capacitor polarity

[frubo]

Hello,

I'm trying to figure out why the digital to analog converter for my embedded systems project wont work. The schematic was copied directly from a microchips pdf. The amplifier part seems to be working but the DAC is not. My guess is that it has something to do with the capacitors with round edge on the symbol. Are these a specific type of capacitor I need to make this schematic work? You can see the schematic here. The capacitors I am talking about are in Figure A-3 Speaker Schematic (C37 and C34). Any help would be appreciated.

 

[mneary]

I don't see a reason why they are different from others like C38, C26, etc. They can be ceramic, but be sure to get C0G or NP0 dielectric. This is not the best type of capacitor, and they can cause some audio distortion, but this circuit isn't audiophile grade.

They are in the low pass filter, and the circuit should work without them (but will have high frequency noise from the DAC).

 

[frubo]

I don't see a reason why they are different from others like C38, C26, etc. They can be ceramic, but be sure to get C0G or NP0 dielectric. This is not the best type of capacitor, and they can cause some audio distortion, but this circuit isn't audiophile grade.

They are in the low pass filter, and the circuit should work without them (but will have high frequency noise from the DAC).

Ok so the circuit should work with ceramic caps in there? I guess that isnt the problem then. Well is there any reason that the output from the second op amp would be getting saturated? I think thats what my teacher said was wrong with it. He recommended just modifying the schematic so it only uses the first op amp but it sounds like crap on the speaker and id rather find a different fix.

 

[ericgibbs]

Ok so the circuit should work with ceramic caps in there? I guess that isnt the problem then. Well is there any reason that the output from the second op amp would be getting saturated? I think thats what my teacher said was wrong with it. He recommended just modifying the schematic so it only uses the first op amp but it sounds like crap on the speaker and id rather find a different fix.

hi,
Is it the A-3 circuit which is giving you problems.?

Do you have a datasheet for the MCP61 OPA that you could post or link to.?
I'll have quick look on the LTspice Sim.

 

[audioguru]

Any amplifier saturates and sounds like crap when its input level is too high. Add a volume control to reduce the input level.

 

[frubo]

hi,
Is it the A-3 circuit which is giving you problems.?

Do you have a datasheet for the MCP61 OPA that you could post or link to.?
I'll have quick look on the LTspice Sim.

yeah the A-3 circuit.

MCP617-I/SN

 

[ericgibbs]

yeah the A-3 circuit.

MCP617-I/SN

hi,
This is what I see on LTspice. Looks OK at 100mV signal input.

What input signal level are you using.?



EDIT:
This is with a 1.7Vppk input signal, looks fine.
Have you checked that you have not crossed over the input pins of the OPA.???

 

[mneary]

Ok so the circuit should work with ceramic caps in there? I guess that isnt the problem then. Well is there any reason that the output from the second op amp would be getting saturated? I think thats what my teacher said was wrong with it. He recommended just modifying the schematic so it only uses the first op amp but it sounds like crap on the speaker and id rather find a different fix.

What is this now? You said it doesn't work so I based my answer on that, as if it was true. Now you say the problem was that it saturates. The changed information makes my answer wrong.

I give up.

 

[frubo]

What is this now? You said it doesn't work so I based my answer on that, as if it was true. Now you say the problem was that it saturates. The changed information makes my answer wrong.

I give up.

Sorry I should have been more specific in how the DAC doesn't work. My teacher told me it was saturating the op amp but we also had a discussion about those capacitors so I asked about that first.

 

[frubo]

hi,
This is what I see on LTspice. Looks OK at 100mV signal input.

What input signal level are you using.?

View attachment 40782View attachment 40783

EDIT:
This is with a 1.7Vppk input signal, looks fine.
Have you checked that you have not crossed over the input pins of the OPA.???
View attachment 40784

So I should only be inputting a 100mV signal? That makes sense why it isnt working. Ive got it hooked up to my microcontroller pwm so its going to be 3.3V. I should add a voltage divider then or is there an easier way? Also at what frequency should I be pulse width modulating?

 

[ericgibbs]

So I should only be inputting a 100mV signal? That makes sense why it isnt working. Ive got it hooked up to my microcontroller pwm so its going to be 3.3V. I should add a voltage divider then or is there an easier way? Also at what frequency should I be pulse width modulating?

hi,
Reading thru the datasheet for the PIC24H shows that the PWM audio signal is at 3.3V/0V and is passed thru the 4th order LPF , which has a cut frequency of approx 4kHz.

The main audio amp is rated at 1.5W.

Looking at the simple frequency sweeps using a 1.8V square wave input to the filter, show a distorted output from both the 1st stage and 2nd stage of the filter at frequencies above approx 4kHz.
I would expect it to sound poor quality on high frequencies especially 'sibilant' sounds.

If I get time later today I will try the LT sim using PWM signals rather than a swept square wave.

Have you rechecked all you connections on the filter.?