts
of QRM/QRN. Just kidding, good luck with it.

-Bill

Mark Rehorst wrote:
I tried adding caps across the rectifiers- no good.
I tried bypassing the main filter cap with a 20nf ceramic cap. No
good.
I tried swapping out the rectifiers. No good.
I added a 3 terminal voltage regulator, reducing the audio amp supply
voltage to about 10V. Buzz all but gone. I have to press my ear to
the speaker to hear any buzz now.

Try replacing that one cap with a larger valued cap. Or make a pi filter
with a resistor and another cap.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On Fri, 31 Oct 2003 16:21:50 UTC, Chuck Harris
wrote:

Mike Knudsen wrote:

I have a cheap AM/FM clock radio that sounds like light-dimmer power line noise
even on FM. I've tried tacking caps all over the place, to no effect. The DC
supply is clean on a scope. You have to wonder how such a sound gets into FM
-- it sounds exactly like the sort of noise that sends you reching for the ANL
switch on a real radio. --Mike K.

A lot of that kind of noise comes from solid state rectifiers in the
powersupply. The HV rectifiers don't switch off quickly enough, and
essentially short out the supply for an instant around the zero crossing
point of the input sinewave. This always sounds like a very raucus
60Hz (half wave rect.), or 120Hz (full wave rect.) buzz, and it comes in
thru some RF or IF stage in the receiver.

This can be solved one of two ways:

1) add a series resistor to each each diode to limit how much current
can flow, 100 ohms, or some such. (Note, one for each diode!)

2) switch to fast recovery diodes.

As to why cheapy FM radios respond to this kind of AM noise, it is all
in the use of a ratio detector vs a limiter/discriminator. Ratio
detectors are pretty good at eliminating AM, but limiter/discriminator
stages are very good at eliminating AM. Ratio detectors are basically
simpler, and as such much cheaper.

-Chuck Harris, WA3UQV

I have an S9+ noise level on all the HF bands. I've suspected my
computers, fluorescent lamps, BPL, but I'm not certain. It
sounds/looks like this.

XxxxXxxxXxxxXxxxxXxxxXxxxXxxxxXxxxxX
0----v----1----v----2----v----3----v----4

The scale is, oh, 40 meters from 7.0 to 7.4, each lowercase x is
S7-8 pink noise. Each uppercase X is a roughly 10 kHz wide cluster
of shot-noise, S9+.

The uppercase X's are about 55 kHz apart. It's all approximate and
seems to be different at different times.

I've turned off all the computers and other appliances, alarm
clocks, etc. There is no change in the noise.

I hear it on all my receivers which include 3 SB-303's and a
Signal/One.

any ideas?

de ah6gi/4

No Spam (ckh) wrote:

I have an S9+ noise level on all the HF bands. I've suspected my
computers, fluorescent lamps, BPL, but I'm not certain. It
sounds/looks like this.

I've turned off all the computers and other appliances, alarm
clocks, etc. There is no change in the noise.

Don't overlook a UPS...my APC makes garbage at periodic intervals across
the band, varies with input voltage

-Bill