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Board for WA1GFZ MOSFET Audio Driver




 
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w9jsw
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« on: February 09, 2019, 11:27:08 AM »

I am generating a new board for this driver design. Attached are the original schematic and my copy of it into EasyEDA so that I can generate the PCB. Please look at it carefully (WA1GFZ ??) to see if I copied it correctly.

WA1GFZ - your PMs are blocked and there is no email address. I tried to respond to your PM. Hopefully you will want to contribute to this thread.

I have most of the footprints correct.

Questions -

50uf non-polorized caps are crazy expensive. I subbed two back to back 27uF electrolytics. Are there any special characteristics I should be looking at for the caps (low ESR, etc)?

The 1000uf at 450V Electrolytics are huge. Can I place them off board so that I can save space on the PCB? Should I add some smaller ones or more bypasses in addition to the off-board caps?

Are there any design updates that are not listed on the schematic?

BTW, found some nice 2.2uf coupling caps - Panasonic ECWFD2J1225J - in stock and $1.5 each...
Put a trimmer in place of R26/R27
NFB Ladder in rig

Comments please...

John W9JSW



* WA1GFZ MOSFET Audio Driver.pdf (39.79 KB - downloaded 45 times.)
* New WA1GFZ Mosfet Audio Driver.pdf (431.69 KB - downloaded 37 times.)
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K1JJ
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« Reply #1 on: February 09, 2019, 12:41:31 PM »

Hi John,

Maybe Frank will see this and comment. I'll send him an email too.

Yes, the beer can 1000uFd @ 450Vs can be off-board.  I did it that way. Just keep the leads as short as possible. If you build the +- 300V supply in the same enclosure, the filter caps should be enough by themselves.  I think the extra caps are a precaution against remote supplies having PS leads of several feet. This is more about suppressing glitches and spikes rather than AC hum filtering and regulation.   IIRC, there is no PS current variation during modulation being in class A, so dynamic regulation is not an issue.


Speaking of RF bypassing, etc:

Yes,  capacitor bypasses are always a good idea to sprinkle around.  I also added ferrite cores, #63, to all input and output leads on the MOSFET driver. The ones that snap on work well. In fact I have about 30 of them all around the shack in critical low level audio and RF leads just to have a more stable environment.   The old days of RF in the audio never plague me anymore. This can become a big problem when running a KW and using antennas with exposed antenna tuners, openwire or antennas in close proximity... as well as unbalanced antenna currents in coaxial  feedlines, etc.  Operating on the higher bands can also bring out issues.  Once we cross 40M and higher, it's a different, squirrely, RF whirl.

T
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« Reply #2 on: February 09, 2019, 02:16:23 PM »

John,   JustRadios.com has 47uF 100v NP caps for $2.99.

I am building the MOSFET driver too.  I will do mine point-to-point I think.   Just putting the order together today.

Steve - KK7UV
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« Reply #3 on: February 09, 2019, 03:15:10 PM »

Two back to back series 27 uf caps equal 13.5 uf.  In parallel of course they'd be 54 uf. 
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w9jsw
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« Reply #4 on: February 09, 2019, 03:40:02 PM »

Two back to back series 27 uf caps equal 13.5 uf.  In parallel of course they'd be 54 uf. 

Right...

I switched it to a non-polarized one. Like it better.

John
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w9jsw
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« Reply #5 on: February 09, 2019, 03:41:58 PM »

John,   JustRadios.com has 47uF 100v NP caps for $2.99.


Thanks for the pointer! That is the type of cap I would prefer to use. Let me know how big it is i.e. how much space do I need between holes. No specs on that site for me to pick the correct footprint.

John
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« Reply #6 on: February 09, 2019, 03:47:41 PM »

I have the board laid out now but have not yet added the traces.

Advice is welcomed.

I was planning to use a 1oz copper board, 1.6mil thick. Traces at 20Mil for signals, more for power. Was going to do a ground plane on the bottom. Minimize vias. Grounded mounting holes on all 4 corners.

Fets mounted under the board with access holes to allow them to be screwed to a heatsink. I have done this before and it works quite well. Easy to install.

Feel free to comment on the above.

Overall board looks like 100mm by around 175mm. A 2 layer board from Taiwan for Qty of 10 is $40. Plus shipping on a slow boat for $15 and DHL for $26.

Not a bad price!

John

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« Reply #7 on: February 09, 2019, 04:16:34 PM »

John, in case you open this project to the public,
I am interested in 2 or 3 boards (so I can manage
populating one that works  Smiley )

Please let me know when the time comes.

Peter
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« Reply #8 on: February 09, 2019, 04:29:07 PM »

I have the board laid out now but have not yet added the traces.

Overall board looks like 100mm by around 175mm. A 2 layer board from Taiwan for Qty of 10 is $40. Plus shipping on a slow boat for $15 and DHL for $26.
Not a bad price!

John


Hi John,

$40 + shipping for ten is a tremendous price. It's not even worth it to do point-to-point anymore. Wish they were available when I built mine.

Anyway, they will sell out quickly.  This is the breakthru step between having average audio to transparant pristine audio.  Especially when the audio negative feedback is added.

I was talking to Frank WA1GFZ and he said he will send you an email. He will answer your questions and also wants a board for his new heavily-modified Valiant project.  Frank even wound his own FB mod xfmr.

Thanks for the excellent effort, John!    " Chirp-chirp -   AMers were only waiting for this moment to arrive..."


BTW, your Easy EDA schematic is MUCH easier to read than the original.  Use that one only - once it is perfected.    (Posted below)


It's important to point out that this driver uses only the famous and inexpensive 11N90 900 volt MOSFET as used in most class E rigs.  A well-proven device.

T

* New WA1GFZ Mosfet Audio Driver (10).pdf (409.88 KB - downloaded 40 times.)
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« Reply #9 on: February 09, 2019, 05:16:38 PM »

Latest schematic - use this one for comments -

Tom - the one you posted is identical to my first one. Did you have any comments for it? If it has annotations I did not get them.

Changes -

NP input cap
Fuse clip components for glass fuses (to get the footprint correct)
Removed beer can caps. Still need to add a few more coupling caps after I do the traces.

Still quite a bit of open real estate. I can go to 100mmx200mm at the same cost and no need to get it smaller/cheaper. I may stretch it out a bit to give more space between the 10W resistors.

Should have a picture of the board in the morning.

John

* New WA1GFZ Mosfet Audio Driver.pdf (431.69 KB - downloaded 39 times.)
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K1JJ
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« Reply #10 on: February 09, 2019, 05:46:50 PM »

OK John -

I would keep the beer cans on the schematic since Frank intended that much capacitance there. You could make a note that these caps are not physically on the board but should have short leads to the board. They can be mounted on the power supply to double as filter caps - as long as the leads are short.  Short means = 6-8" or less to the audio driver PCB?  

Keep the board comfortably big enough to work on parts, remove them, etc. Also, sometimes parts change size in the future, so plan on roomier than too tight.  And remember we are dealing with 600VDC. (+ - 300V)  So keep the trace spacing wider rather than too narrow so not to end up with charred boards.

BTW, I used a Variac on my + - 300V supply. I found I could reduce it somewhat to just what was required to drive the tube modulators. Same results in class A, less heat. Still, the 11N90s barely get warm.  In my case using tetrode configured 4-1000A modulators, I was able to back it down to +- 260 volts.  Big low-mu triode modulators like 833As will require the most voltage.

Important: The driver board's MOSFET gate bias resistors are optimized for +- 300V. When the power supply voltage is changed too low, some of the FET gate bias resistors need to shift in value to get back to maximum output capability. So watch the scope carefully when reducing voltage.

How it works, simplified: The + - 300V supplies basically center tap  and produce  0 volts to the tube grids thru the FETS. You set this DC offset that sets the negative bias and adjusts the idle current for the tubes. Then the audio signal comes thru push-pull and swings the grids up and down around this DC bias point.  So if you need less than  +- 300V of audio swing to fully drive the tubes, you can back down the overall supply somewhat.  Put an audio tone thru at full RF and audio power and you can easily see this + - 300V voltage Variac adjustment threshold on the scope.


I don't have any more comments at this point. Let Frank look over the final schematic and PCB to see what he thinks...

T
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« Reply #11 on: February 09, 2019, 09:28:20 PM »

If you have extra space, may I suggest that you include the qix negative peak limiter.

Of course, switch by passable.

I used it in my own processor design.  Works FB after an all pass rotator and 3 band compressor!

--Shane
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« Reply #12 on: February 10, 2019, 07:34:51 AM »

How much current does the driver need?   I am trying to design a power supply for it.
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w9jsw
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« Reply #13 on: February 10, 2019, 09:07:37 AM »

40-60ma on each leg. Tom has a variac on his supply and runs it slightly reduced.
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« Reply #14 on: February 10, 2019, 10:07:04 AM »

Thanks John.    Is a regulated supply needed?  or perhaps just rectified, cap filter, and perhaps a current limiting resistor?
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« Reply #15 on: February 10, 2019, 11:02:33 AM »

Is it done in LTspice? I would like to play around with it a little. I really like the design and the evolutions.
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« Reply #16 on: February 10, 2019, 11:37:55 AM »

Thanks John.    Is a regulated supply needed?  or perhaps just rectified, cap filter, and perhaps a current limiting resistor?

I would like to see a PS proposal as well.
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« Reply #17 on: February 10, 2019, 12:29:21 PM »

Thanks John.    Is a regulated supply needed?  or perhaps just rectified, cap filter, and perhaps a current limiting resistor?


FB, Steve -

The more help the better.

The driver board runs class A, a steady ~40 mA draw per leg, (two legs) so any brute force type power supply will work fine.   I believe the PS continuous rating should then be at least  100 mA since the two class A legs are running together.  Let me axe Frank about his rating opinion.  No need for voltage regulation.    

I have just one meter on the audio driver board panel monitoring the drain current of R4... a 50 mA meter in series with R4.  This is a steady state current of 40 Ma or so.   I read the modulator tube's grid current separately in the mod deck, since this is a dynamic measurement with audio fluctuation.

Just a heads-up to those ordering parts: Frank, John and I have been emailing and making some minor value changes. A few errors were found in the transition from the prototype to the schematic.... The C12  0.47 uF NFB cap is now 300 pF.  That's what I had in mine all along.

I am using the NP 20 uF cap in the audio input, so that still stands.  John is considering different pot wattage ratings too.

John will be posting an updated schematic.


BTW, yes, the QIX negative peak limiter would be a nice addition. It's really only a few parts, so worth adding.   The circuit can be found within his PWM modulator board. (We should ask for Steve's permission, of course)

** Another point:  I noticed that I never employed a way to key the driver board on and off for T/R. I just leave it running and a step start relay disconnects the audio drive. I probably didn't want to have power supply pops by keying the +- 300V.  Maybe we can find a way to bias the whole circuit off during receive to kill (or greatly reduce) class A current without causing audio pops.


*** UPDATE:  It was decided to keep any negative peak limiting circuitry off the driver board. This would require low level op amps, low voltage and other complications.  Keep NPL with the low level processing and leave the driver board as is...


T  
 
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« Reply #18 on: February 10, 2019, 02:01:53 PM »

Preliminary look-see

BTW - found bi-polar elecrolytics at Digikey for a buck. Fets are $4 due to the tariffs.

John


* boart top.png (58.45 KB, 500x344 - viewed 58 times.)

* board bottom.png (45.09 KB, 500x344 - viewed 36 times.)
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« Reply #19 on: February 10, 2019, 11:51:03 PM »

I bought my NP caps from capacitorworld.net

Was prescribed by someone here, and I've purchased from them a few times.

--Shane
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« Reply #20 on: February 11, 2019, 12:20:59 PM »

You might want to increase C1's value to C9's value (47 uF) since you are working into the same low impedance of about 1.9k.


Phil
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« Reply #21 on: February 13, 2019, 06:59:20 AM »

I have a day job, so slow progress during the week.

Frank or Tom - can you respond to Phil's comment?

We are doing some finalization on the board footprints and making sure the tracks and vias are large enough and well spaced enough to handle +/- 300V. Also adding off board pots to allow for fine tuning adjustment of bias on the final mosfets. Frank is re-running the models to insure we have this right for various voltage applications.

What we will probably do is a small run to build some prototypes then a larger run with errata changes applied from the prototypes to get the final boards right. With the low costs for PCB runs in China we don't have to be perfect on the first run, just close. Takes the pressure off. It is a different world WRT PCB manufacturing than when this was tried back in 2010/2011.

John
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« Reply #22 on: February 13, 2019, 09:23:32 AM »

It is a different world.

And realize.  These boards will be in ebay, alibaba and the rest of the sales sites within a couple days of being produced.

The big 3.5 inch bird meters square you see in ebay?  I had those made up.  And was told I had to come up with 10k dollars for an initial run or they would be in ebay.

They are now sold direct by China and n2yck.

The price we pay for below cost production.

--Shane
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« Reply #23 on: February 13, 2019, 10:53:35 AM »




Hmmmm    what would happen if a few lands were direct shorts to ground on the chicom boards?   Could someone with an exacto knife or a dremmel fix the short?


KLC
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« Reply #24 on: February 13, 2019, 01:11:30 PM »

Don’t know if this is an issue so I asked Frank since he owns the copyright. I can get USA boards for around $12 each
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