diyAB Amp The "Honey Badger" build thread

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Hello Forum Members,
Looking for any feedback that members may feel inclined to offer.
Prior to this test I was able to get DC offset down to 4mV. Powering up the amp this time the CCS adjust settled in nicely at 8.25 VDC accross R14. Bias came down to around 44mV DC as things warmed up.
However, DC offset reads 14 volts DC and R17 seems to have no effect. What the heck?
Nothing smells hot, etc. I'm hoping I'm making a boneheaded mistake somehow with my leads or something. Anyone have any suggestions? I'll include a pic. Thanks.
 

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New board.

First test.

R36 out.

5W amp setup

10R in place

Wiring: amp to DBT to Variac.

I ran the Variac up to 60V AC

LED lights on

These are the readings:

10R: -0.484 DCV
R14: 07.19 DCV
Q 14 & Q15 Loop: 0.794 DCV
Offset: OL

Am I on the right track?

Thanks,
Upon further reflection, is the 10R reading of -0.484DCV too high given that I was at 60 ACV on the Variac and using the DBT given the desired reading is DCV of close to 0? Still trying to avoid toasting a part.

Or can I push on to 120ACV?

Thanks
 
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Hello Forum members,
My conundrum continues. I powered up my HB board this morning hoping to solve the mysyery of excess DC offset at the output. No luck. Again both the voltage accross R14 and the bias voltage settled nicely as the board warmed up. I set my DMM to millivolts to read DC offset but got no reading as the source was out of range. Changing the scale, I again got readings in the 14+ volts DC range.

The only component I am aware of that doesn't adhere to the 2.5.4 schematic is R27. I installed a 51 ohm resistor before becoming aware of the latest scematic. Because there appeared to be some support in the thread for a lower value at this position, I left it in. Could this result in my excess DC output? I do have the recommended 82 ohm components on-hand and will probably install them to see if this helps.

Please note that due to the debate over the proper zener diodes to use for compensation, I chose to go the luxman route. Not sure if this could be a compounding factor.

I have included a pic of my board in case anyone has time to take a look at it for problems. I will continue to study the thread for clues.
So far, no sign of excess heat anywhere. Thank you. Dave M.
 

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Hi Chip,
Yes, at this point I have fuses installed and am off the DBT. I checked the schematic and you are correct. R53 & R54 need to come out. Thank you for catching that. My initial tests were good but 14+ volts of offset seems wildly out of spec. Let me remove those two resistors and see what that gets me.
 
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Hello Forum Members,
First of all a big Thank You to Chip for catching my mistake. I had not removed R53 & R54 prior to my attempt at final adjustments on channel B. I hoped this would solve the problem of excess DC voltage on my output, but the problem actually got slightly worse.

In an attempt to knock off the low hanging fruit (is there such a thing in this hobby) I moved on to the other board. I removed R53 & R54. Changed R27 to the recommended value and installed the output transistors.

I adjusted voltage accross R14 to the recommended value. After several minutes I dialed the bias at TP1 and TP2 up to 44mV and checked my DC output. Much to my delight, I received readings well under the 1-2 mV specified. Although the readings wandered, they did not wander "out of range." I think I have one fully operational board now.

I plan to pull the other board and check for solder bridges or damage, especially in the are of my output circuitry. I also want to devote some time to pondering Chip's dilemma. There must be something that we can help him uncover that will help.
Thanks everyone. Dave M.
 

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UM,

I would first make sure that your input differential pair of transistors (Q1,Q2) are fairly well matched (as the guide suggests), at least for Hfe and preferably for Vbe as well. Also make sure that those transistors are operational.

It’s interesting because the board you are having trouble with was performing just fine on post 5314 for your initial bias measurements.

1709731477317.png


Do you have an inexpensive transistor tester? Such as the Mega328 variety discussed on other threads. Easy to find on Amazon.

Best,
Anand.
 
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Hi Anand,
Thank you for your response. I do indeed have a transister tester. I actually recorded the values I measured on Q1 & Q2 and will dig for them. I can certainly pull them to retest and replace when I get that far. Probably not 'til tomorrow though.
Respectfully, Dave M.
 
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Hello Forum Members,
Update- I looked up the values for the Q1 & Q2 pair I installed on my problematic board.
Q1 ZTX1051a Hfe 408 Vce 571
Q2 ZTX1051a Hfe 409 Vce 568

I would think these would be close enough in value but I will replace them anyway. Unfortunately, I need to order more so I will be at a standstill for a few days.

I plan to study the schematic and the build guide. I want to improve my understanding of the amplifier stages and their proper names. I struggle with that.
 
That’s plenty close. So unless there is failure of one or both of those transistors (unlikely), I don’t think your high dc offset is stemming from Q1/Q2.

You’ll need to slowly start debugging. While you are at it, make sure all the pins of your output transistors are isolated from the heatsink/ground. It’s also unlikely to be the issue since your DC offset would be close to your V+/V- rail voltage if that were the case.

Next, start looking at your VaS stage and since you have a working board, start making comparisons of passive parts. Something might be not installed right or a cold solder joint somewhere. Use a loupe or Yoctosun and look very carefully.

Best,
Anand.
 
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Had a chance to get back to starting up my HB.

Using a DBT and Variac I slowly increased voltage to 120 ACV

These are the readings:

10R: -0.465
R8 7.30DCV
5W loop: 0.797DCV

LEDs on.

No smoke.

I think the coast is clear to adjust bias to:

R7 8.25V
R17 0V
R30 15-20mV

Am I missing anything?
 

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Hello forum members,
Mystery solved! Thank you Anand and jwilhelm for pointing me in the proper direction.
It seems that in my early morning fog, after placing Q1 & Q2 facing each other; I did the same with Q3 & Q4. The result was Q4 installed backwards. I installed my last KSC1845 the proper direction, reattached the board to the heatsink and voilla! Offset within specs. Offset wandered into negative numbers but it is my understanding this is OK as long as the readings stay within the 1-2 mV "window."
Much to my frustration, I also discovered that I installed the wrong value resistor at R19. Because I chose the luxman option my resistor value should have been 68k. I need to change those out. Slow and steady wins the race!
Thank you everyone for your tips and suggestions. Dave M.
 

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