HAND CRAFTED AMPLIFIERS.

Here are some notes and prices for custom built amps.

One 300W push pull mono amp plus power supply......


All *NEW* amplifiers shown at this website have been custom built for customers.
Construction time may be several months.

Many people will not know exactly what will satisfy their needs.
If you wish to purchase a new sound system please email me at
info@turneraudio.com.au

Prices for stereo (two channel) amplification may range between $2,000 to $30,000
depending on the specification of the system which is built.
The price will vary depending on the choice of inclusions.
The choice range :-

INTEGRATED PREAMP....
Stereo two channel, single chassis, including power supply.
For each channel :- RCA sockets, asian made, gold plated, for 6 line level unbalanced inputs,
Record out, 2 signal outputs,
volume control potentiometer, source switch.
All triode operation using single ended mu-follower topology as described at Preamps pages. 
Standard input sensitivity for 5 inputs line in is 0.2Vrms for aux, tuner, tape, TV set, etc.
One input devoted for 1Vrms input sensitivity used for CD.
 
Additional Options :-
Passive preamp with no tube gain stages but with all inputs, source switching,
and 20,000 ohm volume control attenuator, switched or potentiometer.
Remote volume control.
More than two channel for Home Theatre surround sound with multiple
inputs and outputs and multiple control of volume and source selectection. 
Bypassable gain stage and switch.
Bypassable tone control stage with switch.
Dual outputs for each channel with dual volume controls.
Balance control.
Stereo-mono switch.

Notes on 300W monobloc amps in the picture above :-

The amps are nominally 300 watt class AB1 using 12 x 6550 or KT88 output tubes,
2 x EL84 driver tubes, and a 6CG7 input tube.

http://www.turneraudio.com.au/300monobloc.html
300W amp input/driver and output stages. 
300W amp power supply.
300W amp active protection.
300W amp dynamic bias stabilization.
300W amp power vs load graphs.
300W amp images, tubes with blue glow, more blue glow, 2 amps on bench,
umbilical cables, amp end view.

I have one pair near completion, 2011.

Output tubes.
Standard Output stage has 12 x EH6550 fitted.
Optional other tubes can be 12 x KT88, KT90, KT66, EL34, 6CA7, 5881, 6L6, 6V6.
Tubes other than 6550 may need workshop adjustment of bias and feedback arrangements.

Class A1 power.
Class A power depends on the idle bias current in the output tubes and load match
setting of the output transformer windings.
For 99% of listeners, there is **enough** pure class A power to cover all the
power required even with speakers rated for only 81dB/W/M.
Because there is no need to ever adjust the bias, automatic bias setting is achieved
with individual cathode bias capacitor and resistor networks for each output tube.
This means you don't have an absurd 24 bias adjustments for the two channels.
I have set the idle current in each 6550 output tube for = 35mAdc, and the Ea = 460Vdc approx
so each 6550 operates with Pda = 16.1 Watts. The amount of Pda for 12 tubes =  193.2W.
The maximum pure class A power possible is 45% of the idle anode power liberated
in the output tubes = 86.94 Watts, but the speaker load value must be
a fairly high number of ohms to achieve this.

Two load settings are available for speakers, 2.5 ohms and 5.6 ohms.

This may seem strange in a world where people like to see
4 ohms and 8 ohms printed on the rear panel of an amp.

But in fact a speaker with a nominal impedance of "4ohms" may in fact have
impedance which varies between 2 ohms and 20 ohms.
4 ohms may be the "average" impedance between say 100Hz and 1,000Hz.

The next two tables show output power at clipping, and with amounts of pure class A and balance of class AB
possible with two different anode voltage supply and anode current operating conditions....

The THD at a few watts is less than 0.02%.
I doubt biasing output tubes closer to their dissipation limits give better music.

Local Negative Feedback.
The output stage has 20% of the tube signal voltage as local cathode feedback applied
from the output transformer to the output tube cathodes and is equal to about 8dB of locally applied NFB.
This causes the output tubes to behave more linearly than triodes and with a lower plate resistance
while retaining the high output power of a beam tetrode.
Global Negative Feedback is also applied from the output transformer speaker secondary
to the cathode of the input triode in the conventional manner to reduce output resistance
and distortions so that there is a total amount of local 8dB of local CFB plus 8dB of global NFB to total 16dB.

Supply voltages.

Anode supply voltage is +500V, unless ordered to be less before purchase.

Idle bias current in each 6550 is 35mA for long tube life, or slightly more if Anode voltage
is supplied at lower voltage for higher class A1 power.

Cathode biasing and coupling caps, for use with anode voltage = +500Vdc max.
Each output tube has 500 ohms (rated at 15 watts each)  plus 1,000 uF networks at
each cathode to provide automatic Cathode Biasing.
The 12 x 500 ohm resistors regulate the anode current of each output tube with +18Vdc at the cathode.
Fixed bias of -14Vdc is also applied to all grids to allow the highest output power ceiling.
Each output tube grid is separately RC coupled to the
EL84 driver stage anodes with 0.47uF caps plus 120k grid bias resistors.

Matched tubes are not mandatory.
Twelve matched output tubes are not needed since with so many output tubes the sum of
the characteristics of the six tubes on each side of the PP output stage will usually be close.
The self regulation of individual cathode bias for each output tube also makes
it less critical to use matched output tubes.

Dynamic Bias Stabilization is a unique circuit technique used to regulate cathode bias
during high power class AB operation when cathode bias voltages tend to vary and slightly
upset the DC balance in the OPT.
The special cathode circuit with active solid state components do not have any
effect during class A operation.
The net result allows the amp to have the same low distortion advantages
of a 100% fixed bias amp but eliminates any need for
bias adjustments which trouble many owners greatly.

Solid state rectifiers & rail filters.
Solid state rectifiers are used  throughout.
Noise in the dc B+ power supply is well filtered out with a CLC filter using 470uF input cap + 1.8Henry choke
 + 470uF reservoir cap with generously rated following RC filters for the input stages.
Shunt regulation is applied to the input stage rail.
DC is applied to the input tube heaters to ensure hum remains imperceptible.

The driver stage is a differential long tailed pair, LTP, with 2 x EL84 connected as triodes
with balanced CT choke to supply dc to each triode. This enables the stage to produce a
high maximum possible voltage swing of 300Vrms+ grid to grid at low THD.
A maximum of only 160Vrms grid to grid swing is needed.
The output impedance of the driver stage is low and its bandwidth high and
reliance on global NFB to reduce drive amp distortions is minimal.
Each EL84 in triode mode is equivalent to 5 half sections of a 6CG7/6SN7.

The Input stage is a 6CG7 twin triode with both triodes in parallel.
Although set up in common cathode mode, its acts as a differential amp with the signal
input fed to the main high impedance input grid and the second low impedance port is the cathode
to which is applied the global NFB from a low resistance voltage divider from
the OPT secondary speaker connection.
Since 2008, experiments using the input 6CG7 as a true differential amp with constant
current sink for both cathodes and running the EL84 stage as a true balanced amp with
balanced input to each EL84 grid has proven to be slightly superior.

Constant Current Sources.
In the 2008 schematics a constant current source is used for the dc anode supply
for 6CG7 input tube and a transistor constant current sink used for the EL84 common
cathode circuit to to ensure low distortion and excellent drive voltage balance.
The transistors are working as slaves to the tubes involved, and have no active voltage amplifier role,
and hence no sonic signature, and they act as a better alternative to using resistances or active tube elements.

Umbilical cables.
The power amp chassis each have two industrial grade 1.2 metre long umbilical cables
hard-wired to the amp chassis with octal plugs which are plugged
into the color coded reinforced sockets on the power supplies.
See 300 watt images page for umbilical cable details.

Mains power draw.
With Ia = 35mA per output tube and B+ = +500V, each output tube has
about 20 watts of combined anode plus screen input power so total for 12 output tubes is about
240 watts plus about 8 watts in the 12 cathode resistors.
The LTP driver and input stages use about 20 watts of anode power and other losses.
The cathode heaters require 150 watts.
With 4% mains transformer losses, total mains input power is thus approximately 440 watts per channel.

Mains Voltage selection can be made for 100V, 110V, 120V, 200V, 230V and 240V
all at 50Hz or 60Hz because there are TWO power transformer
primaries each for 120V with two taps for lower voltages.

Output transformers have a 110mm stack of 51mm tongue GOSS E&I laminations.
OPT bandwidth is 13 Hz to 270 kHz at 200 watts even with no negative feedback.
With NFB added and with some critical damping networks the open loop bandwidth
and phase shift is tailored to ensure that the amps are unconditionally stable
and able to drive any type of load including the most difficult ESL.
With the NFB the bandwidth is restricted to a safe 84 kHz, -3dB, with resistance loads.
Five primary winding sections and six secondary sections are
used to achieve the flawless high frequency response.
There are 2 secondary windings per secondary section allowing for two waste free and
uniform current density arrangements to give ideal load matches to 2.5 ohms and 5.6 ohms,
thus allowing two ranges of *nominal* speaker loads, one between 1 and 5 ohms and the
other anything above 3 ohms.
Using a speaker with nominal impedance of 8 ohms connected to the 2.5
ohm outlet setting results with nearly all output power being class A1.

Power transformers have GOSS E&I laminated cores and are rated for
1, 900VA with windings rated for 600VA.
Iron losses are only 4 watts, with winding losses less than 5%. Temperature rise and noise are negligible.

Operating bandwidth at 250 watts = from 13 Hz to 84 kHz, -3dB, resistive load.
Input impedance = 80 kOhms.
Output impedance = 0.45 ohms, 5.6 ohm load match, 0.22 ohms for 2.5 ohm load match,
Damping factor = better than 10.
SNR. At 200 Watts into rated load  = -90dB unweighted, and noise at zero signal < 1 mV.
THD at 250 Watts, 1 kHz, 5.6 ohms < 0.25%.
THD at 25 Watts, 1kHz, 5.6ohms, < 0.04%

Note that if you only had 2 x 6550 to make 42 Watts max at 0.25%,
then at 25 Watts the THD = 0.20%, if operating conditions were identical.

Active protection is provided so excessive cathode current in one or more
output tubes will automatically turn off the main anode supply at the power transformer.
There is also inrush current limiting at turn on to allow the use of sensitive mains fuses.

Warranty on tubes is 90 days, and two years for all amplifier parts
but if an amp is dropped or altered or used incorrectly, it would void the warranty.
All parts are able to be replaced after re-making them if needed.
Output tubes are most likely to wear out first but after 4,000 hours cathode electron
emission may only fall 10% which will have an imperceptible effect on music and little effect on measurements.
5 years of tube life is not unusual if the amps are used on 200 days per year for 4 hours each day or evening.
Sometimes there is a random failure of a tube, especially since there are so many
but my experience is that early tube failure is unusual and not inconvenient if a few spares are kept on hand.

Service information.
Full service information is provided with each amplifier complete with 8 schematics and
explanations and amendments if optional changes are selected.

THE POWER SUPPLY AND AMP CHASSIS CONTAIN POTENTIALLY DANGEROUS VOLTAGES.
THEY MAY BE SERVICED ONLY BY EXPERIENCED TECHNICIANS.

Power supply temperatures.
The power supplies have no tube rectifiers and run quite cool, and do not require any special ventilation.

Amplifier chassis temperatures will be warm.
The amplifier chassis do require well a well ventilated area,
and will act as desirable room heating in winter.

Input terminals are standard unbalanced RCA sockets.
Balanced input for XLR is an optional extra and with a balanced 10k:10k input transformer.

Output terminals are 2 pairs of recessed 4mm banana sockets to enable
two pairs of speakers to be connected or bi-wiring one pair.
I do not like binding posts for spade or other screw tight methods because
the screw tight posts inevitably become loose. Protruding binding posts tend to be
broken off the chassis or bent during moves and are a clumsy old fashioned way of
connection and do not contribute anything to sound quality.
If a speaker cable is accidently yanked, you would want it to slip out and away from the amp,
and not pull the amp off a bench onto the floor so I much prefer banana plugs.

Amp chassis size and weight.
Each amp chassis is 630mm long, 250mm wide, and 230mm high and weighs 24Kg,
and made with a welded steel frame, mild steel sheet transformer enclosures and
natural anodized aluminium top plate.
The steel grille over the tubes allows removal of tubes through grille openings.

Power supply size and weight.
Each power supply for each amp chassis is 300mm long, 250mm wide, and 230mm high and weighs 26kg.
The power supply enclosure is mild steel sheeting, with the mains on/off switch for the channel mounted
in the top of the power supply cover.

For any enquiries email Patrick Turner at  info@turneraudio.com.au