BENCHMARK MEDIA SYSTEMS, INC.

Jr. Audio Director™ Plus - Instruction Manual

BENCHMARK MEDIA SYSTEMS, INC.

5925 Court Street Road

Syracuse, NY 13206-1707

(315) 437-6300, FAX (315) 437-8119

BENCHMARK MEDIA SYSTEMS, INC.

Jr. Audio Director™ Plus - Instruction Manual

1.0 Overview..........................................................................................................................................1

2.0 Performance Specifications............................................................................................................1

3.0 Features ............................................................................................................................................2

4.0 Installation and Operation.............................................................................................................2

4.1 Mechanical Installation ......................................................................................................2

4.2 Electrical Installation ..........................................................................................................3

4.3 Setting Levels.......................................................................................................................3

4.4 Circuit Description..............................................................................................................3

4.4.1 Input Switch Banks ...............................................................................................3

4.4.2 Instrumentation Input Amplifiers.....................................................................3

4.4.3 Level Potentiometer and Gain makeup Stage .................................................4

4.4.4 Mono Mixer and Inverting Amp .......................................................................4

4.4.5 Mono and Polarity Control in the Switch Banks .............................................4

4.4.6 The Output Stages................................................................................................4

4.4.7 The Headphone Amplifier..................................................................................4

5.0 Troubleshooting and Repair..........................................................................................................5

5.1 Servicing Techniques..........................................................................................................5

5.1.1 Circuit Board De-Soldering ................................................................................5

5.1.2 Circuit Board Re-Soldering.................................................................................5

5.2 Common Mode Rejection Null .........................................................................................6

6.0 Component Placement Diagram...................................................................................................9

7.0 Schematic Diagram ........................................................................................................................11

BENCHMARK MEDIA SYSTEMS, INC.

Jr. Audio Director™ Plus - Instruction Manual

1.0 Overview

The Jr. Audio Director™ Plus is a width, 1 RU stand alone product with input source select, level controls, mode controls (mono mix & polarity), and a stereo headphone amplifier.

The Jr. Audio Director™ Plus has balanced inputs and outputs. It provides output selection between Left Only, Right Only, Mono, Stereo, and Stereo Reverse by way of the input source selections. Right Channel Polarity is provided to correct polarity inversion in source material. The Jr. Audio Director™ Plus has gain adjustment of both L and R inputs by way of a concentric potentiometer. This feature provides control over the mix in the Mono mode, and a easy level adjustment for the two channels.

The rack mountable chassis includes LEDs to indicate source selections (green) and the mono and polarity functions (red). The mono mode has built-in gain reduction to correct for the natural summing level increase; front panel adjustable with a range of -6 to -3 dB.

Installation is easy, using plug-in Euro-style barrier strips.

Power for the Jr. Audio Director™ Plus comes from a PS-1, the Benchmark wall mount ±16

V regulated supply.

2.0 Product Performance

Benchmark Media Jr.+

0.050

THD+N(%) & PHASE(deg) vs FREQ(Hz) 25 Oct 93 10:15:00

5.000

0.0

0.010

-5.00

-10.0

0.001

.0002

20

-15.0

-20.0

100 1k

Figure 1. Typical Total Harmonic Distortion and Phase

10k

1

20k

-25.0

Benchmark Media Jr.+ Crstlk

-60.00

AMPL(dBr) & AMPL(dBr) vs FREQ(Hz) 25 Oct 93 10:20:00

-70.00

-80.00

-90.00

-100.0

-110.0

-120.0

20 100 1k 10k 20k

Figure 2. Typical Crosstalk, L into R, and R into L

3.0 Features

t 2 MΩ Balanced inputs t CMR >100 dB to 2 kHz, 75 dB @ 20 kHz t

60 Ω Balanced outputs t

THD = 0.00088% @ 2 kHz, 0.0011% @ 20 kHz t Ø @ 20 kHz = -9.9°0.. t

Bandwidth = 160 kHz t

Excellent RF immunity t Mono mode built in gain reduction (reduc- tion is adjustable between -6 dB to -3 dB) t

Eurostyle barrier strips rear connectors t Rack mountable with optional rack mount kit.

4.0 Installation and Operation

4.1 Mechanical Installation

The Jr. Audio Director™ Plus may be operated on a desk or rack mounted as desired. If the unit is to be operated in a rack mount, the unit simply slips into the rack mount kit and four screws (two black 4-40 x 1/4 " pan head, and two black 4-40 x 1/4" flat head) hold the chassis firmly in place.

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4.2 Electrical Installation

Electrical installation is very simple. Connect the outputs from the signal sources to the inputs of the Jr. Audio Director™ Plus. Now feed the outputs of the Jr. Audio Director™ Plus to its destination. If you intend to take the output of an unbalanced consumer or semi-professional device, use the forward referencing method described in “A Clean Audio Installation Guide” by Allen Burdick, a Benchmark Media Systems, application note.

Power is supplied by the PS-1 wall mount power supply included with the unit.

4.3 Setting Levels

The Jr. Audio Director™ Plus is intended to operate at or near unity gain. This is found at approximately the 1 o’clock position of the gain potentiometers. We recommend that metering be permanently placed at the output of the Jr. Audio Director™ Plus. This will facilitate the setting of levels on a day to day basis. 15 dB additional gain is available when the potentiometers are in their full clockwise position, approximately 5 o’clock.

If the incoming source is in mono and there is no need to have the source “on the air” at just this time, there is an easy way to precisely match levels. Set one output level to the approximate desired level. Monitor one of the outputs. Place the Jr. Audio Director™ Plus into the mono mode and invert the right channel input polarity with the polarity control.

While listening to the monitor adjust the second input level until there is an exact cancellation.

This technique will match the two levels to within 0.1 dB or better.

4.4 Circuit Description

The circuitry of the Jr. Audio Director™ Plus is quite simple. It consists of the following major parts. There are two input switch banks, two instrumentation input amplifiers, two level potentiometers, two gain make up stages, a mono mix amplifier and inverting follower, output amplifiers, and a headphone amplifier. While reading the following description, it will be well to have the schematic along side.

4.4.1 Input Switch Banks

Four main inputs and four secondary inputs exist at the rear of the Jr. Audio Director™ Plus.

Internal jumpers cross-connect these inputs. Under many circumstances left and right channel selections will need to be made from the same four sources. In that case the sources are taken to the main inputs only and the internal jumpers are left on their posts. The same sources will then appear at both the left and right 1 through 4 positions on the respective switch banks.

Occasionally, two different sets of inputs may be desired for the left and right positions. In such a case the internal jumpers must be removed and the signals brought to both the main and secondary inputs.

1 kΩ input resistors are in the signal chain prior to the switch bank. This allows two or more switches to be depressed and a passive mix of the signals is then presented to the two instrumentation amplifiers. For this mix to work properly, the input sources should be of the low impedance type, that is ≈ a 50 Ω source. Of course the relative input levels are not controllable in the mix within the Jr. Audio Director™ Plus as they are when an internal mono mix is created.

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4.4.2 Instrumentation Input Amplifiers

The instrumentation input amplifiers consist of two unity gain non inverting buffer amplifiers that in turn feed a highly trimmed differential amplifier whose gain is -6 dB. The input to the instrumentation amplifier consists of paralleled input capacitors. The 100 µF aluminum electrolytic provides excellent low frequency response, while the paralleled 0.1 µF film capacitor overrides the ESR and inductive effects of the electrolytic at high frequencies. The input resistor attenuator network provides isolation to the output stage that drives the Jr.

Audio Director™ Plus in the event of a power loss to the unit. The attenuator also provides very high over voltage protection for the input amplifiers, with the inputs able to take 125 V

AC, differentially, without harm.

4.4.3 Level Potentiometer and Gain makeup Stage

The output of the differential amplifier feeds the log taper level potentiometer. The output of the pot feeds the non inverting input of the gain makeup stage. The amplification factor of the stage is +15 dB. The gain makeup amplifier sets the noise floor of the entire Jr. Audio

Director™ Plus.

4.4.4 Mono Mixer and Inverting Amp

The mono mix amp takes its two inputs from the outputs of the gain makeup amplifier. The gain of the mix amplifier is variable with a gain range of ≈ -3 to -6 dB. This is necessary due to the signal level addition that takes place when summing two signals. When summing two non-coherent signals such as noise sources the addition causes a 3 dB increase in level. When summing two fully coherent sources, the level increase is 6 dB. The gain has been set at the factory to –3 dB. If further gain reduction is necessary, this is easily available at the front panel and can be adjusted with the aid of a green Xcelite

®

screwdriver, or “greenie” as they are often called.

Since the mix amplifier is an inverting amplifier, absolute polarity would be compromised without additional inversion, hence the unity gain inverter.

4.4.5 Mono and Polarity Control in the Switch Banks

The extreme left positions of the Schadow switch banks, are push on push off controls that select both the mono mix and the right channel polarity inversion functions. The Right

Channel Polarity inversion is on the input to the instrumentation amplifier, whereas the mono mix selection is a down stream control.

4.4.6 The Output Stages

The output of the switch bank feeds two paralleled capacitors, the same as on the inputs. The outputs of the capacitors feed the inputs to the balanced outputs. The purpose of the capacitors is to isolate any DC offset coming from the previous stages.

The output stages consist of two amplifiers, an inverting amplifier, and a non-inverting differential amplifier. Both the inverting amp and the differential amp have the same 20 kHz phase shift. As a result, there is no differential phase shift between the two outputs.

The output impedance of the output stages is set to 60 Ω balanced by the two 30.1 Ω output resistors.

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4.4.7 The Headphone Amplifier

The headphone amplifier consists of a TLO72 dual operational amplifier with a very high speed buffer that is functioning as a current boost output stage. The output current capability of the buffer is greater than 200 mA, and typically is around 300 mA. Since the buffer is a very high speed device, it does not degrade the stability of the overall amplifier.

Gain is determined by the setting of the feedback potentiometer and since the device follows the gain makeup stage, any adjustments to the relative levels up stream will be heard at the headphone output. 30 Ω output resistors provide over current limiting, as well as providing some equalization of sensitivity between high and low impedance headphones.

5.0 Troubleshooting and Repair

5.1 Servicing Techniques

Service and calibration can be easily performed at the test bench. Be sure to use current limited power supplies set for a current limit at ≈ ±100 mA.

The first thing to do is to look for integrated circuits that are hot to the touch. Since most passive components are extremely reliable, most circuit failures occur with the active semiconductors. Replace any extremely hot chips. Keep in mind that the NE5532 runs very warm to the touch. In fact, many people cannot keep a finger on a working NE5532

(approximately 135° F).

If you strongly suspect a passive component failure, replace it. Remember, however, printed circuit boards are very easy to damage with excessive heat. Unless you have developed the specialized skills necessary to remove and replace components, we strongly suggest that you leave the task to someone skilled in these techniques.

5.1.1 Circuit Board De-Soldering

When servicing printed circuit boards we strongly recommend the use of a vacuum desoldering station, such as the Hako 470 or various Pace models. The proper technique with these stations is to apply the tip to the area to be de-soldered and wait for the solder to thoroughly melt. You can be sure of a thorough melt by observing the top side of the board.

Only when the solder there has become liquid, apply the vacuum while moving the hollow tip with the component lead in a circular motion. By rotating the lead, with the tip lightly against the board, and without applying pressure to the pad, you are able to remove solder from the plated-through hole. When this procedure is performed correctly the component will often drop out of the board of its own weight. If the solder is not thoroughly removed from the plated-through hole do not pull the component. Attempting to force the component will bring with it plating from inside the hole. This, in turn, will require the repair of the hole with an eyelet. Also, if your attempt to completely remove the solder from the hole has failed, do not re-heat the area with the de-soldering tool. Doing so will overheat the pad, and will not heat the area that is in need. As a result, the board is usually damaged. Rather, re-solder the joint, and then go back and apply the proper technique, by allowing the solder in the joint to thoroughly melt before applying vacuum. This technique uses new solder as an efficient heat conductor to the total area, eliminating hot spots.

5.1.2 Circuit Board Re-Soldering

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Here is an effective technique that ensures highly reliable solder joints. First, if the component that is to be installed has leads that are highly oxidized, it will be necessary to clean them. This may be done with a Scotch Bright® abrasive pad or fine bristle fiberglass brush, among other methods. After inserting the component, apply a small amount of liquid flux directly at the component leads and PCB with a needle bottle applicator or small brush. We prefer to use water soluble flux for ease of cleaning. The added flux is an immeasurable aid in achieving a good solder joint. Next, heat the component lead, since it usually has the higher mass, by applying a small amount of solder to the tip of the soldering iron at almost the same time as you apply the iron to the component lead. The iron should be approximately 1/8" above the board. When the lead has come up to temperature so that it melts the solder when placed against it, slide the soldering iron down the lead and heat the printed circuit board pad while applying a controlled amount of solder to the joint. The solder should flow into the joint with extreme ease and create an excellent fillet on the top of the PCB as well as on the bottom. The entire soldering portion of the procedure should take no more than a couple of seconds.

5.2 Common Mode Rejection Null

The common mode rejection trims on the input stages should never need to be readjusted, once they have been set at the factory. This is a passive bridge, and the characteristics of the operational amplifier used do not affect the accuracy of the balance on this bridge, that is, unless there is a malfunction with the op-amp. When replacing the operational amplifier, measure the common mode rejection before touching the trims.

The process of nulling the common mode rejection should be performed with the overall gain of the Jr. Audio Director™ Plus set to unity. See Figure 2.0

Feed an unbalanced signal with a level of +20 dBu, referenced to ground, into both inputs of the module being adjusted. This signal must be exactly the same on both inputs. This is best achieved by using an oscillator with a single ended output, tying the ± inputs together and, in turn, to the single ended output of the generator. The ground side of the generator, of course, also ties to ground.

Fig. 2 - Common Mode Rejection Adjustment

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Send a 2 kHz signal to the input and adjust the resistive portion of the differential amplifier bridge for a minimum audio output from the Jr. Audio Director™ Plus. Use either a logarithmic level meter with a sensitivity to below -100 dBu, such as the Audio Precision

System One, or a very sensitive linear meter, such as the Amber 3501 distortion and noise meter. It is very helpful to watch the audio signal from the meter output on an oscilloscope, which in turn should be synchronized to the signal source. This will allow you to quickly see the phase/amplitude nulls as they take place. Once a minimum resistive trim has been achieved, null the capacitive trim.

Two or three iterations between these controls should be sufficient to achieve the best broadband null possible. A null of better than 90 dB at 200 Hz, and better than 70 dB at 20 kHz, is achievable. Unfortunately, it is almost impossible to maintain this good a null over the operating temperature range. A degradation of up to 10 dB may be expected when the unit is returned to its working environment.

This completes the Jr. Audio Director™ Plus Instruction Manual

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BENCHMARK MEDIA SYSTEMS, INC.

5925 Court Street Road

Syracuse, NY 13206-1707

(315) 437-6300, FAX (315) 437-8119

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