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The following are circuits to build a stereo leveling amplifier (limiter). This is an amplifier that maintains an average output level. These
amplifiers are used in recording studios and other installations where an average level is required. This unit was used on the air at a FM
broadcast station in Baltimore, Maryland with outstanding results. This unit prompted unsolicited comments on how good the radio station
sounded shortly after its installation. This is a project for the advanced audio hobbyist. Component layout must be done carefully and
some special skills are required such as making your own meter scale. The unit shown here is the unit that was built for use in Baltimore. A newer version is on the drawing board with some minor circuit changes and less rack space. However, actual audio performance will be the same, the main difference being able to operate with -10db consumer levels. The unit controls gain in a dynamic manner. Gain reduction, and release, provides for good average levels while still allowing dynamics through. Gain is controlled at two different points in the audio path. Very slow gain control is applied at the input stage via a light driven resistor (LDR). A faster gain reduction is applied after the input stage via a gain controlled triode tube stage. The tube stage uses a dual time constant arrangement, a fast constant riding a slower constant. Although the unit is a wide band processor; the use of two gain control stages, one slow and one fast, provides limiting close to split band processing without the coloration of split band processing. Gain reduction ratio is variable. As the input level goes up, and the gain reduction increases, the ratio becomes tighter. This unique feature gives the sound a wide range of dynamics. Low, or soft, passages have full dynamics while high, or loud, passages are more restricted but still sound louder. Distortion induced in the triode gain control stage is second harmonic predominate and increases as the triode goes into gain reduction. An interesting effect of predominate second harmonic distortion is that as tube saturation is approached, the sound quality is still good but the apparent loudness goes up. A gain reduction meter is used to show relative gain reduction. The meter indicates gain reduction of the triode tube stage. The slow gain control of the LDR is not directly metered. However, since the slow LDR gain control at the input stage precedes the gain controlled triode stage, the effects of gain control at the input will be seen on the meter. There are 3 controls on the front panel, LDR attack/release adjust, triode attack/release adjust and a test/operate switch. Both attack/release controls set straight up is normal operation. Full counter clockwise is slower than normal and full clockwise is faster than normal. The test operate switch defeats gain control in the test position. The switch is a good tool for checking operation of the unit. Because of the dynamic way in which gain is controlled, it may not seem like there is much gain control. However, the amount of gain reduction is evident when the test/operate switch is put in the test position while the unit is operating and in gain reduction while observing the output level. Although the useful range of this design is only 25db of *dynamic gain control, this is sufficient to maintain a good average level without "sucking" up every sound. The design was intended to sound natural, not compressed. this is not a "brick wall" processor" Both channels have low and high EQ trim adjust controls. They are intended to "trim" the sound of the audio chain and once set, left alone. They have approximately a 10db range. Although not directly coupled, the two channels are linked sufficiently at the triode gain control stage control to prevent center channel shift during periods when one channel has more audio than the other. Although the inputs and outputs are unbalanced, the design provides for connecting to balanced audio connections, the inputs and outputs have capacitor isolated grounds available. The inputs provide a 10K load from both audio leads to ground for a balanced load. The output's have an internal 10K load for proper performance. The design is intended for professional levels in and out. The output level is fixed and is approximately +4db, depending on input levels. To drive the unit into compression range, the input level should be at least 0db, .775V. During experimenting, several different brands of 12AX7's were used including old RCA and Mullard tubes. The circuit values were selected so distortion was lowest regardless of what brand tube was used. The best test results were using recent tubes made in China. However, selecting tubes for best performance may be necessary. |
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The power supply using a conventional vacuum tube power supply plus a DC supply for filaments of noise sensitive circuits. Vacuum tube power transformers are not as common as they once were but some are still available. When selecting a transformer remember that you can substitute windings with a higher current (amps) rating than specified but never use a transformer with lower current ratings. The only effect of using a transformer with a higher current rating is that the voltage will be somewhat higher.
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The input stage with high frequency and low frequency trim. Input is 10K ohms and will interface well with solid state equipment. Input is unbalanced and requires a 0db (.775V) input level. An external high quality input transformer can be added for balanced input. However, connecting to a balanced source using capacitor isolated ground has worked fine. |
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The output stage using a cathode follower output stage to interface with solid state equipment. There is negative 6 volts tapped off the DC filament supply applied to the cathode of the output stage. This unusual bias arrangement provided lower distortion readings on some tubes during testing. The output is unbalanced. However, connecting to a balanced input using capacitor isolated ground has worked fine. |
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The AGC amplifier amplifies the input audio enough to drive a DC rectifier producing the DC voltage control voltage for the gain control stages. |
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The Triode DC amplifier is actually not an amplifier but rather an audio to DC converter. This is where the DC control voltage and time constant for the triode gain control stage is derived. Also included here is the gain reduction meter amplifier that indicates triode gain reduction. As with most older vacuum tube audio compressors; the meter reads full scale, or some reference point, with no gain reduction. Then reads down scale, or towards 0, as gain reduction increases. The full scale reference point is adjustable with R2. R1 adjust the time constant from very slow to faster, this is the attack and release time of the triode gain controlled stage. The time constant is such that dynamic range is intact while still maintaining an average level. |
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The LDR amplifier is the input slow gain control circuit. The LDR in the circuit is no longer available but some may still be around in old stock. However, you may sub a VACTEC VTL5C2, Newark part number 43F886. |
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This is the triode stage that controls gain |
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A complete diagram use stage drawings above for detailed information |
DESIGN by ED JURICH
copyright 1990 - 2008
may be used for home construction
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