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FH9HVX - Budget Conscious 100w Class AB for Lean Times

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#742
Thanks guys,

Good idea to move SSR to the rear panel. I need a bit of room down the center to run the power wires from the IEC module to the transformers (is this a good idea or is it better to route them across and down one side ? ).

Almost have the amp boards for the Xmas amp built. Waiting on a few parts. Lots of fun!

MM
 
#743
Looks like gathering parts is becoming increasingly difficult with various shortages.

The Fqa36 mosfets are out of stock on mouser. I think I can get them on Avnet but have never bought from this company before. Does anyone have another source or substitution? I have a lot Fqa9n25’s on hand but their parameters don’t look close enough to the 36’s.

Also, the Koa Speer 5W resistors are out of stock. Is there a substitute for these? Maybe a 5W wirewound like the Vishal/Draloric?
 
#745
You can use the the common IRFP240/9240 just fine. They may have a shorter lifespan than the Fairchilds which are rated for higher dissipation.

The KOA BPR are bulk metal non inductive resistors. They will have less distortion than wire wound resistors. Feel free to use Panasonic ERX 3W as a substitute. Those are thin film vs wire wound which is inductive.
 
#746
Thanks X. I should have remembered the irfp’s shown in the first schematic. It really pays to re-read post #1. Thank you for always summing up everything so well in those posts, X.

Thanks Zman. I ordered Some of each from Newark. Hopefully they get here soon.
 
#750
Thanks X, I thought so. Are the Gen1 RTR SSR boards electrically isolated or are they the same as Gen2 and need the washers. I assume a nylon washer between the pcb and the bolt and no other nylon washers needed. Correct?

MM
 
#752
Progress pics

While waiting for some parts, here are some progress pics. Please comment if you see something weird. Full view, right and left channels. Sorry about my poor lighting.

X, I noticed that your PE connection on the amp board is doubled wired. Is one of the wires from some other component and you are joining there, before continuing to your ground block.

FH9Full.JPG

FH9Right.JPG

FH9Left.JPG

Regards,

MM
 
#755
In case there is a fault in the amp, you want as free of path back to mains earth ground to reduce electrical shock, fire, etc. Probably a single 16ga line is fine. I am just being extra careful. It also provides a lower impedance path for the RF filtering on the board to pass to earth ground (the 22nF cap between analog zero and PE).
 
#757
Got a couple of hours to work on this again after a few weeks, got it hooked up and playing some initial music, sounds great and really full bodied. Appears that it struggles with really low volumes, as it'll clip the audio a lot until you give it a bit more amplitude. Any ideas as to what could be causing this? Possibly just the preamp?
 
#758
Hi As8912,
Something is not right, it should not clip at low volumes. You might have a damaged active somewhere is my guess. You had smoke earlier? What caused that?

Awesome, thanks for the guidance everyone! The LED was the root of it, I swapped it in for a generic 3mm red LED with the correct polarity and it powered. Cautious of the bias voltage and runaway thermals, I only powered the amp on for 0.5 seconds at a time, initial bias voltages were ~180mV instantly, dialed it back a few times but all 0.22ohm resistors were fried (both on the amp, and both on the All Cees board), will order more resistors and mosfets. Anything else I should be aware of/get spares of before I cause real damage? (blown capacitor, etc)
Click to expand...

Photos and measurements of key voltages at all pins of all actives will let us help debug this.
 
#759
The issue then was the bias voltage was massively off, lead to thermal runaway of the mosfets and shorted the mosfet leads. I've replaced the 0.22ohm resistors on the FH9 and All Cees (they were all cracked), and both power mosfets on the FH9.

Datapoints:
Bias+ PD:
0mV start
peak 52mV 14 seconds
settles 24.8mV after ~90 seconds
settles 20.5mV after 20 minutes

Negative rail: 49.945V
Positive rail: 49.960V

V129
Emitter: 49.000V
Collector: -2.990V
Base: -38.340V

V124
Emitter: 48.920V
Collector: 4.74V
Base: 48.33V

Those were the two most accessible and looked like likely candidates from the schematic, is it worth probing all of the following; V121, V101, V103, V113?
 

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