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Finding the noise-source in a headphone-amp

  • Thread starter Rikard Bosnjakovic
  • Start date
R

Rikard Bosnjakovic

Jan 1, 1970
0
Greetings

I found and built this project:
http://raymondaudio.nl/projects/project7/project7.html

After I finished and tested it out, I found lots of noise in the
headphones. Since my skill of analog electronics is way too low to get a
good guess what might have gone wrong, I took the construction to my
lecturer at the university. He gave me a couple of advices how to find the
errors in such piece of circuit.

Another week passed and after extensive searches, I think I have pinned
the problem. Here's an explanation of what I did to test and pin:

The blocks of the circuit are transformer, rectifier, linear regulators,
sound amplification. Using it plainly, I get lots of noise in the
headamps. Bypassing the transformer and rectifier-bridge (using a DC-psu
and feed the DC into the regulators), the noise goes away. Doing the same
but feeding to the bridge (i.e. "rectifying" my DC), the noise comes back.

I take this means the rectifier bridge (four 1N4005) is broken in some
way, but how? Before building, I tested each diode with an ohmmeter, and
they all showed "infinite" resistance in the wrong direction, and low
resistance in the correct direction.

So, what might it be that's happening at the diodes? Magnetic field
inducing? Short circuits?
 
E

Eeyore

Jan 1, 1970
0
Rikard said:

Is it built on a pre-made pcb or have you made it on stripboard ?

After I finished and tested it out, I found lots of noise in the headphones.

Hiss (white noise) ? Does it sound 'swirly' at all ?

Since my skill of analog electronics is way too low to get a
good guess what might have gone wrong, I took the construction to my
lecturer at the university. He gave me a couple of advices how to find the
errors in such piece of circuit.

Another week passed and after extensive searches, I think I have pinned
the problem. Here's an explanation of what I did to test and pin:

The blocks of the circuit are transformer, rectifier, linear regulators,
sound amplification. Using it plainly, I get lots of noise in the
headamps. Bypassing the transformer and rectifier-bridge (using a DC-psu
and feed the DC into the regulators), the noise goes away. Doing the same
but feeding to the bridge (i.e. "rectifying" my DC), the noise comes back.

I take this means the rectifier bridge (four 1N4005) is broken in some
way, but how?

Doesn't sound much like it.

Before building, I tested each diode with an ohmmeter, and
they all showed "infinite" resistance in the wrong direction, and low
resistance in the correct direction.

So, what might it be that's happening at the diodes? Magnetic field
inducing? Short circuits?

I suspect the op-amps are oscillating at VHF due to an absence of local supply
decoupling. That tends to make them noisy. These 'project designs' on the net
are often very amateurish.

Try adding some plastic film or ceramic 22nF capacitors from both V+ and V- on
the op-amp power pins to ground.

You should really have something similar on the inputs to the LM317 and 337 too.

Graham
 
R

Rikard Bosnjakovic

Jan 1, 1970
0
Eeyore said:
Is it built on a pre-made pcb or have you made it on stripboard ?

It is built on a veroboard / stripboard.
Hiss (white noise) ? Does it sound 'swirly' at all ?

I don't think it sounds swirly or noisy, it sounds more like a constant
"clear tone" ringing all the time.
I suspect the op-amps are oscillating at VHF due to an absence of local supply
decoupling. That tends to make them noisy. These 'project designs' on the net
are often very amateurish.
[...]

A lot other people have told me this as well, that the PSU in the
schematic is really a waste of power. I'm too unskilled to have an own
opinion, but running into problems like these are probably a good idea to
learn more things that aren't in the books.

I will add some caps to the op-amps and the regulators and see what (if
something) happens.
 
C

Chris

Jan 1, 1970
0
Greetings

I found and built this project:http://raymondaudio.nl/projects/project7/project7.html

After I finished and tested it out, I found lots of noise in the
headphones. Since my skill of analog electronics is way too low to get a
good guess what might have gone wrong, I took the construction to my
lecturer at the university. He gave me a couple of advices how to find the
errors in such piece of circuit.

Another week passed and after extensive searches, I think I have pinned
the problem. Here's an explanation of what I did to test and pin:

The blocks of the circuit are transformer, rectifier, linear regulators,
sound amplification. Using it plainly, I get lots of noise in the
headamps. Bypassing the transformer and rectifier-bridge (using a DC-psu
and feed the DC into the regulators), the noise goes away. Doing the same
but feeding to the bridge (i.e. "rectifying" my DC), the noise comes back.

I take this means the rectifier bridge (four 1N4005) is broken in some
way, but how? Before building, I tested each diode with an ohmmeter, and
they all showed "infinite" resistance in the wrong direction, and low
resistance in the correct direction.

So, what might it be that's happening at the diodes? Magnetic field
inducing? Short circuits?

Hi, Rikard. You've pretty much proven that the issue is with your
power supply. But I think it's possible the basic problem is with the
zeners rather than the rectifiers. I'd use a standard resistive
voltage divider (try replacing the zeners with1.2K and the 200 ohm
resistor with a 120) instead of the 12V zeners for the 317 and 337.
Any zener noise is just going to be copied directly onto the output
line, with poor audio resuts.

Good luck
Chris
 
E

Eeyore

Jan 1, 1970
0
Chris said:
Hi, Rikard. You've pretty much proven that the issue is with your
power supply. But I think it's possible the basic problem is with the
zeners rather than the rectifiers. I'd use a standard resistive
voltage divider (try replacing the zeners with1.2K and the 200 ohm
resistor with a 120)

Which increases the noise of the 317 / 337 by ~ 10 times, the gain of the reference diode !


instead of the 12V zeners for the 317 and 337.
Any zener noise is just going to be copied directly onto the output
line, with poor audio resuts.

You're talking complete and utter rubbish.

A. The noise fron zeners isn't enough to be problematic.
B. Op-amps have very significant supply voltage rejection. They don't care too much about
in band supply noise.

Graham
 
R

Rich Grise

Jan 1, 1970
0
It is built on a veroboard / stripboard.


I don't think it sounds swirly or noisy, it sounds more like a constant
"clear tone" ringing all the time.

Like a whine, or a whistle? Then it sounds like you've built an
oscillator. :) Do you have access to a 'scope?

Good Luck!
Rich
 
E

ehsjr

Jan 1, 1970
0
Rikard said:
Greetings

I found and built this project:
http://raymondaudio.nl/projects/project7/project7.html

After I finished and tested it out, I found lots of noise in the
headphones. Since my skill of analog electronics is way too low to get a
good guess what might have gone wrong, I took the construction to my
lecturer at the university. He gave me a couple of advices how to find
the errors in such piece of circuit.

Another week passed and after extensive searches, I think I have pinned
the problem. Here's an explanation of what I did to test and pin:

The blocks of the circuit are transformer, rectifier, linear regulators,
sound amplification. Using it plainly, I get lots of noise in the
headamps. Bypassing the transformer and rectifier-bridge (using a DC-psu
and feed the DC into the regulators), the noise goes away. Doing the
same but feeding to the bridge (i.e. "rectifying" my DC), the noise
comes back.

I take this means the rectifier bridge (four 1N4005) is broken in some
way, but how? Before building, I tested each diode with an ohmmeter, and
they all showed "infinite" resistance in the wrong direction, and low
resistance in the correct direction.

So, what might it be that's happening at the diodes? Magnetic field
inducing? Short circuits?

Each regulator should have a .1 uF cap from Vin
to ground. Whether or not that fixes your problem,
install them.

Ed
 
C

Clint Sharp

Jan 1, 1970
0
Rikard Bosnjakovic said:
So, what might it be that's happening at the diodes? Magnetic field
inducing? Short circuits?
More likely you've got one of the diodes in backwards. Check and double
check.
 
L

Lionel

Jan 1, 1970
0
More likely you've got one of the diodes in backwards. Check and double
check.

Not a chance. Putting in one or more diodes backwards would either
make it buzz or stop it working altogether.

As everyone else has said, it's almost certain to be one or more of
the chips oscillating from the lack of bypass chips - especially
seeing as it's built on Veroboard. I'd solder a 100nF monoblock
ceramic across the power input/output pins of each chip, *as close as
physically possible*. (And maybe a 4.7uF tantalum to the input side of
each LM317.)

Oh, & if I were building that circuit myself, I'd use a 78L12 & a
79L12 & replace each Zener diode with a pair of 1N4001 diodes to jack
the power rails up to 13.2V. (Cheaper & smaller)
 
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