Please excuse the hand drawn circuit presentation of
schematics and large file size.
1010w Integrated amp
One channel shown with 2 x 6GW8, UL, classAB
Two triode phono stage NFB eq
1 x 12AX7, feedback RIAA eq
Three triode phono stage
NFB
eq
1 x 12AX7, feedback RIAA eq, buffer
Three triode phono stage
Passive
eq
1.5 x 12AX7,
passive
RIAA, buffer
Phono
Amp PSU Schematic
LowPower supply for 2 and 3 tube preamps
10 tube Preamp April 2000
5 x twin triodes per channel
1 x j-fet 2SK369 simple
pre-preamp
A test circuit showing THD for a single 2SK369 j-fet
Test filter, Reverse RIAA eq
A simple test filter with discrete RC components
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In Australia there was once what was called the Playmaster Amplifier
series of amps which
were published in the old Electronics Australia magazine, long since
closed down. This circuit is a development of the original and this was
prepared for a client who had a sample of one of these Playmaster amps
on a very poorly home made chassis, with lots of what must have been
1930's resistors and capacitors. Amazingly, it worked, but looked
dangerous,
and was a mess to look at.
The existing transformers were put onto a new stereo chassis, made into an integrated form, and included simple 12AX7 triode phono stages in case he ever got a TT. It packs more punch than any other ten watter that's ever been heard, and shames Krells, and big clunky stuff like that. No pictures available, the project was supplied to a happy client, before I had a website.
Note the transistor voltage regulator used to prevent LF instability when the phono stage is used.
The circuit is very simple, and uses a concertina driver stage for the output tubes, and a triode gain stage for input stage. Less than 20 Db of Global NFB is employed.
Its a real lively fun type of amp, and the owner says that when his wife is in the bathroom, located down a hallway, she can hear when he plugs the amp into the system instead of the solid state amp... "Ah," she says, "that's better."
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Three Triode phono amp, Passive
RIAA eq.

The above phono preamp schematic is designed
for ease of construction, minimise component numbers, allow a
fairly
accurate RIAA eq with the aid of a reverse RIAA network, make use of a
well known
common tube type, have low distortion, a warm sound, high gain, and a
low output impedance for somebody who may want to use it to transfer
vinyl to CD or run long cables to an integrated preamp or power amp
well away from their listening
chair.
What more could you want?
The calculations for passive RIAA eq are
simpler than trying to use NFB and
since triode distortion and noise is
negligible at such tiny voltages involved
with a triode phono stage, then why use
any NFB?
Well some people would say they want extremely
low distortion, lower than
lowish.
I leave the experimenter to decide which sounds the best.
Without NFB, some care has to be taken with the power supply, and I have shown above an absolute minimum shunt regulation arrangement using a 10k resistor, 1 x 470 uF cap, and 3 x 75 volt x 5 watt rated zeners, and I have assumed you will have a +320 volt supply handy.
Making assumptions never got me anywhere in
life, so for a basic power supply, go to
Phono Amp PSU Schematic so that
you know what I am talking about.
The power supply shown should suffice for such
a basic circuit, with some room for use
with tubes which draw a little more anode
current. Since we only want 5.2 mA of
anode current supply
for 3 x 12AX7 tubes for two channels,
and then need 6 mA of current flow in the shunt
regulator zeners, the total anode power
will be only 11.2 mA x 320v = 3.6 watts, using a
240 volt secondary winding on a power transformer. Such a power tranny
would at least also have a heater winding to provide 6.3 volts x 0.9
amps = 5.67 watts, so the total power use is only 9.27 watts.
In practice, we would purchase a 30 VA
transformer, because they are likely to be more rugged.
It could have a 120 volt sec, which could be used
in a doubler circuit to make the +320 and
the heater winding of at least 6.3v could be
used in a doubler as shown to make +16 volts at 0.45 amps, which can be
filtered down with R and C to provide a noise free DC heater supply.
A12V shunt regulator zener diode and
series diode is used to make to make 12.6Vdc.
and they would draw 0.3 amps
The schematic calls for the heater supply to
be biased at +50Vdc, to relieve the pressure on insulation between the cathode and heater in the
cathode follower, only rated for 90V.
This is easily done by making a voltage
divider in the power supply
voltage output bypassed to ground via the
100 uF, or else just taking a 100k from the top of the first 50V zener
diode
to the floating heater circuit, and bypassing to ground with 100uF.
Where heater voltage is 12.6Vdc, the lower side of the 12.6v should be
at +50V.
Bias isn't critical, one could have bias between +50V and +70V.
The idle current in the cathode follower
buffer stage is about 0.8 mA, which doesn't sound like much, and it
isn't much, but even if the load on the output is only 10k only,
we can get a maximum over 5Vrms.
The limit for low distortion output voltage
production from a cathode follower into a low RL is the cut off of current in the tube. In this
case when the grid voltage goes negative
enough to cut off the tube current,
the 10k ac RL and the
100k follower dc RL form a divider, and the
peak negative going voltage is limited to
[ 10k / ( 100k + 10k ) ] x Ek. Ek is the
cathode idle DV which will be about 90 volts,
hence the 10k peak swing is [ 10 / 110 ] x 90 =
8.2 volts. In fact even if we had a 5 k RL, we could get 4.3 peak volts.
Under normal use, the output voltage won't be
this high, as the average 2mV input from
a MM cartridge will produce an
average 0.8 volts output.
We could even afford to use a lower gain second
gain tube, such as a 12AY7 or 12AT7. If we wanted more voltage swing into a low value
ac RL, we would be better off using 1/2
12AU7 for the cathode follower, so we could
have an idle current of 4mA, and the dc
RL would be 22k, and with a 5k
ac RL, we could get a peak voltage swing
of 18V.
I would add that the gain tube before the
cathode follower could easily produce up
to about 45Vrms of signal output, so you won't
ever overload such a preamp with a phono signal.
The output impedance of the follower is about
700 ohms, so cable capacitance and low input impedance will not result
in high
frequency losses.
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Three Triode Phono amp, RIAA eq with NFB

The above phono preamp schematic is virtually
as simple as the two triode phono stage with NFB eq
but has an additional cathode follower buffer output. It should sound
well.
It is unknown if this topology sounds any
better
or worse than the passive eq method,
and I leave arguments about the sound to others,
but my experience is that the
NFB eq method compared to the passive eq method
can be very good indeed,
if the values of the FB network are chosen
carefully,
and the network doesn't impose
too low a load on the tubes driving it, even
at HF. Thr values of R&C in the RIAA feedback network should be
trimmed with the aid of a reverse RIAA network
used to equalize the input signal in the same way as a a record cutter
amp.
The final buffer stage is where the
feedback take off point is for the NFB network.
Any error signal created by the following cable
or impedance will be
subject to NFB correction and will have less
effect
than with any other normal
follower stage output stage.
A normal 1/2 12AU7 cathode follower as used in the above 3 triode amp
with passive RIAAeq will
have Rout = 600 ohms approx. The Rout will be further reduced by the
loop NFB for the RIAA eq. The amount applied
varies with frequency and is only about 6dB at 10Hz so Rout at 10Hz at
the cathode of the 12AU7 will be about 300ohms but at 1kHz it will be
30 ohms and even much lower at 10kHz.
There is a slight danger that with so much NFB that HF oscillations
could be a problem if cable capacitance was high
so to safeguard the amp against any possibility of oscillation a series
R of 270 ohms is connected between the
cathode and output terminal.
The 5k horizontal
R in the NFB network imposes
a HF time constant well above
20 kHz to prevent the likelyhood of
oscillations
at HF when the cathode
of the follower becomes virtually directly
connected
to the input tube cathode at HF,
via the capacitors in the NFB network. We don't
need the NFB
to keep increasing above 30kHz, and we can level
off the amount applied with this
trimmer resistor.
It is unlikely that any recordings have had any
significant HF content added above 25 kHz, ( unless the
recording was for
the shortlived experiment with quadrophonic sound using a multiplexed
signal conaining a 50 kHz modulated
carrier tried back in the 1970s. ) Indeed the
cutting head amps would have
filters
to prevent HF oscillations,
so if the supersonics were not emphasized, there
is no need to de-emphasize
what was never emphasized.
The above amp will have less gain than the 3 triode passive preamp, since the FB is active at LF, but still the gain will be more than adequate for all concerned.
I have an alternative power supply schematic at
Phono
Amp PSU Schematic.
This schematic shows a 225 volt supply, but it
could be easily made up to provide
more voltage, if suiatble 5 watt zener diodes
are found.
I do not have strict rules for power supplies,
other than insisting the voltage they supply be
from a substantially low impedance at low
frequencies,
and free of noise from the
rectifiers.
I do not subscribe to the school of thought which
stipulates that paper in oil
caps are all that are permitted, and chokes
always
must be used. I hear no evidence
in the sound quality that modern capacitors fail
to allow music to come from amplifiers.
For the DIY constructor, feel free to build your
supplies any way you wish but remember phono stage amps need
well filtered supplies.
I have done supplies with tube rectifiers, tube
regulators, and they can all be made to work effectively and reliably
at
such low levels of power. I quite enjoy seeing a nicely tubed
power supply for a preamp, and I have built a
couple for customers on seperate chassis.
But in power amps I build, the use of tube
rectifiers
would cause excessive
inefficiences, and never improve the sound
quality.
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Phono amp PSU
It can't get much simpler than this.
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10 tube Preamp, April 2000

This schematic of one preamp channel was used in my own "prototype" preamp, shown in the picture at the Preamps1 page, and was firmed up in April 2000. It is a good performer with phono provision for moving magnet phono replay.
From left to right, the first tube is a 12AX7 mu-follower with gain
= 88, ( 39 dB ),with a
low Rout to feed a passive RIAA filter network for vinyl use. This
suits MM cartridges with
outputs down to about 1.0 mV. Second tube is a 12AT7 SRPP to provide
extra gain of 40 times ( 32
dB )
to complete the phono stage, to give a low Rout.
The gain of the phono stage is 350 times at 1 kHz, or 51 dB.
The third stage is a SRPP stage with a 12AX7 to provide deletable
tone
control for high and low frequency eq. It uses a Baxandal network in a
variable shunt feedback network,
and has a gain of exactly unity, or 0.0dB.
This allows for seamless insertion into a signal path for accurate
comparison of with or without tone control. Although tone controls have
lost favour amoungst audiophiles,
they are very useful for checking a speaker's basic bass, mid, treble
balance,
and correcting some bad old recordings, ( and some lousy new ones!
)
The fourth stage is another 12AT7 SRPP stage which is also
deletable,
to provide a gain of 8 times ( 18 dB ), for the line level inputs.
There
is shunt feedback used to reduce the 12AT7 gain to a sensible level,
and
to reduce the Rout to feed the balance controls and a pair of dual
ganged
gain pots, seen in series, which allows the tube to see a favourable
loading.
The effective total line stage gain is thus 4.5 times, (13 dB).
Two switches allow or disallow the use of tone control or line stages. In practice, both these stages are rarely used.
The fifth stage is a pair of cathode followers using a 12AU7 which
buffers
the output
from effects of combined losses from interconnect cables and any
following
power amp input circuit. Gain is about 0.92 times, or -0.7 dB.
The output impedance of the preamp is thus only 600 ohms, and it will
power
any loads down to a few k ohms,
and the preamp will be compatible with any SS equipment.
Both left and right channels have two separately variable pairs of
outputs
so that two systems can be set up with different sensitivity power
amplifiers to allow their comparison at an exactly similar level of
signal
output.
It would be easy to use such a preamp to provide filtered outputs
allowing
the use
bi-amping.
The phono stage input suits moving magnet type of cartridges. The
use of moving coil cartridges would require a pre-preamp, or a
step
up transformer,
or an additional amp stage built into the circuit.
The 'Preamps1 page' gives recently developed details for cascode
circuitry in the phono
stage using a j-fet
for low level MC inputs.
Power supply requirements
The tube line up includes
Phono, 2 x 12AX7, 2 x 12AT7, Line gain, 2 x 12AT7, Tone control, 2 x 12AX7, Buffer outouts, 2 x 12AU7.
The total anode current supply required for the 20 triodes involved will be approximately 25 mA. I use a transformer capable of 50 mA. The B+ supply needs to be about +340v to +370v, so about 8 watts should be allowed. In my prototype, I used a seperate potted B+ transformer rated for 30 VA.
The heater supply for 20 triodes requires 12.6 volts at 1.5 amps, so
about 19 watts is needed,
but I use a 30VA transformer, also potted.
DIY folks should allow for future changes to signal tubes such as the
6CG7, or other octal
tubes, which need twice the heater currents as the above listed tubes,
so in fact the heater current rating of the transformer should be for
3 amps.
The two transformers of my amp are mounted inside a mild steel on the
chassis which
is 500 mm long. There is thus sufficient distance between the sensitive
phono amp tubes,
and the power supply.
If the one power transformer is chosen, it should be rated at 50 VA, and be made using a Bmax of no more than 0.8, have GOSS cores, and should be potted.
I used all DC for the heaters, and I had a LM350 regulator, which is a TO3 device to remove the ripple from the heater supplies after the rectifier. There were no chokes in the preamp. The transformer heater windings, rectifiers, and regulator are not connected to 0V directly, but are "biased" up at +70v to allow the use of the SRPP and cathode followers without exceeeding the 90 volt ratings for the heater to cathode insulation. Then to keep hum out from stray transformer capacitance from the mains primary, the whole heater is bypassed to 0V with 100 uF. There was no hum in my prototype amp.
There also has been attention paid to 0V paths in the amp, and the
schematic
doesn't
indicate the optimum 0V routing.
The chassis is connected directly to the earth wire coming from the
wall socket,
but the 0V line is only connected to the chassis via a 5 watt10 ohm
resistor.
Thus it is difficult to get earth loop hum problems, despite the
un-balanced
circuitry used throughout.
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Simple phono pre-preamp using 2SK369 j-fet.

This test schematic could be used for a pre-preamp ahead of a normal MM
phono amp for MC use.
But it was used to get some idea of how linear a high gm j-fet
such as the 2SK369 (or 2SK147) really is, and the answer is that above
0.1Vrms output the THD becomes excessive even with nearly 12dB of NFB
applied.
The THD can be reduced by near doubling of Id to 5mA, when the gm rises
to 40mA/V thus increasing the
open loop gain to 88, and thus increasing the amount of applied NFB.
The THD reduction is only marginal
and compared to a triode the j-fet
has about 25 times more THD for the
same output voltage.
For MC use, I recommend the drain RL be reduced to 1.2k so Id would
be raised to about 5mA.
The same 100 ohms for Rs could be retained and gain overall would then
be about 9.4x
which is enough to raise the 0.3mV from a typical MC at 1 kHz to 2.9mV,
with THD less than approx 0.005%.
The result will not be as quiet as a properly made cascode circuit, but
may be quite sufficient.
Output resistance will be dertermined by the RL of 1.2k and noise from
this R should be just 66dB unweighted
below the signal level of 2.9mV. The fet will not have troublesome
microphony that a tube would have in this application.
However, j- fets like bjts are very prone to stray magnetic fields,
along with any input wiring. Magnetic screening for the enclosure, well
filtered rail supplies and
well routed earth paths and short leads around above amplifiers all all
essential for low hum levels. Such pre-preamps should be set up well
away from any other equipment with a power transformer within
including innocent looking cd players and tuners and tape decks.
I tested small signal SE bjts in the same application and found them
all to give little better THD measurements
and the spectral content of the thd contained greater amounts of odd
numbered harmonics. Noise was worse.
And for lowest noise in a typical small TO92 package bjt the collector
current is usually about 0.2mA,
so the use of 5 parallel bjts is needed to get Ic up to 1mA, and then
the maximum output voltage is limited to
0.6mA rms, or 0.6V rms for a 1k load.
Input impedance is low and the biasing is more difficult and I just refuse to build any pre-preamps
with bjts.
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Test filter, Reverse RIAA eq
This filter is a great way to test the frequency response of a phono amp. The filter boosts all frequencies above very low frequencies in the same manner used during the record cutting process. The filter profile is equal to the RIAA profile used for all LP records.
The filter can be easily assembled on a small peice of plastic board, and fixed to the inside of a metal container, so that it remains well shielded, and fitted with RCA input and output sockets.
A signal generator which has at least 100 ohms output impedance, or lower is then connected to the filter input, and the amp to be tested is connected to the filter output.
An oscilliscope and wide bandwidth voltmeter is placed at the output
of the phono amp being tested, and the signal gene set for 1 kHz, and
the level adjusted for a 1 vrms output at the amp. If the RIAA filter
components in the amp are of the correct value, and have not drifted
over time, then the amplitude of all frequencies between 100 Hz and 10
kHz should appear
to be equal, with a roll off of only 2-3 dB at 20 Hz, and 20 kHz.
The test signal should be able to be changed to 'square wave' and the ouput wave when the gene frequency is 1 kHz should appear as a substantially square wave form, without peaks or troughs along its horizontal parts. This will show that the eq is phase correct, and confirms the accuracy of the amplitude response.
Using a filter like this is far easier than laboriously trying to measure the voltage output amplitudes of test signals accurately set to spot frequencies along the audio band. And any errors in the amplitude of the signal gene can be neglected if the square wave test is used, although I do like to use the sine wave test. A signal gene with a swept frequency facility will also show the response well.
There is a more scientific approach to reverse RIAA filters at
http://www.hagtech.com/pdf/riaa.pdf
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