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.

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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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