Are line harmonic suppression filters ever used for audio amplifiers?

[Hari Iyer]

Line frequency harmonic supression filters are quite common in power electronics for industrial use where lots of AC motors, inverters and welding and other power euipmenets are used. These usually supress the harmonic freunccy from 3rd to 20th harmonic and allows pure sine wave to the load. The fundamental frequency here is 50Hz and harmonic supression will be for 150Hz and above.

Are such kind of filters also relevant for use with audio gear? Actually i tried building one with CFL chokes and capacitors and found the tube amplifier power transformer pretty cold even after 2+ hours of use. Also the sound was free of any harshness and sparkle and the mids very very open and less listening fatigue. Would be glad to hear from experts here. I am not sure if this is how music should be as the extra sparkle from the music has just vanished for the better.

 

[Otpidus]

Hari, for a moment imagine feeding our amplifiers and DACs with 1kHz AC. How it's supposed to sound?

Lets me just touch upon your observations -

1) Amplifier running cold even after 2 hours of operation - The moment AC power is effectively filtered power of harmonics, it's "Power Factor" increases, resulting in more actual power delivery per duty cycle hence power drawing reduces for the same load resulting in cooler operation.

2) Less harsh, less fatiguing and much less extra sparkle - A typical center tapped step down transformer can't effectively filter any kHz level frequencies although it might effectively dissipate MHz level frequencies. Millions of SMPS based devices are injecting kHz and MHz frequencies to our Power Line in form of "Common Mode Noise" and "Differential Mode Noise", these noises reach past the DC conversion in our Audio equipment (specifically analogue) fluctuating the DC Rail voltage and Ground Plane in milli volt order, resulting in noise modulated sound output.

Another observation I had, when the power line is free of high frequency noise, the dependency on Cables and Vibration Management reduces significantly that's because kHz noise can radiate out of the Power Cord causing mutual inductions with nearby cables.

Hope you find my explanations and observations helpful. You might also look at "Active AC Waveform Correction".

 

[Hari Iyer]

Hari, for a moment imagine feeding our amplifiers and DACs with 1kHz AC. How it's supposed to sound?

Lets me just touch upon your observations -

1) Amplifier running cold even after 2 hours of operation - The moment AC power is effectively filtered power of harmonics, it's "Power Factor" increases, resulting in more actual power delivery per duty cycle hence power drawing reduces for the same load resulting in cooler operation.

2) Less harsh, less fatiguing and much less extra sparkle - A typical center tapped step down transformer can't effectively filter any kHz level frequencies although it might effectively dissipate MHz level frequencies. Millions of SMPS based devices are injecting kHz and MHz frequencies to our Power Line in form of "Common Mode Noise" and "Differential Mode Noise", these noises reach past the DC conversion in our Audio equipment (specifically analogue) fluctuating the DC Rail voltage and Ground Plane in milli volt order, resulting in noise modulated sound output.

Another observation I had, when the power line is free of high frequency noise, the dependency on Cables and Vibration Management reduces significantly that's because kHz noise can radiate out of the Power Cord causing mutual inductions with nearby cables.

Hope you find my explanations and observations helpful. You might also look at "Active AC Waveform Correction".

Otpidus, Thanks for your logical explanation. But i am not sure as of now if LC filters can be used in transformer primary as there could be other consequences too of resonance and power / fire issues which i am currently not aware of. Any hint in those directions?

 

[Otpidus]

Otpidus, Thanks for your logical explanation. But i am not sure as of now if LC filters can be used in transformer primary as there could be other consequences too of resonance and power / fire issues which i am currently not aware of. Any hint in those directions?

As far as "X" Capacitors are concerned, anything less than 70 micro F (please use 5% tolerance ones) on the Primary Side is fine provided you have Inrush Current limiter. On the secondary side, it's better not to exceed half the the primary side value. Don't worry too much about Transformer Core Resonance, approximately a few hundred milliHenry of Inductance of a 1kW Transformer with 70 micro F capacitor would never be able to over come Core Hysteresis to resonate.

For "Y" Capacitors, the most agreed range is between, 4700 pico F to 0.1 micro F. Anything more than is would increase the leakage current to ground.

Hari, please keep in mind that as you increase the "X" Capacitor value, you get much much pure AC wave form. This would be immediately evident in your listening sessions and the temptation to go for higher and higher value is quite addictive but kindly don't exceed 120 to 140 micro F on the Primary side and Secondary side combined in order to keep the fire hazard at bay. Hope this helps.

 

[Hari Iyer]

Thanks once again I am using a 15 uF between Line and neutral and 2.5uF between L-E and N-E. This gives perfect resonance at 50 Hz and offers high impedance for the transformer for fundamental 50 Hz, but low impedance for 3rd harmonic onwards. My xsim goes out of range beyond 600 ohms and can't say how much impedance at resonance but looks like it's over 2Kohms. But 3rd harmonic is just 100 ohms and 21st harmonic is just 10 ohms. Hope line frequency won't have harmonics above 9 I believe.

 

[Otpidus]

I have no idea how did you calculate the resonant frequency. Do you know the Inductance of your Transformer? If not then just going by the Capacitor Frequency Chart isn't of much help. If I recall correctly, a 1 Henry Inductor would require about 120 micro F capacitor to resonate at 50Hz.

There are two things at play here, the changing load (Inductive, Capacitive and Resistive) and the changing noise level (both frequency and amplitude) as well as power factor in power. Using a fixed value can only mitigate some of it but never fully.

One more thing, using 2.5 micro F as "Y" Capacitor is way beyond the required value. it would drain a considerable amount of power and also can conduct noise from the ground line into the power line.

 

[Hari Iyer]

Otpidus, I have measured the inductance of my transfomer with an LCR meter as 720mH and resistance as 13 ohms. This I simulated with xsim and replaced the speaker with transformer inductance + series resistance. I then placed a 15 uF capacitor in parallel to the transformer to get resonance at 50 Hz. So this high impedance at resonance will allow 50 Hz to pass easily but will ground everything above 100 Hz. So harmonics will be suppressed. As rightly pointed out by you, this does not consider other loads in parallel to the capacitor even those of my neighbours and apartment, so resonance is actually theroticsl not considering them. So this ckt can work only with an isolation transformer I believe

 

[Otpidus]

Otpidus, I have measured the inductance of my transfomer with an LCR meter as 720mH and resistance as 13 ohms. This I simulated with xsim and replaced the speaker with transformer inductance + series resistance. I then placed a 15 uF capacitor in parallel to the transformer to get resonance at 50 Hz. So this high impedance at resonance will allow 50 Hz to pass easily but will ground everything above 100 Hz. So harmonics will be suppressed. As rightly pointed out by you, this does not consider other loads in parallel to the capacitor even those of my neighbours and apartment, so resonance is actually theroticsl not considering them. So this ckt can work only with an isolation transformer I believe

Yes, it would work more effectively with either an Isolation Transformer or a Balanced Transformer.