Augustica
®
T e c h n o l o g i e s
Page 3 of 9
I
NTRODUCTION
Tube circuit experts know that even power tubes that can deliver several hundred
milliamperes of anode current can never drive a loudspeaker with an impedance of 8 Ω,
since the internal impedance of a tube is several kiloohms. The two impedances are thus
almost always matched using a transformer. In principle, there is no any objection to
using this form of impedance matching, but every transformer tends to degrade the sound
quality. Keeping this effect to a minimum requires very careful and complicated coil
winding techniques, good-quality transformer iron and large core cross-sections.
However, if the load impedance is not just a few ohms but instead several hundred ohms,
as is the case with many types of headphones, and if in addition the required power level
is not overly high, an amplifier with no output transformer - sometimes referred to as an
‘output transformerless’ (OTL) amplifier - can be a feasible option. In such a case, the
load is driven directly by the tubes.
The headphone amplifier kit Frigate
®
allows building a headphone amplifier with
outstanding sound. Frigate
®
omits the output transformer, avoids overall negative
feedback and uses only high quality audiophile grade coupling capacitors, resistors and
other components. The Electro Harmonix
®
6922 and 6H30Pi tubes in left and right
channels are closely matched, therefore, the amplifier guarantees a highly linear
frequency characteristic and low crosstalk between the left and the right channels. The
headphone amplifier Frigate
®
employs a two-stage design suitable for use with a single or
dual headphone, with each headphone element having impedance between 15 and 300 Ω
for the total impedance of 30 to 600 Ω.
A
MPLIFIER
- T
HEORY OF
O
PERATION
The circuit of the headphone amplifier Frigate
®
is shown in
Figure 1
(Frigate's schematic
also appears on the website www.Augustica.com) uses the readily available Electro
Harmonix
®
6922 and 6H30Pi double triodes to provide amplification. For the purposes of
this discussion, we will be considering left channel of the amplifier. Right channel of the
amplifier Frigate
®
is identical in its construction and operation to the left channel and
therefore its description will be omitted. A preamplifier stage is necessary to generate
signal amplitudes sufficient to drive 6H30Pi which in turn will drive a headphone. The
first section of 6922 with base pins 1, 2 and 3 and the second section of 6922 with base
pins 6, 7, and 8 are used for this purpose. The input signal arrives to the circuit board via
a 100-kilo Ω logarithmic potentiometer R1 that serves as a volume control and is directly
coupled to the preamplifier stage via capacitor C1. Resistor R2 provides the necessary
negative grid bias. Resistor R5 determines the gain, while resistor R4 determines the
maximum input voltage. The value of the resistor R5 is selected to assure that the
quiescent anode current is situated in the most linear possible portion of the characteristic
curve for 6922.
