Breve Tufvassons PFS 250 protective isolation transformer test – any good for audio?
1. Introduction
There is a lot of misunderstanding on use of isolation transformers for audio. It should be noted that such device purpose is to get rid of possible or existing ground loop issues in audio, especially in case of single ended cable connections between class I instruments. For this reason, from many types of 1:1 isolation transformers, only class II isolation transformers make sense for audio.
1a. A brief explanation of the differences between transformer classes I, II and 2
Class I transformers are designed and constructed so that protection against electrical shock hazards relies not only on basic insulation but which includes an additional safety feature to provide earthing, (grounding), of accessible conductive parts in the fixed wiring of the installation. In the event basic insulation should fail, the conductive parts cannot become live as they are grounded or earth protected.
Class 2 is a classification referring to the NEC – National Electric Code. To avoid potential cable overheating due to excessive currents and electric shock, the output of the power supply is limited to 60VDC or 100VA, (100W when used with an AC-DC power supply).
Class II transformers are designed and constructed to prevent electrical shock hazards by relying on double insulation or reinforced insulation, (terms common to many IEC and EN standards, including IEC 60664, IEC 61140 and IEC 61558-1)
Double Insulation is defined as insulation comprising basic insulation plus supplementary insulation.
Reinforced Insulation is defined as a single insulation system applied to hazardous live parts, which provides a degree of protection against electric shock equivalent to double Insulation.
To protect against electric shock, a Class II transformer does not rely on basic insulation only but includes additional safety precautions such as double insulation or reinforced insulation.
Class II transformers have no provision for grounding, protective earthing, or reliance upon installation conditions.
A Class II power supply rating label will show this symbol:
For audio, to avoid ground loops, it only makes sense to use class II isolation transformers, without continuous protective PE circuit, with plug and double contact sockets, 2-wire power cord. It is the only way how to avoid ground loop via mains power cables, in case we are connecting two class I audio components.
2. Breve Tufvassons PFS 250 isolation transformer
Encased protective transformer suited predominantly for those applications that require protection against electric shock and high IP grade. PFS is a portable version, fitted with a handle, power cord, and an output socket with a plug. The transformer is made with class II insulation and protection grade IP44, thermal class of insulation B (130°C). Manufactured in compliance with EN61558-2-4 and EN61558-2-6. It is a 1:1 230/230V transformer rated at 250VA. It looks like this:
I have measured 120 pF stray capacitance between primary and secondary and maximum of 8.6 uA (microamperes) leakage current from secondary to mains earth. This makes its use absolutely safe and in case of a single-pole touch of its live output against earth there is no danger of electric shock for the user. In fact it makes the connected appliance safer.
3. Audio test
The test circuit was connected as per the following schematics:
From PC notebook the DAC (Topping D10s) was connected by a 50 cm of single-ended link cable to the stepped attenuator and from the attenuator by 1.5 m of single-ended link cable to a power amplifier laded with 8 ohm resistor load. The output voltage from the load was connected to the balanced input (+IN, -IN, no ground) of the ADC USB soundcard connected to the PC. The PC was alternately supplied from:
a) directly from 230V mains which made the PC a class I component
b) via the PFS 250 isolation transformer to 230V mains which made the PC a class II component
So there was a ground loop from mains PE through the PC, USB, DAC, stepped attenuator, link cables and power amp in case (a). This ground loop was interrupted by PFS 250 in case (b).
Measurements
Noise voltage at amplifier output – class I (a) red, class II (isol. tr) green
We can see at least 10 dB lower noise floor with PFS 250 and more importantly the mid and high frequency audible spurious components have disappeared.
Distortion at amplifier output – class I (a) red, class II (isol. tr) green
Same result, distortion is considerably lower with the PFS 250
4. Conclusion
The class II safety isolation transformer may make a huge improvement if the user has troubles with ground loop hum, buzz and whistles in his system. It is a safe way to fix the issue.
1. Introduction
There is a lot of misunderstanding on use of isolation transformers for audio. It should be noted that such device purpose is to get rid of possible or existing ground loop issues in audio, especially in case of single ended cable connections between class I instruments. For this reason, from many types of 1:1 isolation transformers, only class II isolation transformers make sense for audio.
1a. A brief explanation of the differences between transformer classes I, II and 2
Class I transformers are designed and constructed so that protection against electrical shock hazards relies not only on basic insulation but which includes an additional safety feature to provide earthing, (grounding), of accessible conductive parts in the fixed wiring of the installation. In the event basic insulation should fail, the conductive parts cannot become live as they are grounded or earth protected.
Class 2 is a classification referring to the NEC – National Electric Code. To avoid potential cable overheating due to excessive currents and electric shock, the output of the power supply is limited to 60VDC or 100VA, (100W when used with an AC-DC power supply).
Class II transformers are designed and constructed to prevent electrical shock hazards by relying on double insulation or reinforced insulation, (terms common to many IEC and EN standards, including IEC 60664, IEC 61140 and IEC 61558-1)
Double Insulation is defined as insulation comprising basic insulation plus supplementary insulation.
Reinforced Insulation is defined as a single insulation system applied to hazardous live parts, which provides a degree of protection against electric shock equivalent to double Insulation.
To protect against electric shock, a Class II transformer does not rely on basic insulation only but includes additional safety precautions such as double insulation or reinforced insulation.
Class II transformers have no provision for grounding, protective earthing, or reliance upon installation conditions.
A Class II power supply rating label will show this symbol:
For audio, to avoid ground loops, it only makes sense to use class II isolation transformers, without continuous protective PE circuit, with plug and double contact sockets, 2-wire power cord. It is the only way how to avoid ground loop via mains power cables, in case we are connecting two class I audio components.
2. Breve Tufvassons PFS 250 isolation transformer
Encased protective transformer suited predominantly for those applications that require protection against electric shock and high IP grade. PFS is a portable version, fitted with a handle, power cord, and an output socket with a plug. The transformer is made with class II insulation and protection grade IP44, thermal class of insulation B (130°C). Manufactured in compliance with EN61558-2-4 and EN61558-2-6. It is a 1:1 230/230V transformer rated at 250VA. It looks like this:
I have measured 120 pF stray capacitance between primary and secondary and maximum of 8.6 uA (microamperes) leakage current from secondary to mains earth. This makes its use absolutely safe and in case of a single-pole touch of its live output against earth there is no danger of electric shock for the user. In fact it makes the connected appliance safer.
3. Audio test
The test circuit was connected as per the following schematics:
From PC notebook the DAC (Topping D10s) was connected by a 50 cm of single-ended link cable to the stepped attenuator and from the attenuator by 1.5 m of single-ended link cable to a power amplifier laded with 8 ohm resistor load. The output voltage from the load was connected to the balanced input (+IN, -IN, no ground) of the ADC USB soundcard connected to the PC. The PC was alternately supplied from:
a) directly from 230V mains which made the PC a class I component
b) via the PFS 250 isolation transformer to 230V mains which made the PC a class II component
So there was a ground loop from mains PE through the PC, USB, DAC, stepped attenuator, link cables and power amp in case (a). This ground loop was interrupted by PFS 250 in case (b).
Measurements
Noise voltage at amplifier output – class I (a) red, class II (isol. tr) green
We can see at least 10 dB lower noise floor with PFS 250 and more importantly the mid and high frequency audible spurious components have disappeared.
Distortion at amplifier output – class I (a) red, class II (isol. tr) green
Same result, distortion is considerably lower with the PFS 250
4. Conclusion
The class II safety isolation transformer may make a huge improvement if the user has troubles with ground loop hum, buzz and whistles in his system. It is a safe way to fix the issue.
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