LYNN OLSON:

THE GEORGE ZEN WASHINGTON
OF PUSH/PULL CIRCUITS

A COMPASS IN THE THERMIONIC STORM

Dr. Gizmo's Secret Agenda Revealed

You have been reading my articles and many of you are scratching your butts wondering if I am totally nuts because my views are so anti-orthodoxy. So I can think of no better way to make you comfortable doing the most difficult thing possible for a man…which is to change your mind…then to enlist the help of one of America's most esteemed thermionic savant…who I have absolutely no influence over….who is three thousand miles away….and whose unique vision of the audio arts, which is expressed in his new Amity amplifier, will aggravate you…make your eye twitch. This is a good thing, because there is too much mealy minded information being spewed forth about tube amplifiers. Study what follows very closely. This is not only a fascinating exploration of Lynn's amplifier, in his own words, but the best tutorial in the righteous future of push/pull amplifier design.

I asked Lynn to describe the features and benefits of his Amity amplifier. Of course you are going to surf to his web site for more information aloha-audio.com

Do you have any friends who love their push pull amplifiers? Be a friend and email this article. Print this article and study it in the potty, in your office, and in bed. You are gazing at the PRIMAL BIG BANG of a new aural matrix…a new form of musical beauty.

1) Topology and active devices selected for absolute minimum

distortion, particularly with attention to reducing upper harmonics.

No pentodes, no beam tetrodes, no transistors, no MOSFETs, no IGBT's,

no nonlinear triodes such as 12AX7, 12AU7, 12AT7, or 6DJ8. All

devices selected for absolute minimum upper harmonics and

well-behaved and moderate proportions of 2nd harmonic. This tends to

favor mid-Thirties DHT's and early-Sixties high-transconductance

triodes.

The mid-Fifties favorites such as 12AX7 and 12AU7 replaced the

superior octal equivalents 6SL7 and 6SN7 during a period of universal

feedback, so the threefold degradation in THD went unnoticed. The

12AT7 has a kinked set of curves that were OK for TV video amplifiers

(which it is designed for), not so good for audio. The 6DJ8 is a

color-TV cascode RF amplifier and was never intended for audio. The

so-called subjective "detail" of this tube is simply a high

proportion of 3rd harmonic, which is very undesirable in a triode and

even more undesirable in a PP circuit, which cannot cancel even

harmonics.

2) Circuits that optimize the linearity of each active device. For

triodes, this means active, choke, or transformer loads, no

RC-coupling anywhere. RC-coupling, although essential for the

extended HF bandwidth required by global feedback, also degrades the

distortion of triodes anywhere from 2 to 4 times relative to active,

choke, transformer loads.

I personally favor transformer or choke loads in favor of active

loads due to simplicity, reliability, and avoidance of active-load

coloration. I have not had good luck mixing solid-state with

high-voltage vacuum tubes, and prefer the reliability and

predictability of all-tube circuits. In an all-tube circuit, you can

tell if it's working just by looking if it's lit up, and you can

debug just by swapping parts.

An additional advantage of no RC coupling between the driver and

output tubes is immediate recovery from overload. RC coupling usually

requires hundreds of milliseconds to recover the correct bias point

for the output tubes. This is great for guitar amps, not so good for

hifi applications, where instantaneous recovery is much more

desirable.

3) Zero feedback in either local or global circuits. Some would say

all triodes have large amounts of local feedback, but I would say in

response that the proportion of upper vs. lower harmonics undercuts

this. Feedback of any type has no effect on the proportion of the

harmonics, and simply changes their magnitude. The unique hallmark of

DHT's and good IDHT's is their absence of upper harmonics. This is

*not* true for other devices, such as the types mentioned in (1)

above. Feedback does reduce the absolute amount, but has no effect on

the *proportions* of the remaining harmonics.

Since the circuit is intrinsically linear, feedback is not required,

which sidesteps problems with phase margin and stability with complex

and nonlinear loads ... which pretty much describes all loudspeakers.

In particular, complex back-EMF currents from the drivers stop at the

plates of the output tubes, instead of being mixed at the summing

node with the incoming signal. This avoids load-dependent distortion

terms being generated in the feedback circuit.

4) Adequate driver design. Most commercial tube amps only have 1 or

2dB of headroom in the driver circuit, so the entire amp clips at

once. This leads to longer recovery times and exaggerating the

audibility of clipping. I prefer 3 to 6dB of headroom, so the driver

can retain its (voltage) linearity even when the output stage is deep

into clipping.

Perhaps more important is adequate current and low output impedance

in the driver. Much of the amplifier coloration is actually in the

driver, and is a result of not enough current to properly charge the

grids of the output tubes. I give Arthur Loesch credit for pointing

out that the "sound" of different DHT output is greatly exaggerated

by not enough current in the driver. With enough current, they become

more transparent sounding and begin to lose the characteristic

colorations they are known for. This would imply that grid current is

present during much of the duty cycle and is quite nonlinear. The

more current available and the lower the source impedance, the less

important this grid-current nonlinearity will be. (The same

considerations of linear current deliver apply with greater force to

transistor-amp design, where driver sections tend to limit the

performance of commercial high-end designs.)

5) Absence of rectifier switching noise. The noisiest circuits of all

are solid-state bridges driving large values of electrolytic

capacitors. (As found in almost all transistor gear and the DC

supplies for heaters and filaments.)

The commutation noise of the diodes shock-excites the RLC of the

stray L in the cap bank and the stray C in the power trans secondary.

The resonance of this tank circuit is typically anywhere from 4 to

20KHz and the Q's are large, anywhere from 5 to 100, depending on the

DCR of the caps. This is why paralleling large values of

electrolytics with "better", faster polypropylenes can frequently

result in worse sound. It is also the reason power cables are audible

... they act as antennas for the small Tesla coil that most power

supplies resemble. The supply radiates noise into the chassis, the

power supply B+ lines, the audio circuit, and the power cable. This

broadband noise can be filtered and shielded (at considerable

trouble), but it is much easier to eliminate the commutation

switch-noise right at the source.

Choke-fed supplies are much quieter due to the choke slowing down the

rate-of-charge of the main cap bank. I use a hybrid

choke-fed/pi-filter to minimize the shock-excitation of the main PS

choke (this tip directly out of the RDC IV).

In terms of ragged waveforms, solid-state diodes are the worst,

followed by Schottkey diodes and HEXFRED's, followed by conventional

tube rectifiers, followed by TV damping diodes, which are the

smoothest of all in terms of the AC waveform on the power trans

secondary. This, along with 2 amp peak current, is why I use them.

The 30 second warmup is just a bonus.

6) Capacitance is Everywhere. Most mid and HF colorations in

commercial amplifiers are due to unforeseen stray capacitive effects.

Circuit boards are generally made of low-quality dielectrics (poor DA

and DF), absorb moisture, heat up and dry out, and are microphonic as

well. If a circuit board is a necessity, Teflon is by far the best

dielectric, but it is very expensive and not very strong

mechanically. Point-to-point wiring has the great benefit of air

dielectric.

It should be remembered that *any* capacitor with a polarizing

voltage is a low-quality microphone ... much of the benefit of oils

are simply due to mechanical self-damping. This is why solid-state

components with no vacuum tubes at all are still somewhat

microphonic; the numerous power-supply bypass caps are microphonic,

and pick up ambient vibration (including buzz and vibration from the

internal power transformer). In addition to picking up ambient noise,

caps also self-excite, and create their own resonant noises.

The residual HF imbalance in PP transformers is due to capacitive

imbalance ... you have to get a "geometrically balanced" PP

transformer in order to get AC *and* DC balance. Even some very

expensive transformers are not balanced at 50kHz ... don't worry, the

trans in $20,000 commercial amps are a long way from being the most

expensive. They're usually made offshore to a very low price.

The Miller capacitance in power tube grids leads to various

slew-limiting mechanisms (current delivery from the driver becomes

nonlinear), and the Miller C in the driver tube unbalances the

traditional split-load inverter at high frequencies. The

common-cathode long-tail inverter has unbalanced Miller C on its own

inputs; one side is a grounded-cathode, while the other side is

actually a grounded-grid circuit. The various tube phase-splitters

need to very carefully analyzed to see if they are balanced at high

frequencies.

7) Symmetry is Everything in PP. One of the most difficult aspects of

PP, or differential, or bridge-mode, or whatever you want to call a

balanced circuit, is the cancellation of even harmonics. Note this is

cancellation of internally generated distortion, not the music

itself, as a writer asserted in a magazine some time ago. It would

take a *very* sophisticated DSP system to cancel the harmonics in

music. Instead, what analog amplification does is add *additional*

harmonics that don't belong there.

The problem with PP harmonic cancellation is that it isn't stable in

most PP circuits. This is a serious issue, since it means the

harmonic "signature" of the amplifier is unstable with signal level,

frequency, and power-line conditions. This is the reason for the

opaque and cloudy sound of complex PP amplifiers, and the much more

direct and immediate sound of SE.

Any time there is a summing node (as is the case with PP and

feedback) the two circuits that drive the summing node have to be

examined very carefully to see if they are drifting away from the

ideal textbook condition. With PP, this means the phase-splitter for

the two PP "halves" has to be fully stable; as far as I can tell,

only transformers meet this requirement. It also means that when two

PP circuits are cascaded, the PP output of the first stage needs to

be re-combined before passing it on to the next stage, otherwise gain

mismatch will build up. Remember, these mismatches are dynamic, and

very undesirable.

This is what happens in the traditional RC-coupled Williamson, for

example, and is a subtle advantage of the simpler Hafler/Dynaco

circuit, which does not pass on gain errors from the driver to the

output. Instead, the large impedance mismatch of the split-load

inverter is directly presented to the output tube grids. (No driver

stage at all!) Out of the frying pan into the fire.

The best solution is to use a transformer to re-combine the outputs

of the first PP stage before presenting it to the second set of PP

grids. This removes the mismatch of the first stage, preventing it

from being compounded by the second stage. The IT needs to

geometrically balanced, and the presence of an IT also means that the

first (driver) stage needs to have a low plate impedance so the trans

will have adequate bandwidth. Unfortunately, this rules out a PP 6SN7

driver, which is the obvious choice for linearity and large signal

swing. The alternative choices are NOS 5687, 7044, 7119, and the new

JJ Electronics ECC99 and Sovtek 6H30, which all have pretty good

linearity and 3 times lower plate impedances.

Another awkward aspect of PP DHT amplifiers is the difficult

requirement for very large, very linear grid drive voltages. Since

the output DHT's are so linear (less than 0.25% distortion at 16

watts for PP 300B's), the drivers need to be equally good, or

preferably better. It's poor design practice to let driver distortion

predominate over output distortion. In my circuit, I bias the VV32B's

at 85V each, which translates to a requirement for 120V rms between

the two grids for full output. If I used a step-up transformer, that

would also increase the current requirements on the driver stage in

direct proportion, which is not desirable, since the driver needs to

have high current in the first place to minimize DHT coloration and

minimize slewing. Another undesirable aspect of a step-up transformer

is that the impedances are multiplied as the *square* of the

turns/voltage ratio, so a 1:2 step-up would present the paired 300B

grids with *four* times the Rp of the single driver tube (30K for a

single section of a 6SN7).

As mentioned above, I selected PP 5687, 7044, 7119, or Sovtek 6H30

drivers to maintain linearity in the drive stage. A more over-the-top

approach would be using the new Vaic Valve DHT drivers in PP, such as

the AV 5, AV 8, or AV 20. In addition to a significantly higher cost

(similar to the output 300B's), there is the tricky requirement for

DC (or heavily filtered AC) on the driver filaments. Unfortunately,

my experiences with regulated DC heating on DHT filaments has not

been positive, although I hear that DC *current* sources have worked

well for those who have tried them.

8) Subjective Tuning. With all the restrictions described above, what

if it doesn't sound good? What's left to tune? Well, there's always

the good old power supply, along with the traditional favorite, the

wire itself. Small changes in the corner frequency of the RC and LC

networks in the cathode bypasses and the power supply (in the 3 to 4

Hz region) can have a surprisingly large effect on the sound in the

300 Hz to 1 kHz region; this can alter the entire spectral balance of

the amplifier, even though there are no obvious circuit changes, and

essentially no change in the measurements.

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