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My 4Mhz amplitude modulated audio sstc

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LAN_403

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Mon Jun 16 2008, 04:40PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

its almost impossible to try everything out. thats why i used a "normal" ferrite core for the choke. from my expierience with this circuit that material worked very well without too much heating. i dont know the exact type of material though. just some cheap ferrites. the drain choke is getting hot after a few minutes, around 40Â°C and remains at this temperature, . thats ok i think. i just dont wanted to try more out, cause i was worried that the wrong core material and inductance can distort the audiosignal modulated on the dc voltage. and the streamers are long enough. i wanted to build a high quality plasma speaker and thats it.. if you think, that other core material can improve efficiency in audio quality and volume, iÂ´ll try it out for sure. but longer streamers can distort the audio so im not aiming for long sparks.

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Mon Jun 16 2008, 04:44PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Sorry for doubleposting. heres the improved shematic i use.. the layout is the same, just a few changes in resistor values and gdt and choke

teslacoolguy

Mon Jun 16 2008, 09:46PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

What can the bc546 be substuted for? (I am assuming that a bc550 will work just fine.)

Edit: Also i was wondering what awg is the secondary? And what are the specs, diameter, hight and also what kind of heatsinks do you need for the fets.

Marko

Tue Jun 17 2008, 12:45PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

It depends on the power level you are running at Marko. Type-26 is the cheapest crapiest grade of iron-powder for use in very high-volume low-cost SMPSUs operating the tens or low hundreds of kHz. If it suffices for any other particular application then great, because it is dirt cheap, but there are many better materials for RFCs.

Oh, OK then. Still with low cost and availability of those toroids, why not use them as long as they work?

As far as I think I understand, the *ripple current*, that is, the AC component of current, is what makes the core get hot. To reduce the ripple I want higher inductance, but inductance is limited by max ampere turns before the core goes into saturation.

So, if I need more power I can use several cores stacked or as separate inductors in series to reduce cookage.

Material 2 has mu relative of only 10 though, bug I heard it's praised for RF use. Can it also be used as AC current choke or transformer core at it's rated(2-20Mhz) frequencies?

GeordieBoy

Tue Jun 17 2008, 03:25PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

By all means use type-26 for any application that you can get away with!

Just watch out for any initial signs of heating because the organic binding material is susceptible to thermal degradation. This aging further increases core loss and can ultimately lead to thermal runaway.

You can increase inductance by stacking iron-powder cores, although keep in mind that this increases the volume of magnetic material in the field and therefore increases total core loss again. Micrometals design software is excellent for quickly assessing these tradeoffs although it does contain a few bugs.

Although the drain choke is designed to approximate a constant current source, "more inductance" isn't always better. I would specify the drain RFC to have a ripple of about +/-5% of the DC supply current at the chosen operating frequency. If you make the RFC too big you can run into problems with squegging and sub-harmonic oscillations in the power amplifier.

The inductance of the drain choke also plays a very important role in limiting how quickly you can modulate the PA in designs like this with high-level Amplitude Modulation. Too large an RFC will act too strongly to keep the drain current constant and will prevent rapid modulation of the amplitude of the RF output. The drain choke and power amplifier essentially form a series RL network from the point of view of the modulator, so the choice of L affects the bandwidth of the audio that can be modulated onto the carrier.

Type-2 could in theory be used at any frequency although you would need a lot of turns to get a useful inductor for operation in the kHz or Hz. Such a device would have high copper loss and be indicative that a better material choice should be made for more balanced copper vs core losses.

-Richie,

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Tue Jun 17 2008, 05:02PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

The primary is 4 turns with 6cm diameter and 1,5mmÂ². i dont know the awg cause here in germany every wire thickness is given in "mm" or mmÂ²"

the secondary is 5cm in diameter and 8 cm in height, wound with 0,5mm enameled copper wire.

for the bc546 you can use just any small signal npn transistor you have. it should stand up voltages up to 80v and currents of around 100mA, nothing special. you can use 60v types for sure

teslacoolguy

Fri Jun 20 2008, 02:47AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

What kind of heatsinking is needed for the mosfets?

Firefox

Sun Jun 22 2008, 05:25AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I was trying to translate the writing on the schematic and there were only two things I couldn't understand on it, one was the description of the heatsink; Q5, Q6 mount on a good heatsink At least 3.2K/W?. What is the 3.2 K/W? Also, on C11, what does the description 4.7N 630V FKP1 mean? I assume its 4.7 nF 630V, but what is FKP? I hope my questions aren't too much trouble.

teslacoolguy

Sun Jun 22 2008, 05:28AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Im using a very big heatsink but im having problems with the audio coming into the coil. The bias pots work but i dont get any audio modulation or breakout.

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Sun Jun 22 2008, 11:44AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

The coil is difficult to tune, so its out of question that you need some time to make things right. youÂ´ll need some time to get the right resonant frequency.

my tip. use a long copper wire, around 10cm on top of your secondary. measure the voltage on drain of the first mosfet and adjust the pot for maximum output voltage. the first thing you have to do is to get the right frequency, otherwise its extremely difficult to get breakout.

put the copper wire on top of your secondary and watch out for the current being drawn on the 50v line. then cut the copper in say 1cm pieces and watch the current. it should get higher to around 1,5A-2A where you should get breakout.

the grounding of your secondary coil is another tricky part. in one of my circuits it was enough to ground it on the mains, but for some reason it can be better to connect the secondary ground to the circuit ground.

the complicated part in this circuit is to get breakout. when youve done you can adjust the bias pot to around 30v and then you can adjust the pot for the amplifier. thats it.

the circuit has no feedback or something else in cause of simplicity, so tuning the secondary to 4mhz is the tricky part.

the heatsink of 3,2k/W is a standartised formula for heat dissipation here in germany. it means that for a given dissipation of say 1watts into the heatsink the heatsink is getting 3,2kelvin warmer. for 10watts its 32kelvin or 32degree celsius.

try reversing the primary. that can help too. the primary coil is difficult too. i had best performance with the primary put 1,5cm above the first secondary winding

fkp1 is a pulse capacitor from a german capacitor facility. you just have to use a good cap here cause the whole current is floing through it. just use a good foil cap
hope that helps

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