KISS 125 by Andre Jute
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This text is copyright Andre Jute 1996, 2005 and may not be reproduced
except in the thread KISS xxx on rec.audio.tubes. If you have arrived
late to this project, you can get an overview at
http://members.lycos.co.uk/fiultra/The KISS Amp INDEX.htm
or by finding a file called KISS 100.
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Fighting capacitances lurking with malicious intent in your amp:
Slew rate current, Miller, stray dogs and bandwidth
by Andre Jute
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Just after WW II the PR man at Mahatma Gandhi's ashram was giving a
group of American and British journalists the short tour. The Americans
were interested in the two nubile virgins the loincloth-clad old sage
slept with every evening to keep him warm and to test his willpower.
The British, still hungry after war shortages, wanted to know what
Gandhi ate.
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"Only the simplest food," the PR man said. "A little lemon and
honey with soda water."
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"Wow," said the Brits and moved on. (This was before their tabloids
became the scummiest in the world.)
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Afterwards the Time-Life reporter hung back. "Aren't lemon and
honey and soda water very great luxuries in India?" he asked
cynically. "How can you be so hypocritical as to describe that as
simplicity?"
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The PR man, who had been an Inner Temple barrister with Gandhi in
London a few decades before, knew when he was caught out. "Quite,"
he said smoothly, "but I never said it didn't cost a great deal to
keep Ghandiji in simplicity."
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***
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Ultrafi tube amps are like that. The simpler they appear, the greater
the mental effort required to win the benefits of that simplicity.
There hangs about the apparent simplicity of The KISS Amp 300B a whole
miasma of invisible capacitances which can test one's willpower far
more than two naked nubile virgins.
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Although we commonly say that "a Class A power tube draws no current
on its grid", a power tube requires current from the driver to
overcome various capacitance which loiter with malicious intent in your
amp. A useful shortcut to half a good answer is slew rate current
(which tells us how fast a capacitance is charged up and discharged)
and as usual experience provides the other half of the answer. The
first half of the answer can be calculated with slew rate formulae and
one empirical assumption generously suggested to me by Gordon Rankin
(who isn't responsible for what I do with the information) on 15
October 1996 while I was working on an 845 amp:
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***
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The Slew Rate
SR =3D 2*Pi*Bandwith*Vmax in V/uSecond
Where
Bandwidth =3D 20kHz or whatever your design will be good for
Vmax is the maximum voltage the stage will deliver
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The input capacitance
Cin =3D (A + 1) Cgp + Cgc
Where
A =3D the gain of stage for which the input capacitance is being
determined
Cgp is the Capacitance of the Grid to Plate
Cgc is the Capacitance of the Grid to Cathode
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The Slew Rate Current
SRC =3D Cin*SR
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Erno Borbely, Jung and Ron Gunzler (according to Gordon Rankin) suggest
that the stage current should be 5 times the slew rate current to
overcome the input capacitance of the next stage.
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The Stage Current
Scur =3D SRC * K
where
K =3D 5
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***
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Bear with me while I put in some numbers:
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Desired Stage Current is therefore 5*Cin*SR.
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For a 300B,
Cin =3D9 + (1 + 3.85)*15 =3D 81.85pf
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For a fullrange amp with 80V signal voltage,
Slew Rate =3D 2*3.14*20000*80 =3D 10,048,000
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therefore
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Desired Stage Current =3D 5*81.85*10,048,000
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and after moving the decimal we get
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4=2E1mA
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***
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Mmm. That is of course an absolute minimum requirement. Notice
something above? No one has yet counted stray capacitances. In a simple
300B amp strays usually amount to between 15 and 25pf. They too have to
dealt with, which is why the constant is a multiple.
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We know from experience that a 300B likes more current on the plate of
the driver than 4mA. I think it was the excellent Steve Bench who first
suggested to me that 7 or 8mA is the right minimum number for a driver
for 300B. In fact, this is less of discrepancy between theory and
practice than at first appears. We would normally design the amp out to
at least 40kHz, not just 20kHz, so the calculated current requirement
would match the 8mA from experience without altering the famous
constant of 5. In practice, say your transformers are good beyond a
43kHz bandwidth you have chosen for good reasons to be explained below,
then the constant of 5 gives the right result of about 8.2mA. (The
constant of 5 always attracts flack but its precise level is
irrelevant: it is just a guide to be adjusted in the light of
experience.)
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I suggest that if you use the standard formula above, with the audio
band in the bandwidth position, you use double the standard constant,
10 rather than 5, and you will get something nearer the right answer.
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Do the calculations for an 845, which has a difficult signal
requirement of around 150V in the more common designs, and you will
discover that 20mA is just about a minimum on the driver, which
accounts for why some of us laugh when we see designs for 12AX7 driving
845. It also accounts for why I like to drive kilovolt transmitting
tubes with a 300B booster amp, or at least a power tube for a driver.
The KISS Amp 300B is just such a booster design, requiring only a
switch to turn the primary of the output transformer into a choke load
on the plate of the 300B and a polyprop cap in series on the output to
the grid of the main kilovolt amp. In other words, the 300B is used as
a preamp (control amp) tube and as a weapon to absolutely murder
Miller.
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***
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You can use the capacitances loitering in your computer constructively
if you think laterally. For instance, the bandwidth of an amp should be
balanced. Any extension below a rather high level, say 50Hz, should be
matched by an extension at the top end. If the bottom end is being
deliberately sloped off early to protecthorn drivers (which rapidly
become unloaded below Fs), the HF should not be designed out to
infinity, whatever the iron may be capable of; it should instead be
balanced with the LF you are plotting. One of the tools under your
control is the amount of current you put on the driver, and the slew
rate concept is your tool to calculate it.
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***
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The question arises, why do you want an amp that has HF extension so
far beyond your hearing? What is this "balance" good for? Does
something lie beyond the achievement of "balance"? The answer is
that there is a subliminal effect, which ultrafidelista sometimes refer
to as "speed" or the "fast amp" syndrome, and which novices
think is only about undersizing power supply caps. Unfortunately
building a "fast" amp is far, far more complicated than that and
definitely requires attention at both ends of frequency scale.
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The interrelationships of these factors are not well understood but
they are likely to be complicated by the usual difficulties of
psychoacoustics. That is one reason I somewhat dislike the sound bite
of "a fast amp" and prefer the more sober phrase "a responsive
amp".
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The implication of this caution is that net gossip that "mo' driva
current is betta current" is not necessarily true. There is a correct
bandwidth for every speaker/amp combination, and particular correct
lower and related correct upper frequencies for each application. If
you just throw current at the driver because you have it to spare and
the iron can make the frequency, that can easily be as harmful to the
sound of your finished amp as not enough current.
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Don't skimp. Don't overdo it. Calculate thrice, solder once. An
elegant sufficiency will give you the right sound.
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Copyright =A9 Andr=E9 Jute 1996, 2005

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On 31 Mar 2005 19:46:33 -0800, "
wrote:

KISS 125 by Andre Jute

snip truly unbelievable wads of irrelevant waffle

Although we commonly say that "a Class A power tube draws no current
on its grid", a power tube requires current from the driver to
overcome various capacitance which loiter with malicious intent in your
amp. A useful shortcut to half a good answer is slew rate current
(which tells us how fast a capacitance is charged up and discharged)
and as usual experience provides the other half of the answer. The
first half of the answer can be calculated with slew rate formulae and
one empirical assumption generously suggested to me by Gordon Rankin
(who isn't responsible for what I do with the information) on 15
October 1996 while I was working on an 845 amp:

***

The Slew Rate
SR = 2*Pi*Bandwith*Vmax in V/uSecond
Where
Bandwidth = 20kHz or whatever your design will be good for
Vmax is the maximum voltage the stage will deliver

This requires some clarification and correction:

For this calculation to be correct, 'Vmax' is Vpk, i.e. 0.5*Vpk-pk or
1.414*Vrms. The result is of course in Volts/sec, not Volts/usec.


The input capacitance
Cin = (A + 1) Cgp + Cgc
Where
A = the gain of stage for which the input capacitance is being
determined
Cgp is the Capacitance of the Grid to Plate
Cgc is the Capacitance of the Grid to Cathode

The Slew Rate Current
SRC = Cin*SR

Erno Borbely, Jung and Ron Gunzler (according to Gordon Rankin) suggest
that the stage current should be 5 times the slew rate current to
overcome the input capacitance of the next stage.

The Stage Current
Scur = SRC * K
where
K = 5

***

Bear with me while I put in some numbers:

Desired Stage Current is therefore 5*Cin*SR.

For a 300B,
Cin =9 + (1 + 3.85)*15 = 81.85pf

For a fullrange amp with 80V signal voltage,
Slew Rate = 2*3.14*20000*80 = 10,048,000

therefore

Desired Stage Current = 5*81.85*10,048,000

and after moving the decimal we get

4.1mA

If anyone wants to know how to put the decimal in the right place
(i.e. to be sure that it's 4 and not 40mA), do the calculations in
Farads, Hertz and Volts. For those without scientific calculators, you
won't run out of digits if you work in MHz and nanoFarads, where
40kHz=0.04 and 82pF=0.082. The answer will be in mA.

snip of more reams of waffle where we find that a fudge factor is
introduced for the sole purpose of getting to the 8mA or so which
experienced designers know is about right for a 300B

This is the 'ultrafidelista' design process? Just make up numbers
until you get to the value that every reasonable designer uses?

You can use the capacitances loitering in your computer constructively
if you think laterally. For instance, the bandwidth of an amp should be
balanced. Any extension below a rather high level, say 50Hz, should be
matched by an extension at the top end. If the bottom end is being
deliberately sloped off early to protecthorn drivers (which rapidly
become unloaded below Fs), the HF should not be designed out to
infinity, whatever the iron may be capable of;

Why? What has this to do with bass extension?

The question arises, why do you want an amp that has HF extension so
far beyond your hearing? What is this "balance" good for? Does
something lie beyond the achievement of "balance"? The answer is
that there is a subliminal effect, which ultrafidelista sometimes refer
to as "speed" or the "fast amp" syndrome, and which novices
think is only about undersizing power supply caps. Unfortunately
building a "fast" amp is far, far more complicated than that and
definitely requires attention at both ends of frequency scale.

Utter hooey, as expected. The simple fact of the matter is that an amp
with a -3dB point of 20kHz will sound dull in the treble, whereas one
with a -3dB point of 40kHz will be only 1dB down at 20kHz, probably
inaudible in comparison with one which is ruler flat.
--

Stewart Pinkerton | Music is Art - Audio is Engineering