Amplifier AC Power Supply Requirements

So today I learned a thing. Like why linear amp manufacturers spec their AC requirements so high. Like the Elecraft amp calls for 20A at 240v and a bunch of others say 15A etc. The reason why I am thinking about this is because I have been thinking about upgrading my license and then doing some sums to work out the costs of adding an amp to the line up.

If these things really did require ALL THE AC AMPS, I would have to pull a dedicated 20A line from the sub panel in the garage. Lucky there is room on the board to do this, but, its going to cost about $1500 to have a elechicken come and take half the roof off the house to pull a cable to where it needs to go and that is on top of the costs of an amp and the like.

My radio room already is on a 20A circuit that it shares with the water pump, bathroom and laundry, so leaving some overhead for the freezer and washing machine, there is room to spare, but not 15A, maybe 10A and then you would not want the wife to dry her hair in the bathroom. LOL

So why do these things need so much current? Inefficient power supplies. If they are using linear power supplies, because hams are scared of switch mode supplies, they might only be at best 60% efficient compared to say 80% efficient for even the worst switcher from MeanWell. And there is where I had my eureka moment. Math will give me a rough idea of the actual AC current requirements for any particular amp.

AC AMPS == DC Power (watts) / (Power Factor * AC Volts * Efficiency%)

So for your typical LDmos amp these days with 50V at 30A a maximum of 1500 watts, with linear power supplies 1500/(0.8*240*60%) you are looking at 13 Amps AC (guessing the power factor here). Compare that to a fairly efficient switch mode supply with power factor correction and your total AC current draw is going to be around 8 Amps or better.

This got me thinking, legal limit here is 400w and those numbers are for something closer to US power limits. So in theory, without knowing what the actual DC Power requirements are, but lets say 50V at 10A, thats 500 DC Watts and with a 90% efficient switch mode power supply we are looking at only around 2.5A AC.

That is not going to stress the existing household circuitry at all, in fact there is more than enough capacity to not even stress things even a tiny little bit. So maybe this upgrade and build and amp, yeah build one not buy it because I have the skills to design the boards and write the code to control it all, could be done for a reasonable cost, not in the 5 to 10K kind of mark.

So maybe this stupidity is actually within reach and I should give a little more thought to upgrading the license. Currently I am really close to finishing off 40m and 10m dxcc which will give me 4 bands and for the 5th band we are talking 80m and for that having 400w on tap with crappy dipole antennas might make dxcc a lot less painful.

So who knows, but watch this space. 🙂


A Story Of Two Crystal Filters

While waiting for parts and boards to arrive for the main things I am working on, I thought I would turn my attention to the age old problem of crystal filters. Its not like you can easily buy filters off the shelf these days and even finding suitable crystals can be a problem. The low profile half cut crystals can be hit and miss and full size crystals can be expensive. Hayward wrote a few papers not long back on using 9Mhz crystals in simplified designs and pretty much if you buy 9Mhz 30pf crystals you should be able to duplicate his work with minimum effort. Problem is,  9Mhz crystals are non existent in Australia even in the low profile cases and they are $1 or more each in quantity on mouser. Screw that Jacko.

I have had mixed results making home brew filters, but this one looks the business. Other than being 4K wide, it looks like a filter should look. Fixing the width is just a matter of changing the capacitor values. The second filter is 600Hz wide for CW, thats just about where I like it, but its got -10dB loss, not sure the issue just yet but my gut feeling is that its one of the crystal is off from the others. I did measure them all, but you know, failure is always an option. I am going to suck all the crystals out and start with a fresh batch of 8 and see if that is the problem and then change the caps on the SSB filter to narrow it up some. I hope to have 2 very usable filters.

Typically i fix things until they are broken, but hey this actually looks alright. I need to narrow up the caps some, but now its not all loss and no stop band. -60dB rejection is more and good enough for the kinds of girls I go out with.


New Broken Parts

Ever get that sinking feeling when you realize that the parts that you have been hoarding for 5 years are actually broken? Well, i have had these TFT displays in my parts draw for who knows how long. Long story short, I am working on a project that will probably use one of these displays. So i grabbed them out and a micro to test them with to ensure their is no library issues when I build one of these into the project. Good news is that there are no library issues, bad news is that both of my displays are broken, one more so than the other. This one worked just enough to show me that the code libraries worked with this display. So that is a good thing, now i just need a display that is not broken. To the ebay mobile batman.


Prototyping Board V2.0

So after playing with the old version of my prototyping system for a while and building some modules for it and making some noises with it, i started to get a handle on its short comings and there were many. But i think that I have addressed them in this latest version.

The main differences here is that I have moved all the power stuff to the back, added in a audio amp and 2 speakers on the board. You can just see them poking out from under the LCD, they might be tiny but are rated at 2w each. The audio amp has a 2n3904 preamp followed by a TDA2003 amp and should be fine for 5w of output.

All the digital stuff has been moved to the main board now, rather than on a card and everything important has been broken out. The ESP32 is connected to WIFI and i can program it over WIFI so there is no more connecting cables and the like. Extra pads near each daughter boards means routing signals between modules is now much easier.

All around the improvements should mean I will be able to burn though a ton of though hole parts over the xmas break and get rid of them from my life HAHA. Most likely what will happen, I will build something I like and end up keeping it LOL. Once i have finished testing the board, I will add the schematic, PCB files and the code to get the VFO running, so everyone can have at it, if they choose to.


I did it again. Designed something and then used the wrong part. In this case I designed the audio amp using a TDA2003 and I have 30 of them coming from LCSC. I thought that I had a couple of them here already, but NO NO NO. I have TDA2050 which is a totally different part requiring different components. LOL, Anyway, i have 1 TDA2003 that i got gouged on from an Ebay seller in Sydney and it will be here probably early next week. So the final testing is going to have to wait until then.


Everything is working and tested except for the TDA2003 audio amp. I have no reason to think it wont work, so downloader beware, use at own risk. Or wait a week until i have parts in my hand to test it.


MC3361 Mixer IC

What can I say, I like to trawl through parts catalogues and find somewhat obscure parts or ultra cheep parts to play with. So, when i found the MC3361, which is an FM Cordless phone IC, I thought why not. It costs 25 cents in one offs, and has some conversion gain and I assume its a gilbert cell in there, so its way cheaper than an SA612 and others.

LO in is on pin one, you can also add an xtal here for the LO. RF in is on pin 16 and the IF out is on pin 3. Now i did not try and match the impedance’s which are around 1800 ohms, i just fed in 50 ohms from my function generator into the LO and RF ports and then attached the spectrum analyzer to the IF port.

Other than some DC blocking caps and a 50 ohm termination resistor on the input there is not much to add to this. And as you can see from the plot below from the spectrum analyzer its working doubley balanced. RF in is suppressed and the sum and difference are the dominate signals. RF in was 7Mhz and the LO was 1Mhz for the sum and difference of 6 and 8mhz IF frequencies.

Is it as good as other mixers? I don’t know, but it works and its cheap, so i think its worth a go. Oh and it also has an FM de-modulator, AF amp and a op-amp for filtering in there also. I am yet to play with the other bits, i just setup the board for mixing only and well, mixing works.


Just A Few Vias

I have been sitting here today working on the PCB for the WSPR amp and got to thinking about cooling for these transistors. Sure they run cool enough to touch, but im sure over time things are going to warm up some so I threw in a million vias around the transistor and though the source pad to wick the heat away. Then i can bolt this direct to some kind of heatsink and have more than good enough cooling to let her run for forever.

I have got a few boards here almost ready to send off for production. A new BPF board using shielded can inductors. an updated LPF board using SMD caps, an experimental audio filter board where I designed the CW and SSB filters, while I like the hypermite/SSBmite filters I probably cannot really in good conscience release my gerbers as they are a product sold by 4state qrp, so if these work, I can then release my gerbers. Finally I have this test board for the WSPR amp and another PA board for 2m/70cm to send for testing. So much happening at Rob’s Lab its not funny. 🙂