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Old 25th August 2019, 06:28 PM   #1931
zman01 is offline zman01  Bangladesh
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xrk971 Pocket Class A Headamp GB
Cute looking blue boards X.
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Old 25th August 2019, 06:45 PM   #1932
ItsAllInMyHead is offline ItsAllInMyHead  United States
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X - I also realized that I may not understand the layout of the board as well as I should. Just so I'm not running you around in circles and wasting your time...

The points in RED are where I was measuring the voltage between pin 2 and ground. When I look at the board itself vs. the trace diagram, Pin 2, Pin 4 (the large back pin) and the vias all seem to share the same copper pad.

Have I measured pin 2 completely incorrectly? I can't understand how I'd get a voltage the way I measured it if my observation re: the board construction is accurate.

I measured pins 1 and 3 similarly, with one probe on the "leg" and one in the via.

I'd hate for you to waste effort if my measurements were not taken properly.

As always, with thanks!

Jumper.png

Below is from post #665
"On MOSFET just these 3 measures (relative to GND) will tell you if amp is working:

1. D (pin 2 or tab) says you have the main power applied to the amp (15 to 18v)
2. G (pin 1) says the gate voltage set by JFET is working (if off then check volts across R3) (7v to 9v)
3. S (pin 3) says bias current is flowing through resistor array and you have Class A action (6v to 8v)"
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Old 25th August 2019, 08:02 PM   #1933
xrk971 is offline xrk971  United States
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Voltage should always be measured relative to ground. Ground is the pin on battery connector on opposite end of positive or the pin closest to edge on LED.
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Last edited by xrk971; 25th August 2019 at 08:05 PM.
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Old 25th August 2019, 08:46 PM   #1934
ItsAllInMyHead is offline ItsAllInMyHead  United States
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Quote:
Originally Posted by xrk971 View Post
Voltage should always be measured relative to ground. Ground is the pin on battery connector on opposite end of positive or the pin closest to edge on LED.
Well, don't I feel silly. I won't make that error again. My apologies. Once again, thank you for your patience.

Here are the correct measurements.

Alpha - Right channel working
Legs to Ground
1A Gate - 8.15V - OK
2A Drain - 15.80V - OK
3A Source - 5.08V - Below goal of 6-8V

1B Gate - 8.60V - OK
2B - Drain - 15.80V - OK
3B - Source - 5.61V - Below goal of 6-8V

Across R3
A - 7.69V & 8.49mA
B - 7.25V & 7.99mA



Beta - Neither channel working
Legs to Ground
1A Gate - 9.73V - Higher than goal of 7-9V
2A Drain - 15.80V - OK
3A Source - 6.62V - OK

1B Gate - 8.78V - OK
2B - Drain - 15.80V - OK
3B - Source -5.65V - Below goal of 6-8V

Across R3
A - 6.1V & 6.83mA
B - 7.00V & 7.75mA
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Old 25th August 2019, 10:48 PM   #1935
xrk971 is offline xrk971  United States
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Your measurements show that the amp is working fine. Compare the voltages to the simulation at the various points. They are not far off - it should play music. Do you have the Input jack and output jacks reversed? Input is on left and output is on right side when SMT side is up. I have made that mistake. No sound.

Click the image to open in full size.
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Old 26th August 2019, 12:31 AM   #1936
ItsAllInMyHead is offline ItsAllInMyHead  United States
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Quote:
Originally Posted by xrk971 View Post
Your measurements show that the amp is working fine. Compare the voltages to the simulation at the various points. They are not far off - it should play music. Do you have the Input jack and output jacks reversed? Input is on left and output is on right side when SMT side is up. I have made that mistake. No sound.
Hi X - You have incredible patience and a skill for teaching. I can't thank you enough. I had that printed out. At least I'm on the right path. I've measured all points on one of the amps (the Beta amp that had one channel working). As I was fiddling with the amp, the channel came in and out. I am going to re-check all joints, physical legs of caps etc. before troubling you further and taking more of your time. As a reference though, here are the measurements.

For the next batch of "amps with friends", I clearly need to try and learn your sorting and biasing process. The two channels are not electrically matched to your expectation. My goal was music - then tweak and learn further.

Pausing for now, and will report back tomorrow when I'm less on edge and have a bit more time. Such a silly mistake to goof up two functioning amps by hurrying.

If I make it to see family in the DC area, and you're up for it - at the very least a nice meal is on me.

With thanks!

Pocket-Class-A-Schematic-v8-voltages.png

Edited to Add - I forgot to add the point between R4 and R5 to the picture. A = 2.08V B=1.95V

Last edited by ItsAllInMyHead; 26th August 2019 at 12:34 AM.
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Old 26th August 2019, 12:48 AM   #1937
xrk971 is offline xrk971  United States
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You are welcome to stop by lab for a visit if in DC area. If you want to match FETs - use one of your boards all populated with resistors and caps but leave the FETs unpopulated. Connect amp to power - preferably Alkaline batteries so they don’t shut down. Solder some small wires across R3 and connect DVM. Take a JFET and place on pads and use a pencil with eraser to apply pressure to JFET and measure the voltage across R3. Record that with JFET. Do this for maybe 12 to 20 and you should have a pair that matches within 2%. Use those. Now install those matched JFETs and solder some small test leads across the the 4x 470R source resistors and connect DVM. Do same thing with MOSFETs. Record voltages for about a dozen and pick two that are within 5% or better. If you have lots of them aim for 2% or 1%. When you buy Uber hand matched JFETs from me they are matched to 1% or better. Takes a lot of stock to do that though. Now when you build them up, both channels will have almost the same bias current and same sensitivity.
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Old 26th August 2019, 12:41 PM   #1938
ItsAllInMyHead is offline ItsAllInMyHead  United States
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X - Thank you for the kind offer. I hope to take you up on it.

Noted re: the process. I have it in my build notes. When I get the next set of boards, I'll dedicate one for matching and then use it for a build after all the parts have been measured. Related to that, is it okay to simply swap R4 for unmatched MOSFETs? i.e. have one value for A and another for B?

On to the good news. I now have two perfectly functioning amps. Two things (at least I think) contributed to the situation.

1) I don't know if it's related to the situation or not, but one channel on my test earbuds was out. Perhaps it got smoked during the light show. They were disposable buds you get on an airplane. If it's not related, then it's an odd coincidence. After the re-flow and visual check on Beta, it still had a channel out. On a fluke, I swapped to another set of buds - MUSIC!. Done.

2) On Alpha - I tried the new buds and got one channel. Progress. I did the same reflow, cleanup and visual check. There is a solder bridge between C4 and C5. On the side of the pads it was on, it should not affect the circuit. However, after the reflows and checks, and rechecks, it now functions perfectly again. I truly have no idea what caused the channel to go out. For anyone's interest - the measurements are attached below. Note - batteries were charged over night.

Again, I can't thank you enough.



The twins. I think it's time to do some other tins.

IMG_1431.jpg
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Old 26th August 2019, 01:04 PM   #1939
xrk971 is offline xrk971  United States
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IAIMH:
Congratulations! Based on your measurements, I knew that the amp was still good and should have made music. You may have had a solder bridge form or a cold solder joint break when the arcing happened. You can adjust the R4's individually - make them higher value to increase the bias current across the MOSFET. Change them in small amounts. Note that if they are too far apart, their harmonic profile balance between H2 and H3 will not be the same, but you may or may not hear that. You don't want to go over 57ohm. You can use a 57ohm and parallel (on top) another resisitor that is large like 470ohms etc to slowly bring the parallel resistance down to match,

Enjoy your amps!
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Old 26th August 2019, 01:41 PM   #1940
zman01 is offline zman01  Bangladesh
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Nice pair of headphone amps IAIMH!
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