[vgjako]

Hi again,
 
I once again have a question, the standard configuration of C5 on the PPA V2 uses a 1 uF capacitor, this gives a corner frequency of 16 kHz. I've would guess that a lower corner frequency is better, say I use a 2,2 uF capacitor instead of 1 uF, this will give me a corner frequency lower than 16 kHz.
 
I know that I could instead raise the value of R8, but as tangents guide says: "Theoretically you could instead raise the value of R8, but as its value rises, so also does the current-modulated rail ripple due to the op-amp’s varying current draw. We decided during development that 10 Ω was the best trade-off here."
 
Is there a formula for calculating the corner frequency?
 
Regards
vgjako

 

[vgjako]

bump

 

[tcpoint]

 

[FallenAngel]

I don't get it, C5 is a filtering cap for the opamp, there is no "corner frequency" in relation to sound as you're dealing with the power supply, not audio input.

 

[mugdecoffee]

While R8 and C5 do form a low pass filter, the idea of corner frequency and that formula don't really apply since there's current being drawn out of the resistor in effect making the resistor and capacitor not a simple RC filter.  However, the basic trend still applies in that the larger the cap, the less higher frequencies (which are in this case noise) are passed.

 

[vgjako]

Quote:

While R8 and C5 do form a low pass filter, the idea of corner frequency and that formula don't really apply since there's current being drawn out of the resistor in effect making the resistor and capacitor not a simple RC filter.  However, the basic trend still applies in that the larger the cap, the less higher frequencies (which are in this case noise) are passed.

 
 
Thanks mudgecoffe, that really cleared a lot of things up for me.
So the higher the cap value, the less noise you get.
 
 
What gain do you think I should be running on, 3, 4, 5...? I will use the Burson Audio HD Op-amp (single) which is unity gain stable. I would guess that a gain between 4 and 6 would be best, what do you think
 

 

[whitelabrat]

So I've had some issues with one of my channels with an OPA637 getting cranky. If you look at post #748 you'll see that I used cheap electrolytics and ceramic caps for power smoothing near the opamps.  So I reclaimed four Elna Certifines and some 4uf film capacitors and replaced the cheap stuff for the left and right channels hoping that would get the oscillation under control.  In the process I appear to have hosed up something.  I replaced the stepped attenuator with a ALPS blue velvet pot and that made no difference.  I pulled out the 637's and replaced them with a pair of OPA37's for testing.  No luck.
 
So things sound OK, until I turn up the volume.  Then the right channel gets very distorted and ugly.  I also measured about 10ma of dc offset on the right channel.  Something must be blown.  Any guess where i should start pulling parts?
 
I should also mention that I'm using a TREAD power supply and a doorbell ringer transformer that I got for $3.  Gives me a solid 24v.

 

[ShinyFalcon]

What gain is your PPA set to? OPA637 are not stable at gains of 5x and below. Same with the OPA37.

 

[FallenAngel]

That is one funky looking amp.  Put a finger on all the transistors and see if anything gets hot.  Check output stage current to make sure it's reasonable.

 

[whitelabrat]

I have a 10k resistor for R4.  My guess is gain would be 10k/1k = 10?  Or is my math wrong?  Left channel is a happy camper.  I could drop in OPA27's for testing.
 
A few parts are barely warm.  Nothing hot.  The capacitors all check out so nothing weird there.  The silicon parts are suspect.  I'll need to probe for shorts.
 
Yes.  It's even funkier looking now.  I tried to use only parts that I had already.  I think the build of this only cost me $50 including the PCB.

 

[tangent]

Did you jumper S2?

 

[vgjako]

Is there a formula for calculating the buffer bias? I am going to use a 3,3 ohm resistor for R24 and R34, I want 30 mA of buffering. What will R12 be?
 
Thanks

 

[whitelabrat]

Quote:

Did you jumper S2?

Absolutely.  I don't care for bass boost.

 

[tangent]

Then the only things I can suggest are the standard ones: post larger, clearer pics (both sides!), and the results from going through the troubleshooting guide.

 

[whitelabrat]

I didn't find any shorts, so I figure I'll put a 1k sine in and test the signal path with an oscilloscope.