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AU-111 Engineering 1965

Sansui Electric KK Technology Division

45w / 45w power Sansui AU-111
Stereo pre-amplifier

By  Engineer Chiwaki Kubo ( Sansui Electric KK Engineering Section )


Tone control  circuit that can change the ascending and descending points 


The sources for FM broadcasts, records, cassettes and hi-fi are increasing every year. This shows that Hi-Fi stereos are widespread. It seems that most hi-fi enthusiasts use separate amplifiers instead of integrated amplifiers.
This is because the separate type is less technically inadequate, so it can be designed with a rim and has excellent properties.
This device has been designed with an emphasis not only on features but also on easy of use with the intention of making it an amplifier that will add flavor when used.
In the following I will explain the new product AU-111 from Sansui .

=== Design and Mechanism ===

A quiet, heavy and mechanical front panel with a functional layout and extrusion .
The color is based on black, and we appreciated a sense of reliability and a more special feel not found in the past, and made it sober without an appearance that matched the performance of this machine.
Regarding the mechanism, rollers and side handles have been installed to make it easier to move or carry.


=== Preamplifier section ===

Input circuit :
It comes with Phono 1 (low level), Phono 2 (high level), tape head 19cm/s, tape head 9.5cm/s, aux,  tuner and tape monitor.
With Phono 1, MC type cartridge can be used by attaching an input transformer. As you know, the Moving Coil (MC) type cartridge is superior to the Moving Magnet. (MM) type and the Variable reluctance  type in terms of frequency characteristics, distortion, transition characteristics, etc., but it also has disadvantages.
It has a lower output voltage than other cartridge and cannot be used with available MM amplifiers with few mV inputs. Therefore, a method should be used to amplify this low output voltage without degrading the signal-to-noise ratio and distortion.
If you use a transistor, etc., it will generate noise by itself.
Therefore, it is difficult to remove the noise generated by resistors and electrolytic capacitors.
When using an input transformer, no noise is generated by itself, and when the shield is completely shielded, there is almost no reception from outside. MC cartridges can be used with the Sansui.
A-604 and A-603. The assembly legs can also be inserted into the 9-pin socket as they are, so that assembly and replacement can be carried out in the same way as with vacuum tubes. I will. Refer to Fig.1 and Table 1 for the characteristics and standards of the input transformer. Phono 2 can be used with general MM cartridges and Varirera cartridges.


Tape head input circuit :

The equalization circuit can be switched according to the speed of the tape, and both those recorded at 19cm/s and those recorded at 9.5cm/s can be played back. Head


Head amplifier part :

I made an hybrid eclectic shape of a transistor and a vacuum tube out of this. S / N, distortion, dynamic range, etc. must be good around the head amp.

This machine uses up a low-noise silicon transistor 2SC402, and the equalizer is the NF type, so S / N, both tribes have good properties. To increase the dynamic range, a high voltage must be applied so that the high output voltage can be taken out without distortion. The 2SC402 has a maximum nominal voltage of 85V between collector and base and was able to apply a voltage sufficient for the dynamic range.

Since  it is an  NPN type, it can be supplied with electricity via the B network part.
It is not necessary to install a separate power supply unit, which is advantageous in terms of cost .

Intermediate amplifier part :
It consists of two 12AX7. The first stage is the cathode follower. Since the output impedance is low, it is an ideal circuit as a pre-stage of the audio circuit.
The tape monitor connector is also connected to the cathode follower , there is almost no need to worry about the output impedance of the tape recorder.

Tone control loop :
This only applies to this device. The rise and fall points of the lower range can be changed to 250c/s and 500c/s, and the rise and fall points of the upper range can be changed to 2.5kc and 5kc .
The sound defeat  circuit  is not a circuit that  defeats the entire area.

But the low range  (20c/s to 1kc ) and the high range (1kc to 20kc )

can be independently controlled .
In HiFi playback, these vary considerably depending on the sound equipment, the sound conditions of the listening room , the age difference of the listener, etc.
Audio devices are cassettes, amplifiers, speakers, etc., but the characteristics of cassettes, speakers, etc. are inherent to the devices.
Once the acoustic conditions of the listening room are set, they are considered almost fixed.
In order to perform a reproduction that corresponds to the taste of the listener, the acoustic conditions of the acoustic equipment or the listening room must be corrected.
There is volume control, tone control, presence control, low cut filter, high cut filter, etc. to make this correction, but the tone control circuitry plays a particularly important role. , The sound circuit should be able to make as many corrections as possible.
In other words, you need to have as many change curves as possible.
Of course, the quality of the HiFi amplifier must be good in terms of frequency characteristics, distortion, signal-to-noise ratio, damping factor, etc., but the other is how it suits the situation and the listener. Can you play it?

 I often keep the tone flat and tune the amplifiers, but to really know the amplifier's worth I think it is possible to actually reproduce on site the sound that suits the listener's taste .
Therefore, this machine was designed with particular emphasis on this tone control circuit. Fig. 2 shows the low frequency tone control characteristics and Fig. 3 shows the high frequency tone control characteristics.


Fig. 4 shows each of these change curves (setting range for the sound quality) in an easily understandable triangle.


The triangles AEI and BFJ are tone circles of conventional products, and this device has additional triangles CGK and DHL via tone selector. The triangles AEI and CGK indicate the bass, and the triangles BFJ and DHL indicate the treble range. Abc represents a point on the line where tone selection is disabled. Let's compare the sound adaptation range between the traditional and this device with this combination of triangles.

                                 Ascent           Descent         Ascent            Ascent
Conventional           A-b-B             E-b-F              A-b-F              B-b-F

This machine           A-b-B             E-b-F             A-b-F              B-b-F
                                  A-b-D             E-b-H             A-b-H             B-b-C
                                  C-b-B             G-b-F             C-b-F              D-b-E
                                  C-b-D             G-b-H             C-b-H             D-b-C

Each is reinforced 4 times. There are 16 combinations in this alone. In addition, there is a defeat circuit for bass and treble.

a-b-B             a-b-A             a-bc
a-b-D             b-c-C
a-b-F             b-c-E
a-b-H             b-c-G


You can see that the above combination is possible and the adjustable range of the sound is greatly expanded.
Fig. 5 shows the ascending and descending operations of the bass and treble of the tone control circuit used in this unit for the first time with the SAX-300. This is a mixed type of LF type and CR type and has excellent characteristics as well as frequency characteristics and distortion.


If you look again at Fig. 2 and 3, you will understand.
Compared to the CR attenuator type and the BAX type, this tone circuit has a smaller number of CR parts, and the cost can be reduced accordingly. When the bass and treble are cut off separately from the sound as in this device, the value of the capacitor for treble adjustment is small in the case of the BAX and CR types, and the treble is quite high due to the influence of the shield wire .

Flat features cannot be obtained. The low-cut filter and the high-cut filter use the CR field back method. The feature is that it doesn't drop off like a CR filter but is sharp. At around 100c/s there is almost no attenuation and at 20c/s it is attenuated by 26dB. There is no sense of lack of bass (see Fig. 6).


In addition, an output connection for 3D is provided so that the three-dimensional (3D) method can also be used. There are several different 3D methods, but the method used with this device is the 3 amp 3D method.

There are two methods of doing this. One is to insert a filter between the preamp and main amplifier, and the other is to take out the center channel signal from the speaker terminal. The former method is used in this unit. The 3D method originally applied the fact that the human ear has a very dull sense of direction for frequencies below 200c/s to 300c/s. This frequency changes depending on the person. For those who want to cut with 150c/s or 250c/s  a flat connector is provided. And a high-cut output terminal 200c/s. ..

In the general case. This frequency is almost okay. Since this is a high cut filter, there is no need to install a separate filter.
The output level of the center channel can also be controlled over the entire surface. Once the main amp is set up this comes in handy as you can use this knob to adjust the bass.

If this VR is not used as a 3D method and this VR is minimized, there is no problem with the Cross Storm in this regard. You can see this by looking at the 50dB crosstalk from AUX.

Even when used as a 3D method there is no problem as the output impedance at this point is only 710Ω.

Next, the tone control feature that can change on this unit is that the preamp output and main amplifier input are provided.

The amplifier can be used for many purposes.
Usually it connects to a PM (preamp, main amplifier) ​​connector.
By providing these two input connections, a great many uses can easily be carried out. For example, if you are using a mixer amplifier and only want to use the AU-111 main amplifier. If you have a very popular preamplifier and want to use this preamp in combination with the AU-111 main amplifier. If you want to use another main amplifier, if you want to use a hall amplifier between the preamp output and the main amplifier input, or if you want to use a channel filter, etc. You can use it in many ways.

Accessory circuit :
In addition to the tone selection, there is volume control, tape monitoring, presence control, etc.
The presence control provides a time constant in part of the audio circuit and changes the audio volume in the low range. That is, while the low range is raised from near the frequency where the low range of the overdamping begins to fall. The damping factor is reduced and the low range of the loudspeaker is corrected. The turnover frequency is lower than the tone control, and the low frequency correction is performed for the speakers only. In Fig. 8 you will find a circuit example of the NF filter for 3 channels.


=== Main amplifier section ===


The main amplifier section consists of 1/2 6AQ8 and 12BH7A. When a pentode is used for the upper tube, the high frequency characteristics often deteriorate. So for this device I used 6AQ8 for audio. Cathode coupling is used for phase inversion. Compared to the Altec type, this has higher gain, less distortion, and when the load resistance is set to a suitable value, the balance between both outputs is good. By directly connecting the first stage and the phase inversion, the time constant in the lower area becomes two stages.

In the  high range, as mentioned above, a triode  was used and de  load resistance is only 51k Ω. We have improved the high frequency characteristics.

Although a cathode feedback is performed. This feedback amount is about 5dB.

And about 1.8 times the input voltage required to drive the output tube.

Therefore, 12BH7A is used for the driver tube to get sufficient excitation voltage.

A cathode feedback is performed by providing a dedicated cathode winding.

The external negative feedback (NF) is applied from the load terminal of 8 or 16Ω. Normally, when applying NF, a correction circuit is required so that NF is sufficiently stable. In this machine.  Because there is a direct link stage in the cathode coupling method, there are only two low frequency time constants, including the out put transformer (OPT).

In the two cases, by increasing or decreasing the time constant of either one, so that the time constants of both do not overlap.Even a considerably large amount of NF can be applied stable.

So that the time constants of not overlap both. A considerably large amount of NF can be applied stable. However, the time constant in the high frequency region cannot be reduced no matter how directly it is connected. It is difficult to calculate the high frequency characteristic including OPT, but it is also poor in practical use.

In general, when the correction is not made, the characteristics are measured and the correction is made based thereon. Alternatively, if no correction is made, oscillation occurs. If the characteristics cannot be measured at that time, the correction

is determined by cutting and experimentation.

Applying  cathode NF improves the characteristics.

The improved characteristics also mean that this correction circuit is included to some extent.

Furthermore, in order to obtain the same characteristics as those external NF amount

Compared to the one without the Cathode NF, the amount of NF can be reduced.

Therefore, the amount of correction is also reduced.

The deterioration of characteristics due to correction (especially distortion in the high frequency range) is reduced.

If the high frequency characteristics including OPT are poor, this correction constant will inevitably increase. The fo will enter the audio band.  Deteriorating the frequency characteristics, distortion factor, power bandwidth, etc.

Power bandwidth is 20c/s to 20kc.

In order to improve the power bandwidth, we paid special attention to the OPT, which has a big point.

The core uses S-114 and Z-13, and the product thickness is as large as 58mm.

I used a good quality Orient Core, so I was able to make the OPT quite small, but it's still quite large.

See Fig. 9 for power bandwidth.


Power bandwidth also affects cross-modulation distortion.

Cross-modulation distortion is when two or more frequencies are applied to an amplifier, and if the amplifier has non-linear distortion, the sum and difference frequencies will be generated in addition to the added frequencies.

This means that one frequency has been modulated by another frequency, which is called cross-modulation distortion (IM distortion). If there is a lot of this cross-modulation distortion, the sound will be muddy and the crisp and clear sound will not be produced.

IM distortion also occurs in cartridges and speakers, so if there are many IMs.

It is premature to immediately determine that it is due to the amplifier, but it is wise to reduce the IM in the amplifier first.

The IM disortion measurement method in audio is performed by adding a mixture of two frequencies as the input signal of the amplifier, and the lower frequency is mixed at a ratio of 4 and the higher frequency is mixed at a ratio of 1.

For example, the lower frequency is 60c/s, the higher frequency is 6kc, 60c/s is 4, 6kc is 1, and so on.

It seems that IM distortion meters have not been manufactured much in Japan until now, but as each company has come to pay attention to IM distortion and the demand has increased, it seems that the number of domestic measuring instrument manufacturers has also increased.

Since the frequency values of such low and high frequencies are not close to each other, you can see that the IM distortion is reduced by using the multi-channel method described above.

Fig.10 shows the IM distortion characteristics.


It is within 0.8% up to 45W. Fig.11 shows the power character characteristics.


This is also similar to power bandwidth, but while power bandwidth measures with constant distortion, it measures distortion at each frequency with constant output.

It shows that 22c/s to 22kc is 1% or less and 27c/s to 18kc is 0.5% or less at 45W output.

In addition, the total harmonic distortion characteristics are shown in Fig.12, the presence characteristics are shown in Fig. 13, the tape equalizer characteristics are shown in Fig. 14 and Fig.15, the music power characteristics are shown in Fig. 16, the loudness characteristics are shown in Fig.17, and AUX crosstalk.

The characteristics are shown in Fig. 18, but detailed explanations are omitted due to space limitations.


⭐︎ Output (at a distortion rate of 0.8%)
     Effective output: 45W x 2
     Stereo effective output: 48W x 2
     Music output: 48W x 2

⭐︎ Frequency characteristics (main amplifier)
     10c / s to 50kc (± 1dB)
⭐︎ Input sensitivity (relative to effective output)
     Phono 1: 0.06mV (3Ω), 0.2mV (600Ω)
     Phono 2: 2mV (100kΩ)
     Tape head: 1.8mV (100kΩ)
     Tuner, AUX: 220mV (500kΩ)
     Tape monitor: 250mV (500kΩ)
⭐︎ Output voltage, output impedance and sensitivity Tape recording:
     41dB from phono 2
     Center channel: 1.1V (48kΩ)
     Preamplifier output impedance: 710Ω
⭐︎ S / N, phono, tape head: 55dB
     AUX: 61dB
     Main amplifier: 92dB
⭐︎ Power consumption: Maximum 280VA

≪ Technical specifications ≫

(Sansui Electric KK Technology Division)

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