MISCELLANEOUS  SCHEMATICS 1
 
WARNING!
TUBE AMP CIRCUITRY REQUIRES EXTREME CARE
WITH POTENTIALLY LETHAL VOLTAGES.
YOU HAVE BEEN WARNED.

Here are some more archived schematics of interest.

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                                Low Power 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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1010 watt ULAB1 integrated amp

Schematic 1010 PP amp
          6GW8.

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 taken
from old radios. Amazingly, it worked a bit, not very well, but was dangerous,
and was a mess to look at.

The existing transformers were put onto a new stereo chassis, made large enough
for a source select switch, volume and balance control. I 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 in 1998,
well before I could type, or produce 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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Two triode phono stage with NFB eq. 
Schematic
          phono amp 2 x 12ax7.
This is the simplest phono amp I know of. Where someone has a reverse RIAA eq network
and sig gene the values of C5, C6, R8, R9 can be trimmed to get a very good square
wave at 1 kHz and a level response within +/-1dB with sine waves between 50Hz and 16kHz.
This type of phono amp was used routinely in thousands of phono stages fitted to budget
hi-fi gear in the 1950s and 1960s.
The downside is the high output resistance of about 20k at low frequencies because there
is only 6dB of NFB at 20Hz and not much more at 50Hz so the input resistance of anything
connected after this stage needs to be above 200k for unattenuated low bass.

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Three Triode phono amp, Passive RIAA eq.
Schematic 3
          x 12ax7 phono passive eq.

The above phono preamp schematic is designed for ease of construction, minimize
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 zener diodes, 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 further down this page.

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 zener diodes, the total anode power
will be only
11.2 mA x 320Vdc = 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 Vdc which will be about 90Vdc, hence the 10k peak swing is [ 10 / 110 ] x 90 = 8.2 volts.
In fact even if we had a 5k0 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
Schematic 3
          triode phono stage.

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. The 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 RIAA eq 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 likelihood 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 short lived experiment with quadrophonic sound
using a multiplexed signal containing 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.

See power supply schematic at  Phono Amp PSU Schematic.
Note, the PSU shows simple zener diode shunt regulation of the B+. Clever people will
use a series pass element voltage regulator to avoid any wasted B+ Idc current. 

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 must be used,
along with chokes. Use them if you must, but
modern high value electrolytic capacitors allow
RC filters to be used.

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 separate chassis.

But in power amps I build, the use of tube rectifiers would cause excessive inefficiencies, and never
improve the sound quality.

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Phono amp PSU
 Schematic phono amp PSU

It can't get much simpler than this.

NOTE ABOUT POWER TRANSFORMERS.
You will never be able to buy suitable PT with a 240Vac sec for the HT for B+ of +300Vdc
for a tube preamp from your local electronics parts dealers except
EVATCO in Qld,
who may stock a suitable Hammond PT imported from Canada.
Other local electronics dealers with many stores around Oz are are RScomponents, Jaycar,
Dick Smith, wescomponents.
Jaycar do have some 1.4VA to 60VA PTs which have 240Vac primary
and up to 30Vac secondaries with multiple taps to suit most low voltage needs for most
people's solid state projects.
See the Jaycar online catalog page for ac - ac transformers at

https://secure.vividcluster.crox.net.au/jaycar2005/productResults.asp?MID=12&SSUBID=838&SUBCATID=1002&keyform=CAT2#12

Then scroll down to the 240V:15V transformers, catalog number MM2004,
240V : 15V at 2 amps max, 30VA rating. Sec taps are at 6V, 9V, 12V and 15V. I've always found the sec
voltages are a little higher.

1. Buy TWO of these trannies, and each is priced at $18.95 ( 2012 price ).

2.  Use one of them with normal mains 240V primary. Its becomes T1.
The secondary is not grounded, as it will be biased at about +60Vdc. From the Com to 9Vac
tap you can produce a rectified voltage of about +12Vdc using one 1N5408 diode and a
4,700uF cap. Then use an R&C filter to get a wanted 6.3Vdc at up to 2Amps dc for the
cathode filaments. If you have 4 x 12AX7, you need 1.2Amps, so the RC filter will be 4.7
ohms rated for 10W, plus 2 x 4,700 caps rated for 25Vdc. This gives 50Hz ripple of about
85mV, low enough. The RC filter could two RC sections each 2.2r + 4,700uF, and ripple
slightly lower at about 60mV. 

3.   Use the second transformer, it is T2. It is connected "back to front" with its 15Vac winding
connected right across the T1 15Vac winding of the first transformer. There will be approximately
240Vac produced across the T2 240Vac winding which can be used to produce up to about
+320Vdc at 15mAdc. This 240V winding is doubly isolated from the mains.
Don't allow the B+ Idc to be higher than 15mA, its about 5 Watts of power.

4.  CHECK TOTAL POWER DRAWN. The first 30VA tranny must not draw more than 30VA from mains.

There MUST be a mains fuse, I suggest 0.25Amp, slow blow type 2AG.

There must be a 3 amp slow fuse in filament winding circuit.

There must be a 1 amp slow fuse in T2 15Vac winding circuit.

Secondary of T1 loads can be:-
filaments = 9Vac at 1.7A = 16VA
Max load of T2 sec = 240Vac x 20mAac gives 320Vdc x 14mAdc = 5VA, then add 10% losses.
load at T1 sec = 5.5VA, so current = 0.37Aac.
Total maximum current in T1 sec = 1.7 + 0.37 = 2.07A which is OK as winding has 2A rating.
VA used = filaments, 16VA, HT = 5.5VA,
TOTAL VA = 21.5VA which is less than the 30VA rating, so OK.

5. Hopefully, you won't make clouds of smoke as the fuses should protect you against
your mistakes, which I confidently predict that you will make.
Get a tech to check out your work before turning anything on.

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10 tube Preamp, April 2000
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 favor amongst 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 favorable 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 outputs, 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 separate 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 exceeding
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.
Schematic of
          preamp with 1 x 2SK369
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 determined 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.

Where one uses 3 x 2SK369 j-fets each with 5mA of Id the total gm = 120mA/V.
Therefore where you had RL = 1k, the open loop gain, A, with unbypassed Rs = gm x RL
= 0.12 x 1,000 = 120.
If you have 100 ohms for Rs, then the gain with NFB , A' = A / 9 1 + [ A x ß ] )
= 120 / ( 1 + [120 x 100/1,000] ) = 9.23.
NFB = about 22.7dB and THD with any phono signals will be low.
Does it sound better with 3 or more paralleled j-fets? its easy to say that it would but who
really knows? For high gain, Rs for each of 3 j-fets can be 18 ohms for each j-fet unbypassed
from source to 0V to eliminate the need for the negative supply so Egate can be kept at 0V bias.
The separate Rs for each j-fet source is necessary to make sure each shares the drain current
equally.
 
Gain will then be = 120 / ( 1 + [ 120 x 6/1,000 ] ) = 70, and only 4.6dB of NFB is involved.
With higher gain there is higher Miller capacitance. But because MC cartridges have very
low output impedance of below 50 ohms the Miller C will not affect their output and I have
placed a whopping 0.1F across the input in parallel with a 1k loading R for a Denon 103R
cartridge and heard no HF losses. My CRO showed me that 0.1uF reduced the
easily visible THD from a test record above 8kHz. 0.1uF has 80 ohms of impedance at 20kHz.
If the MC cart has low Rout, then the 0.1uF shunt C has little effect.
 
So a 0.3mV input signal gives 21mV output at 1kHz but remember that at 10kHz with RIAA emphasis
there would be 12dB higher input signal so output = 84mV and THD is a little higher but then
still way lower than what the cart gets from the record at that F.
( THD tends to be high at higher F.....maybe it leads to harsher sound.
So hence I don't like to have the j-fets doing very much.)
 
The more j-fets paralleled the less noise you are supposed to get. Noise is halved for for each
quadrupling of devices. With x 3 devices, noise will be 1 / sq.root of 3 compared to just one device,
ie, 0.58 times less or roughly - 4dB.
This is offset by the noise of the source resistances; the lower Rs the lower the noise.
Usually the gain x noise from the input is higher than any noise in the drain load resistance.

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.
BJT 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

 schematic RIAA
            reverse eq filter for RIAA.
 

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 piece 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 oscilloscope 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 output 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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