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The WA1GFZ MOSFET Audio Driver Board - Working modern version




 
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Author Topic: The WA1GFZ MOSFET Audio Driver Board - Working modern version  (Read 1612 times)
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K1JJ
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"Let's go hiking in the woods, Tommy!" - Yaz


« on: July 10, 2020, 02:24:19 PM »

After some serious testing and optimizing I have the latest PCB version of Frank's MOSFET audio board working on my 813s X 813s rig, Hollywood.  It is dialed in with just a wee bit of  about 4 dB of negative feedback which gives me an excellent, clean response between about 15Hz to 12 KHz.  My DSP audio filter limits me to 40 Hz to 6.5 KHz by choice, so only about 4dB of NFB is used.  Some rigs don't need much NFB if they are already clean.

I found that a single section "brute force" capacitor input power supply filter was not enough 120Hz filtering. There was a slight hum on the output of this 50 dB+ gain audio driver.  I did up a simple pi-section  C-L-C  + - 300 VDC supply and the hum fell into the noise.  Before, it could easily be covered up by a little blower noise like in my 4X1 rig using this same driver, but the 813s are dead quiet and I needed all the hum and artifacts to a minimum. So now, there is virtually no hum or 120 VAC sawtooth harmonics. This needs to be addressed when running a quiet shack.

The new PS and driver pictures are below. I did a lot of parts replacement and experiments on the audio board, so it looks a little beat up. I am going to have the rig on the air very soon if not this weekend. It is working FB into a dummy load right now.

Anyway, I am very pleased with the new audio driver.  It shows flawless looking tones and effortless 150% voice audio peaks.  Be sure to build the audio board on a big aluminum ground plane /heatsink  like you see mine.  Also, I had trouble with wire wound resistors used in the HV NFB ladder. Use carbon or metal film resistors. I was getting ultra sonic resonances for a few days until I figgered it out.

The printed circuit board, as designed by John, W9JSW usually works right away after building. There are some minor mods like larger wattage power resistors and other stuff since he laid it out a few years ago.  I think there were at least 15 boards sold and I know of Frank's and mine in operation. John and Bob/W1RKW are working on theirs. So there are some more boards out there if someone wanted to look.  Or, build it point to point, WTF it's a simple circuit and it would be easier to service..

I built my first one point to point and still use it with the 4X1 rig since 2010.  I now have two systems working.

I included the links to the latest circuits and notes by John.

If you have a plate modulated rig, there is nothing better than this board to give you flawless transformer-less audio with a touch of negative feedback.... clean, powerful SS audio (with very little phase shift)  at least up to the modulator grids. It's then up to the modulator tubes and mod iron. It is worth the effort and can be transferred from rig to rig in the future.

Tom, K1JJ

** The MOSFETS are mounted underneath the PCB directly onto the 11" X 11"  aluminum plate as a heat sink. It just gets warm after 2 hours at + - 275V.  The voltage regulation is excellent under full drive with all this filter capacitance.  


* DSCF0012.JPG (320.67 KB, 1280x960 - viewed 139 times.)

* DSCF0009.JPG (327.9 KB, 1280x960 - viewed 147 times.)

* DSCF0008.JPG (326.63 KB, 1280x960 - viewed 156 times.)
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
K1JJ
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« Reply #1 on: July 10, 2020, 02:25:38 PM »

More:

**  I might add that the entire 813 rig is now rock stable for RF, audio and NFB.  It takes time with a new rig and new design - and there are a lot of failures, frustrations and heartbreaks...I had my share with this project. But eventually it will work flawlessly when you come into the shack and flip the switch. There is no excuse not to refine it to perfection. Thanks to Frank / WA1GFZ, Chuck / K1KW, Jeff / W2NBC, John / W9JSW and others who have given me mucho e-mail help along the way.  Frank, especially, has put in a lot of time on the simulator tweaking his original design. Whenever something doesn't work right for me he is there modeling it to answer the questions.

T


* DSCF0014.JPG (322.69 KB, 1280x960 - viewed 116 times.)

* DSCF0006 (1).JPG (317.09 KB, 1280x960 - viewed 112 times.)

* DSCF0007 (1).JPG (319.59 KB, 1280x960 - viewed 113 times.)
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
K1JJ
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"Let's go hiking in the woods, Tommy!" - Yaz


« Reply #2 on: July 10, 2020, 02:26:47 PM »

More:


* DSCF0005 (1).JPG (313.8 KB, 1280x960 - viewed 105 times.)

* DSCF0003 (1).JPG (320.33 KB, 1280x960 - viewed 97 times.)

* DSCF0011.JPG (324.91 KB, 1280x960 - viewed 111 times.)
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
K1JJ
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"Let's go hiking in the woods, Tommy!" - Yaz


« Reply #3 on: July 10, 2020, 02:29:44 PM »

John, could you post your notes and other info here?

Thanks.

T


* Mosfet Circuit with test load[383].jpg (86.78 KB, 1228x620 - viewed 143 times.)
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
WBear2GCR
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« Reply #4 on: July 10, 2020, 05:37:42 PM »

If that is the "as-built" circuit, I'd suggest putting a suitable resistor (based on the current draw)
breaking the B+ line(s) to the input stages, followed by a largish capacitor... don't think I saw
this. Something like a 100ohm resistor (or a diode!) and a 470ufd cap or similar to ground.

That will decouple the input stages from the gyrations of the power stage...
...could be before the phase splitter or after, or both places, before and after...

The other thing that would help the 120Hz ripple is to "resonate" the choke in that
pi filter... use the SPICE to find the best value, then tweak it based on the actual load
in the actual circuit. It's a pretty sharp notch, so it needs to be centered at the 120Hz
spot or it may not be terribly effective. It's worth 60dB of "knock down" in some cases.

Also, use SPICE, again, as the same value of cap on both sides of the choke is far less
effective than dissimilar values - again SPICE or a PS calculator will show best values.

I've done this myself, quite effective.

                   _-_-bear

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_-_- bear WB2GCR                   http://www.bearlabs.com
K1JJ
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« Reply #5 on: July 10, 2020, 07:10:57 PM »

Bear,

Those are some good suggestions and I'm sure Frank will give me some feedback.

We did talk and do a simulation trying to isolate the earlier stages from the 120 Hz hum. The decoupling resistor and cap might help as well as the resonant 120 Hz filtering technique.

OK on the dissimilar caps in the pi filter. I seem to remember that from somewhere.  Actually I am using four 1400 uF filter caps.  Beer cans. Killing the hum at the source.

I just had the rig on and the 120 Hz hum is actually below the room noise, so I will probably not get involved with further mods but future guys building this should take notice. Frank may also see something easy to do from your suggestions and let me know.

Tnx, OM!

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
w9jsw
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« Reply #6 on: July 11, 2020, 08:28:11 AM »

docs

My bom specs 10W resistors. It is suggested to upgrade them to 20W versions that will still fit fine.

John

* Design Notes - 20190510-1.pdf (2549.64 KB - downloaded 57 times.)
* New WA1GFZ Mosfet Audio Driver.pdf (61.55 KB - downloaded 58 times.)
* BOM_WA1GFZ Mosfet Audio Driver.xls (10 KB - downloaded 47 times.)
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W1RKW
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« Reply #7 on: July 11, 2020, 01:13:59 PM »

thanks for the update, John
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Bob
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His fear was when I turned it on for the first time life on earth would come to a stand still.
N4LTA
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« Reply #8 on: July 11, 2020, 02:38:58 PM »

Any boards available? I laid out a board and had one built but it was destroyed in the fire.

Pat
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w9jsw
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« Reply #9 on: July 12, 2020, 06:59:23 AM »

I sold all of the boards from the only run we did. There are around 25 of them out there. I even sold my extra one that I was saving for another rig. Maybe post on the wanted board and someone may want to sell rather than build.

John
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KC2ZFA
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« Reply #10 on: July 17, 2020, 06:28:19 PM »

John, could you post your notes and other info here?

Thanks.

T

what are the diodes D6-D7 in the test load ? I've seen something similar with 1N645...
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KC2ZFA
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« Reply #11 on: July 17, 2020, 06:34:15 PM »

these resistors are R25, R5, R6 and R9, R2, and R4 ?


docs

My bom specs 10W resistors. It is suggested to upgrade them to 20W versions that will still fit fine.

John
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w9jsw
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« Reply #12 on: July 18, 2020, 07:44:43 AM »

correct
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KC2ZFA
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« Reply #13 on: July 20, 2020, 01:22:33 PM »

tried replying to 'GFZ's PM but I'm blocked (?)...so here's my reply:

ah, OK, thanks for this. I was looking for a realistic "dummy load" to test reverse connected output xfmrs to drive class B grids with an audio amp.
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N4LTA
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« Reply #14 on: July 21, 2020, 10:35:49 PM »

I found a board. Are the resistors not sized properly in the original design? Do they fail from overheating?

Using a larger wattage resistor will not reduce the heat dissipated.

Thanks

Pat
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K1JJ
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« Reply #15 on: July 22, 2020, 12:02:34 AM »

Hi Pat,

The 10 watt resistors are OK, but they get hot.  I used 10 watters for 10 years on the prototype with no failures. But for the new PCB version I used 20 watters and they stay just warm to the touch.. a better situation.

Yes, the heat dissipation is the same.

I just finished fabricating the front panel and chassis for my new pair of 813s in linear service. About 2200 VDC or so.   It will be the main RF driver in the shack.  I decided to go GG for about 10-13 DB gain which will drive a big linear or be a driver for the AM class C rigs with a carrier-   using only about 2-5 watts from the FT-1000D.

Does anyone know the gain of an 813 in GG linear service with a tuned cathode input and 2KV?  

I'll post some pics soon.

I need to decide on a front panel color and a name.


** Interesting on using the GFZ board as a tester for output transformers.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
K3YA
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« Reply #16 on: July 22, 2020, 12:58:13 PM »

Hello Tom.   I got about 600 watts out of a pair of GG 813's at 2KV with a 100 watt ricebox driving it.  They play better at 3kv plus.
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KC2ZFA
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« Reply #17 on: July 22, 2020, 01:24:33 PM »

Quote
** Interesting on using the GFZ board as a tester for output transformers.

not the board, the dummy load shown on the spice circuit.
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K1JJ
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« Reply #18 on: July 22, 2020, 01:47:34 PM »

Hello Tom.   I got about 600 watts out of a pair of GG 813's at 2KV with a 100 watt ricebox driving it.  They play better at 3kv plus.

Hi Chuck -

Thanks for the info.  Yikes, so is that  100w in, 600w out = 7-8 DB gain?    I need at least 10 DB of gain, and preferably 13 DB  so that with 5 watts in = 50 watts out.  I might have to go with regulated screen and grid voltages using a more complex grid driven circuit then.

My question is, is your GG linear amp tuned at the cathode and if you put in 5 watts dead carrier, what is the output?   And what is your peak audio plate current reading under maximum ssb strap so I can pick a good meter range to use?  500 mA about right?  This will help me a lot in the design phase.

I have a friend who ran four 813s in parallel, GG with 4KV that put out 2KW pep.  Hand grenade but worked... :-)    I just need it as a mild-mannered IPA driver.

Thanks.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
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