• Help with low power Amplifier..

Posted: **Sat Nov 07, 2015 7:18 pm**

Just received this today, and although everything else looks normal I noticed a Capacitor missing (C2) at the input end.

To those in the know, does this look like it's fallen off, or do you think it's been deliberately omitted?

What purpose does it serve and is it needed?

Also, should I leave the little Variable Resistor alone if I run this at 12 > 13.8V?

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Posted: **Sat Nov 07, 2015 9:40 pm**

I'm guessing it's not needed as there's already C1 there and C2 would just be another one doubled up with it so I'm guessing it's not used on this board.

Posted: **Sat Nov 07, 2015 10:31 pm**

Probably, just a placeholder if you need to add capacitance for better match. Or maybe if you want to go down into Band 1 (ie 70MHz band).

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Posted: **Sun Nov 08, 2015 12:07 am**

Thanks as always guys.

Is it OK to leave that variable resistor where it is if dropping the supply voltage down to 12 > 13.8V?

Also, I should turn supply to amp on before applying any RF input or doesn't it matter? I ask as the driver and amp will sometimes be run off separate power supplies.

Posted: **Sun Nov 08, 2015 12:36 am**

From the looks of it the capacitor has been left off on purpose, probably as pjeva said sometimes people leave space for components at the design stage which are not needed during final manufacture. The variable resistor I guess would have been set already so no need to change that amd it shouldn't matter if its powered with or without drive, and vica-versa.

Posted: **Sun Nov 08, 2015 2:05 am**

Cheers Analyser and all for answering my basic questions..

Posted: **Sun Nov 08, 2015 10:49 am**

That looks like the 15W amp Enigma sell. The guy knows what he's doing and tests everything so you'll be reet.

Posted: **Sun Nov 08, 2015 11:21 am**

Variable resistor controls bias voltage of RD15 fet. You could turn it down completely if you want and then turn it up when you apply rf drive until you have wanted power output. Anyhow, for this setup, this DC voltage at the gate of RD15 should not exceed 2.8V. If everything else is ok, that should give you 15W output with aprox 0.5W input. You can go higher with this voltage but if you do not apply rf input, RD15 will get very hot as it passes current through himself all the time. If this is set up correctly, it doesn't matter what you turn on first, driver or amp.

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Posted: **Sun Nov 08, 2015 12:29 pm**

shuffy wrote:That looks like the 15W amp Enigma sell. The guy knows what he's doing and tests everything so you'll be reet.

Yes it is mate, correct.

Been chatting to him over eMail and he's a sound and legit guy, and got a response about the missing Capacitor just after I'd posted on here. He confirms that it's deliberately missing as others here said.

Anyone know if the owner of Enigma is on here?

Posted: **Sun Nov 08, 2015 12:42 pm**

Double post deleted

Posted: **Sun Nov 08, 2015 12:43 pm**

MiXiN wrote:
pjeva wrote:Variable resistor controls bias voltage of RD15 fet. You could turn it down completely if you want and then turn it up when you apply rf drive until you have wanted power output. Anyhow, for this setup, this DC voltage at the gate of RD15 should not exceed 2.8V. If everything else is ok, that should give you 15W output with aprox 0.5W input. You can go higher with this voltage but if you do not apply rf input, RD15 will get very hot as it passes current through himself all the time. If this is set up correctly, it doesn't matter what you turn on first, driver or amp.

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Nice one for the info Pjeva.

In your opinion, what's the highest drive this'll take? I know it says 1W max on the amp, but I think my driver does around 1.2W and I'm not sure how to attenuate it down apart from dropping the supply voltage. How I could drop my driver to 500mW I've no idea.

Posted: **Sun Nov 08, 2015 1:31 pm**

You could use resistor attenuator at the input. Google it to see how many db is 700mw and then google for apropriate resistive attenuator resistor combination. If you are not sure, i'll draw you one.

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Posted: **Mon Nov 09, 2015 5:03 pm**

Yes, Pjeva, please draw one if you'd be so kind.

I have a pile of resistors here, so can knock one up straight away.

Posted: **Mon Nov 09, 2015 6:01 pm**

MiXiN wrote:Yes, Pjeva, please draw one if you'd be so kind.

I have a pile of resistors here, so can knock one up straight away.

Here you go.

http://www.random-science-tools.com/ele ... uator.html

Use 270R for 1 & 3
USe 18R for R2.

If your driverboard does 1.2W, this will take it down to 600mW.
If you want slightly less attenuation go for 470R for R1 & 3, and 12R for R2.

Posted: **Mon Nov 09, 2015 6:04 pm**

It looks like amp you have has an attenuator on it anyway, so the input power limit is probably to stop you burning those SMT resistors out. I think you'll get away with feeding it 1.2W, but occasionally pop your finger on those SMTY resistors to see if they are getting really hot, if you can't hold your finger on it for more than a few seconds, it's too hot.

Posted: **Mon Nov 09, 2015 8:16 pm**

Analyser wrote:It looks like amp you have has an attenuator on it anyway, so the input power limit is probably to stop you burning those SMT resistors out. I think you'll get away with feeding it 1.2W, but occasionally pop your finger on those SMTY resistors to see if they are getting really hot, if you can't hold your finger on it for more than a few seconds, it's too hot.

That's correct! R1, R2 & R3 are attenuator. But these resistors are on the very edge of handling 1,2W. Maybe you should attenuat that driver a bit more before input. Try to turn it on for few seconds and then turn it off and quickly touch attenuator so you don't feel rf burning, just plain heat.

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Posted: **Mon Nov 09, 2015 11:13 pm**

TBH i think the amp will handle the extra 0.2watt? maybe a small fan will be incorporated into his design??
I would just reduce the supply voltage to the driver board a couple of volts? aslong as he keeps within the dropout voltage of any regulator and PLL tuning range? it should be fine!

Posted: **Mon Nov 09, 2015 11:54 pm**

If you making your own attenuator then use Carbon resistors, dont use wirewound.

However that amp already has one on it, it will handle your pwer easily. Tried and tested design wink wink

Posted: **Wed Nov 11, 2015 12:11 am**

Nice one for the help necks.

I'll power it up in the next few days, first trying a slightly lower supply voltage on the driver, and if I can't get the output power low enough I'll knock one of the resistive attenuators up.

Posted: **Mon Nov 16, 2015 5:34 am**

As expected, this amplifier worked fine.

I got around 13 > 14W out of it at 1w drive at just over 12v, but this was using a few metres of lossy RG58 and a cheap power meter so my readings might be off.

The attenuator network SMT Resistors got slightly hot as mentioned, but I could hold my finger on them permanently so I think they'll be alright?

Help with low power Amplifier..

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