Reference power amplifier
                 with the variable output resistance

 

  SK1200 Studio3

 

         

                    

               Technical descriptions:

 

 number of channels

2

 input sensitivity

2.15 (0.775) V

 balance inputs

10 kOhm

nominal sine power  (1% T.H.D)       mode 

of lowered power

of complete power

4 Ohm

600 W

150 W

 8 Ohm

320 W

80 W

T.H.D.

0.005%

0.005%

I.M.D.

0.004%

0.004%

range of  frequencies   ( -1dB 1W)

5Hz-300kHz

range of complete capacity

7Hz-200kHz

slew rate

100V/mSec

damping factor

controlled

signal/weighed noise (A)

120 dB

power voltage

230 V (50-60Hz)

possible power range of voltage in 

170-250 V

overall sizes

482x450x88 mm (2HU)

weight

16 kg

price

1150 $

 

                               (Note: nominal power is measured at voltage 230V)

 

 

Brief description:

 

    The amplifier SK1200 Studio3 was developed as a further development of the previous model SK1200 Studio2, saving the basic scheme technology and parameters of amplifier. It is intended for use in domains, where the high quality of sound transfer in combination with the high capacity is required - studios of audio recording, cinema halls, etc. The construction of the amplifier was exposed to improvements - the radiators of the other construction were applied (with the greater area) that gave the possibility to lower the speed of rotation of ventilators and accordingly the noise from them (see a photo down). There are applied the more informing indicators of the output signal, done to order by the "Jiann Wa firm" (Taiwan). There is improved defense from short circuit of the output of the amplifier. The possibility of connecting of 2 pairs of monitors with possibility of their operative switching is left. The use of  vacuum tube in a “driver” stage allowed to decrease distortion, arising up because of the change of capacity of semiconductor transitions at the change of voltage on them, and the application of the field transistors on the output with the use of scheme technology of direct communications of input and output stages allowed to attain the high indexes on speed, range of frequencies,  minimum of intermodulative and other harmonics. The important feature of the amplifier is regulation of size of the output resistance that allows obtaining more optimum and qualitative work of AS on comparison with the traditional mode of the constant low output resistance of the amplifier. Switching of launch power allows working safely with the monitors of small power. 

 

 

 

On the picture on the left is shown spectrum of distortion (THD), taken off by the Spectra Lab program at launch capacity 50 W. A value 0,00277% contains  distortion of  the amplifier and distortion of the sound card Wave terminal 192X, that possess very  high  descriptions (24 bits, 192 kgts, 123 db signal-noise, THD 0,00075% on input and output).

 

 

 

 

 

 

 

 

 

 

 

The results of measuring with the use of the high-fidelity professional measuring complex System Two Stages Plus of the Audio Precision firm are shown below (settling ability 0,0001%).

 

 

Dependence of nonlinear distortion on frequency at capacity 50W:

 

 

  Frequency and phase response in range 20gts-20kgts:

 

 

 Frequency and phase response in range 20gts-200kgts:

 

 

 

 Spectrum of distortion at  capacity 50W:

 

 

the amplifier is intended for control and high-quality listening of phonograms, where at first thought capacity 600 W/canal is of no need. However it is necessary to remember that the middle peak-factor of the musical programs makes 1/3, i.e. near 10db, consequently the peak-factor on capacity at the average makes 1/9. I.e., if an amplifier gives in a nosedive 450 W, the middle capacity will be only 50. This is only in the case of loading in 4 Oh. And in the case of loading in 8 Oh the middle capacity of the amplifier, when it is possible to carry conviction, that a peak will not be distorted (or cut), will be no more  than 30-35W! This fact is illustrated also by measuring, conducted on the loud speakers by a diameter 12, 15 and 18 inches, the results of which can be looked here <http://www.sakevich.ru/dempfer/dempfer.htm> (russian).

 

On the picture below is shown the amplifier without the upper lid.

 

 

The amplifier is a simple in exploitations device executed on the original scheme technology. It is necessary to remember that in composition of the amplifier there are the vacuum tubes that possess the increased fragility that is why during the exploitation and transporting of the product it is necessary to protect it from the influence of shock loading.

 

 

Schematic location of  elements of the front panel (setting of  element is marked red):

 

 

 

 

1 - combined input socket  jack-XLR of the left channel

 

2 - regulator of  level of the left channel

 

3 - indication of  level of the output signal of the left channel

 

5, 6 - indication of readiness of the amplifier and of inclusion of AC defense of the left and right channels

 

8 - indication of  level of the output signal of the right channel

 

9 - inclusion of feed

 

10 - inclusion of the 1st  monitors pair, output on terminals

 

11 - inclusion of the 2nd monitors pair, output on the sockets  «speakon»

 

12 - switch of the sign of the output resistance (negative, the block of regulation of the output resistance is turned off, positive)

 

13 - switching of the type of the cushion-frequency description (linear, bend on frequency of 80Hz, bend on the frequency 250Hz)

 

14 - switching of the type of cushion-frequency description (upwards - combined with the negative output resistance that is lower than frequency of  bend and positive that is higher than frequency of bend or on the contrary - combined with the positive output resistance that is lower than frequency of bend and negative that is higher than frequency of bend and down - the output resistance either only negative or only positive, with a small deviation of the output resistance from 0 that is lower than frequency of bend and with the great controlled  output resistance that is higher than frequency of bend)

 

15 - switch of the size of resistance of loading

 

16 - regulator of the change of the size of the output resistance

 

17 - regulator of  level of the right channel

 

18 - combined input socket  jack-XLR of the right channel

 

19, 20 - handles of carrying

 

 

 Schematic location of elements of the back panel (setting of element is marked red):

                                    

 

 

 

 

1, 11 - ventilators of cooling of the channels (have the removable dusters, a right ventilator is shown without it)

 

2 - output (terminals) of the right (B) channel

 

3 - output speakon of the right (B) channel

 

4 - automatic safety device (early versions)

 

5 - net socket

 

6 - terminal of grounding

 

7 - switch of grounding

 

8 - switch of  maximal launch power

 

9 - output (terminals) of the left (A) channel

 

10 - output  "speakon" of the left (A) channel

    

 

The flow-chart of the amplifier is shown below.

  

 

 

where: J1 - is the input combined socket Canon XLR-Jack, R1 - the regulator of  level of signal, V4A - the lamp of driving, Q1Q2 - the output stage, IcVs2, IcVs3 and OPTOISO - the system of checking and control of  idle current of the output stage, VcIs1 - the transformer of signal  «tension - current», S2 - the defense of the acoustic systems from the constant voltage on the output at  inclusion  and of the amplifier (while a lamp will not be warmed up), the emergency disconnection of AS, and also the switching of the working AS pairs. So, the lamp is switched according to the chart with a common net for achievement of its great fast-acting and for liquidation of influence of interelectrode capacities. The system of spying upon idle current is based on direct measuring of current flowing through the output transistors, and  on  control by  current of rest through the opticalpairs, that enables to react instantly on the change of the external terms, and also requires no turning at any throwing of parameters of the output transistors. At the lowered voltage of feed of the output stage its current of rest is considerably multiplied, transferring it in the mode A, at the complete feed - the classic AB. The feed of ventilators is carried out through the element of control, watching after the temperature of the output transistors, and is changed depending on their heating, i.e. at small launch power the turns are minimum, with the purpose of reduction of acoustic noise. The element of control of the output resistance measures current acoustic system  on the sensor of current R3 and sets the necessary type of the damping-frequency characteristic DFC (not to mix up with amplitude-frequency characteristic).

 

 

Regulation of the output resistance

 

           Before description of principles of use of the element of regulation of the output resistance it is necessary to show some explanations about the necessity of use of this element.

 

             As is generally known, displacement of the mobile system of the loud speaker is proportional to current in a sound coil (rather than to voltage, and also to the value of magnetic induction in a gap and length of wire). At near to zero output resistance of the amplifier current in a coil relies on its resistance. However the resistance of a coil during its work has the inconstant value - for example, on the partial leaving of a coil from a magnetic gap its inductance and instant resistance falls down. Also takes place the thermal  change of the value of resistance of a coil during the period of its oscillation. So, it causes the change of the value of resistance of a coil during the period of  sound signal, which can arrive at  some occasions 10-15 %, that  causes the proper change of current in the coil of the loud speaker, bringing about the proper growth of coefficient of the intermodulative and nonlinear distortion (it is natural that every loud speaker has its own value, but in principle there is no speaker system which have not these problems, just speaker system of high quality have less problems).

 

The decision of this problem is the transfer of the amplifier in the mode of generator of current.

 

 Really the intermediate mode is optimum, when the output resistance is not endless, but has some certain value which is optimum for the given loud speaker. It is explained by the fact that a dynamic head must be cushioned not only acoustically, but also electrically, by braking by its magnetic field, for removal of mechanical resonances and false sounds that has any mobile system.

 

           However it is justly for domain of the middle-low, middle and high frequencies. In theory it is justly also for a LF-range, but there are the other problems there. In order to follow  electric signal more or less exactly, without arising of vermin vibrations on the frequency of basic and other resonances, diffuser of the loud LF-speaker in a low frequency range must be cushioned acoustically enough. But as a rule acoustic cushioning is not sufficient for this purpose, especially for the loud speakers of compression type, that are applied in the most home and studio speaker system, and the loud speakers are cushioned additionally by the low output resistance of the amplifier, removing on this grow the problem of «buzz» and the other vermin vibrations resulting in worsening of legibility of LF-signals. (It caused by the way arising of notion the «damping-factor» at one time. The «damping-factor» is a value, which is equal to division of the value of resistance of loading on the value of the output resistance of the amplifier). But the given circumstance of necessity of cushioning on LF by the low output resistance falls into contradiction with the aforesaid one. Therefore in the given amplifier is realized the possibility not only of the linear change of the output resistance in a plus or minus, but also it is possible to receipt the low or even negative output resistance (for the specifically difficult cases that require very strong cushioning on frequency of resonance), and positive in area of frequencies lying higher than frequency of bend (which is switched 80 or 250gts).  Amplitude-frequency characteristicof the amplifier in all the cases remains the linear, but for this purpose it is necessary to set the switch 15 in position which is proper to the nominal speaker system resistance.

 

There are possible 6 types of damping-frequency characteristic (DFC):

 

 

 

 1 - the graph of the linear dependence of DFC from frequency. Here and hereinafter is shown the output resistance of the amplifier on an axis Y, on an axis Õ - the frequency,  the line «s» corresponds to zero output resistance, and line «a» - to frequency of bend (on the given and following graph the line «a» has no value). The given graph shows that the amplifier has the positive output resistance, where the colored lines correspond to different position of regulator of level of the output resistance (16 on the chart of the front panel). The given graph corresponds to the following position of switches of element of control by the output resistance: 12 - to the right (+), 13 - to the left (line), 14 - has no value.

 

 

 

 

 

 

2 - the graph of the linear dependence of DFC from frequency. The given graph shows that the amplifier has negative output resistance, where the colored lines correspond to different position of regulator of level of the output resistance (16 on the chart of the front panel). The given graph corresponds to the following position of switches of element of control by the output resistance: 12 - to the left (-), 13 - to the left (line), 14 - has no  value.

 

 

 

 

 

 

3 - the graph of the nonlinear dependence of DFC from frequency. The given graph shows that the amplifier has positive output resistance which relies on frequency, and is approximately equal to 0 in area of low frequencies and rises in accordance with setting of regulator of  level 16 that  is higher than frequency of bend (line «a»), where the colored lines correspond to different position of regulator of  level of the output resistance, and the line «a» indicates the frequency of bend. The given graph corresponds to the following position of switches of element of control by the output resistance: 12 - to the right (+), 13 - 80Hz or 250Hz,  14 - down.

 

 

 

 

  

 

 - the graph of the nonlinear dependence of DFC from frequency. The given graph shows that the amplifier has negative output resistance which relies on frequency, and is approximately equal to 0 in area of low frequencies and falls down in accordance with setting of regulator of level 16 higher than frequency of bend (line «a»), where the colored lines correspond to different position of regulator of level of the output resistance, and the line «a» indicates the frequency of bend. The given graph corresponds to the following position of switches of element of control by the output resistance: 12 - to the left (-), 13 - 80Hz or 250Hz,  14 - down.

 

 

 

 

 

 

5 - the graph of the nonlinear dependence of DFC from frequency. The given graph shows that the amplifier has positive output resistance in area of frequencies that is higher than frequency of bend and  negative that is lower than frequency of bend, where the colored lines correspond to different position of regulator of  level of the output resistance, and the line «a» indicates the frequency of bend. The given graph corresponds to the following position of switches of element of control by the output resistance: 12 - to the right (+), 13 - 80Hz or 250Hz,  14 - upwards.

 

 

 

 

 

 

 6 - the graph of the nonlinear dependence of DFC from frequency. The given graph shows that the amplifier has positive output resistance in area of frequencies that is lower than frequency of bend  and  negative that is higher than frequency of bend, where the colored lines correspond to different position of regulator of  level of the output resistance, and the line «a» indicates the frequency of bend. The given graph corresponds to the following position of switches of element of control by the output resistance: 12 - to the left (-), 13 - 80Hz or 250Hz,  14 - upwards.

 

 

     For practical application there is enough to have the form DFC that is proper to the graphs 1, 3 and 5. It is desirable to apply the graph 1 with speaker system, that have the wideband loud speakers in the composition, or professional speaker system of great volume, that have in the composition the LF-speaker of direct radiation 12", 15" or 18", which don't require additional electric cushioning. It is desirable to apply the graph 3 with speaker system, that have in the composition the compressive loud LF-speakers which are cushioned well enough. It is desirable to apply the graph 5 with speaker system, that have in the composition the loud LF-speakers which are not cushioned acoustically enough. By the switch of frequency of bend and by the regulator of level of the output resistance it is necessary to pick up such a mode of the speaker system operations, when in the area of the middle and high frequencies distortion and vermin vibrations of speaker system are minimum (the most «transparency» of the sound), and in the area of low frequencies – are maximally diminished the low frequency «tails» and vermin vibrations as «rumblea».

 

It should be remembered that the possibility of regulation of the output resistance is no panacea, but only a possibility to improve the sounding of Your speaker system. I.e. by no regulations of cushioning you will reach the sounding of low cost speaker system as the sounding of Quested or Dynaudio :-)

As for the question of acquisition in Russia (Moscow) - apply to the "LONG” firm (Mihail Chernetskiy)  +7 495 471-56-67, site - http://long.ru, mail  - long@long.ru. 

In Saint Petersburg - piter@long.ru mailto: piter@long.ru

The developer and the manufacturer - "SK Sound Electronics", Lugansk. Fone +38 091 308 19 63, Sergey Sakevich.