Though it is still an indispensable
necessity in solid state hi-fi power amplifier designs, should all
hi-fi audio power amplifiers be designed to work with out negative
feedback to achieve a more natural sound?
By: Ringo Bones
The zero negative feed back movement
on hi-fi audio amplifier design first gained both widespread and
worldwide notice during the hi-fi scene of the 1990s when many hi-fi
enthusiasts, especially in the do-it-yourself front, noticed first
hand that vacuum tube based audio amplifiers achieved their most
natural sound when negative feed back was minimized to near at or
reduced to zero. Although even now no solid state transistor hi-fi
audio amplifier design has been able to work with minimal – never
mind zero - negative feedback, should all hi-fi audio amplifiers,
whether tube or solid state, be made to work without the use of
negative feedback in the future in order to achieve a more natural
sounding music playback result?
Historically, negative feedback in
audio frequency power amplifiers was formulated to alleviate the
problem of harmonic and frequency distortion that made voices
unrecognizable and unintelligible during the early days of long
distance telephony. The only way to solve the problem back then was
the design of inherently linear output vacuum tubes that work without
the aid of negative feedback – like the PX25, 2A3, the 300A and
later on the 300B. But such very linear vacuum tubes are very
expensive to produce – even now – thus the necessity of negative
feedback. Harold Black who graduated from Worcester Polytechnic
Institute back in 1921, he later took a post as a research scientist
at Bell Labs was one of the more famous electronic engineers who
conceived of the negative feedback due to his frequent contact with
the electronic engineering academia at the time.
Though the British electronics engineer
Paul Voight also deserve credit for inventing the concept of negative
feedback for use in electronic audio amplifiers by patenting it back
in January 29, 1924. Given the cost-effectiveness of negative
feedback in making vacuum tube based audio amplifiers work linearly
during the period, it eventually gained widespread acceptance and
made the 300B the only widely manufactured directly heated triode
vacuum tube for audio output power amplifier use that would work
linearly without the aid of negative feedback. So the manufacture of
inherently linear vacuum tubes becomes non-economic and later more
modern vacuum tube designs with multi-element components in its glass
envelope that where maximized for gain via negative feedback merely
paved the way for the solid state transistor.
Given the runaway success of negative
feedback, virtually all vacuum tubes still manufactured today
intended for preamplifier use can be very hard to implement in a
design that works without negative feedback. The only widely produced
hi-fi audio preamplifier design to date that could work without
negative feedback is the Art made by Conrad-Johnson using 6DJ8 / 6922
triode preamp tubes though critics said Conrad-Johnson could have
used “inherently” more linear preamplifier vacuum tubes in the
Art – like the 6SN7 / 6SL7 family of vacuum tubes given that 6DJ8 /
6922/ ECC88 vacuum tubes were originally designed for electronic
measuring instrument use like 1950s era fast sampling oscilloscopes.
From an electronic engineering
standpoint, hi-fi audio power amplifiers designed with negative
feedback – whether thermionic vacuum tube or solid state bipolar
transistor or MOSFET based – tend to have lower parts cost despite
of the circuit complexity and tend to make the low frequencies / bass
frequencies rhythmically taught and enables the power amplifier to
drive a hi-fi loudspeaker with a widely-varying impedance curve. But
from a musical standpoint, the time smear induced in the application
of negative feedback tend to make the power amplifier to sound
harmonically threadbare – i.e. that “gray electronic sound” –
and tend to become rhythmically reticent especially in the vital
midband section when most of the music happens.
Hi-fi solid state amplifiers –
whether bipolar transistors or MOSFET based – tend to be very
impossible to work and maintain overall stability and a total
harmonic distortion below 1 percent when the overall negative
feedback is reduced below 30-dB. Most thermionic vacuum tube power
amplifiers – even ones operating in push-pull ones using more
modern beam tetrode (EL34 and 6L6) and pentode tubes (7591A) can be
made to work with as little as 5-dB of overall negative feedback
though the bass frequencies may become softer and nebulously blobby.
Despite the caveats, the inherent
naturalness of the midband of zero negative feedback single ended
triode thermionic vacuum tube hi-fi power amplifiers is just too hard
to ignore. Sadly, mainstream solid-state amplifier designs may not be
able to achieve such superb sound quality before the year 2050 until
the top consumer electronics manufacturing firms starts making solid
state bipolar transistors and MOSFETS that are as linear and good
sounding as the 300B directly heated triode thermionic vacuum tube
and amplify and playback music without the help of negative feedback.
