how to reduce the 120hz harmonics in 40dB gain amplifer

[Andrew Xiang]

for my inverting amplifier, the gain is 100v/v 40dB. the noise floor
is about 100uV=0.1mv, after 40dB, it is about 10mv and noticable on
the speaker, worse yet, there is 120hz, 240, 480hz harmonics present,
very noticable, The white noise level is actually acceptable, I would
still appreciate it if someone can help me reduce the 100uV noise. But
I really want to remove the 120hz harmonics.


I used 3 stage linear regulator. 15v->5v->3.3V. the last one is a low
noise LDO, noise is about 30-40uVRMS, I guess 100uV is reasonablely a
good design for the ground plane? The opamp is TL074 jfet input
low-noise opamp.

Anyway, the 120hz harmonics is the key, how can I remove thme?

thanks
Andrew

 

[John Popelish]

for my inverting amplifier, the gain is 100v/v 40dB. the noise floor
is about 100uV=0.1mv, after 40dB, it is about 10mv and noticable on
the speaker, worse yet, there is 120hz, 240, 480hz harmonics present,
very noticable, The white noise level is actually acceptable, I would
still appreciate it if someone can help me reduce the 100uV noise. But
I really want to remove the 120hz harmonics.

I used 3 stage linear regulator. 15v->5v->3.3V. the last one is a low
noise LDO, noise is about 30-40uVRMS, I guess 100uV is reasonablely a
good design for the ground plane? The opamp is TL074 jfet input
low-noise opamp.

Anyway, the 120hz harmonics is the key, how can I remove thme?

thanks
Andrew

What device is connected as a signal source to your amplifier?

Almost certainly these harmonics are not getting through your
regulation (probably not through the first stage, unless you have very
marginal filtering on your rectifiers). It is either direct field
pickup by unshielded or unguarded input lines from the rectifier
section, or, more likely, by rectifier filter currents passing through
your ground plane and causing small resistive voltage drops that are
getting added to your input voltage. Think about your layout, trying
to account for the current paths in everything connected to ground.
Then consider any nodes that are exposed via line of sight to any 120
Hz ripple source.

 

[Tweetldee]

for my inverting amplifier, the gain is 100v/v 40dB. the noise floor
is about 100uV=0.1mv, after 40dB, it is about 10mv and noticable on
the speaker, worse yet, there is 120hz, 240, 480hz harmonics present,
very noticable, The white noise level is actually acceptable, I would
still appreciate it if someone can help me reduce the 100uV noise. But
I really want to remove the 120hz harmonics.


I used 3 stage linear regulator. 15v->5v->3.3V. the last one is a low
noise LDO, noise is about 30-40uVRMS, I guess 100uV is reasonablely a
good design for the ground plane? The opamp is TL074 jfet input
low-noise opamp.

Anyway, the 120hz harmonics is the key, how can I remove thme?

thanks
Andrew

If the power supply shows no sign of 120 Hz ripple, then the problem must be
in the signal path. A few questions to ask yourself about the design and
layout:
Are the power pins on the opamp well bypassed? Are the signal lines in/out
well shielded and are the shields connected to a single point ground?
Is the amplifier and power supply on the same PCB? Are the AC and
unregulated DC paths kept well away from the opamp? Is the power transformer
nearby (magnetically induced hum)?
Cheers!
--
Tweetldee
Tweetldee at att dot net (Just subsitute the appropriate characters in the
address)

Time is what keeps everything from happening all at once.

 

[Ben Bradley]

What device is connected as a signal source to your amplifier?

Almost certainly these harmonics are not getting through your
regulation (probably not through the first stage, unless you have very
marginal filtering on your rectifiers). It is either direct field
pickup by unshielded or unguarded input lines from the rectifier
section, or, more likely, by rectifier filter currents passing through
your ground plane and causing small resistive voltage drops that are
getting added to your input voltage. Think about your layout, trying
to account for the current paths in everything connected to ground.
Then consider any nodes that are exposed via line of sight to any 120
Hz ripple source.

I totally agree with the above, they are all good suggestions, but
the first thing I would do is put a 0.1uF cap (of appropriate voltage
rating) directly across each and every rectifier in the power supply
(might also consider one across the power transformer secondary, and
in parallel with the first filter cap - caps are not that expensive,
especially if they fix a problem like this). This by itself may reduce
the buzz to acceptable levels, but if not, continue on as above. Once
you see a reduction in harmonics of the powerline frequency due to
adding caps like this, you will want to make it standard practice in
power supplies for sensitive circuits.

And before anyone asks, no, I don't work for a company that makes
capacitors, but my resume is available for the asking.

 

[N. Thornton]

And before anyone asks, no, I don't work for a company that makes
capacitors, but my resume is available for the asking.

I've found a capacitor on the TF secondary is a simple way to
attack pops and clicks on basic unreg supplies.

Regards, NT