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FH9HVX - Budget Conscious 100w Class AB for Lean Times
FH9HVX - Budget Conscious 100w Class AB for Lean Times
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Old 26th April 2020, 11:51 PM   #1
xrk971 is online now xrk971  United States
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FH9HVX - Budget Conscious 100w Class AB for Lean Times
Default FH9HVX - Budget Conscious 100w Class AB for Lean Times

There was a very nice little Class AB amplifier I worked on years ago based off of Apex Audio's wonderful little FX-8 lateral FET amp. Back then I did not want to use the lateral FETs because they were pricey. So I set off to make a hexFET based version. It was called the FH9, and it sounded very good. If one used generic resistors and caps, you could build the whole amp for under $10. The layout was done by several people, Sonal K and Prasi, but the one that caught on was the Sonal K layout.
Here was the Sonal K layout:
Click the image to open in full size.

Many people have built this amp since then and it has been a favorite. At one point, I swapped out the transistors for some high voltage ones and hooked it up to a $20 Abletec +/52v SMPS. Running a pair of IRFP240/9240's I was able to eek 150w into 8ohms with it.
Click the image to open in full size.

That's 34.5Vrms into fan cooled 8ohm load:
Click the image to open in full size.

Over the years, I continually get requests for help with this amp or people asking for a PCB etc. I have noticed that lately with the Covid19 quarantine going on, many people are not having as much cash on hand for premium parts. So I thought it might be neat to revisit this amp and see if we can make a nice performing budget 100w amplifier. Naturally, I have enlisted the help of JPS64 to do the layout, and Vunce to help with the beta testing. Vunce has since gotten the original FH9 to work and is about to get stereo sound any minute now. In the meantime, I have put the circuit back on LTspice (it was on Tina in the early days). And I have tweaked it for 52v rails and nominal output up to 100w into 8ohms. No fancy parts and not even four MOSFETs. Just 1 pair per amp if you use Fairchild with a higher power rating, should be fine. No matching to do. The amp is super simple and very pleasant to build in one evening. If you happen to have an old Abletec 225w SMPS, dust it off and you can have one of these amps playing. It has a very powerful bass and nice overall presentation. One of my favorite simple amps to make and listen to. Many people can vouch for its sound if you check on the Apex thread.

On this amp, one can use nice Dale resistors, boutique input coupling caps, or you can use no name Aliexpress resistors - it won't matter. I just ask that you get either genuine Vishay IRFP240/9240 or genuine Fairchild FQA40N25/FQA36P15's (about $7/pair). You can run it at +/-35v of course as a 50w amp, but I suggest going for the +/-52v version and getting it to crank to 100w without a sweat. THD predicted at 100w is only 0.005% and at 25w is less than 0.003%. Dominant second/third order harmonic distortion, and a semi-monotonic descending higher orders (ok, so it's no SE Class A amp in profile...) But what it has is some great bass dynamics, and killer foot tapping immediacy. A very fun amp to listen to and play with. Cheap fun.

If you don't want to go with a cheap +/-52v SMPS, a single 300VA 38v trafo will work nicely or dual 200VA 38v trafos and a simple CRC PSU with four 10,000uF caps each will work fine.

Here is the LTspice schematic (in JPS64 part notation):
Click the image to open in full size.

Here is the LTspice file for the simulation.

Here is the predicted distortion at 25w into 8ohms of 0.0027%:
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 1.984e+01 1.000e+00 -0.72 0.00
2 2.000e+03 3.797e-04 1.913e-05 134.57 135.29
3 3.000e+03 2.438e-04 1.228e-05 -115.92 -115.21
4 4.000e+03 1.048e-04 5.279e-06 173.49 174.21
5 5.000e+03 1.811e-04 9.124e-06 -110.61 -109.89
6 6.000e+03 8.789e-05 4.429e-06 173.30 174.02
7 7.000e+03 9.443e-05 4.758e-06 -114.64 -113.92
8 8.000e+03 7.359e-05 3.708e-06 172.29 173.00
9 9.000e+03 4.741e-05 2.389e-06 -123.30 -122.58
10 1.000e+04 6.369e-05 3.209e-06 170.96 171.68
11 1.100e+04 1.842e-05 9.282e-07 -149.71 -148.99
12 1.200e+04 5.589e-05 2.816e-06 169.47 170.19
13 1.300e+04 1.325e-05 6.676e-07 127.79 128.50
14 1.400e+04 4.934e-05 2.486e-06 167.92 168.64
15 1.500e+04 2.346e-05 1.182e-06 94.55 95.26
Total Harmonic Distortion: 0.002677%(0.008486%)
FFT for 25w into 8ohms:
Click the image to open in full size.

Here is a sneak preview at the 100mm x 100mm PCB that JPS64 is working on. We will have optional flying leads with quick connects for the MOSFETs.
Click the image to open in full size.

The pre-proto FH9HVX built by Vunce is working very nicely. Here it is in an old donated chassis and powered by the Abletec SMPS. There is not much to this amp. But it packs a punch.

Click the image to open in full size.

Here is the pre-proto FH9HVX all buttoned up:
Click the image to open in full size.

Edit May 9, 2020: Vunce has made Mouser shopping carts for this project here. There is a basic version for $32 and the premium version for $56.
FH9HVX - Budget Conscious 100w Class AB for Lean Times

The prototype schematic is here:
Click the image to open in full size.
Attached Images
File Type: jpg FH9HVX-Schematic-v001.jpg (307.9 KB, 935 views)
File Type: jpg FH9HVX-FFT-25w-8ohms.jpg (141.9 KB, 1055 views)
File Type: jpg FH9-Sonal-K.jpg (384.9 KB, 1009 views)
File Type: jpg FH9HVX-PCB-Top-v001.jpg (238.1 KB, 1044 views)
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Last edited by xrk971; 9th May 2020 at 07:38 PM.
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Old 27th April 2020, 01:59 AM   #2
gannaji is offline gannaji  India
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Congrats for the new project !
Please post the .asc file also for completeness !
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Old 27th April 2020, 03:30 AM   #3
Corloc is offline Corloc  United States
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Very cool! Following!
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Old 27th April 2020, 06:39 AM   #4
ballisticz4 is offline ballisticz4  South Africa
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Looking forward to the end result.
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Old 27th April 2020, 07:19 AM   #5
meanie is online now meanie  Singapore
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Tempting!
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Old 27th April 2020, 07:30 AM   #6
xrk971 is online now xrk971  United States
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FH9HVX - Budget Conscious 100w Class AB for Lean Times
To help you visualize the amp (With some premium components):
FH9HVX - Budget Conscious 100w Class AB for Lean Times-96c1c680-5244-4858-841c-4ead662dc855-jpeg

No boutique cap and on-board traditional MOSFET mounting. Can be underhung from PCB as well:
FH9HVX - Budget Conscious 100w Class AB for Lean Times-0b1011c0-27b4-44cf-b57b-94014b83dd4d-jpeg
Attached Images
File Type: jpeg 96C1C680-5244-4858-841C-4EAD662DC855.jpeg (295.6 KB, 905 views)
File Type: jpeg 0B1011C0-27B4-44CF-B57B-94014B83DD4D.jpeg (348.0 KB, 876 views)
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Last edited by xrk971; 27th April 2020 at 07:34 AM.
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Old 27th April 2020, 01:44 PM   #7
xrk971 is online now xrk971  United States
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FH9HVX - Budget Conscious 100w Class AB for Lean Times
Quote:
Originally Posted by gannaji View Post
Congrats for the new project !
Please post the .asc file also for completeness !
Here is the LTspice simulation with all models embedded.
Attached Files
File Type: asc FH9HVX-with-models-for-DIYA-v001.asc (15.4 KB, 30 views)
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Old 27th April 2020, 06:51 PM   #8
xrk971 is online now xrk971  United States
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FH9HVX - Budget Conscious 100w Class AB for Lean Times
Would anyone be interested in a simple CRC PSU board to go with this amp? I know many examples are on eBay, other GB's, etc. so not sure how much interest. It would be designed by JPS64 so you know the layout will be done with low impedance traces and attention paid to keeping the system low noise.

FH9HVX - Budget Conscious 100w Class AB for Lean Times-crc-psu-render-jpg

For the output Thiele network inductor, I am specifying a premade 27A 15uH shielded flat copper wire inductor from Coilcraft. Yes, I know, it is $7 but saves you from winding your own from magnet wire (which you are still free to do). It also looks way cool. I use similar inductors on the output filter of my TPA3255 and I think they are part of the reason why my amp measures half the distortion of the TI reference amp which uses toroidal inductors.

Click the image to open in full size.
Attached Images
File Type: jpg CRC-PSU-Render.jpg (77.6 KB, 873 views)
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Last edited by xrk971; 27th April 2020 at 06:57 PM.
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Old 27th April 2020, 07:20 PM   #9
pinnocchio is offline pinnocchio  Canada
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Hi X,

I build the FH9HV a few years ago and it really sounded nice. Like what you've done with the layout and love the output inductor style!

Do
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Old 27th April 2020, 07:32 PM   #10
spind is offline spind  Canada
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FH9HVX - Budget Conscious 100w Class AB for Lean Times
Looks very interesting, nice job all around. Was wondering why such a large output inductor, aren't most around 2 uH or so?

Steve.
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