Step-up transformer is one of the most critical parts of electrostatic loudspeaker. Most high quality transformers are designed for hybrid use. They cannot drive high load capacitances and their core can be easily driven into saturation. Lower quality transformers have problems with resonance peak. There are several transformers commercially available for hybrid use but it is difficult to find true full range transformers.
Two major parameters of ESL step-up transformer are resonance frequency and core saturation current. Those two will set high and low frequency limits. Load capacitanse together with spread inductance and capacitance of transformer and external wiring will set LC-resonance frequency. By using more turns or larger core improve operation at low frequencies, but simultaneusly add spread quantities and resistance and decrease resonance frequency. Tight interleaving will diminish spread inductance but increase spread capacitance and make transformer impossible to be driven with normal amplifiers.
Frequency response has 12dB/oct roll off after resonance peak. So it is obvious that transformer which has resonance in audio band is useless in high quality full range ESL. Another important factor that is not so self-evident is nonlinear hysteresis curve of the core. It brings harmonic distortion to low frequencies and intermodulation distortion to high frequencies thus spoiling more or less the whole spectrum. Bass loses its accuracy and high frequencies sound irritating. Core saturation would result primary short circuit and extreme stress to amplifier.
In mid and high frequency use it is possible to use small cores and keep spread quantities low with ordinary techniques. It is even possible to use standard mains transformer. Core material is though poor and resonance frequency rather low, usually in audio band. In some applications there is separate transformers for low and high frequencies.
True full range transformer
I have used partially self would toroidal transformers in hybrid panels that are based on mains transformer. Two transformers with 1:45 turns ratio are used with secondaries in parallel to lift resonance above audio band. Requirements are not very high because panel has less than 1nF capacitance and its crossover frequency is around 150 - 500 Hz. Some custom toroidal transformers have been ordered for some earlier full range experiments that have been done at TUT but they variation between different units was too large to be useful. Portal full range panels were originally driven with tube amplifier output transformer.
New transformer were developed for Portal panels. Head designer was Harri Raittinen. Design principles, core structure, wire insulation, and wounding structures are somewhat different than what is normally used. Transformer is based on the idea of an inverted thorus, with high quality core material.
Design objectives were to create a transformer that could drive high load capacitances without band limiting and to be able to handle high currents at low frequencies. Both objectives were accomplished and even exceeded.
High saturation current and very low primary resistance quarantee low harmonic distortion and low intermodulation distortion. High load driving capability allow any realistic panel size to be driven with one transformer. (Portal panels has total capacitance 2.2 nF) Normal load will result flawless phase and frequency response.
|Useful frequency range:||20Hz - 40 kHz|
|Transform ratio:||1 : 60|
|Size (H x W x D)||11 x 11 x 30 cm|
|Self resonance frequency:||70 kHz (no load)|
|40 kHz (2.2n load)|
|20 kHz (10n load)|
|Saturation voltage:||100V @ 50Hz|
|(1250W @ 4ohm)|
|Minimum load impedance||0.5ohm @ 20kHz|
Portals had little irritating midrange with previous transformers. I thought that it was due to directivity and different tonal balance than small speakers typically have. Some music sounded excellent but many recordings were too bright to be pleasant to listen. By changing the transformer those problems were mostly gone. Excessive brigthness at midrange was removed and bass received more strenght and accuracy. Transformer made no significant difference on stereo image. Instruments are still in those exact places they used to be.