The following are a few circuits for enthusiests to try out.
All of the circuits work and have been built and auditioned at
Black Art Audio. Black Art transformers have been used throughout
, but we have included enough information for the builder to source
components from elsewhere if desired. The circuits do not rely
on the use of our products and will work with other suppliers
but will sound different.
( but this is half the fun )
NOTE :
All these circuits contain potentially lethal voltages and should
only be built by a suitably experienced personell. Black Art Audio
and its employees accept no responsibility for damages to person
or equipment that may arise from using these designs.
The following circuits will be updated as time allows. A brief
guide to the components quality is given with each design, this
is for guidance only - experiment. Full iron kits including output
transformers, input chokes and power transformers can be obtained
from Black Art if required.
Circuit 1 - Simple 417A Power Amplifier
A very simple design for a 0.5 Wrms power amplifier ... yes thats
right 0.5W, we measured 1.0 Wrms @ 5% THD ( mainly 2 nd ) decreasing
to 4% at 20 kHz. Dont tune out as this will drive 91 dB speakers
to mild listening levels. However the design is really aimed for
those of you lucky enough to own efficient speakers c. 95 dB/W/m
and above, such as Lowthers, Corals, Fostex, Edgar horns, Altec
etc.
Incredibly simple using only 4 resistors and 2 capacitors, it
will take longer to build the cabinet than wire the circuit. The
low component count means the best quality components can be used.
We used silver foil for wiring, Holco/Vishay resistors and polypropylene
power supply capacitors.
Notes :
The decoupling capacitor on the 417As cathode has a great effect
on the sound - experiment with size and type.
All grid pins on the 417A must be used, use very short pieces
of wire between each pin to a "star" grid point.
For those of you without an oscilloscope to check this baby out
add a ferrite bead to the grid pin.
Different recifiers can be used ie 5AR4,5R4 etc but sound will
change, voltages are for type 5V4.
The 100 uF power supply cap is Ok for 92 dB od speakers but users
of 95 dB+ should try 200uF or another choke section
Make sure ALL signal earths go seperately to a single "star"
point.
Needs about 2 Vrms for full power out, Ok for direct CD and pre
amp out connection.
Can be used as a pre amp to drive very long cables ( you must
lower the imput resistance on the following power amp )
Addendum : We have been recently experimenting and have determined
that sonics improve ( cleaner ) with minimal power loss if the
primary load the 417A sees is increses to 6000 Ohms.
Click here for the 417A power amp circuit
diagram
Circuit 1A - Fixed bias 417A Power Amplifier
Well it just gets easier, throw out the bias resistor and bias
decoupling capacitor ( the one that changes the sound so much
in the previous circuit ) and replace them with a lead acid battery.
The battery provides fixed bias and charges at the same time.
We used a "D sized" 2 V lead acid battery as this provided
the exact voltage and looked about the right size ?. The battery
is a surprisingly good bias source and doesnt really loose the
plot until somewhere around 300 kHz. Most would admit that a 300kHz
bandwidth is good enough for audio. Noise is very low and better
than some of the alternative fixed bias methods ie LEDs and grid
bias.
Click here for the fixed bias 417A power
amp circuit diagram
Circuit 2 - 2A3 Single ended power amplifier ( SRPP Driver
)
A relatively simple direct coupled 2A3 power amp to give around
3.5 Wrms. This is no a Loftin-White circuit, I have not included
it as there are plenty of variations around on the net. Check
out the excellent Sound practices magazine on 2A3s. ( cant remember
the issue ).This circuit seeks to address some of the problems
with the LW circuit mainly getting enough volts for linear operation
of the driver stage. The penalty for this is having to mess with
higher voltages on the B+, and the big hot cathode resiston on
the 2A3 ( heatsink this ). I have included two variations of driver
tube for experiementation. The 12AX7 for loads of gain and a slightly
smoother sound and the 6DJ8/6922 for less gain with a harder edge.
Click here for the SRPP DC 2A3 power amp
circuit diagram
Circuit 2A - 2A3 Single ended power amplifier ( Anode loaded
driver )
Another relatively simple 2A3 power amp to give around 3.5 Wrms.
This time its a fairly conventional anode loaded driver stage
using 6SN7 tubes. The only departure from norm is to split off
the power supply and pi filter it so the driver stage has a decent
voltage to run off. Use soft recovery diodes in this application.
The driver stages pi filter resistor ( the 2 k 2 W baby ) could
be replaced by a small choke to throw away some of the solid state
diode sound. Sonics are a bit smoother and rounder than the other
circuit mainly due to the anode loaded topology.
Click here for the anode loaded 2A3 power
amp circuit diagram
These are some curves that we have developed because we couldnt
find the actual curves. Pinouts are determined by looking through
a strong light and power disapations guessed but the valve curves
are generated using these pinouts , so they must be pretty right.
Parameters are calculated of the valves so experimental error
may mean that mu does not = Gm x Ra so dont get upset.
CV 4079 MEDIUM TRIODE mu =4 Ra = 1000 Gm
= 4000 9 Pin 6.3 V Click here for curve
50 kb
CV 4079 available from Retrovox see Retrovox
- surplus valves & stuff
6DN7 MEDIUM TRIODE Used in ultra cheap experimental
SE amps see Valve magazine for details. Can make a good driver
for power triodes. Click here for curve
53 kb
10 - VT25 family of DHT POWER TRIODES - hard
to impliment , low power and virtually extinct. This valve can
give you 1.6 W of awesome midrange vocals if your prepared to
go the distance with the circuit design.
10/VT25 GE mu =8.5 Ra = 4500 Gm = 1800 4 pin
7.5 V Click here for curve 57 kb
10Y Sylvania mu =8.5 Ra = 3500 Gm = 2400
4 pin 7.5 V Click here for curve 64 kb
VT-25A Hytron mu =8.5 Ra = 3000 Gm = 2800
4 Pin 7.5 V Click here for curve 66 kb
