RM Italy HLA 300 linear HF amplifier, schematic HLA 300 (PDF).


All the technical details of this amplifier can easily be obtained from the internet. In this survey some practical experiences were written down, that might become handy in case you're contemplating purchasing such an amplifier.

Attention, there are two versions of this amplifier with the type-id HLA 300 on the front-cover of the amp.; with or without air-ventilators. If the HLA 300 V is mentioned, we are talking about the version with the ventilators. In my opinion this cooling is a necessity and operates very well, so it would be a wise decision to choose the latter. On stand-by the vents operate allmost noiseless, and on transmit they immediately will kick-in.

Transmitting power according to the manufacturer (RM Italy www.rmitaly.com):

CW: 300 W output at 1 - 15 W input,

SSB: 550 W output at 2 - 20 W input.

It is expected that somewhere in the first quarter of 2009 RM Italy will launch a more sturdy type of amplifier into the market (BLA 600 or 700: Basic Linear Amplifier 600 or 700 Watt). Daniël, PH4U, has been told so by a RM Italy spokesman.

Daniel, did test on my request, this HLA 300 transistor amplifier, an all-band/all-mode type which runs at 12 - 14 V/40 A. His practical test-results, in my opinion, do seem of importance to others too.



In contrast with many similar products, which are lacking any exit filtering (low pass filters), this amplifier has 6 low pass filters installed, covering 9 HF bands.




Amplifier: HLA 300 (4 × SD1446, forced cooling by triple ventilators); power consumption is about 550 Watt.

4 × SD1446.

Power supply:


Low Pass filter:

SWR/PWR meter:

Dummy load:


SSB mode:

SPS-9600, 13.8 V (switching PS, 0.8-15.4 V/60 A).

Kenwood TS-570D with its own power supplies.

Kenwood LF-30A.

Daiwa CN-101L, 1.8-150 MHz 15/150/1500 Watt.

MFJ-264, 1500 W for 10 seconds, dries load type.

Each cables about 3 ft Aircell 7 with PL connectors.

Speech at a normal level.

A speech-processor wasn't used, and is not advised because of the bandwidth of the amplified signal. Daniël commonly doesn't exceed 20 Watt of input power in SSB.




K6AER also reviewed in detail a HLA 300 and his power output test is showed in the middle table. Please note that this power supply voltage is higher. A higher voltage as well as the diameter of the supply cable can make quite a difference in voltage and/or amperage.

Meanwhile PH4U was able and willing to lend me his HLA 300, to tinker around with. At that moment his SPS 9600 power supply had some troubles. For the time being I had to use a car-battery of 12 V/70 Ah, which produced, fully charched and under load of the amplifier, exactly 12 V. The HLA 300 was driven at 10 W, first with its internal attenuator on, later without the attenuator. Then the battery voltage was taken-up to 13 V under load. On average the output power did increase with about 50 W (see fig.) At a later point in time the SPS 9600 functioned adequate and was set for 13.8 V., and the test was done again, this time with 5 W and 10 W drive. The bottom lines of the measurements show the results.

A 1-Volt drop can easily occur with these high currents (here 45 - 55 Amps) when (to) long or (to) thin leads are used.


The average performance increased with the deleted 100 pF capacitor.

With 5 W drives the unchanged (except a deleted 100 pf cap) amplifier gave the best results.



Almost immediately after publishing the Dutch version of this article, questions from abroad arose, such as: is there a way to drive this linear to its full potential with an input of let's say 5 Watt (FT-817)? Because I don't own a HLA 300, it's hard to predict.

For a better overview the two positions of relays RL1 are drawn (fig»), including the converted resistors. For a drive of 5 W, one can replace all the original resistors, but putting-in parallel resistors instead is much easier to experiment and getting things back in original condition again. The capacitor of 100 pF wasn't drawn in the schematic, however it was mounted in the amp. While testing it showed that the 100 pF capacitor in this specific amplifier better could be taken out to improve performance.




Because the equipment (PA + PS) was lent to me, I could experiment further on it. The attenuator was changed several times, to make lower drive input (5 W) possible, but the SWR that comes along with it changed to much with even bad to worse readings; once rectified with other resistors, power dropped even beyond the point of the original attenuator. With 5 W drive the best output I have seen, I did achieve with the unchanged attenuator.

Conclusion: 5 W are too low to drive this HLA 300 to its full potential. The only solution is to use an extra amplifier stage, which can lift the FT-817's power level up to 20 W.




The HLA 300 is neatly of construction, has adequate internal safeguards which will warn you by a buzzer, and what is more important: the amplifier is able to withstand excessive drive-power for a short period (a couple of seconds) of time.

This amplifier has two pairs of transistors working in balance and both pairs are coupled with splitters.

How harmful this is or could be for the equipment, I don't know, but after a few minutes transmission with a FM or CW carrier, the ferrite-tubes (fig») of both output transformers got warm, up to hot, and you could burn your fingers on them. Remarkably, at both transformers, the right tube gets the warmest. On that side the five windings are connected to earth on one side only. So the exit per stage is not really symmetrical and could explain the difference.

If it were my own homebrew PA, I would have changed the tubes for a bigger size.

The SWR on the entrance of this tested device could still be slightly improved by adding an extra capacitor parallel to C50 (620 pF) near T4 and one to C55 (620 pF) near T5.

Looking at the transmission-power, the difference between 10W or 20W input is of null and void, and quite soon the point of compression is reached. An efficient operation can be obtained by using a clean input-signal in combination with a switched-on speech-processor and limit the input-signal to 10 Watt.

At 10 W drive the current at one band was about 45 A, on the other bands lower.