Reach The KISS Amp through
http://members.lycos.co.uk/fiultra/
or directly at
http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm
on which KISS 190 is an index of schematics and illustrations.
All text and illustration is copyright property and may not be
reproduced except in the thread KISS xxx on rec.audio.tubes

KISS 124
Shaping the Ultrafi driver stage
by Andre Jute
By the time we come to design the driver stage of the ultrafi 300B amp
there are so many hard points settled by the process elsewhere in the
amp that the stage almost designs itself. Let us look at some of those
hard points.
For historical reasons I decided on:
-a triode
-a Western Electric tube
-a single tube to do duty both as input and driver
-a 300B output tube which has a particular signal voltage requirement
All of this together fixes the input tube to a choice between the 417A
and the 437A, which, given their relative availability and price is no
choice whatsoever: the 417A is as WE tubes go inexpensive. Choosing the
417A is no hardship: it is a gloriously musical tube, a little wonder.
For reasons of taste I have chosen to use:
-a tube rectified input, which drops much more voltage over the
diodes than silicon rectifiers
-a choke input power filter, which requires two chokes rather than
one, so doubling the voltage drop in the filter, which further requires
-a big bleed because swinging chokes are not available these days; I
would anyway want the big bleed for stability and safety
For reasons of common sense, aesthetics and cost, I have chosen;
-to buy my power transformer the same place as my output transformers
-all of that fixes the voltage available before the power isolation
filter for the driver to 396V
From consideration of the necessary slew rate current and the Miller
effect, I want to put a good deal of current on the plate of the driver
for the 300B. Experience leads me to the same conclusion: high current
is a club to linearize even recalcitrant tubes and the 417A, an RF
tube, is not among the best behaved, though it is linear enough. But I
selected it for its sonic qualities (if linearity were my only concern,
I would have chosen the 6SN7 which itself sounds uncommonly fine.) So
we have,
-high current; from experience 20mA is pretty linear though the
hardliners prefer 24mA
-I need high current in any event because there will as a matter of
principle be zero negative feedback
For sound sonic (hee-hee) reasons, I want battery bias on a 417A used
as a driver. Remember, we are coming to this KISS ultrafi amp from the
KISS standard reference of 6SN76SN7300Bhigh impedance, an
ultra-silent amp. While we are satisfied that we can back away from
that perhaps overhigh standard a little, there is no call recklessly to
embrace noise. I despise regulated power on the filaments of high-class
amps for the bleached out, lowest common denominator sound it produces.
(One of the reasons those big American amps sound like silicon is
because they use beautifully designed regulated power supplies.)
Battery bias is one way of avoiding noise. It eliminates a major
sound-shaping element, the cathode bypass cap, leaving us enjoy the
sound of the 417A unadorned. The bass boost of fixed bias can be used
by the selection of other
elements to shelve and tilt the driver-output tube transfer curve just
so.
-battery bias fixes the quiescent or zero-signal operating point at
2=2E4V because that is what we can get ni-cad batteries for, 1.2V being
too little and 3.6V taking us outside the tube's permitted parameters
(too much voltage on the plate-check the Eb-Ia-Eg curves)
At this point we thus have a tube operated at 20mA (arbitrary) and
-2.4V (battery availability) and 175V on the plate (read off the
transfer curves). 6K8 is conveniently close to our preferred multiple
of four of the 417A's plate resistance of about 1750--1800 ohms. All
that remains is to calculate that a 6K8 load resistor will drop 136V at
20mA and now we know the power supply must deliver 311V at the takeoff
for the driver.
The grid leak resistor of the tube following, the 300B, which forms
part of the load on the 417A plate, thereby becomes a response-shaping
element in precisely the same way as its associated coupling capacitor
(with which it forms an HF filter). For the time being I shall merely
tell you the grid resistor is 47Kohm and the coupling capacitor 0.1uF
because we want matching time constants on the RC filters to the grid
of the 300B and to its cathode. In a later chapter I will show how I
deliberately used these two elements to shift and tilt the transfer
function of the 417A/300B combination, what the pretentious call
distortion cancellation; they're in for a shock in just a moment...
Remember what I said about the relationship of wu to the swan? The way
all this falls together so tidily has nothing to do with luck and a
great deal with years of hard work on drivers for 300B over more than a
decade.
The only remaining design decision is the value of the stepped
attenuator across the input. The QUAD 67 CD player I favour demands to
see at least 10K from the next device in the chain. But the 417A, which
never stops reminding the clumsy that it is an RF tube, doesn't like
more than 10Kohm attached to any part of it. The compromise I choose is
a 20K DACT stepped attenuator with very short tracks (it is built with
surface mount resistors) plus a 220 ohm grid stopper soldered right to
the socket pins of each of the four grid inlets of the 417A. (Use the
centre pin of the socket for a common input end.)
Now we have a complete signal section for The KISS Amp 300B "Ultrafi"
from volume control through driver, power tube and output transformer
to loudspeakers. By now the power supply is essentially designed by
reference to the parameters guiding the signal section; we shall
discuss the general principles of the power supply in another chapter.
***
At this stage you may compare what I have done with the transfer
function in the audio band with the standard reference amp of 2x 6SN7
working with 300B to what I have done with 417A in the Ultrafi working
very substantially against the linearity of the 300B. When it was put
to me by someone, who hadn't yet seen the schematic for the circuit,
that I would of course use the 417A for harmonic cancellation on the
300B, I burst out laughing. A 300B that requires linearization is
incompetently implemented.
Make no mistake. I deliberately used the superb warmth of the 417A to
subvert the blameless linearity of the 300B.
I did it only for a little way, and very selectively, of course, so you
might say I did a reverse harmonic cancellation job.
There are several good reasons for what I did. First, I wanted highish
and very quick rolloff below 36Hz to protect my expensive Lowther
drivers fitted to horns because horns simply don't load the driver
below the fundamental resonance. This in turn requires that the top end
be very carefully limited perceptually to balance the response around
700Hz or 1000Hz; this is a psychoacoustic matter with no electronic
justification.
Then I went one step further and tilted the system response so that,
instead of a sharp peak in low presence range like most SE amps (the
source of the silly myth that SE amps must necessarily possess "added
euphonics), there is a shelf that runs from 36Hz to 17kHz, that is from
the fundamental resonance of my Lowther loudspeakers to the fourth
(natural) harmonic of the highest notes on a piano. The purpose of this
is to boost the frequencies which carry most of the energy in the sound
(which is not, repeat not, the fundamental) especially since they fall
neatly in the specially pleasing territory of my horns; thus, without
endangering my horns, I have boosted the frequencies which most
strongly by subsonics suggest to the ears that fundamental which nature
has made weak or which by electronic legerdemain I (or more usually the
recording engineer before me) have written out of the script.
These are artificial means to attain a sound professional chamber
musicians will recognize as "more natural" than the sound made by
amps which measure better. It is a key example of electronics serving
taste and culture. Of course, before one can demand that electronics
serve taste and culture, one must first earn taste and culture.
In a later chapter I will show how all this is based on very hard
science, a nasty wodge of tricky math straight out of the RCA
Institutes by hand of Mr Julian L Bernstein.
Mr Bernstein was Associate Dean of the RCA Day School.
Here Andre Jute on the left receives The Compleat Tranfer Formula
directly from his hand.
Muscle tone like that requires a lot of soldering!
Illustration by Michael O'Dwyer after Michelangelo
I shall also present the spreadsheet in which by optimization search I
discovered the correct component values to achieve this advantageous
transfer function; trying to handle a complicated complete transfer
function formula on a slide rule or calculator will drive the most
patient of men nuts. As it was, this driver stage took me many months
to design, test and optimize. Without the help of a computer it is the
sort of development only a big team can do, beyond the resources of a
hobbyist to handle manually.
Of course the Ultrafi 300B sounds warmer than the 6SN7 driven amp. I
designed it to do precisely that while casually waving a fig leaf of
engineering respectability ("within 3dB over the entire audio band
covered by the intended speakers") in the approximate vicinity of its
impressively big-- er-- sound every time puritan silicon
curtain-twitchers approach primly. It is a musical hedonist's amp and
sound like it.

JUTE ON AMPS =B7 KISSmain =B7 KISS190index
HOME =B7 JUTE ON AMPS =B7 CLASSICAL JUKEBOX
THE WRITER'S HOUSE =B7 THE TRUTH =B7 OTHER MATTERS ARISING
THE VOLTAGES IN THIS AMP WILL KILL YOU.
GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP

All text and illustration is Copyright =A9 Andre Jute 1996, 2005
and may not be reproduced except in the thread KISS xxx on
rec.audio.tubes

On 22 Apr 2005 13:33:05 -0700, "
wrote:

Of course the Ultrafi 300B sounds warmer than the 6SN7 driven amp. I
designed it to do precisely that while casually waving a fig leaf of
engineering respectability ("within 3dB over the entire audio band
covered by the intended speakers") in the approximate vicinity of its
impressively big-- er-- sound every time puritan silicon
curtain-twitchers approach primly. It is a musical hedonist's amp and
sound like it.

What happened to 'ultrafidelity'? This amp is 'designed' to sound
overwarm, and is severely band-limited, which is not a problem only
because it's being used with speakers that have no deep bass or high
treble. This is 'ultrafidelity'? Sheesh! Whatta maroon!
--

Stewart Pinkerton | Music is Art - Audio is Engineering