DE
Posted: Sat Dec 03, 2022 5:38 pm
German limiters
U73 manual translation by google translate.
Purpose of use
Limiters *) are variable-gain amplifiers whose gain is negatively regulated at input levels above the point at which the regulation starts, as the input level rises, so that the limit value of the output level is exceeded only very briefly or insignificantly. At lower input levels, the gain is constant.
Compressors *) are control amplifiers whose amplification level depends on the input level over the entire dynamic range. A high level of amplification at a low input level is steadily reduced as the input level increases.
*) Definitions see also DIN 45 568, currently still draft.
The control amplifiers described here are used for automatic level monitoring in front of systems that are sensitive to overload, such as sound recording devices, postal lines and transmitters, as well as for increasing the average volume of strongly impulsive, low-energy modulation (speech) in order to match it to the high-energy music modulation. The U 73 in the "Compressor" operating mode also causes a reduction in dynamics due to the relatively stronger boosting of small input levels.
The U 73 can be switched both as a limiter and as a compressor
use. However, the limiter characteristics also remain in the mode
"Compressor" received. It is designed for an output level of + 6 dB.
In "limiter" mode, the gain is below the
Limiter use constant 0 dB. At input levels between + 6 dB and
+ 22 dB, the output level can increase by a maximum of 1.5 dB above the limit of + 6 dB.
Dynamic control is also performed in the "Compressor" operating mode. A constant gain of +10 dB that is effective at input levels below -14 dB steadily decreases with increasing input level down to 0 dB at +6 dB input level. With overdrives up to
+ 16 dB, the signal is limited to a maximum of 2 dB above the nominal value of + 6 dB at the output.
The U 83 is only designed as a limiter. It is primarily used to protect VHF transmitters and can be set to a limiter level of +6 dB or +6 dB.
+ 12dB can be switched. The clipping protection ranges up to an input level of +22 dB, regardless of the selected setpoint of the output level.
The control effect can be observed on a display instrument (J 83, J 83a), which can be connected to the outside of both devices and is available in two sizes. The beginning of the red area of the instrument scale is reached at a control deviation of 10 dB. This corresponds to input levels of + 6 dB for the U 73/comp., + 16 dB for the U 73/limited. and U 83/+ 6 dB as well as + 22 dB with the U 83/+ 12 dB. The instrument can be set to these values with a control on the front panel of the devices. No binding values for the corresponding input levels can be assigned to the end of the red range because of the different deviations in the characteristics of the control tubes (see also sheet 18).
In order to avoid distortion of the stereophonic
In order to prevent the sound impression from being adjusted differently for the two channels, it is possible with both devices to mutually couple the control voltage in such a way that the control amplifier with the stronger response controls the other to the same extent.
With both devices in the "none" position of the operating mode switch, the amplifier section can be disconnected and the signal path can be switched through from the input to the output.
Manufacturer
Fa. Tonographie Apparatebau v. Willisen & Co., Wuppertal-Elberfeld.
time of start-up
U 73: 1960
U 83: 1962
Mode of operation and technical details Principle circuit
The input level is fed to the input transformer (item 70) via a voltage divider (item 31, 32, 33) and a symmetrical low-pass filter that cuts off above 15 kHz (item 68, 69, 5, 6, 34). From its secondary side, the signal is routed to the control grid of the E 99 F control tubes (items 1 and 2) which work in push-pull, which receive their control voltage via the center tap of the secondary winding. The output transformer of the control stage (item 76) is heavily damped on the anode side by a resistor (item 40) in order to eliminate its frequency response and to keep click pulses as small as possible when controlling. The downstream amplifier section with the E 88 CC tube (item 3) amplifies the regulated AF voltage and outputs it via the output transformer (item 73) with low resistance and symmetrically.
Both devices work with feed-forward control. The signal taken from a second secondary winding of the input transformer is first amplified by the pentode system of the tube E 80 CF (item 4). The following triode system has a low internal resistance due to the anode base circuit, which is necessary for the short response time of the control process. The cathode circuit feeds a full-wave rectifier with highly blocking silicon diodes (item 84 and 85). These charge, with practically no time delay, the capacitor item 82, which is discharged through the resistor item 55 with a time constant of 0.5 s.
In the limiter mode, these diodes receive a positive bias voltage stabilized by a Si Zeher diode. This achieves the required sharp transition between the areas of the unregulated and the limited signal. The preload and thus the kink in the control characteristic can be adjusted with the controller item 63 (see also the operating instructions).
In compressor mode, the diodes are operated without bias. Because of the non-linearity of the on-state characteristic of Si diodes, the control process only starts in this case when the voltage at the diodes exceeds a value of approx. 0.5 V (threshold voltage). The control characteristic curve is very rounded between the start of control intervention and the limiting case.
With an RC element (items 38 and 13) in the secondary circuit of the intermediate transformer (item 76), an additional bass reduction of approx. 6 dB at 40 Hz, independent of the control, can be carried out during compressor operation. This measure causes an adjustment of the affected frequency response
reaches the ear curve at volumes between 70 and 80 DIN-phon. By raising the lower limit frequency, there is an apparent increase in the measured non-linear distortions at the low frequencies. This RC element can be rendered ineffective with a solder bridge.
Partial suppression of the disruptive control effect that occurs during compressor operation (breathing of the background noise) can be achieved by shortening the control time constant. For this purpose, a resistor (item 80) is connected in parallel with the RC element (items 55 and 82) via the operating mode switch, which reduces the time constant by half to 0.25 s. With this measure, however, the non-linear distortions at 40 Hz are more than twice as large, so that a decision must be made on a case-by-case basis with which time constant the device should be operated. By unsoldering the resistor (pos. 80) the small distortions can be maintained.
Bass reduction and a reduced time constant are built in as standard by the manufacturer from device no. 766 onwards.
Remote switching compressor/limiter of the U 73
In order to increase the range of medium wave transmitters, a U 73 is often connected in front of them in the "compressor" position, provided that it can be guaranteed that the same program can be received uncompressed over VHF within the coverage area. However, it must then be possible to switch the operating mode to "limiter" by remote control, because compression would compensate for the intended reduction in the reception volume of program parts that were fed to the transmitter at a reduced level, such as pause signals and identification signals. For this purpose, a negative bias voltage should be applied from the outside to terminals 4a and 4b for the duration of the compression cut-off. Switching occurs without clicking if the auxiliary voltage is applied with a time constant of more than 0.1 s.
The pre-emphasis in the control path
(Curves see sheet 19)
With VHF transmitters, the signal to be transmitted is subjected to a treble increase (pre-emphasis) before the modulation stage, which is canceled out again in the receiver of the listener by a mirror-inverted reduction. This increases the effective signal-to-noise ratio of FM transmission. The course of the pre-emphasis curve of the transmitter corresponds to the conductance course of an RC parallel circuit with a time constant of 50 /las and is shown on sheet 19. The increase starts at about 1 kHz, is 3 dB at 3.15 kHz, and 13.5 dB at 15 kHz.
If the signal in the control path of the limiter amplifier U 83, which is connected before the modulation section of a transmitter, is subjected to the same high-level boost of 50 /us before rectification (control voltage generation), i.e. the limiter starting point for high frequencies is lowered, this can
Output signal of the limiter amplifier only accept limit values that drop in the upper frequency range according to 50/us (curve 1). After the subsequent treble increase in front of the transmitter's moaulation section, the limitation of the signal voltage and thus of the frequency deviation at the same level sets in over the entire frequency range. This pre-emphasis in the control path does not affect the frequency response of the useful signal because the gain of the limiter amplifier is reduced independently of the frequency, regardless of which spectral portion of the program content triggered the control process. If the U 83 is operated in front of a VHF transmitter with the mode switch in the + 6 dB position and the 50/us pre-emphasis (pos. 30, 17, 80) switched on by solder bridges, the frequency-independent limitation of the transmitter excursion occurs according to curve 1'. The transmitter is thus protected against clipping, but the limitation will be used unnecessarily often, since the clipping reserves of the transmitter, which is set to around 42 kHz deviation at + 6 dB input level and 1 kHz and allows a maximum deviation of 75 kHz.
If, on the other hand, the U 83 is set to an output level of +12 dB and 50 rus pre-emphasis, the limiter starts according to curve 2' and the maximum deviation of the transmitter is at curve 2', independent of the frequency. In this case, the level of the transmitter must be adjusted so that this limit is at a maximum excursion of 75 11. A limitation now only occurs if the maximum deviation of the transmitter would otherwise be exceeded.
Finally, a third mode of operation is to be used when both a VHF transmitter and a transmission line with a maximum level of 6 dB are to be protected at the same time. This is the case, for example, when a U 83 is located at the radio station output in front of a transmission line and the connected VHF transmitter does not have an additional limiter. For this case, a pre-emphasis is provided in the signal acquisition, which results in a limiter use according to curve 3. At an output level of + 6 dB for low and middle frequencies at the highs, it transitions to the 50 rus curve of the + 12 dB setting with a sharp kink. This course can be set on both the ΓΌ 83 and the U 73 (items 35, 7, 36, 37, 8, 12, 15, 16, 58 and 59).
The deviation limit to be set after the 50/us preemphasis of the transmitter is set in curve 3'. Due to the required protection of upstream 6 dB lines, the limitation starts earlier than for curve 2, but compared to curve 1, the transmitter's overload reserves in the high frequencies are fully utilized. This setting is also suitable for protecting transmission elements with a modulation limit that falls at higher frequencies (e.g. magnetic recorders) in special cases where the spectral energy distribution of the modulation does not correspond to the amplitude statistics of natural sound patterns for which they are designed, e.g. with electronic ones Music.
In the "Compressor" operating mode of the U 73, the set pre-emphasis characteristic curve is also effective, but the control action is then at a 10 dB lower input level.
In both devices, the pre-emphasis can be disabled in the normal path by resoldering the coupling link (item 30 on the U 73, items 30 and 17 on the U 83).
grounding
The 0 V line and the housing connection are routed separately to the connector strip. The housing connection must be connected to the protective earth.
mechanical data
Executed in cassette size 2, blocking positions no. 12 (U 73) and no. 20 (U 83), weight approx. 5 kg.
assembly
2 tubes 1 tube 1 tube 1 G-fusible link
1 signal bulb E 99 F
E 88 CC or E 188 CC
E80CF
T 0.2 B DIN 41 571
7V/0.18A plug blade
Operating Instructions AllEemines
Before putting a device into operation for the first time, the desired pre-emphasis and, in the case of the U 73, the control time constant and bass roll-off must be set accordingly by means of soldered connections.
If a J 83 or J 83a display instrument is used, this is adjusted to the start of the red scale range using the "Instr.Eichen" controller, which can be operated from the front panel with a coin slot button. With the U 73, this setting is at an input level of
+ 6 dB ("Compressor" mode) or + 16 dB ("Limiter" mode). With the U 83, the input level is +16 dB when the operating mode switch is in the + 6 dB position or with an input level of +22 dB in the
+ 12 dB position adjusted. (In the "+ 12 dB" operating mode of the U 83, the start of the red scale range already indicates the maximum permissible input level!)
setting instruction
Balancing (applies to U 73 and U 83)
After replacing the E99 F control tubes, the device must be rebalanced after a burn-in period of approx. 48 hours in order to avoid cracking noises with impulsive modulation. For this purpose, pulses of a sinusoidal shape are applied to the input of the test object
15 kHz signal with a level of + 22 dB. The pulse duration should be 50 ms, the interval between two pulses should be 1...2 s. At the output, the interference signal caused by the control process is measured via a 5 kHz low-pass filter with a level meter and set to minimum with the controller item 61. The values measured with the level meter should not exceed - 10 dB with compressor operation and - 20 dB with limiter operation.
If it is not necessary to record these values numerically, the adjustment can also be carried out by ear without using a low-pass filter or level meter. In this case it is convenient to increase the input frequency slightly above 15kHz. With optimal symmetry, the low spectral components of the click pulse disappear.
With both methods, it must be checked in any case whether the signal source used itself, in particular the pulse generator, is sufficiently click-free. This can be done with the same measuring arrangement, only the bet mode switch of the DUT to the "none" position.
The ear curve filters of the noise voltmeters J 77 and J 78 are not suitable for this adjustment.
Balancing should always be carried out before the adjustment work described below.
Gain adjustment
(The control characteristics and their tolerance limits are shown on sheet 18).
U 73, "Compressor" operating mode
(The adjustment of the U 73 always starts in this operating mode! )
Set the operating mode switch to "Compr.n", input level pi = - 20 dB, measuring frequency f = 1 kHz. Set the output level with controller item 62 to p2 = - 10 dB.
Input level pi = - 4 dB, measurement frequency f = 1 kHz. The output level should be p2 > + 2 dB.
Increase the input level steadily in the range pi = + 6...+ 16 dB, measuring frequency f = 1 kHz. The output level must not exceed the maximum value P2 max = + 8 dB at any point. If necessary, set the maximum value with controller item 64.
This completes the adjustment of the U 73, operating mode "Compressor". Items 62 and 64 must not be adjusted during the subsequent adjustment work.
U 73, operating mode "Limiter"
Operating mode switch to "Limited." set, input level pi = - 10 dB, measurement frequency f = 1 kHz. Set the output level with controller item 67 to p2 = - 10 dB.
Set controller item 63 to middle position, input level pi = +16 dB, measuring frequency f = 1 kHz. Set the output level with controller item 79 to p2 = + 7 dB.
Input level p1 = + 6 dB, measurement frequency f = 1 kHz. Set the output level with controller item 63 to p2 = + 6 dB ?'1 dB.
Increase the input level steadily in the range pi = + 6...+ 22 dB, measuring
frequency f= 1 kHz. The tolerance limits of the output
level p2 - +6 dB dB are not exceeded at any point.
If necessary, adjust with controller item 79 to ensure that the control characteristic curve is within these tolerance limits.
The adjustment processes to be carried out according to the two previous paragraphs with the controllers items 63 and 79 influence each other. They must be repeated alternately until the control characteristic lies within the specified limit values. If this requirement cannot be met, the two control tubes E 99 F must be replaced.
U 83
Set the operating mode switch to "+ 6 dB", input level pi = - 20 dB, measuring frequency f = 1 kHz. Set the output level with controller item 62 to p2 = - 20 dB.
Set the operating mode switch to "+ 12 dB", input level p1 = - 20 dB, measuring frequency f = 1 kHz. Set the output level with controller item 67 to p2 = - 20 dB.
Set the operating mode switch to "+ 6 dB", control item 63 to the right stop, input level p1 = +20 dB, measuring frequency f = 1 kHz. Set the output level with controller item 64 to p2 = + 6 dB.
Input level pi = + 4 dB, measurement frequency f = 1 kHz. Turn controller item 63 to the left until the output level has dropped by L,p2 = - 0.2 dB.
Increase the input level steadily in the range pi = + 6...+ 22 dB, measuring
frequency f =1kHz. The tolerance limits of the output
level 1 0
level p2 = +6 dB _ '5 dB are not exceeded at any point. If necessary, adjust with controller item 64 to ensure that the control characteristic curve is within these tolerance limits.
U 73 / U 83 Issue 1 Sheet 9
The adjustment processes to be carried out according to the two previous paragraphs with the controllers items 63 and 64 influence each other. If necessary, they must be repeated until the control characteristic is within the specified limit values.
Set the operating mode switch to "+12 dB", constant input level
increase in the range pi = + 12...+ 22 dB, measuring frequency f = 1 kHz.
The tolerance limits of the output level p2 = + 12 dB +1095dB
are not exceeded at any point. If necessary, adjust with controller item 79 to ensure that the control characteristic curve is within these tolerance limits.