Ampleon has launched the ART2K5TPU LDMOS RF power transistor, offering a 2.5kW output and 1-400MHz frequency range, outperforming its predecessor with a higher supply voltage and better thermal performance due to lower thermal resistance. It includes dual integrated temperature sensors for improved protection, housed in Ampleon's OMP-1230-6F-2 package. Availability and pricing details are pending.
Discover the Enhanced Experience: Icom IC-7300 Firmware v1.42 Unveils Upgrades for Improved Control and Functionality The Icom IC-7300 recently received a firmware update to version 1.42, bringing a host of exciting changes. This update, available initially on the Icom Japan website, (use google translate for english) introduces several enhancements and features poised to revolutionize user […]
In April 2012 I reported on this site that Acom had announced a new amplifier. At the time, I had an Acom 1000 and in the comments, I said “I don’t think I’ll be swapping my Acom 1000 for an Acom 1500 any time soon!”. Technically I was correct, it wasn’t any time soon. In […]
Figure 1: The amplifier - with heat sink. Click on the image for a larger version.
On EvilBay, you can find a number of sellers of a device described as:
"1-930MHz 2W RF Broadband Power Amplifier Module for FM Radio HF VHF Transmission".
This unit has SMA connectors for both input and output and is constructed on a circuit board and heat sink that measures just a bit under 2" square (50mm x 50mm).
Asis so often the case with these sorts of things, the sellers likely have no idea what this actually is - and their listings are often sparse on details other than general operating parameters.
In the case of the device depicted in Figure 1, the parameters given in the listing are:
Type: RF Amplifier Module: RF Broadband Power Amplifier Module Dimensions: 50*50*15mm (L*W*H) Working voltage: 12V (DC) Frequency: 1-930MHz Working current: 300--400mA (determined by output power) Type 1: 1-930MHz 2W Working frequency: 1-930MHz
There are a few "tells" here that this data was simply copied from some source - notably that line beginning with "Type 1" which probably means something only to the original supplier, in the original Chinese - but likely means nothing at all to anyone else. Unfortunately, you are unlikely to get more information that this from an EvilBay listing and this hardly counts as "detailed technical information" about exactly how it works by virtue of describing its design in detail.
I do not have one of these (yet?) but based on the above web page, it looks to be pretty much identical electrically aside from the output transistor being what appears to be an older variant of the one described below.
* * *
What is it, really?
From the picture in Figure 1, it's apparent that there are two active devices, but what are these - and will identifying these devices give a clue as to how one might really want to use one of them?
With a bit of magnification and Google-Foo, I was able to determine the nature of both of the active devices - and reverse-engineer a schematic diagram, below in Figure 2.
Figure 2: Reverse-engineered schematic diagram and component layout of the amplifier. The component designations are arbitrary as they are not marked on the PC board so a layout is provided. Click on the image for a larger version.
This amplifier is about as simple as it gets: A broadband MMIC with approximately 20 dB of gain is coupled into the gate of a VHF/UHF N-channel MOSFET amplifier - which itself has 10-15 dB of gain - with no matching.
What this means is that it will take just a few milliwatts input to obtain about a watt of RF output across the intended frequency range - the precise amount of drive depending on the frequency, the supply voltage, and the desired output power.
A 5 volt regulator (U2) provides about 1.68 volts of gate bias on Q1 while supplying U1 with a stable 5 volt supply (at about 90 milliamps).
With no drive, the total current consumption is likely to be in the
area of 130-150mA, but it could exceed 500mA at higher operating
voltages and saturated power output levels.
Inspecting this data sheet we can see that its rated for operation from 50 to 850 MHz - although these types of devices typically have no problems operating at much lower frequencies (even down to DC) - and they can typically operate at quite a bit higher frequency than the specification, albeit with a bit of roll-off in gain and output power capability meaning that this stage of the amplifier should have no problem operating up to its 930 MHz stated range - or even higher.
Right away you'll spot a bit of disparity between the EvilBay listing and the manufacturer's specifications - the former stating 2 watts at 12 volts. Taking a close look at the specifications in the data sheet we can see that we should easily (and safely) be able to get about a watt out over the range of at least 100 to 930 MHz (and likely down to a few MHz) with a drain voltage of 7.2 volts on this device - perhaps a bit more or less, as there is no attempt at impedance matching on the output of this amplifier.
Looking further at the specifications, you might also note that the maximum drain-source voltage of this transistor is 25 volts: If it is operated at 12 volts into a highly reactive load, it could be expected that the peak voltage could reach or exceed twice the supply voltage - and this does not take into account that the drain current, which is specified as an absolute maximum of 600mA - could also be exceeded.
What we can conclude from this is that operating at 12 volts or greater - particularly under conditions where the load to which the amplifier is connected might be mismatched (e.g. high VSWR) is probablynot a good idea!
The device overall:
It should also not escape the attention of the reader the comment on the schematic relating to inductors L3 and L4 on the drain of Q1: Together, these have a DC resistance of a bit more than an ohm. With an expected drain current of 300-400mA in normal operation, one can expect at least a half-volt of drop across these two components which actually can work to our advantage in reducing the power supply voltage a bit.
Finally, looking closely at the data sheet you'll note that there are graphs that show operation to 10 volts drain current (or about 11 volts supply voltage, considering the drop of L3 and L4) that show outputs exceeding 2 watts. If you feel that you really need more than 1 watt - or wish to have a bit of extra headroom for 1 watt operation (e.g. to preserve linearity) then operating at that voltage (10-10.5 volts) may be possible with the caution that you may be sacrificing reliability.
Considerations of linearity and stability:
This amplifier will generate significant harmonics, so it should never be connected directly to an antenna without appropriate filtering! At a power output of 1 watt, if its second harmonic is 20-25 dB down (a reasonable value) that will represent several milliwatts of power on its harmonics which can easily carry for several miles/kilometers line-of-sight.
Particularly when this amplifier is operated from a supply of greater than 8 volts, care should be taken that the output is resistive (nominally 50 ohms). Now this may sound pretty easy as antennas and filters are nominally 50 ohms, but one should consider frequencies other than that on which the amplifier may be operating: Being an inexpensive device from EvilBay, it's hard to be sure of the quality of the components that one would use to make it operate in a stable manner (capacitors, board layout, inductors) - and since this amplifier has a rather high gain of around 30dB, it may not be unconditionally stable.
Take, for example, this amplifier being used to boost the output of an exciter on the amateur 6 meter band - around 50 MHz. We should assure that at 50 MHz that the load (low-pass filter plus antenna) provides a reasonable match to 50 ohms. What is not easily knowable with this sort of device is how it will behave at other frequencies: If you move below 50 MHz, the match (SWR) will get terrible because the antenna is out of its design range - and if you move above 50 MHz, the SWR will also be terrible not just because of the antenna, but because the low-pass filter itself will start to reflect energy. Again - in this example - the amplifier will see a match only at the antenna's design frequency - but it will be terrible everywhere else.
While an ideal amplifier wouldn't really care about off-frequency mismatches, a poorly-designed amplifier - or one that may be well-designed, but has not been designed to be intrinsically stable under all load conditions - might be prone to oscillation at some unknown frequency if it is connected to a load that presents to it just the right conditions that its built-in instability may cause oscillation.
This last point - the possibility of an amplifier oscillating at a frequency other than at where it is intended to operate - can be difficult to diagnose: Worst case, this will cause the amplifier to die randomly and in the best case, it will output power that is lower (or randomly varying) than expected and - possibly - have spurious outputs related to the mixing of the desired frequency and that at which it is oscillating. If the frequency at which it is operating capriciously is above that of the low-pass filter, you may not even be able to detect that it is behaving in an untoward manner unless you were to do a broadband analysis by probing the amplifier's output directly - a process that could, itself, change the results!
This sounds like a lot of conjecture, hassle and trouble - and sometimes it is - but there are a few things that one can do to make the device work more reliably and also detect that something may be amiss.
Do not operate it at a higher voltage than needed to obtain the desired output power. In the case of this device, 1 watt is about all you should reasonably expect in terms of long-term reliability. Period.
If, under certain conditions, you see the power output randomly fluctuating - but the input drive and power supply voltage is constant - you likely have a spurious oscillation occurring within the amplifier. A redesign of the filtering to change the off-frequency characteristics (e.g. the impedance well above the cut-off frequency of a low-pass filter, for example) may improve things: Consider the use of a diplexer-type circuit with the "other" port (e.g. that which passes frequencies other than the desired) terminated.
A reasonable question would be: "If I blow up Q1, where can I get another RD01MUS2B transistor to replace it?" The answer - albeit a bit glib - is to simply buy another of these amplifier modules: Unless you buy a lot of them at once, it will probably be cheaper to get another amplifier than just that transistor!
Like many, I enjoy the recorded sounds of — and feel inspired by the Gibson GA-50. That warm tone, full mid range and brown suitcase look epitomizes an American jazz guitar amp classic. What do I like about the GA-50? Its simplicity, the non-mega-scooped mid range, and of course, the warm, thick low and lower middle tones. Feeling inspired by these attributes, I embarked on a design journey to make a GA-50 inspired preamplifier to go with my simple experimenters PA and its offspring such as this PA.
I used JFETs & op-amps instead of octal tubes. Perhaps, now, even op-amps are getting outdated as digital algorithms simulating some old amp of lore push the bleeding edge of design. I’m not a fan of using tubes in jazz guitar amplifiers – give me solid state any day for clean signals. Perhaps 1 day, I'll possess the skills for digital amp design. I’ve studied hundreds of solid-state, analog designs and it seems that many just copy someone else's solid-state guitar amp design. Most of the ingenuity in solid-state design seems to go into the distortion circuitry. I found a patent by someone who made a so called 'tube sounding' clipping circuit by putting zener diodes in the feedback loop of an op-amp. Really?!? They were doing this in the late 1960s albeit for other reasons such as op-amp voltage limiters or zero crossing detectors.
It's more than the 'lack of tubes' that make some solid-state amplifiers sound poor. Design elements such as putting electrolytic caps in the signal path, along with brash, tinny-toned, picofarad coupling capacitors. Add in toxic sounding distortion — and not to mention the grinding noise from mega-high, tube-era, resistor values, and sometimes poor gain distribution that often compromises the noise performance and/or headroom.
The classic, passive Fender, Marshall et al. mid scooping bass/middle/treble tone stacks looms prevalent in solid state guitar amps. You will find them in countless guitar amps from as many different companies. While they do sound good in many designs, they depart from the desired GA-50 tonality and boy they exhibit loss.
My whole adventure focused on the study and testing of tone circuitry. I will blog more about that in future posts, however, as much as I wanted to keep the passive tone circuit of the original tube GA-50, it performed with lackluster results in my solid-state versions. Yes, these simple tone controls work, but such passive tone circuits seemingly lack versatility for getting a consistently warm tone with different guitars, speakers and speaker boxes.
That left active tone control circuitry and about 3 things to play with: [1] shelving bass or treble circuitry (basically these are variable low-pass or high-pass tone controls respectively) [2] peaking/resonant circuits [3] choosing a Q for my tone control circuits that gives the natural sound you hear in amps like the GA-50.
I spent ~6 weeks on the bench & computer working on my tone circuits along with the basic gain stages. I also studied non-linear design including distortion circuitry, switching, line-out circuits and speaker box emulation circuits for DI purposes. I filled 2 notebooks and learned much.
I burnt thru 3 soldering iron tips and tons of parts on my tone quest. I’ll start by showing you my latest design as of Dec 11, 2021. I strove to keep my signal path resistor values down in value to reduce Johnson or thermal noise arising from these resistors.
Above — The complete preamplifier schematic. Click on the image for a better view since it's 1121 X 821 pixels with lossless compression. This is actually version 7 & sounds sweeter, plus uses less parts than the previous 6 versions. This design was built around the glorious 5532 op-amp. I measured no instability.
Buffers. Isolation buffers lurk everywhere !
In your lab, you're likely not making a cost-conscious design for the production line. Therefore, use as many op-amp buffers as you like to enhance stage isolation and to prevent the loading of your tone circuit at extreme settings of the potentiometer wipers. I love buffers and isolation — almost to a fault. Go ahead and remove some of these voltage follower buffers if you wish.
The input 10K resistor adds noise at a low level spot in the preamp. However, a series input resistor follows normal practices. My listening experiments yielded my personal preference for this resistor to lie between 10 and 12K ohms. That resistor and the 150 pF shunt cap are wired right on the input jack provide RF filtration in a critical spot.
The 47 nF input cap comes right from the GA-50 which uses a 50 nF cap. This capacitor value works perfectly as a high pass filter pole to attenuate the low frequency rumbling noises emitted from an arch top guitar. Further, it simplifies the design by allows readily available for a reasonable price 1 µF signal chain caps for DC blocking in a clean signal preamplifier design. My 1 µF caps = Panasonic metalized film polypropylene jobs — chosen for their low distortion.
Active volume and master volume controls help with noise and headroom management. In many solid state guitar amps you'll see a stage of massive voltage gain immediately followed by passive attenuation via a 100K or so volume potentiometer that's shunt to ground. That voltage gain stage runs at its maximum gain (and noise) all the time. Why boost — then right away attenuate the signal if you don't have to ? I tried to copy the GA-50 and just provide 1 volume control, however, found too much compromise in headroom and noise performance. Distributing the gain stages and adding an active master volume control improves the preamplifier and 1 extra volume pot on the front panel does not seem too onerous.
Tone Circuitry
I tried many tone control circuits for bass, treble, lower mid and high mid range. I made variable Q parametric stages; EQ style stages with an op-amp gyrator to simulate the inductor at Qs ranging from 0.7 to 5.1; Baxandall circuits with shelving and/or peaking; and also Wien bridge type treble and mid range circuits. Some of these Wien circuits were fixed while others went variable frequency. I also explored some passive circuitry like those found in Fender, Marshall, Hiwatt, Pignose .....etc. amplifiers as well as circuits from old hi-fi amps. I even copied an old Hughes and Ketner design that plys passive middle and treble controls, then an active, op-amp bass stage that worked pretty well.
After listening to these circuits over many weeks I came to a few conclusions in the context of a GA-50 inspired amplifier:
I disliked EQ style tone controls. If the Q was 0.7 to 1, they seemed a bit more tolerable
In general, circuits with a Q > than about 1.5 tend to sound unnatural and may trigger listener fatigue
I'm not a big fan of deeply scooped midrange
Shelving bass and treble sounded better than peaking bass and treble, however, variable frequency shelving seems quite desirable for versatility
Combining shelving low and high; plus peaking or resonant type mid-range controls is common in mixer boards and other professional gear and sounds OK. Again, when the Q is <= 1.5, the peaking/resonant mid range tone circuit seems more natural sounding to me
Based on my conclusions above, you now know why I went with the variable frequency shelving low and high frequency controls that forms the heart of my preamplifier. I struggled with potentiometer interdependence in early shelving circuits. When running both a high and low shelf, the low boost/cut pot may affect the high shelf and visa versa. Further, when the shelf frequency pot is rotated to lower resistance to get a higher frequency, distortion, weird behaviour, and/or noises might creep in a some settings of the boost/cut pots. Hence voltage follower isolation amps help make all that pain go away — and provides reasonable potentiometer separation. My chosen circuit still needs work.
Having 2 low and high frequency pots allows you to find a sweet spot for each respective shelf with respect to the room, guitar and whatever speaker your running at the moment.
Ultimately, I opted to not put in a mid-range peaking/resonant tone control for my GA-50 inspired amp. That just seems wrong. On the other hand, the 2 shelving frequency controls allows some alteration of the middle tones which boost versatility. Shelving networks exhibit a Q of less than 1 and subtly, gently alter your guitar's tone. I'll blog more about my tone controls circuit experiments in the future — I've got lots of interesting material to show you.
Choosing Time Constants
In many low + high frequency shelving circuits, the designer chooses the same capacitor value but up a decade for the low frequency network. For example, .0082 + .082 µF. This allows a bit of overlap between the 2 shelves. I opted to run my high frequency network a little higher and get some calculated response slightly above the 10 KHz zone with the 20K high-pass potentiometer set to 0 Ω.
The standard calculation applies. Frequency = 1 / (2 pi * R * C ). So, for the low-pass network with the 20K pot set to maximum resistance: Freq = 1/ (6.28 * 22200 Ω * .082 E-6) = 1/.011432 = 87.4 Hertz. If you seek mega bass, try swapping in a 100 nF capacitor instead.
Above — An early version of my preamplifier using nJFETs as amplifiers. I found IMD and clipping on the output of the Q3 source follower. I later replaced it with an op-amp buffer, and perhaps another after the volume control before abandoning JFETs altogether. The low frequency shelf circuit is by Douglas Self in his book Small Signal Audio Design. I've got 4 of his lovely books and Mr. Self is my favourite audio design book author. I noticed that in his latest version of Small Signal Audio Design, he's added a chapter on guitar amplifiers. Yay!
This schematic includes my favourite simple passive treble network. The 20K pot is used to variably shunt high frequency to ground. I also added a "fatten" switch to roll off less low frequency for use in 10 inch and 8 inch speaker applications. I would normally switch this capacitor in or out using a front panel switch that remotely controls some DC voltage to a JFET switch using the J111 or PF5102.
Above — A whimsical idea that I tested and discarded when I tried to keep the original passive GA-50 tone circuit topology in my preamplifier. When I don't like the result of a circuit I write FAIL on it so that I never make it again.
Above —A schematic excerpt from the solid-state Gibson G-35 guitar amp. This amplifier kept the basic topology of the GA-50 tone circuit. By that time, most of their solid state amps had abandoned this style of tone control circuit and were running a passive Baxandall tone stack instead.
Modules
At first, I built my preamps on small boards that I lifted in and out of my experimental chassis to work on. I did not bolt them in — for they were just held in place by the DC voltage, ground and wires going to and from the various potentiometers. This worked OK, but soldering and unsoldering all those wires became tedious. Later, I moved to modules that sat in front of the chassis that contained integral pots. I only had to solder or unsolder just 3 wires: B+, B- and the ground wire. Much easier.
Above — 2 identical modules that contain all the circuitry from input to U1b pin 7 of the inverting amp in my Dec 11 schematic. The circuits were tested with a signal generator + DSO. Then I began installing the low frequency shelving network boost/cut potentiometer as shown in the closer or proximal module.
Above — The completed Dec 11, 2021 module with the DC voltage and ground wires attached. An alligator clipped lead connects the output to the PA input. A Boss digital reverb permanently lives on my bench for testing circuits with reverb. I'm addicted to plate reverb. The speaker cable runs just to the left of the reverb pedal. I've currently got five 8 Ω speakers in my lab for amp testing.
Above — A closer photo of the final module. The master volume pot lies on the extreme right hand side. I chose a 2.2 µF output cap since it lies in series with a 1 µF capacitor that's soldered to the PA input. In my final, proper build, I would just likely use a single 1 µF capacitor. My 2.2 µF polypropylene caps are huge 630 volt jobs. I've got a few 100 volt caps @ 2.2 µF, but save these for final builds. Brand name capacitors are not cheap.
Speakers
No doubt
some of the sonic signature tones from the GA-50 come from the Alnico magnet 12-
and 8-inch speaker pair. I’ve learned that speakers and perhaps even more
importantly, the box they’re mounted in prove crucial to realizing your desired
guitar tone. I’ve tried 2 speaker pairings: 12 +
8 inch, 12 + 10 inch and 8 + 8 inch. The wide sound field somehow disturbed me.
The only combination I seemed to tolerate was the 8-inch pair, however, they need
to go in a box designed to boost the bass response for my taste. Thus, I just
use one 8-ohm speaker with this preamp + my PA.
With my
preamplifier, if you drive a speaker mounted in a cabinet designed to give bass
extension, the available low-end response might amaze you. This is definitely a warm, low-end focused amp – lacking the brash high-mid & raunchy treble
response associated with some “transistor amps”. However, the high end is still
there. Sometimes it’s pleasant to put a bit of shimmering > 8 KHz treble
into your tone.
Above — An early breadboard using Ugly Construction. The power supply is a simple interface to my bench +/-15 volts regulated, 5 Watt power supply. Islands are carved out in the circuit board for temporary connection to the power supply when developing circuits prior to putting them in the guitar amp. I measured DC voltages, DC current, AC peak-peak voltages and usually hook up a signal generator and look at the FFT also. I use the grounding techniques shown on this web page so despite all this wire, I hear no hum unless I use a single coil pickup.
Future Experiments
Above — Future things to add to my GA-50 inspired amplifier. Thanks to the books of Douglass Self and from studying many schematics, I've got confidence to work out ways to record my amp without speakers + a microphone and/or add software-based digital effects. The features found in many newer amps such as integral DI patching, speaker cabinet emulation and ground loop noise isolation circuitry will help take the GA-50 inspiration into new realms. I feel that if Gibson's Seth Lover and Ted McCarty were alive today they would embrace the latest technology including digital signal processing and of course, op-amps.
I'll continue to work on my basic GA-50 inspired guitar amp from time to time. I've got parts coming to make a small, complete low-power amp Dec 11 version for practice. I've already tweaked the improved Polytone inspired PA from my last posting to lower its noise floor & distortion a little.
I only wish I had more time to spend on the bench.
For optimal performance, the heatsink should have a thermal resistance of 0.1C/W or better at full fan speed. For the prototype I used ABL Components 180AB1500B, it is available in many countries from RS Components. However, the thin anodised black layer had to be removed for better thermal contact transfer (although it improves heat transfer […]
I have recently made available a new version on the A600 amplifier based on MRF300 LDMOS transistors. Here is the list of changes and improvements that come with v2.0. 1. Redesigned and improved PCB All components are now on the top side, while the bottom side is a continuous ground plane in direct contact with […]
Update: this project is evolving. See this page for the latest version. Kits are available in the shop area. The announcement of the MRF300 and MRF101 transistors by NXP in 2018 has generated quite a spark of interest in the amateur radio community and as soon as I learned about them I wanted to get some […]
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