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FH9HVX - Budget Conscious 100w Class AB for Lean Times

Founder of XSA-Labs
Joined 2012
Paid Member
Edit June 5, 2020: Here is my completed FH9HVX in dual mono using a cast iron griddle heatsink:
848644d1590823434-fh9hvx-budget-conscious-100w-class-ab-lean-times-griddle-fh9hvx-mosfets-setup-closeup2-09-jpg


849180d1591003996-cast-iron-griddle-amp-budget-conscious-heatsink-solution-griddle-amp-final-02-jpg

More info on the Griddle Amp here.

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:
838214d1587945663-fh9hvx-budget-conscious-100w-class-ab-lean-times-fh9-sonal-jpg


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.
558782d1467949449-100w-ultimate-fidelity-amplifier-fh9-hv-heatsink.png


That's 34.5Vrms into fan cooled 8ohm load:
558783d1467949449-100w-ultimate-fidelity-amplifier-fh9-hv-stress-test.png


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):
838205d1587944992-fh9hvx-budget-conscious-100w-class-ab-lean-times-fh9hvx-schematic-v001-jpg


Here is the LTspice file for the simulation.

Here is the predicted distortion at 25w into 8ohms of 0.0027%:
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:
838206d1587944993-fh9hvx-budget-conscious-100w-class-ab-lean-times-fh9hvx-fft-25w-8ohms-jpg


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.
838215d1587945663-fh9hvx-budget-conscious-100w-class-ab-lean-times-fh9hvx-pcb-top-v001-jpg


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.

839187d1588236461-fh9hvx-budget-conscious-100w-class-ab-lean-times-6a61a91e-64bb-49b1-b631-61882fabb0fc-jpeg


Here is the pre-proto FH9HVX all buttoned up:
840003d1588438460-fh9hvx-budget-conscious-100w-class-ab-lean-times-fbe748cb-859b-45ab-bff9-748ec85e4ba7-jpeg


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

BOM in Excel format v01a here:

FH9HVX - Budget Conscious 100w Class AB for Lean Times

The prototype schematic is here:
842132d1588999708-fh9hvx-budget-conscious-100w-class-ab-lean-times-fh9hvx-v001a-schematic-jpg


Here is the All Cee's PSU that is recommended for use with the FH9HV:
FH9HVX - Budget Conscious 100w Class AB for Lean Times
850135d1591375406-fh9hvx-budget-conscious-100w-class-ab-lean-times-cees-psu-3d-render-v001-jpg


Edit Aug 2, 2020: please be careful of the orientation of the LED in the input stage CCS. The Anode goes to ground - not cathode.
 

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Founder of XSA-Labs
Joined 2012
Paid Member
To help you visualize the amp (With some premium components):
attachment.php


No boutique cap and on-board traditional MOSFET mounting. Can be underhung from PCB as well:
attachment.php
 

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Founder of XSA-Labs
Joined 2012
Paid Member
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.

attachment.php


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.

AGP2923_DSL.jpg
 

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Hello x,
Not sure if putting a big antenna of a cap and long signal traces near V+ is good idea...
Supply rails go all around the PCB , I would try to keep them as close as possible to the big current components.

here is another version of a compact CRC made by me in the APEX thread.

100W Ultimate Fidelity Amplifier
100W Ultimate Fidelity Amplifier

my 2c.

regards
prasi
 
Founder of XSA-Labs
Joined 2012
Paid Member
Hello x,
Not sure if putting a big antenna of a cap and long signal traces near V+ is good idea...
Supply rails go all around the PCB , I would try to keep them as close as possible to the big current components.

here is another version of a compact CRC made by me in the APEX thread.

100W Ultimate Fidelity Amplifier
100W Ultimate Fidelity Amplifier

my 2c.

regards
prasi

Hi Prasi,
Thanks for your comment - noted. Your CRC layout looks very nice too.
X
 
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Member
Joined 2013
Paid Member
Hello x,
Not sure if putting a big antenna of a cap and long signal traces near V+ is good idea...
Supply rails go all around the PCB , I would try to keep them as close as possible to the big current components.

here is another version of a compact CRC made by me in the APEX thread.

100W Ultimate Fidelity Amplifier
100W Ultimate Fidelity Amplifier

my 2c.

regards
prasi

Thanks Prasi - I guess I missed you posting this. Looks well done and compact.
 
Founder of XSA-Labs
Joined 2012
Paid Member
Some updates on pre-prototype testing by Vunce using old Sonal K PCBs with KSC3503 for LTP CCS and revised resistor settings on LTP. Test were performed using a 38v linear trafo and CRC PSU show that amp has clean low noise background, does not have oscillations, hiss or hum. Also, use of KSC3503 and revised setpoints reduced the high temp condition of previous BC546 used on the CCS. JPS64 has also come up with a totally revised layout with shorter traces for the power rails, and optional solder bridges that can be used to locate the power inpouts on either the left, right, or both edges of the PCB. A new Wurth 1.5uH inductor is now specified (or one can make one with magenet wire wrapped on a 12mm dia mandrel).

Here is an O-scope shot showing the amp output with a small signal sinewave input to check for signal response cleanliness:
attachment.php


Here is sinewave generator turned off showing the clean low noise background at 100mV scale:
attachment.php


Zooming into 5mV scale still looks really clean. No hiss, no hum, no oscillations:
attachment.php


Here was the test rig using a 38v trafo and a CRC PSU (Prasi 4x22,000uF):
attachment.php


An Abletec +/-52V SMPS was also tested and the output was totally hiss and hum free. Measured 0.4mV rms on Fluke 101:
attachment.php


Here is the totally revised layout of the FH9HVX by JPS64:
attachment.php


I really like the new layout! Looks way cool. Super flexible with options for power connectors on either or both edges. Nice symmetry and super cool looking 1.5uH ouput inductor rated for 95A! Yes, overkill but you have to have a cool part in here somewhere to make it special. :)

A big thanks to Vunce for doing the pathfinder studies with the pre-prototype board and a big thanks to JPS64 for working his magic with the layouts.

I can't wait to build this little amp! :D
 

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