When it comes to audio, in my opinion, quite often simple is good, sometimes best. It's very easy to overcomplicate designs and introduce additional problems, only having to add additional parts to ameliorate basic flaws.
A perfect amplifier, as someone once said, is a piece of straight wire with gain. While there are plenty of line level ICs such as opamps capable of vanishingly small levels of measurable distortion, achieving the same with much higher levels of current and voltage is much more taxing.
As has been said, running in Class A gives a head start in terms of low distortion, as it avoids the problem of
crossover distortion in Class B amplifiers, where the amplifier switches fully from sourcing to sinking current, or vice versa. To lessen the problem, Class B amplifiers are often offset biased to become a Class AB, so that the amplifier operates effectively in Class A for a fraction of its rated power. This is very effective at reducing crossover distortion, but as Class AB amplifiers tend to be designed for much higher powers (and therefore, typically run on higher rail voltages), the bias amount tends to be fairly small to keep consumption down.
For the sake of a single clean audio watt, I believe that (with mains power at least!) Class A is probably the best way of achieving the goal... as an added benefit, Class A circuitry tends to be simpler which means from my perspective, less componentry to sully the sound. An excellent reference on the subject of Class A amplifiers is the
Class A amplifier site, run by Geoff Moss. The site focuses on variations of an amplifier developed by John Linsley Hood, or JLH as he is often referred to.
JLH came up with quite a brilliant little Class A design which was the subject of a
Wireless World article back in 1969, and is still a reference today. It uses just four transistors, one input level PNP, a mid powered NPN for phase splitting and two beefy NPN power transistors for the output. As was common for the time, this is a single rail non-complementary design which means you can really go to town on just one rail of power supply and not worry about how complementary your output pair really is.
One of the reasons why the design needs so few active parts and can run single rail is due to the simplistic biasing and that both the input and output are AC coupled (so coupling capacitors in the signal path). Some audio enthusiasts run a mile at the thought of capacitors in the signal path, but I am not one of them. Providing the cap is of high quality, I'll happily take one over a far less linear active device which will leave a much bigger sonic imprint (to my ears, anyway). They also provide a degree of safety over that inevitable time when then the bias "wanders" and there is an unpleasant amount of DC going where it is not desired...
I won't go into the circuit operation in detail as far wiser heads than me have debated it in great detail over the years, but it can be read as a simple three stage amplifier. While I've grown to have a fondness for FETs over the past few years, the sound of a JLH amplifier has always stuck with me, so thought it was an excellent bipolar-based design to start with.
As someone who's designed a few bits of audio kit in their time (and
still do, when time permits!), it feels a little lazy to use someone elses' PCB, but you have to value your time when it comes to these things... there is often little point in reinventing the wheel, particularly when going after a fairly faithful recreation.
I decided to start with the first iteration of the JLH design, as this is the simplest (and you could argue, purest) form of a bipolar Class A that you could wish for. I found a seller on eBay that does what appeared to be authentic looking 1969-design PCBs with the added bonus of supporting more modern component pinouts if you wanted to try different parts.
On the whole, I was keen to stick close to the original transistors, with the exception of the output transistors... by all accounts, the OnSemi
MJE15003 are considerably superior to the originals in this design and so could be used without hesitation. TO-3 packages are a pain to mount compared to the more modern TO-247/TO-3P as they usually need an angle bracket when using with a PCB... this is then thermally coupled to the primary heatsink.
For extra security against misalignment, I put PTFE sleeving around the TO-3 pins... this ensures even if the TO-3 packages somehow wiggle their way to making contact with the metal, no shorting should occur. Fancy alumina ceramic shims (about 1.5mm thick or so) were used to thermally couple but electrically isolate the TO-3s from the heatsink, with liberal use of good quality
thermal paste in the sandwich. This naturally gives a bit more lead inductance but reduces stray capacitance.
I chose to keep the original
2N3906 PNP (using a Magnatec part which should be very close as a second source to the original Motorola version) for the input transistor, though for the NPN splitter, I ended up going with a
2N1711 branded part with lovely-looking gold plated leads - the 2N1711 was endorsed by JLH as a superior replacement so feels authentic enough. All transistors were tightly Hfe matched, across both pairs and channels. I would have preferred higher Hfe parts but out of 16 power transistors, the "best" were around 50ish, with a lot of them much lower.
There didn't seem much point to throw exotica at this first attempt, so you won't see any teflon capacitors or tantalum resistors here - the input cap is a salvaged WIMA Polypropylene from an amplifier refresh, and the output capacitors are effectively "no name" Forever-branded units of basic merit. The decoupling capacitors are good quality Rubycons, bought in for the job
I socketed R5 in the picture so that I could tune the output to be half the rail voltage as JLH recommends, but the fixed 100K was so close to half rail already that I took the trimmer pot out and put the 100K back in.
For the sake of getting things up and running quickly, I wanted to skip the regulated supply and try one of the many simple unregulated supplies floating around here of many voltages - unfortunately never quite the *right* voltages, it seems! Commandeering a set of 2x25V 160VA toroids from Antrim, back when Maplin used to sell more interesting componentry, I put the secondaries in parallel, rigged up a simple full wave rectifier and threw a big Elna Cerafine on the output. Hm... a bit high... off-load DC voltage was something like 45V!
This design is meant to run off 27V for 8 ohm loads... more could certainly be tolerated by the parts in question, but that it going to get properly toasty at that, never mind being worried about the health of the some of the parts from a voltage point of view. A bit of thought, and I remembered that there were a couple of 100VA toroid cores spare which I'd intended to wind chokes with... a spool of 30A wire and a patient Anna resulted in two simple chokes of approximately 2mH each, and very low DCR. These were put in series with the rectifier, and kept well away from the mains toroid in use.
As the angle brackets were rather oversized for the job, it was thought that it would function as a basic heatsink for now - after all, it should be only 30W or so per channel.
Both amplifiers were gingerly powered up, and gladly showed signs of sane biasing the first time around, rail being between approximately 32 and 35V. The bias starts off fairly low and stabilises at a considerably higher point, being similar for both channels... the bias appears to be very sensitive indeed to temperature - just grabbing the heatsink with your hand is enough to affect the bias to a significant degree, which does sound like an element of the design that will benefit from slightly more complexity!
Ok... enough of this faffing about, let's get them into the main system and see how they sound. First night impressions were very positive indeed... while I wouldn't call the resulting sound "airy", it certainly came across as more beguiling and of fine definition. I've experimented with many amplifiers over the years but have usually come back to my humble Arcam
Alpha 8Ps... a quite traditional (and relatively complex) Class AB amplifier with a complementary MOSFET output stage - the JLH was certainly bringing something new, though quite hard to define.
Let's see what some measurements show. For sake of brevity, I'm just going to show some normalised 24/96 65536-point FFTs of a 1kHz input... load is a 25W wirewound power resistor of either 8.2 or 15 ohms. This lets you see the harmonic spectrum, and give a great deal more information that any single THD figure will. While these traditional measurements only give, IMO, a small insight into the sound quality of a device, the order and shape of the harmonics can be quite revealing. There have been
alternative tests proposed for quite some years, but this a reasonable starting point.
Let's start with a 15 ohm load first. This was done at considerably less than a watt as the MF+HF units will rarely get anywhere near a full watt... if they do for any length of time, I'll probably have my fingers in my ears, and perhaps the neighbours might want a word...
The first thing that is immediately obvious is that the JLH output is being modulated, probably from mains harmonics... at a relatively low level, but nevertheless impacting on the sound. This is likely to be a combination of the very primitive unregulated supply and the primitive biasing arrangement.
The second thing that struck me is how much (relatively speaking) high order harmonic distortion the Arcam has. While this amplifier will no doubt measure very well in terms of a THD figure, the harmonics of this single test tone are spread across the whole frequency range. Second order harmonic distortion in my experience is fairly benign and is generally overwhelmed by the speaker contribution, so isn't worth worrying about unless of a very high quantity... third order is a bit more concerning, and I'll generally like to see it below -80dB on the reproduction front, and fifth and above odd harmonics preferably below the noise floor.
A high noise floor is evident on the Arcam, possibly down in part due to the high gain that this amplifier offers, considerably higher than that of the JLH - I suspect when the JLH is given a better power supply and better biasing, the noise floor will drop further to go with the lower modulation.
Let's have a look at an 8.2 ohm load now, at close to a watt - this is considerably harder than I'm putting on the amplifier at the moment due to a resistive attenuator network for gain matching, but is interesting for comparison purposes.
Note that with the more difficult (albeit still only mostly resistive) load, the modulation on the JLH output has dropped considerably. The JLH second harmonic is quite high (which would lend a rather poor THD score, for what that's worth - not very much) and slightly higher on third harmonic, but aside from a few odd glitches, higher harmonics are pretty much absent. Odd order harmonics on the Arcam are visible all the way up the 15th, and this isn't even with extra averaging.
So the Arcam isn't very good objectively at low power levels... consistent, yes, but not particularly low in even basic distortion tests. The JLH is already sounding good, although these results do suggest that the modulation will be impacting on the sound - hard to know if this is being perceived positively or negatively at the moment.
In any case, there is more work to do on the JLH... whether I will attempt to modify the existing boards or start afresh, I'm not sure, but it's certainly worth some more listening hours! :)
