High power 12v amplifier

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OgreVorbis
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High power 12v amplifier

Post by OgreVorbis » Tue Sep 19, 2017 3:32 am

I've been thinking about building a high power 12v amplifier. I already tried to build one based on the rd70hvf1, but it didn't work well and I've also read that it is pretty difficult to get even 50w out of that device with a broadband match. I have, however seen amplifiers based on the rd30hvf1 with broadband match, produce up to 40w. I was thinking, would it be possible to match two of those to make 80w. Also the rd70 would need like 6w to make the 50w and the rd30 needs only 1w.

What about the type of matching used. I dont know too much about this. What's the difference between a coaxial balun type matching vs inductors type? Would one type be better for this situation? What I'm imagining is an amp that looks like one of those push pull type fet configuration except I'd use 2 rd30s. Is this a good idea and could anyone help me with this?
Thanks!

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Re: High power 12v amplifier

Post by Radio Garfield » Tue Sep 19, 2017 8:55 am

Hi there.
Here is the copy of text that Albert wrote on another topic:

...I then discovered the Mitsubishi low voltage FETs and found out how to parallel them up. I've just done a PA with 8 of the 70 Watt jobs in parallel for 400 Watts out. The efficiency of these things is insane, and I only had to get rid of just over 120 W of heat. The only problem is the huge supply current, but I distribute so that each device has its own power input. The main supply comes in on a big piece of 16mm² twin & earth! The 8 FETs cost much less than Motorola "Gemini" device or a big Philips BLF....

The combiners are just inductors, emulating ¼-wave lines. The splitters are more hassle - they're small trifilar wound ferrites. They're fiddly to wind, but they work. The two PAs we built have left for the Middle East, but I expect to make some more for a client closer to home shortly. When we do the next ones, I'll make sure that I have some suitably anonymous photos to put up here.

The FETs are individually powered and their bias adjusted so that each stage develops 50 Watts - in fact they're done in pairs, for 100 Watts, but the principle's the same. There's a further benefit to using distributed power like this - you can "lose" a FET or two, without massive degradation of the output power - this was the principle used with the old BBC 300 Watt pallets with the pairs of TP9383s.

I find that the biggest problems at power beyond 150 Watts or so are the output filter capacitors. We've been making our own, out of brass and the Teflon™ insulation material. When they're trimmed to the exact value that we want, they're dipped in acrylic paint to stop them corroding. LB had the smart idea of painting stripes on to them denoting their values in pF! We used to just write on them with a Sharpie, but found that it could get wiped off quite easily.

The other problem is power supplies. I've never really been comfortable with running rigs from Switched-Mode Power Supplies. The problem comes when you want to do a really big linear supply - the transformers cost £100 and the smoothing capacitors are almost as expensive. You need heavy supply cables to prevent volt drop and heating issues..... I'm really going to have to have another look at the "Advance" and "Meanwell" supplies. These seem to be the best of a bad lot.....

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Re: High power 12v amplifier

Post by thewisepranker » Tue Sep 19, 2017 10:53 am

The main problem is that the output impedance of the FET has to be so bloody low that it's beyond practical and difficult for the home-brewer without some decent test gear to align it properly. Just because there's no spurs on the output, doesn't mean that the filter is properly matched.

I'm assuming you're asking about running from 12 V because you want to use it in your car or from a car battery... You'd be far better off boosting your voltage to something nicer, i.e. 24 V or 48 V where the usual suspects are.

I don't agree with Albert's thoughts about SMPSUs. Yes, when the cheapest no-brand power supply is fitted, it will result in a mess but it's 2017, there are plenty of commercial bits of equipment running with switching supplies and only switching supplies, running on much higher frequencies than we are dealing with. The VNA on my desk goes up to 3 GHz, the signal analyser (a fancy spectrum analyser) on my desk goes up to 13.5 GHz - both have off-the-shelf switching power supplies in them (the VNA is a Windows computer in a custom box) and both can measure down to about -130 dBc - the signal analyser probably down to -140 dBm.

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Re: High power 12v amplifier

Post by Analyser » Tue Sep 19, 2017 6:55 pm

My preferred solution to this problem would be a push-pull amplifier using coax transmission lines for the baluns and matching networks. As TWP says, the impedance at this voltage and power level would need to be very low (around 2ohms for only 30W at 12V). This makes it more complex to design a broadband matching network using lumped elements.
If you tried a push-pull amplifier using either 4:1 or 9:1 transformers, this effectively doubles the output impedance of the transistor pair and also has the advantage of lower even-harmonic content and (usually) higher efficiency.

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Re: High power 12v amplifier

Post by Albert H » Tue Sep 19, 2017 11:45 pm

I still have a bit of a love / hate relationship with SMPSUs. I have an Italian rig on the bench at the moment that has a "Meanwell" open frame SMPSU - so it's a reputable brand (as far as I'm aware!). It came here because of the spurs every 100kHz either side of the carrier. The biggest ones are -48dBc and are at 200 kHz either side of the carrier, There are several more that are visible either side at diminishing levels, but they're enough to cause interference in the vicinity of the TX site. There is huge 100kHz switching crap on the DC supply to the PA board. The "spare" PSU does the same thing.

I wondered if it was a screening issue, with the PSU suffering from RF getting into it.... Nope. The rig is spectacularly clean at full rated output (800 Watts) when run from my bench supply. The PSUs are marked as being good for 2kVA output, and since we're only drawing a bit over half that, they should be loafing along.

I borrowed a "Coutant"-brand 2kVA SMPSU (that's too high to fit into the box!) and we have a clean carrier. I'm now debating what to do - build a nice, reliable, clean linear supply, or try and find an SMPSU for greater efficiency that fits inside the box!

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Re: High power 12v amplifier

Post by OgreVorbis » Wed Sep 20, 2017 12:18 am

The FETs are individually powered and their bias adjusted so that each stage develops 50 Watts - in fact they're done in pairs, for 100 Watts, but the principle's the same.
Albert, I would greatly appreciate if could you supply any schematics for one of these dual-fet amplifiers?

What is the RF input power for one of these?

Also, if you still have any available, I'd be interested in purchasing.
I'm assuming you're asking about running from 12 V because you want to use it in your car or from a car battery... You'd be far better off boosting your voltage to something nicer, i.e. 24 V or 48 V where the usual suspects are.
I don't really like the efficiency loss by converting the voltage even if minimal. I also just find it intriguing to build something like this which I've never seen before in pirate radio (seen ham designs like this), but I think there'd be a lot of demand for something higher power that runs at 12v.

Ideally, I'd like to use multiple rd30s because the gain is higher than the 70 and I'm not really interested in anything much over 100w.

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Re: High power 12v amplifier

Post by Albert H » Wed Sep 20, 2017 1:40 am

I'll scribble down what we did. It's based on one of the Granberg designs from the earliest days of the power FETs. He designed a 144MHz amplifier that used a pair of MRF150s. It went like a rocket if you got the drive and the bias right!

The input match is always the tricky bit!
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Re: High power 12v amplifier

Post by Analyser » Wed Sep 20, 2017 9:16 am

Albert H wrote: Tue Sep 19, 2017 11:45 pm I still have a bit of a love / hate relationship with SMPSUs. I have an Italian rig on the bench at the moment that has a "Meanwell" open frame SMPSU - so it's a reputable brand (as far as I'm aware!). It came here because of the spurs every 100kHz either side of the carrier. The biggest ones are -48dBc and are at 200 kHz either side of the carrier, There are several more that are visible either side at diminishing levels, but they're enough to cause interference in the vicinity of the TX site. There is huge 100kHz switching crap on the DC supply to the PA board. The "spare" PSU does the same thing.

I wondered if it was a screening issue, with the PSU suffering from RF getting into it.... Nope. The rig is spectacularly clean at full rated output (800 Watts) when run from my bench supply. The PSUs are marked as being good for 2kVA output, and since we're only drawing a bit over half that, they should be loafing along.

I borrowed a "Coutant"-brand 2kVA SMPSU (that's too high to fit into the box!) and we have a clean carrier. I'm now debating what to do - build a nice, reliable, clean linear supply, or try and find an SMPSU for greater efficiency that fits inside the box!

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All Meanwell supplies are not equal!

They're all pretty reliable but as you've found the performance can vary quite a lot. I've found it's generally always necessary to wind the PA supply wires a few times through a large ferrite core to calm things down. This mostly helps with RF getting back in to the supply and messing with the regulation but also a little with the switching noise going back the other way.
Have you had a look at the secondary-side filtering on the supply? Could you add some more L-C networks on the output to get rid of the 100KHz noise?

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Re: High power 12v amplifier

Post by Albert H » Wed Sep 20, 2017 10:31 am

I looked at the conducted crud from the original PSU in both directions. The manufacturers had taken great pains to keep the mains supply clean, and there was little rubbish going that way. However, the output side of the PSU was a nasty mess! I tried to use additional filtering on the output of the PSU to no avail.

I have another Coutant supply on the way - physically small enough to fit inside the rig. The manufacturers say that it's entirely "RF-proof"..... We'll see!
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Re: High power 12v amplifier

Post by thewisepranker » Wed Sep 20, 2017 10:57 am

OgreVorbis wrote: Wed Sep 20, 2017 12:18 am I don't really like the efficiency loss by converting the voltage even if minimal. I also just find it intriguing to build something like this which I've never seen before in pirate radio (seen ham designs like this), but I think there'd be a lot of demand for something higher power that runs at 12v.
Well, the more continuous current you draw from a battery, the further away you get from achieving the full nameplate capacity:

Image

Running high power rigs off of large car or leisure batteries doesn't get you much runtime, even for very large batteries. Your conversion losses in a DC-DC converter are going to be negligible compared to the internal resistance losses when drawing high current.

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Re: High power 12v amplifier

Post by Albert H » Wed Sep 20, 2017 10:46 pm

Years ago, we used battery powered rigs because they could be used where there was no mains supply. They had the advantage that you could go up a hill and find the biggest tree that you could climb, and get great coverage at relatively low power.

In my Californian days, we ran (roughly) 40 Watts from a solar / wind charged leisure battery, and got overnight service from it for months on end. Again, the gear was up a big hill with an unimpeded view of the target area.

Powerful batteries tend to be big, heavy and expensive, and require lots of maintenance. Other than that, they're great!
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Re: High power 12v amplifier

Post by OgreVorbis » Thu Sep 21, 2017 7:25 pm

I'll scribble down what we did. It's based on one of the Granberg designs from the earliest days of the power FETs.
Thanks!

I've been looking at some newer MOSFETS. This one looks really nice:
https://www.nxp.com/products/rf/rf-powe ... FT05MP075N

It's cheaper than the RD70, has more gain (should only need about a watt for 70w out at 12.5v) and has 65:1 SWR tolerance. I really wish I could design amp matching networks myself. Unfortunately, they only provide a 450mhz reference circuit for this one. Their 30W model (AFT05MS031N) has a 136-174 reference circuit which could probably be adapted for FM broadcast. I don't know how you're meant to mount it though because it has tabs, but no screw holes.

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Re: High power 12v amplifier

Post by sinus trouble » Thu Sep 21, 2017 10:38 pm

I think its just not a good idea to run high power amplifiers at such a low voltage! I doesn't make sense as explained already here! I get so many people with HUGE car audio amplifiers wondering why they have alternator or battery failure!
With battery technology these days, 18v, 24v even 36v is easily sourced with 5Ah or more!!
Wattage will most likely calculate at a similar value? But will be far more efficient throughout!
I am as stupid as I look! :|

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Re: High power 12v amplifier

Post by OgreVorbis » Tue Sep 26, 2017 1:39 am

So I contacted freescale about the AFT05MP075N 70W mosfet and whether they had any broadband application schematics. There is one for the 400-500mhz band in the datasheet, but that's not very helpful for FM broadcast as the frequencies are so different.

However, they responded and provided me with a schematic for 136-174mhz broadband match which I think could probably be altered for FM. It may not even need to be because often these ranges are arbitrary and the amps can go much lower.

Anyway, I thought I'd attach it here if anyone wants to have a look or suggest what components might need to be slightly altered.
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Re: High power 12v amplifier

Post by Analyser » Tue Sep 26, 2017 10:47 pm

OgreVorbis wrote: Tue Sep 26, 2017 1:39 am So I contacted freescale about the AFT05MP075N 70W mosfet and whether they had any broadband application schematics. There is one for the 400-500mhz band in the datasheet, but that's not very helpful for FM broadcast as the frequencies are so different.

However, they responded and provided me with a schematic for 136-174mhz broadband match which I think could probably be altered for FM. It may not even need to be because often these ranges are arbitrary and the amps can go much lower.

Anyway, I thought I'd attach it here if anyone wants to have a look or suggest what components might need to be slightly altered.
To make that circuit work at FM, try increasing the capacitor values first. You could add trimmers over the top of the original capacitors so you're able to tune for best peformance. I also see they have a coilcraft inductor on the output which you should replace with a larger value.
Start with the output first as it will be more critical.

One word of warning though, try and find out which PCB material they are using for the reference circuit before building it. Often manufacturers design circuits using low-loss materials to get the best performance and therefore make the products look good. If, for example, you build it using standard FR4 and the manufacturer has specc'd it using Rogers or Taconic high frequency laminate then all the microstrips won't work and you'll be on a hiding to nothing.

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Re: High power 12v amplifier

Post by Analyser » Tue Sep 26, 2017 10:49 pm

I've just noticed they're using FR4, so you should be okay.

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Re: High power 12v amplifier

Post by OgreVorbis » Fri Sep 29, 2017 1:36 am

Thanks!

This is a bit tangential, but...
How do you calculate the needed wire gauge for an RF inductor at a given power level? I'm curious because I have some prewound inductors here, but I don't know how many watts they'll handle.

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Re: High power 12v amplifier

Post by BroadcastersWarehouse » Sun Jan 09, 2022 3:19 pm

I found this thread on a Google search for high power 12V amplifiers using this particular FET and noticed the last post above was 4+ years ago! OgreVorbis, Did you ever progress with this project? On a similar theme, I came across this project which I thought was pretty interesting for 3 x HF inexpensive FET's > https://70mhz.org/equipment/amplifiers/ ... CR63h3crSg

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Re: High power 12v amplifier

Post by Albert H » Mon Jan 10, 2022 10:09 am

Low supply voltage really isn't the way to go. We gave it up in favour of Voltage step-up after a couple of supply melt-down events back then. The losses at low voltage soon add up - the impedance conversion up to 50Ω from a couple of ohms on the drains of the FETs is relatively lossy, certainly when compared to the losses incurred in switching the supply voltage up to 50V from a battery voltage supply. There's also a further advantage with going with the step-up supply approach - you can prevent the rail voltage diminishing as the battery discharges.

The supply current for 100 Watts at 48 V is minute compared to 100 W at 12 V. The power cables from the supply to the PA can be much cheaper! Incidentally, we used to use cut-up car battery jumper cables as the supply leads for our battery-powered rigs - Halfords is a relatively cheap source of high current, low cost cable!

Way back in the 70s, we ran 40 and 50 Watt rigs from car batteries, and some of them would suffer from instability at lowered supply voltage, so you'd have to time your broadcasts so that the battery never quite reached that critical point! You really don't want that particular hassle.
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Re: High power 12v amplifier

Post by kevycorsa » Thu Jan 20, 2022 1:51 pm

What about something like this ? 100w 12v around 8a
https://www.pcs-electronics.com/shop/fm ... er-12-15v/

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