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Kit Upgrades |
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Use Terminated Interconnect Cables
A preamp feeding a power amp with interconnect cable is shown in fig. 1. Normally the output impedance of the preamp is in the order of 50 to several 100 Ohm. The input impedance of the power amp on the other hand is in the tens to hundreds of kilo Ohm. Connecting these together with a shielded cable means that the cable sees a low driving impedance and a very high load impedance. The result is that the signal you are sending from the preamp bounces back from the other end resulting in echoes. And the signal will be smeared by the time it arrives at the power amp. This is practically independent of whether you are using a cheap shielded cable, a professional coax cable or a 1k$/m fancy silver cable. The reason is that the cable is not terminated with its characteristic impedance.
Consequently significant improvements can be achieved by using terminated cables between pieces of audio gear. This is especially important when the distance is several meter. Typical example is a lineamp sitting next to your armchair and drives a 10m cable going to the power amp. In fig. 1 I have designated the output impedance of the line amp driving the cable with R and the other end of the cable is also terminated with the same resistor. This resistor has to be equal to the characteristic impedance of the cable. If you are using a 50 Ohm coax cable then R=50 Ohm, if the cable is a 75 Ohm video cable then R=75 Ohm. Should you use a 150 Ohm microphone cable, then R=150 Ohm. Naturally the connector at the output of your lineamp and at the input of your power amp has to have the same impedance as the cable. If you are using R=50 Ohm then you need a 50 Ohm connector. In case of R=75 Ohm the connector has to be a 75 Ohm one. Fortunately there are very good 50 Ohm and 75 Ohm BNC connectors on the market and they are much better than any RCA connector you can buy anyway.
If you terminate the cable at BOTH ends with its characteristic impedance, then it appears to be a purely resistive cable and there will be no reflections (echoes) back from the load. This is not new; the RF people have been using this for ages. The real advantage of this method is that you can use relatively inexpensive cables, which give you better results than the expensive ones without termination.
Also digital signals are transmitted with terminated cables. If you have an outboard DAC with your CD player you are connecting the digital signal from the CD player with a 75-Ohm coax cable to your DAC and this cable is driven from a 75-Ohm source and is terminated in your DAC with 75 Ohm.
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 |  | | Figure 1. Terminated Interconnect Cable |
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Naturally there is no free lunch in life and this method also has some drawbacks. As you see the load on the line amp is 2xR. This means that your lineamp has to be able to drive low impedance. If R=50 Ohm then the load will be 100 Ohm on the lineamp. With R=75 Ohm its 150 Ohm, and with R=150 Ohm its 300 Ohm. Not all line amps on the market can drive such loads. But all of our ALL-FET line amps can!
As you see there is a 6 dB voltage loss between the lineamp and the power amp, because of the two resistors. However, this is usually no problem, especially if you are using CDs. In fact most of our customers are asking how to reduce the gain in the lineamp, because it’s too much. In any case you can also increase the gain in the lineamp, should that be needed.
The terminated cable approach is not limited to unbalanced interconnects; it can also be applied to balanced ones. Fig 2 shows how it works. In this case both lineamp output have to have an output impedance of R and both inputs of the power amp has to be terminated with R. The Neutrik XLR connector has an impedance of 110 Ohm, so the easiest way is to use a 110 Ohm balanced cable and make R=110 Ohm.
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 | | | Figure 2. Terminated Interconnect Cable for balanced system |
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Coming now back to an Upgrade of the 114 lineamp, the output impedance of the amp itself is very low, so you need to connect the appropriate resistor in series with the output to terminate the cable with the right impedance. You can access the output of the amp at the opposite end of R23 seen from the point marked OUT. Make this resistor 50 Ohm if you want to use a 50-Ohm coax, 75 Ohm if you use 75-Ohm cable. Naturally, the cable has to be terminated with the same resistor at the other end. Use the terminated interconnect system shown in fig. 2 if you use two 114 for a balanced configuration.
The result of this Upgrade is a significant improvement in clarity, and mid-high frequency smoothness. You have also saved a lot of money on the interconnect cable!
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