Why did Shunyata Research create DTCD and the DTCD Analyzer?
Author: Caelin Gabriel
Date: August 20, 2010
Shunyata Research designs and manufactures high-end power cabling and power distribution devices. We devote a great deal of time and resources toward researching all the materials and parts that go into our products. We perform many tests in an actual audio system to determine if there are audible differences between materials. Many other tests are performed with precision test equipment to see if there are objective scientific measurements that can be made to make correlations between the subjective tests and the measured results. We are constantly looking for differences between materials that might provide a performance advantage in our products, even if that difference is very small. Below is a list of some of the many items that have tested:
- conductor metal types (i.e. copper versus silver)
- plating materials (i.e. gold, silver, rhodium, etc)
- conductor stranding (number of conductors, strand size, etc)
- conductor metal purity (i.e. 4-9s, CDA-101, ETP, single-crystal, etc)
- conductor size (i.e. 16AWG versus 18AWG, etc)
- dielectric materials (TPE, PTFE, PE, PU, PVC, etc)
- conductor geometries
- fuses types
- breaker types
- contact types
- termination methods (crimp versus solder, etc)
While many of these materials can be objectively measured, there are many that have verifiable performances differences in subjective testing that cannot (or are very difficult) to measure with test equipment. There is a variety of test equipment that is conventionally used in the testing of wire and AC power applications. These include multi-meters that can test continuity, resistance, voltage and current. There is more advanced and specialized test equipment such as power analyzers that can test THD distortion and power factor. Other test equipment includes HI-POT testers, impedance analyzers, insulation testers and many others. Shunyata Research owns all of these types of test equipment and we use them in our research. Unfortunately, we have found them to be incapable of resolving differences between some types of materials that are clearly different in subjective testing. This lead to our experimentation with other types of test equipment not commonly used for cable testing in the context of AC power delivery. For instance, we experimented with TDR (time domain reflectometers) and network cable testers (CAT6) testers in an attempt to identify objective differences between materials. While some of these tests have proven to be useful, there have been none that could objectify the difference between (as a single example among many) a six foot AC power cable -- one being a 16 gauge version and the other being a 14 gauge version. With that relatively short length of wire, the test devices listed above could not distinguish between the two. However, in subjective listening tests the differences were obvious.
In our years of testing, we have found that there was a very strong and direct correlation between conductor size and subjective performance. Generally speaking, a power cable with a larger gauge wire will sound better to the test subjects. Additionally, we have found strong correlations between cable impedance and performance. As cable inductance is increased there is a corresponding negative response from the test subjects. Common knowledge states that that a high inductance should be beneficial to AC power delivery. The argument being that AC power is a low frequency transmission (50-60hz) and that a high inductance would block high frequency noise from reaching the equipment. How do we reconcile the subjective testing with the seemingly obvious engineering principles?
Note: cable inductance can be increased with the use of coils and ferrites.
At this juncture we began to look at the operation of the power supplies within consumer audio and video products. We knew from power analyzer tests that a typical power supply could generate current harmonics above the 50th harmonic of the power line frequency. This implied that there may very well be high frequency events in the current domain that could be causing audible differences between cables.
We used a high powered audio amplifier as a test subject. We looked at the AC input to the power supply and at the current and loads across the rectifiers (electronic switch) using current probes and spectrum analyzers. To our surprise we found that the rectifier "ontime" was different when different types of power cables were used. Further, we found different spectral signatures with different cables. After many months of testing we found that the results of the test comparisons were not always consistent and repeatable. The input voltage from the wall outlet to the amplifier could vary based upon time of day and other loads within the building. The test could vary depending upon the load on the amplifier and the specific heat that the amplifier was generating. For these and other reasons we decided that it would be imperative to create a precision reference test rig that would simulate the power line voltage source and to create a test load that closely simulated the action of a typical power supply that was under load. If we could build such a device it could provide us with test results that were repeatable and calibrated. We could then test a power cable or other AC power device directly without having to run tests on an amplifier and then interpreting the results indirectly. Thus, the concept for the DTCD Analyzer was born.
The DTCD Analyzer was designed to test the primary function of a power cord or AC power transmission device. Specifically power cords carry current to the component power supply. Therefore, the analyzer was designed to detect instantaneous current delivery in the context of a typical power supply as the load device. Why instantaneous current; because all modern power supplies pull power in current pulses. (See the conduction period diagram)
Consumer electronics devices have power supplies that use rectifiers to convert AC current to DC. Since these supplies use capacitors to store energy, the rectifiers only switch on when the input AC voltage waveform exceeds the stored voltage across the capacitors. This means that the power supply pulls current in a pulsed fashion -- only turning on for a fraction of the total waveform period. The actual time period that the power supply is pulling current is what we refer to as the "conduction period". It is only the conduction period that we are concerned with in the context of measuring current and voltage. We do not want an averaged reading of the current drawn during the entire power cycle, because that would obscure the actual performance of the tested items.
The DTCD Analyzer is designed to test a single current pulse through the DUT (device under test). Since the primary purpose of DTCD Analysis is to measure devices that inherently have very low impedance, it becomes challenging to create test equipment that can accurately measure the differences between what amounts to differing wire types and sizes or different switch designs. Further, it is important that the differences (if they can be measured) have some relevance to the task that they are intended to be used for.
With that in mind, the DTCD Analyzer needed to simulate the AC electrical power grid, with its characteristics as a constant voltage source. The DTCD Analyzer's voltage source consists of a capacitive array whose characteristics include ultra-low impedance, resistance and ESR (measured at 0.0016 ohms) with the ability to provide peak currents of several hundreds of amps with minimal voltage drop.
The second major element of the DTCD Analyzer is the load. It was designed to mimic the action of a typical power supply when the rectifier turns on and off to fill the power supplies' storage capacitors. It is for this reason that the source voltage is 30VDC, which is a reasonable voltage differential between line input voltage and the stored voltage level across the power supplies storage capacitor array.
Third, a detection circuit was created to accurately sense the voltage and current waveforms that occur during periods as short as only a few microseconds. The detection circuit in the DTCD Analyzer consists of a precision current sense, non-inductive resistive array.