It Is rugged and road-worthy, conservatively rated, and can handle reactive loads with ease. The engineering design approach stresses the optimization of each stage, allowing high slew rate and relatively low loop gain. Overall feedback has been held to a minimum and Is employed only to stabilize the gain and the operating point. This design approach results In an amplifier with excellent performance under the most demanding dynamic Input and load conditions. As evidence of the stress on dynamic rather than static or steady-state distortion mechanisms, transient I ntermod u I at I on distortion measures less than 0. This JBL amplifier uses multiple watt output devices In complementary configuration for high reliability and low distortion.
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It Is rugged and road-worthy, conservatively rated, and can handle reactive loads with ease. The engineering design approach stresses the optimization of each stage, allowing high slew rate and relatively low loop gain. Overall feedback has been held to a minimum and Is employed only to stabilize the gain and the operating point. This design approach results In an amplifier with excellent performance under the most demanding dynamic Input and load conditions.
As evidence of the stress on dynamic rather than static or steady-state distortion mechanisms, transient I ntermod u I at I on distortion measures less than 0. This JBL amplifier uses multiple watt output devices In complementary configuration for high reliability and low distortion. The benefit Is high reliability and long component life.
Reliable operation Is ensured through the following protection modes: current Is limited under Improper load or drive conditions; output relays with front panel Indicators protect the loudspeaker under conditions of DC offset or excessive low-frequency transients.
The relays also provide power-up, power-down, and "brown out" muting to protect loudspeakers from AC power transients. Clip Indicators allow for system optimization.
The JBL amplifier may be operated In the normal stereo mode, dual mono mode, or bridged mono mode. These modes are switch selectable on the rear panel. Active differential Input circuitry offers the benefits of balanced operation without the use of Input transf ormers.
The barrier strip has separate terminals for audio ground and chassis ground. The rack ears and heatsink of the amplifier are made of high grade aluminum extrusions; the chassis Is fabricated of heavy gauge steel.
All Internal components are easily accessible through removal of top and bottom panels. Front panel graphic details are Incorporated on the rear-side of a polycarbonate laminate which Is virtually Indestructible. Maximum total harmonic or intermodulation distortion measured at any power level from milliwatts to rated power is less than 0. Midband Power is maximum output power at onset of clipping, both channels driven with 1 kHz sine wave, THD 1?. Bridged Mono and Dual Mono Modes: 20k ohms used as balanced input; 10k ohms used as unbalanced single-ended I nput.
At Idle approx. Short circuit protection Is clean with no degrading characteristics. If the amplifier overheats, the relays open until the amplifier has cooled down. Independent detented level controls. Heavy-duty Illumin- ated rocker-type power switches. May be wired balanced or unbalanced. Chassis ground and audio ground connected with removable shorting strap on rear panel barrier strip.
Nevertheless, we recommend careful examination of the shipping carton and Its contents for any sign of physical damage which could have occurred In transit.
If damage is evident, do not destroy any of the packing material or the carton, and Immediately notify the carrier of a possible claim for damage. Shipping claims must by made by the customer. Save the carton and packing material In the event the unit must be returned for service. See Section 5. If the amplifier Is Installed in an equipment rack, adequate ventilation must be provided In order to assure longest component life.
Keep any obstructions such as wire bundles and rear mounted connector panels away from fan Intake area. Again, do not mount other products above or below the amplifier In such a way that would block off air flow to heatsink area. It is normal for the amplifier front panel and rack ears to become warm during operation since it Is part of the heatsinking system of the amplifier. See Section 3. Since the amplifier is quite heavy, be sure the rack is capable of supporting it. When a rack is to be transported with a portable sound system, the amplifier should be supported from the rear and from below; a few pieces of angle Iron secured to the sides of the rack should suffice.
It is also recommended that the amplifier be placed low in the rack to minimize any tendency for the rack to tip over. Amplifiers wired for operation on any other voltage are identified as such with a sticker on the rear panel and a tag attached to the amplifier power cord. To comply with most electrical codes this amplifier is supplied with a three-conductor AC power cable, the grounding pin of which is connected to the chassis.
In some instal lations this may create ground loop problems when an AC potential exists between conduit ground and audio ground. This will be evidenced by hum or buzz in the amplifier output. If this should occur please refer to Section 2. Proper grounding of the amplifier Is important for both noise and safety reasons.
Be aware that unless the amplifier is properly grounded, a safety hazard can exist. J B L accepts no responsibility for legal actions or for direct, indirect or consequential damages that may result from violation of e I ectr i ca I codes. The input connector goes to Channel A. The signal is internal ly routed to Channel A and Channel B, with both level controls active.
There is no connection to Channel B Input. Input is to Channel A. I 2. See Figures , and Since all three connectors are wired in parallel, however, only one should be used at a given time unless it is specifically desired to loop a signal through the amp I I f i er 1 nput.
The amplifier will not unbalance floating or balanced Input sources since the input circuits consist of balanced differential amplifiers.
To use an unbalanced source, wire the signal carrying conductor of the cable from that source to XL-type pin 3 phone plug tip , and wire the shield to XL-type pin 1 phone plug sleeve. The unused connector terminal, pin 2 ring , should also be connected to shield ground.
Unbalanced connections are simplified by using two-conductor standard phone plugs because they automatically short the ring and sleeve together when inserted In the input jacks.
Set the mode switch to Dual Mono. Do not apply signal to the Channel B input. That signal is applied "in phase" to Channel A, and internally connected, with inverted polarity, to Channel B. No signal should be applied to the Channel B input, but the Channel B level control must be at the same setting as Channel A for proper operation. See Figure The preferred connection method Is to use a dual banana plug for each speaker cable.
In the absence of a dual banana plug or two single banana plugs , loosen the plastic terminal nut, wrap the stripped and twisted wire end clockwise around the terminal, and secure It by tightening the nut. NOTE; Tin the wire ends with solder to prevent unraveling, but avoid excess solder as it can promote cable breakage.
Smaller speaker cable can be pushed through the hole In the binding post shaft, but we recommend heavier gauge cables to wrap around the shaft. If a lug Is Installed on the cable, loosen the terminal nut, push one "leg" of the lug through the hole In the shaft, and tighten the nut.
The speaker Is connected between the two red binding posts the black posts are not used In bridged mono mode. In this case, a positive-going signal applied to the Channel A Input appears as a positive-going signal at the Channel A red binding post and as a negative-going signal at the Channel B red binding post.
Long audio transmission lines, like their video counterparts, must be properly sourced from and terminated in equipment which matches their character i st i c impedance if optimum frequency response and noise rejection are to be achieved. However, transmission line theory and techniques are not only unnecessary but impractical within modern recording studios, broad- cast studios and other local audio systems where transmission circuits are seldom more than several hundred feet in length.
The advent of negative feedback circuitry and solid-state electronics has spawned modern audio amplifiers and other signal processing devices having source impedances of only a few ohms. They are essentially indifferent to load impedances and, by varying their output current inversely to changes in load impedance, maintain the same output voltage into any load Impedance above a rated minimum, with no change in frequency response. Modern audio systems, therefore, utilize amplifiers and other active devices which have very low output impedances and high 10k to 50k ohm Input impedances.
These products may thus be cascaded operated in series , or many inputs may be connected to a single output of a preceding device, without regard to impedance matching.
Switching and patching is simplified because double loads and unterminated bugaboos are essentially eliminated. Floating ungrounded transformer outputs minimize ground loop problems, and differential transf ormer I ess input circuitry or input transf ormers minimize common mode noise or interference which may be induced into the interconnecting wires or cables. Where audio must be transmitted through cables or wire pairs of more than several hundred feet in length, however, transmission line termination practices should still be observed.
This amplifier has input impedances of 40, ohms when used in a balanced, differential input configuration, and 20, ohms when used unbalanced one side grounded.
This makes the amplifier suitable for use with any normally encountered source impedance, low or high. Therefore, there are only two situations which will require an input load at the amplifier; 1 when the source requires a ohm load, such as a passive equalizer, older vacuum tube equipment, etc.
In some Instances it may prove beneficial to treat the input feed to the power amplifier as a transmission line to lower its impedance and its susceptibility to noise p i ckup. I Input load resistors, If required, may most conveniently be attached to the barrier strip connector of the amplifier. We realize this Is not always easy to determine, especially since speaker power ratings have not been standardized.
Fuses may be Inserted in series with the loudspeaker to protect against overpowering, though the fuse value must be chosen with care. Ideally, the value will be high enough that the fuse does not limit the peak power capability of the loudspeaker. On the other hand, the fuse value must be low enough for the fuse to actually do its job. It takes a period of time to heat the fuse element enough to cause it to melt and break the circuit.
If it takes too long, the loudspeaker may go first. Obviously, delayed action slo-blo fuses are not acceptable for use here. If fuses are used, consider- ation must be given to their location. Put them where they are accessible for ease of replacement, provide clear labelling of the replacement fuse value, and place spare fuses nearby.
If the amplifier power Is only recommended, or if a power rating is given without mentioning the amplifier, then the question as to whether the amplifier is "safe" becomes more difficult to answer.
Loudspeakers usually fail due to one of two factors: thermal or mechanical overload. Thermal overload means overheating, and is almost always caused by applying too high a level of sustained, average power; the voice coll insulation may burn and short circuit, the coil may deform due to the heating and scrape in the gap, and.
In some cases, speaker cones have been known to burst into flame. A single very high power transient, especially at lower frequencies, can literally tear a loudspeaker apart.
The power supply caps and troublesome electrolytics in the speaker relay circuit all looked to have been replaced already. Connecting speakers and pre-amp I found that I only had output on one channel and that seemed a bit soft. I reflowed the solder on the relay pins on the back side of the board, as well as a number of the other components in that area but still no change. At that point I was thinking I might have to replace the relay, so I removed it from the board completely.
Urei Jbl 6260