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PPA v2 construction discussion - Page 70

post #1036 of 1062

Looks like a 2-layer board?

 

-chris

 

Quote:
Originally Posted by tangent View Post


I have no plans to remove those pages from my site.

 

Since discontinuing it, I've put up the ExpressPCB design files, too, as was also always the plan. If one were interested in trying to revive the board availability, one would be welcome to as far as I'm concerned. One attempting that should ask ppl and Morsel, too, however.



 

post #1037 of 1062

Yes.

 

Definitely no good for hand-etching in its current state. We knew we were going to make professionally-etched PCBs from the start, so we took full advantage of the tighter margins that affords.

post #1038 of 1062

yeah, looked a bit tight for that.  The expressPCB 'standard' process isn't completely cost prohibitive but I'd loose the silkscreen and the solder mask... if I did it I'll let you know so I can send something  your way.

 

-chris 

post #1039 of 1062

The first several rounds of PPA prototype PCBs were made on that process. Of the three of us involved in the PPA development, I'm pretty sure I assembled and tested more prototype boards than the others, so I feel comfortable assuring you that that silk and mask are very much optional with the PPA. Nice, but not actually necessary for a design with this component density.

post #1040 of 1062

Way back when I built my PPAv2, I had bought 8x 220uF 35V Panasonic FM caps for C1, figuring that more smaller caps was better than a few bigger caps.

 

I've purchased some 480uF 50V and 680uF 35V FM caps, for some other purpose, and have enough extra that I can replace the old caps too.  I was wondering if it worth the effort to swap out the old C1 caps for a set of the new ones.

 

With the 8x 220uF caps, the total capacitance is about 1760uF.  With 3x 680uF, that would be 2040uF; with the 4x 480uF caps, about 1920uF.  I was thinking of using the 4x 480uF caps, but their rating is 50V vs 35V of the others.  Does the voltage rating affect things much besides being above my 24V supply?

 

As a secondary benefit, the old caps are 8mm in diameter versus 12.5mm of the newer ones (a mistake I made).  I'm not really that concerned about the diameter though, as the amp is essentially immobile its whole existence.

 

Thanks!

post #1041 of 1062

I think you should leave it alone.

post #1042 of 1062


Originally Posted by tangent View Post
 
Actually, it looks like an RC charge curve to me, but the C1 bank will do that, and it could be masking the test results. Ideally you'd power up the amp and ground channel separately.

You could do this while you play with Q3G: desolder them, jumper Q3G- (big loop so it's easy to remove), and put a switch across Q3G+. Then you'll get one of two results when you power up the ground channel: either the amp will function again, or you will get the same result, which rules out the Q3Gs being a problem.

 

Just wanted to let you know it turned out to be Q3G-. I shorted across the pins with a screwdriver and amp came back to life. I've since replaced the part.

 

Unfortunately I have a new problem, the computer at my new job is causing an issue with I plug my usb-powered AlienDAC into the PPAv2. :(

I run the PPAV2 off a STEPS that i'm pretty sure is constructed properly. Everything works fine if I plug a diffrent source (like a cellphone) into the PPAV2 (but goes to hell again if I put the cell phone on a usb charger hooked to same PC)

 

Seems like a ground problem, But i'm at a loss how to solve it. Anyone have any tips?

 

p.s. anyone have a link to a tutorial/video on how to trace a signal though a ground amplifier like the PPAv2 uses? I've been able to use the math function on my scope to do L/R without differential probes, but no luck on Ground :(

post #1043 of 1062

Radio Shack sells an audio ground loop isolator. If it fixes the problem, then you know you do have a ground loop. It effectively turns the PPA into a transformer-coupled amplifier.

post #1044 of 1062

Hi everyone,

 

i finally have my PPAv2 running, after sitting in the basement for almost 2 years waiting for its enclosure :(

 

So far so good, everything is fine, the sound is awesome! I'm using OPA627s and running 30mA buffer bias and 2mA op-amp bias (2V drop).

 

The only "problem" i have is that if i turn up the volume from nothing, the right channel gets louder first... if i have the volume set to my listening volume, i am not a 100% sure, but it seems to be still out of balance, meaning that the right channel seams to be louder.

 

All biases are exactly the same and i don't know what could cause that problem...

 

Anyone knows whats going on?

post #1045 of 1062

It sounds like the volume pot is dirty

Try rotating it back and forth many times

and see it the problem goes away.

post #1046 of 1062
Quote:
Originally Posted by Avro_Arrow View Post
 

It sounds like the volume pot is dirty

Try rotating it back and forth many times

and see it the problem goes away.

Thanks for the fast anweser, i already did that trick a couple of hundred times with no difference.

 

EDIT

I just switched my input channels to cancel out the source when i noticed, that if i only have one channel input attached, i still hear it on both channels, but only quiter on the missing channel. is that normal?!?!

 

EDIT2

I just tested it with test sound files, even if only the right channel is playing, i hear it on the left channel too, but much much quieter... i do not have a crossfeed or anything installed. i play back the files using foobar+asio and my asus xonar st...


Edited by nini_knoxville - 10/31/13 at 1:53pm
post #1047 of 1062

Do you have a fairly sensitive meter? If so, give the AC voltage measurements of the input signal -- both before and after the pot -- and both output signals. Also give the R3 and R4 values. Do this with a simple sine wave.

 

High crosstalk is usually a grounding problem, which often comes down to the power supply. What power supply are you using?

post #1048 of 1062
Quote:
Originally Posted by tangent View Post
 

Do you have a fairly sensitive meter? If so, give the AC voltage measurements of the input signal -- both before and after the pot -- and both output signals. Also give the R3 and R4 values. Do this with a simple sine wave.

 

High crosstalk is usually a grounding problem, which often comes down to the power supply. What power supply are you using?

R3: 1k

R4: 10k

 

How should i measure this? If i measure from ground to either input r/l before the pot, i get exactly 0.7V AC on both channels. After the pot, from ground to R1, i get nothing realy? Just tipping it with one lead, i get 1.49 on left and 1.44 on right. i don't know if this is a valid measurment. same if for the output.. from ground to l/r nothing, just tapping r or l 1.39 V AC. But that cannot be a correct measurment. All done with a 400hz sine wave.

 

for the psu i use your yjps at 16.7V output. i could not find a problem within the psu, but U1 gets pretty damn hot.

 

EDIT

I just checked some of the voltages.. from ground to the op amps v+/v-, each rised by 0.1v per chip. left: 6.11V each, ground: 6.21V each, right: 6.31V each. Also.. from ground to the left output i get 1.7mV, right: 1.6mV and ground... 180mV.. something is wrong with that but what? I also swapped the op-amps... still the same issue.

 

 

EDIT

I found an error! I'm feeling kinda stupid now, but i forgot to add C6. I don't know why, populated the board 2-3 years ago. Now each channel is about 1.6mV. The crosstalk is gone and so is a grounding problem (touching the headphone metal 3.5 to 6.3mm adapter). Whats left is that left/right problem. I mean that the right channel is "louder"/starts earlier.

 

Some pictures of my build:

 

 

 

 


Edited by nini_knoxville - 10/31/13 at 3:29pm
post #1049 of 1062

Pots are imperfect, increasingly so at low settings. If you decrease the gain to something more appropriate, you will get past the bulk of these imperfections.

 

All I can see in your pics is that you're using some BB chip, but not which.  If it's the OPA637 or OPA228, you can't go below g=5. Most other BB op-amps popular for audio will let you go down further. A lot of people use g=2 happily.

post #1050 of 1062
Quote:
Originally Posted by tangent View Post
 

Pots are imperfect, increasingly so at low settings. If you decrease the gain to something more appropriate, you will get past the bulk of these imperfections.

 

All I can see in your pics is that you're using some BB chip, but not which.  If it's the OPA637 or OPA228, you can't go below g=5. Most other BB op-amps popular for audio will let you go down further. A lot of people use g=2 happily.

I'm using the OPA627. I guess with g you mean the op-amp biasing voltage?

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