This is a matter of phase distortion. Simply put, every transformer (and any other signal processor, unless DC-coupled) behaves as a 6 dB/octave high-pass filter. This filter creates phase distortion that’s significant at about 10 times its ‘cutoff’ (−3 dB) frequency and gets worse as frequency decreases. Therefore, to avoid significant phase distortion at 20 Hz, the cutoff frequency must be 2 Hz or lower. In many Jensen designs, this frequency is well under 1 Hz. Again, this is a matter of time alignment of musical overtones. For example, a kick drum, when heard through a Jensen transformer will actually sound like a live kick drum because the fundamental note and all its overtones have arrived at your ears at the same time!
The intensity of the magnetic field in a transformer increases as frequency decreases and signal level increases. Therefore, the most demanding job for the transformer is to pass a high-level, low-frequency signal with acceptably low distortion. For example, if too little core material is used, the transformer may magnetically ‘saturate’ (producing very high distortion) at bass frequencies that are at normal signal levels, let alone the higher peak levels associated with instruments like kick drums. This means that even the tiniest, poorly-designed transformer can exhibit low distortion at 1 kHz. In general the ‘level handling’ of a transformer is inversely related to frequency. Jensen rates its transformers very conservatively at 20 Hz, so they will handle even higher levels and have less distortion at higher frequencies. In most of our designs, distortion becomes unmeasurable at frequencies above a few hundred Hz!
The most common phono ‘pickup’ or ‘cartridge’ is the moving-magnet type and the vast majority of phono preamplifiers are designed for their relatively high output voltage … generally about 5 millivolts (at standard stylus velocity of 5 cm/sec). However, extremely high-performance moving-coil pickups are preferred by many audiophiles. These pickups have extremely low output voltage … most often in the 0.1 to 0.5 mV range … and a correspondingly low impedance … most often in the 5 to 50 Ω range. It is extremely difficult to design amplifiers for these pickups that have acceptably low noise floor. Therefore, step-up transformers are commonly used to provide essentially “noiseless” gain to these tiny signals. The benefit is a very low noise floor. Of course, preserving audio quality (low distortion and wide bandwidth) is very important … and why you should consider Jensen.
If maximum elimination of ‘ground loop’ buzz is the goal, an input type isolator located at the receive end of an interface cable is always the preferred solution. However, if the signal destination is unknown or may change with setup, an ISO-MAX isolator with an “O” in the model number (PO-2XR, for example) may be useful at the mixer’s balanced output. The isolator output can then drive either balanced or unbalanced equipment inputs with no risk of damage or misbehavior in the mixer, regardless of what kind of output stage it may use. There is generally a reduction in ‘ground loop’ hum but little effect on buzz.
A major benefit of Faraday shielding is high attenuation of RFI (radio frequency interference). ISO-MAX microphone isolators and “splitters” (model MS-2XX, for example) have outstanding rejection of RFI, especially at troublesome AM radio broadcast frequencies.
The laws of physics dictate that a transformer cannot pass DC and, in fact, this guarantees that there is no “offset” voltage at its output. Such offsets are one of the main causes of clicks and pops when audio signals are switched. When inserted at the appropriate point in the signal chain, an ISO-MAX isolator can eliminate such noises.
All transformers inherently behave as low-pass filters. The characteristics of a filter depends on its type or “alignment.” A Bessel filter is optimized for uniform time-domain response. This means all the overtones of a musical instrument stay properly time-aligned, preserving the instrument’s true timbre. In square-wave tests, a Bessel response is free of overshoot or ringing in the waveform. In frequency response plots, a Bessel response produces an unusually gradual high-frequency roll-off. All Jensen transformers are intentionally designed to have Bessel response curve.
There is no “free lunch” in transformer design. In ISO-MAX models with an “I” in the model number (CI-2RR and PI-2XX, for example), the Faraday shields responsible for extremely high noise rejection unavoidably makes their output sensitive to capacitive loading. Since the cable that connects isolator output to equipment input is the main source of capacitance, it must be kept reasonably short. Capacitance up to 200 pF, about 3 feet (1 meter) of standard cable, will preserve the rated high-frequency bandwidth. There is no restriction on length for the cable that connects equipment output to isolator input. Special-purpose isolators (PC-2XR and DB-2PX, for example) are more tolerant of longer cables.
Most ISO-MAX isolators use 1:1 transformers that simply reflect the load connected to their output back to the driving source, so there is no additional “loading” of the source. Because the copper wire used in transformer has resistance, a small signal loss is unavoidable. When used as directed, this “insertion loss” is generally under 1 dB for most isolators. However, some special-purpose isolators (PC-2XR and DB-2PX, for example) intentionally change signal level and impedance.
ISO-MAX isolators with an “O” in the model number (PO-2XR, for example) are most useful when driven by a balanced equipment output and a good choice when driving long cables. The isolator output can then feed either balanced or unbalanced equipment inputs with no risk of damage or misbehavior in the driving equipment, regardless of what kind of output stage it uses. These provide a reduction in low frequency “ground loop” hum but little effect on high frequency buzz.
All ISO-MAX® isolators with an “I” in the model number (CI-2RR and PI-2XX, for example) use transformers with internal Faraday shields. Faraday shields, sometimes called “electrostatic” shields, improve rejection of high frequency buzz by a factor of 100 to 1,000 (40 dB to 60 dB) over transformers without them.
The turns ratio of a transformer describes the number of wire turns on the input versus the number of turns on the output. A 1:1 transformer would have the same number of turns on each coil while a 1:2 transformer would have twice as many on the secondary.
The common mode rejection ratio spec is used to measure the transformer or circuit’s ability to reject noise. A CMRR of 85dB would mean that the transformer will reduce noise by 85 dB at a given frequency, usually 60 Hz.
Steel has a permeability factor of about 100 while mu-metal or perm-alloy can reach 200,000.
Deane Jensen figured out that the permeability of steel caused severe audio artifacts in transformers, particularly at very low signal levels. Jensen spent years developing a recipe for the types of nickel alloys that are used in Jensen Transformers which along with lamination stacking and winding techniques play a significant part in their construction.
MuMETAL® is actually a trademark of Magnetic Shield Corporation. It is a metal alloy consisting of 77% nickel, 16% iron, 5% copper, and 2% molybdenum. It has extremely high magnetic permeability of about 100,000. Magnetic permeability is a measure of how easily magnetic fields flow through it, compared to air with a value of 1 and ordinary steel with a value of a few hundred. It has other very desirable magnetic characteristics which can be optimized by careful annealing (heat treatment) processes. Jensen uses this material both for transformer core material and for magnetic shielding (the housing “can” of all input type transformers). The special heat treatment of Jensen core material results in the ultra-low distortion figures for low-level signals.
A Faraday shield is usually a copper foil that is layered between the primary and secondary coil. It is used to reduce high frequency buzz.
Unlike electronic circuits that distort when the incoming signal level exceeds the rail voltage or power limits, transformers saturate. The saturation rounds off or smoothes the transients, creating a natural limiting effect that is often referred to as vintage sounding. It is no winder… vintage audio gear generally employ transformers.
By employing very precise winding techniques and optimizing the core material by using higher grade 84% nickel alloy and then testing these to ensure the permeability meets Jensen’s stringent requirements.
Phase deviation is the effect of having certain frequencies be offset from others after the signal passes through the transformer. As phase shifts, the information begins to muddy up and detail is lost. By minimizing phase deviation – particularly at the low end – the all important mid band becomes more solid and transparent.
There are several factors one should consider when choosing a transformer. The first is aligning the application with the level handling. This would dictate the distortion spec. Next, one would consider if the transformer is being used for music or for voice. If music, then low frequency content is critical. The musicality of the transformer relates to both linear performance and phase shift. Finally, one would consider noise rejection or the importance thereof.
A basic transformer is made up of two coils, a primary and a secondary and a core. When an electrical current flows into the primary coil, it generates a magnetic field. The field is concentrated via the core which in turn excites the secondary coil and regenerates electrical current at the output. This creates a magnetic bridge between the input and output without any physical (electrical) connection. This allows alternating current such as an audio signal to pass, while blocking DC.
Good sounding transformers are extremely difficult to manufacture. The sound or performance is relative to the frequency response, phase shift and distortion. Jensen is one of the only companies that actually publishes all of the specifications so that a serious circuit designer can make engineering decisions based on the true values, not merely posted specifications. Very few companies show their noise rejection (CMRR) which for all accounts, is the primary reason to incorporate a transformer into a circuit design or isolator into a PA, broadcast, hi-fi or recording system.