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Time To Get Theoretical

I know some this will be a little heavy going for some, but don't be put off, the conclusions are interesting.

There are still quite a few things that have influenced the development of this project, both the commercial JLTi and these two DIY versions – primarily the Tube version.

Some may ask, what is the differences between the JLTi and the DIY (Tubed) version?

For a start, the Tube Buffer in the JLTi is much more enhanced, the so-called SLCF enhancement. For a discussion of the SLCF and other good things, may I again refer you to my on-line essay:

Tubes & the Gainclones

Also this enhanced Tube Buffer needs higher voltages to work comfortably. Thus it is plus and minus 55V. It is also regulated, using an adapted version of the SuperReg, developed by the Vacuum State Electronics team headed by Allen Wright.

If you wish to regulate your Tube, then by all means go ahead. I have stuck to simplicity and if you want to go further, you have my blessing.

Let it rip...

Gazing Into The Dark

What follows may be theory and attempting to gaze into the dark. Nevertheless I hope it Is of interest.

First Subject: Lynn Olson’s “The First Watt”

For a couple of years the thoughts and writings of Lynn Olson has intrigued me, especially as they agree with much of my own, indeed I wrote a series of articles for the Audiophile Society of New South Wales in the 90's that turned out to parallel many of his views. In March and April of 2000 Olson wrote two articles in Glass Audio magazine. If you can acquire them, please do read them. I have a PDF file containing both of them, but for Copyright reasons I don’t feel right posting them on one of my web sites or that of  www.vacuumstate.com. Besides being 3.5MB in size it would make for costly traffic costs. But if you have the opportunity to read these articles, don’t miss out.

Lynn Olson has a web site:


But I digress. From his site I have borrowed this clip showing a distortion measurement of his Amity amplifier:

Caption: Amity PP Tube Amp. This is a 1Khz sine wave, 1.6W into 8 Ohm load.

The vertical scale is 10dB/division and lateral 1KHz/division. From this we can see that the 2nd harmonic is-70dB (0.03%) and the 3rd –65dB (0.05%). This is quite respectable, but what is noteworthy is the absence of higher-order distortion. There is just a hint of 5th harmonic at –82dB (0.008%). (The rest is either the noise floor or the limitation of measurement). This is commendably achieved without feedback. Compare this to more modern Push-Pull Ultra-Linear Tube amps (look at his web site as he provides sample measurements) are much more elevated in higher order distortions.

This is an important issue because, in general, as the order of harmonic distortion increases, it becomes more evident and displeasing to our ears. It's not the actual THD figure that is important but rather how it is spectrally distributed. For example, if 1% of  2nd harmonic distortion is compared to an amplifier producing 1% of 7th harmonic distortion, both amplifiers will measure the same THD reading - but will those amplifiers sound the same? No way! They will sound very different. In fact 1% of 2nd harmonic may well seem to improve the sound BUT same amount of 7th  harmonic will sound very unnatural. So the spectral distribution is paramount and especially avoiding high odd order harmonic distortion.

Another key to understanding THD is that while global feedback may well reduce the complete THD figure, of what benefit is that if, in the light of above, the higher order distortions become elevated?

Let's get back to Lynn  Olson, what Olson proposes is:


It kinda makes basic sense. After all this is the part of the amp we are listening to virtually ALL of the time. Higher power levels are more transient and shorter in duration.

Olson applies this to Tube Amps, but what about Solid State or Hybrid Power Amps or Gainclones.... ?

Solid State do behave differently but feedback still tends to reduce overall THD, but at the expense of higher orders being emphasized?

There is another difference; at 1 Watt virtually all Tube amps are still in Class A, whereas Solid State will likely be beyond its Class A range and switching into B (certainly the case with Gainclones). So we expect Solid-State to increase distortion as we go DOWN in power (getting closer to the AB switching point), whereas Tube tends to reduce as level goes down.

Going the other way, Solid-State likely decreases in distortion as power goes up (until it approaches clipping) whereas Tubes increase. Tubes, especially pure Triodes, are preferable in this, indeed both, respects.

So at 1 Watt crossover distortion will not be a factor in tubes, but with Solid-State Gainclone IC we would still hope for acceptable results at the same power. But a big BUT! What happens when we go below 1 or 2 Watt becomes critical with Solid-State. Distortion WILL increase; you can count on it.

As we approach the point at which Class AB switching occurs (crossover) the reduced amplitude will not suppress these distortions (so they may not show up as well at 1 or 2 Watt and even less at 8 Watt etc). Will very low power levels see an increase in high order distortions?

This is what we will investigate here.

How does our project fare with these factors in mind?

We start with 1 Watt:

Caption: This 1W into 8 Ohm load.

Distortion is primarily EVEN order, the only odd order showing up is the 3rd, and this is about 80dB down or 0.01% out of the total THD (Total Harmonic Distortion) of 0.114%.

Caption: This 2W into 8 Ohm load.

Much the same. Distortion is again primarily EVEN order; the 3rd has barely changed. Almost no difference except the note noise floor has been slightly suppressed as would be expected.

It is apparent that at these power levels, the First and Second Watt, our amp is very well behaved. There is a notable absence of ODD higher order distortion.

Before going on, I decided also to do a IMD (Inter-Modulation Distortion) test at 1W using 1KHz and 2KHz combined test signal:

Caption: This 1W approx into 8 Ohm load. IMD = 0.046%.

Again this looks pretty good, high order irregularities minimal.


But what happens when we REDUCE power. This is a Class AB amplifier and so is prone to crossover type distortions. We need to examine THD at much lower power levels.

Caption: This 125mW into 8 Ohm load.

This level means still Class AB (thus switching). As expected the noise is being suppressed less (this is a combination of the amp Signal To Noise Ratio and that of the test equipment) but around –90dB at this low level quite acceptable. No major increase in THD, indeed the slightly higher THD percentage is probably caused by the higher S/N. This is better than I would have expected. Some pushing up of odd orders was anticipated and it has hardly happened.

Let us go much lower still:

Caption: This just 25mW into 8 Ohm load.

I also did 60mW and it was comparable to 125mW. But at 25mW it is most likely operating in Class A. Again S/N will be contributing to THD =  0.151%. An extremely small increase in 2nd and 3rd orders, and 4th unchanged.

Caption: This only 10mW into 8 Ohm load.

Definitely in Class A now. This seems confirmed by almost total lack of odd orders, but THD figure definitely influenced by S/N. But at this very low level indeed so no reason to complain.

I am very happy with these results. Especially when you consider the fact that, as the noise floor increases it potentially mask higher order distortions. This makes the 10mW more difficult to interpret, but at 25mW and 125mW problems should have showed up already because here the noise floor is still well suppressed. Yes, I must say I'm happy with these results.

I have a question and I'm not sure what the answer is? The effect of the noise floor acting as dither if higher order distortions remains buried at low power levels. The other thing, if the amp is well behaved after feedback (indicating good linearity before feedback i.e. good open-loop characteristics) and that these higher order distortions remain constant in absolute level while the dynamic level (caused by the music), then dither may have a beneficial effect not just at low level but all levels. Keep this in mind, and I know it may be difficult to grasp if one isn't familiar with these types of measurements, that the noise floor is constant in all the above measurements even if they look like they are not. The reason is simple, the reference 0dB represents a different voltage in each graph. Since the main culprit (I suspect) is crossover distortions, then these may well be effectively masked by dither because these crossover distortions are as constant as the noise floor? I am not sure about this but the thought has to be entertained?

Pseudo Single-Ended Operation?

But that is not all. It is apparent that this amplifier outputs little in Class A, hence I would like to make this statement, one that is open to argument (I welcome debate) but nonetheless needs to be stated: If we can control potentially nasty crossover induced distortions, especially high order, then it becomes apparent to me that either an amplifier should output high level of Class A and must at the same time have near perfect symmetry (very very difficult) OR it should have very little Class A so that once it switches into Class B the amplifier is largely operating single-ended during those moments in time (and thus avoiding symmetry problems since it isn't symmetrical). This thinking would be considered heresy by some.

So I conclude that Class AB amplifiers must be either highly symmetrical with significant Class A output power OR have very little Class A provided crossover distortions are superbly suppressed (and possibly dithered) giving a pseudo single-ended operation.          (Added 27th May 2003.)

All Power Measurements

I did a series of these measurements as high as 47W and as low as 10mW and the consistency was remarkable. Here are the THD figures from all of them.

10mW              0.177%

25mW              0.151%

60mW              0.139%

125mW            0.125%

250mW            0.120%

500mW            0.113%

1W                    0.114%

2W                    0.111%

4W                    0.110%

8W                    0.109%

As expected, distortion continues to drop gradually with increasing power, yet harmonic distortion pattern is always the same. The distortion harmonics largely ‘cascade’ like a waterfall, something that was recognized by insightful research decades ago (and subsequently forgotten) as sonically desirable. The name Jean Hiraga comes to mind.


Indeed, looking at all THD graphs (up to 32W) the results are consistent, 5th order or higher harmonics are well suppressed. In all cases the 3rd harmonic is equal or lower than the 2nd (cascading) and the latter is repeatedly the dominant factor in the percentage of THD. This lack of odd order distortions is very welcome and the predominant EVEN order only tends to add very little ‘sweetener’ to the sound.

Max Power Test:

What if we push it to the limit, like more than 40 Watt?


Caption: 47W   & THD = 1.324%

WHOA! What has happened here? Looks kinda ugly but actually not all bad news. The scope shot next explains all:


Caption: 19.5V RMS into 8 Ohm = 47W.

Notice that the scope shows that the amp is NOT clipping? So what is happening to the top and bottom of the waveform? It is actually 100 Hertz ripple intruding (double the Mains frequency) when the power supply is required to deliver peak current. You can see this modulating the 1KHz waveform and if you look carefully the lines are lateral and low frequency in content. Because we are using smaller than usual 1000uF reservoir caps, these caps are depleting, creating the above effect. But music signals are NOT continuous like our test signal. So we would expect that short music peaks might well be left unaffected. So the amp is easily capable of  35 Watt RMS (continuous) and short-term music peaks of 50 Watt.

The use of such small reservoir caps may seem controversial, yet using higher values may well be lowering the time constant (slower) caused by the combined AC source impedance of the power transformer and rectifier bridge. The problem is that this is a complex dynamic inter-face as it is governed by both the charging cycles and interfered by the DC current requirement of the amplifier itself. I would love to see someone model this mathematically, could make for some interesting reading.

This is not exactly new thinking. I recall some fifteen years ago that Martin Colloms (of HFN & RR) writing on this very subject and he demonstrated to himself, as I recall it, how he could make sonic changes by varying the wire gauge between the Bridge and Reservoir caps. As far as using small caps in our Gainclones, I do know this, it has a great agility in the mid-bass that makes it as good as anything when playing acoustic or even electric bass, and other instruments that require good sense of pitch, pace and tonality.


So here are by reason some good pointers to the more than acceptable sound quality of these little wonders. I have said to myself many times over that they don’t deserve to sound as good as they DO. After all, this is a feedback amp, indeed an Op-amp device where Feedback is THE controlling element as to gain. It is also Push-Pull (derision from certain quarters), Class AB (more derision) and not much of it is Class A (probably about 0.03W) as the idle current is only slightly more than 30mA.

So while it breaks the mold in many ways there are also some good indicators. The use of Current Sources (six minimum, probably seven) and Class A in all stages up to the Quasi Output Stage (NOT complimentary). The Constant Current Class A Driver (to the Quasi out) used internally must be very good in absorbing crossover artifacts as they look so well suppressed, no real increase with descending power, in 3rd order but also 2nd order which continues to dominate. At all levels it is the 2nd and 4th combined even orders that define the measurements. Excellent!

I came across this quote from another manufacturer who uses the 3886 version as stating:

One common question is whether the output section of the amplifier is biased into Class-A operation.  Technically, the answer is “yes,” as the exceptional linearity of the output section design achieves the advantages of pure Class-A operation.  Using the “Intelligent Power Transistor with Gain” system keeps the output section properly biased at all times to eliminate crossover distortion caused by the output transistors during turn-on and turn-off, as opposed to the brute force Class-A biasing techniques used in other inadequate amplifier designs to compensate for poor linearity that result in low efficiency, excessive heat, electrical waste, and the need for power management.

Imagine calling the Gainclone IC Intelligent Power Transistor with Gain”.

What a Hoot!

Added 17th May 2003.

But measurements are not everything, it is the SOUND that is the final arbiter! That is why the commercial equivalent is called JLTi. Guess what that stands for?

So all I can say is Just Listen To it.

Join Us On




Send mail to joeras@vacuumstate.com with questions or comments about this web site.
Copyright © 2003-10 Joe Rasmussen & JLTi
Last modified: Monday June 08, 2015

Just had a terrible thought. If "intelligent design" is unscientific, then who will design our audio equipment?