[EdibleStereos]

Anybody know why the buffer bias reading would drop across one channel when hooked up to headphones without sound. The right and ground channel only vary from no load readings by 3mv. while the left channel drops about 20mv.

With another set of headphones it is the opposite. Left and ground stay stay around 63mv, while the left channel drops about 40mv to 24mv. Also, DC offset shoot up on the Right channel, to 2.2mv. While the Left channel stays at under 1mv.

 

[fc911c]

Hi all

I just built a PPA v2 that I had for years with my spares. I cleaned the board off it had some oxidation. I am having a problem with the ground channel no voltage when trying to adjust buffer bias. Both the left and right channels are fine and voltages to the ground op amp are correct. I checked all the obvious things bad joints re flowed the ground circuit, wrong parts, all are correct, nothing gets hot either. The only thing I did wrong was to have a 10K resistor in (R4G gain ) in stead of a 3.32K. Any ideas what could be wrong could you give me some test points to try to track down the problem. I tried changing a few of the transistors still no voltage.

I have a few Pimeta that I love that both sound great.

Thanks for the help

 

[tangent]

There are a whole bunch of tests given in the docs, here. What are the test results?
 
What do you mean by "no voltage"?  No voltage across what, and at what setting on the meter? I can give a wild guess that you mean DC voltage across R10G, but you aren't actually saying that.

 

[fc911c]

Hi sorry for being so vaig . I was measuring dc volts at R24. Anyway it turns out I had a 1M resistor in R13 instead of 100R not sure how that happened. I changed all the transistors just to be sure and all is working fine.

I noticed that there is an additional .01uf cap on c2 why was this added?

Thanks for the quick reply

 

[MisterX]

The C2 positions are bypass caps for the output stage.
IIRC there is detailed explanation in the parts guide page.

 

[fc911c]

Thanks I missed that. I am waiting for the 50k pot to come I have a 100k pot on hand is there any harm in using it till the right one comes?

 

[tangent]

If you're going to use the 50k anyway, and your 100k pot sits down on the board's existing pads, don't bother installing the 100k; wait for the 50k to arrive. Swapipng a 6-pin DIL board-mount device without exposed pins is a royal pain. You stand a good chance of destroying your board.
 
If the 100k pot would be connected via wires, that changes everything. Now your replacement problem is six independent wires, which is a much simpler task.

 

[fc911c]

Yes I am hooking it up with wires it's not a board mount. I agree it wouldn't be wise to install it on the board only to remove it in a few days. So my question remains is it ok?

 

[tangent]

It's already explained in the docs.

 

[MisterX]

And in the section about R2.....

 

[fc911c]

Hi Back again not having much luck with this build. The amp stopped working I am getting a loud hum. I think the problem is in the ground channel my supply voltage to opamps is 13.6v which checks out on pins 2,4 and 7 except for pin 2 on ground which is high at around 19v, pin 3 and 7 is 13.6 on ground. Pin 2 is 13.6.
I changed the tele on the ground channel no change. Voltage across R 24 is 46mv on all three Chanel's. Any ideas?

Thanks

 

[fc911c]

Never mind sorted it out bad power supply ground.

 

[fc911c]

Hi I just completed basing to class A. I am having a few issues one is not being able to get down to 1 mv across R10, the lowest is around 2mv with 627/637 and around 4 mv with ad 8610. Amp does not sound right. Here are some measurements and parts used. I am also using bass boost alps 50k pot. The heat sink on the steps supply is hot.

637 l&R 627 gnd I also tried Ad8610
Q1 2N5484 Q2 2N5486
R9 2K pot
R24/34 2.2K buffer bias 46mv

Power supply voltage 29v
Left opamp pin 4 to pin 7= 23v
Pin 4 to in gnd 11.6
Pin 7 to in gnd 11.7

Right
Pin 4 to 7 23.5v
Pin 4 to gnd 11.6v
Pin 7 to gnd 11.87v

GND
Pin 4 to 7 24.2v
Pin 4 to gnd 12v
Pin 7 to gnd 12.1v

S1 to v+ L&R 26v gnd 26.5v
S1- to V- L&R 26v gnd 26.5v



Thanks

 

[tangent]

not being able to get down to 1 mv across R10

 
The documentation doesn't tell you to expect that you can do that. It says, "If R10 is 1 kΩ, then each volt of drop across R10 equals one milliamp through the cascode." 1 mV across a 1 kΩ R10 would mean one microamp of current through R10.
 
This is a simple application of Ohm's Law: I=V/R, in this case.
 

Amp does not sound right. 

 
So go through the troubleshooting guide, as directed by the docs.
 

The heat sink on the steps supply is hot. 

 
What are the measurements on its test points? Specifically:
 

1. AC voltage between TP1 and TP2
2. DC and AC between TP3 and TP5
3. DC between TP4 and TP5

 

S1 to v+ L&R 26v gnd 26.5v
S1- to V- L&R 26v gnd 26.5v 

 
That makes no sense on its face, and is inconsistent with your earlier test results. The DC voltage drop between S1+ and V+ should only be one diode drop, and there should be nearly zero drop between S1- and V-.
 
More useful is DC voltage between either V+ and V- or S1+ and S1-. The two pairs should differ, again, by only one diode drop.
 
The drop between VH+ and VL- and between VH- and VL- is expected, and is why you are getting 26-ish volts at VH+/- and 24-ish volts down at the op-amp power supply pins. The minor difference in supply voltages to each op-amp is also expected, owing to process variations in the JFETs.

 

[fc911c]

Hi thanks for the reply.

I should have some time this weekend to go through trouble shooting guide. I will report back with my findings.

Thanks