Hardrock-50 vs. KXPA100

The following is an attempt to compare and contrast two external linear RF power amplifiers, primarily in the context of using them with an Elecraft KX2 or KX3. The amplifiers are the Elecraft KXPA100 and the Hardrock-50.

This is intended to list objective facts only, without making subjective judgments about which is better quality, better value, or what amp would better match a certain type of operator. Those sorts of subjective value judgements are left to the reader.

There may be errors in the list below, and there are certainly some omissions. Even if the list is correct when first published (and it may not be), the development of either or both products may alter certain points of comparison after this list is produced. Any prospective purchaser is urged to verify all points which are important to him. It is also probably a good idea to try to consult a variety of reviews showing the experiences of various users of each product.


  • Both amplifiers work on HF amateur 160m through 6m bands (except that HR-50 has no support for 60m unless the ATU is installed), with drive power and output power in a similar neighborhood. The drive power of both is well suited to the KX3/KX2. The output power offers a significant increase over the barefoot KX3/KX2.
  • Both support automatic band switching using data from the KX3/KX2.
  • Both have an automatic internal antenna tuner (ATU) available as an extra cost option.
  • Both amplifiers can support other radios.
  • Both are powered by 13.8VDC via Anderson Power Poles.
  • Both have roughly similar size, weight, and shape (KXPA100 is a bit bigger and heavier, see below).
  • Both cases are dominated by a large finned heatsink on the top, with no fan.
  • Both are FCC certified.


  • The KXPA100 has twice the output power, 100W vs 50W.
  • The HR-50 is cheaper than the KXPA100, though the varying options available mean that there are a few possible ways of doing a price comparison. In as near to an “apples to apples” comparison as practical, with both amps in kit form with QSK and with no antenna tuner, the KXPA100 costs twice what the HR-50 costs. That’s $700 for KXPA100, versus $350 for the HR-50. That works out to $7.00 per watt in each case.
    Price Comparison

    HR-50 kit KXPA100 kit KXPA100 Factory built
    Amp with relay switching 300 n/a n/a
    Amp with QSK diode switching 350 700 750
    Amp with relay switching and ATU 480 n/a n/a
    Amp with QSK & ATU 530 1020 1120

    All prices rounded to nearest $10, and do not include sales tax, shipping, or cables to connect a radio. Prices current as of November 2014.

  • The KXPA100 integrates seamlessly with the KX3 or KX2 — you just turn the power knob on the KX3/KX2 down to zero, up to maximum, or anywhere in between, and the user interface behaves as though the KX3/KX2’s internal circuitry had the ability to be turned up to 100 watts of output power, while the firmware of the KX3/KX2 and the amp cooperate to figure out how to deliver the requested output power. The amp will be bypassed and the KX3/KX2 will provide all the power when very low output is requested. With the HR-50, you use the KX3/KX2 power control to control the drive of the amp, and then you can read the output power using the amp’s display while transmitting (the amp itself has no gain control). If you want to bypass the amp, you push a button on the amp. Other features of the integration between the KXPA100 and KX3/KX2 are that turning on the power of the KX3/KX2 can turn on the amplifier, the automatic antenna tuner can be activated from the KX3/KX2’s controls, and faults, PA temperature, and performance information from the amplifier can be displayed on the KX3/KX2’s display. In normal operation with the KX3/KX2, using the right cables, the operator does not need to touch or see the amplifier. All functions, including switching power off and on, can be controlled through the KX3/KX2. It’s as though you had a single 100 watt radio, just split into two boxes. Related to the integration topic, the KXPA100 and KX3/KX2 support bidirectional communications with each other via their serial data link. The amp sends data to the radio, and the radio sends data to the amp. The HR-50 has a data link that goes one direction only. The KX3/KX2 can tell the amp what frequency is being used, to help the amp select the band filters and ATU settings, but the HR-50 amp does not send any data back to the KX3/KX2. With either amplifier, you can connect a computer to the serial line to talk to the KX3/KX2 while using the amp, to support logging software, rig control, or any other thing you might use the KX3/KX2’s serial port for.
  • The KXPA100 has a built-in frequency counter that can sense the band being used for transmit, and can select the proper bandpass filter automatically with any tranceiver, as soon as it begins transmitting. With the HR-50, one must either select the band manually, or with certain supported trancievers (including KX3/KX2 and Yaesu FT-817), one may use a data cable to have the radio send data to the amplifier to automatically select the proper band.
  • The HR-50 has a “Carrier Operated Relay” (COR) mode, where it switches to transmit mode as soon as it detects RF power on the input, and then switches back to receive mode shortly (menu configurable delay) after the applied RF is no longer sensed. This allows interfacing with a tranceiver that does not provide a PTT signal to key the amp. The only absolutely required interface cable between a radio and an HR-50 is a coax to carry the RF. The HR-50 also can be placed in a mode where it will use a conventional PTT signal to switch between transmit and receive instead of the COR. On the other hand, the KXPA100 always requires at least a PTT signal, and requires a cable to carry that signal in addition to the RF.
  • The HR-50’s ATU also includes a bandpass filter for the 60m band. The HR-50 amp without the ATU does not support 60m. The KXPA100 always supports 60m, with or without ATU.
  • The HR-50 has somewhat reduced power on the 6m band compared with its power on the other bands, especially when equipped with the QSK option.
  • The HR-50 can eliminate the QSK option, substituting a slower mechanical transmit/receive relay, for a $50 price savings, but the KXPA100 comes with QSK standard.
  • The HR-50 assembly involves significant soldering and toroid winding, while the KXPA100 is a bolt-together kit that does not require any toroid winding, and does not require a soldering iron at all. The soldering for the HR-50 is all through-hole; the boards come with surface mount devices pre-installed and soldered. It’s difficult to make precise assembly time estimates, but it’s safe to say that a typical ham will probably be able to assemble a few or several KXPA100 amplifiers in the same time it takes him to assemble one HR-50. Those who want to know more details about the assembly should be aware that the assembly instructions for each amp are available on the websites of each manufacturer. Those instructions also provide some photos and insight into the makeup of the amps.
  • The KXPA100 is available fully assembled for an extra $50 (or an extra $100 for amp plus tuner). I have not been able to find a commercial offering of the HR-50 pre-assembled.
  • The KXPA100 has LED bar graph displays while HR-50 has a lighted LCD alphanumeric text display.
  • The KXPA100 and HR-50 have different appearances. The KXPA100 has a black finish, while HR-50 comes in raw aluminum finish.
  • The KXPA100 is a little less than twice the volume of the HR-50. The KXPA100 is 10″ x 5.125″ x 4.25″ versus 7.5″ x 4.25″ x 3.5″ for the HR-50. If you do the math, that’s about 218 cubic inches for the KXPA100, vs 112 cubic inches for the HR-50. In both cases, we’re relying on the manufacturers’ published dimensions, and it’s not clear whether that includes all connectors and controls.
  • The KXPA100 is around twice the weight of the HR-50. The KXPA100 is around 5 to 7 lbs, vs 3 lbs for the HR-50. This comparison is also based on the manufacturers’ published data, and it’s not clear whether this would vary significantly based on optional features that might be installed (QSK module, antenna tuners). The KXPA100’s weight is listed as 7 lbs with the ATU installed, according to Elecraft’s product description¬†http://www.elecraft.com/KXPA100/kxpa100.htm. However, the owner’s manual claims 5lb 5oz on the specifications list on page 38.
  • The KXPA100 has two output antenna jacks (when equipped with optional ATU), controlled by KX3’s ANT switch.
  • The HR-50 has an available extra-cost add-on for a preamplifier that allows it to be driven to full power with as little as 0.5W drive. Note: this would cause its total gain to exceed part 97.317(2) limits for commercial certification. Such a modification can be performed by a licensed amateur for his own personal use, but cannot be certified for commercial sale or import. This modification would be of little use with a KX3, since the KX3 is already more than capable of fully driving the HR-50, but it might be useful in combination with lower powered radios. One final note: the preamplifier is designed to go into the same spot inside the case as the antenna tuner, so an amp cannot have both an antenna tuner and an internal preamp simultaneously.
  • The KXPA100 has a factory-made cable available which connects the amp to the KX3, providing the keying and data connections to support fully integrated operation. This connecting cable is an extra cost option (approx $40) for the KXPA100. The HR-50 requires that the user build a simple interface cable if he wants to support the keying and data interfaces.
  • The HR-50 firmware is open source and may be modified by the end user. The KXPA100’s firmware is closed source, commercially developed by Elecraft.
  • The two units are purchased from completely different organizations, with completely different support networks. Elecraft is bigger.


In many ways, the HR-50 might be considered to be approximately half a KXPA100. It’s about half the power, half the weight, half the volume, and half the price (depending on options). The KXPA100 is more seamlessly integrated with the KX3. There is a significant difference in the amount and type of labor and skills required to build the two different amps.


More information on the KXPA100 is available on Elecraft’s website,¬†www.elecraft.com.

More information on the Hardrock-50 is available on the HobbyPCB website, hobbypcb.com.