LC networks in typical audio PSU applications

[random007]

Just curious: what's the reasoning behind typical amplifier PSU designs not including large inductors to from noise suppressing LC networks? Even RC networks are more popular and the R reduces efficiency (a bit).

[Richard Ellis]

I am familiar with one reason right off the bat why inductors are not used in SS designs. Note that tube amp PS' still use inductors for linear supplies. The current draw is so very low for tubes....low current, high voltages. SS designs low voltages, high currents. A typical 10H inductor used in a tube PS will "hold" say 800V, but only run at 300mA. This inductor is about fist sized & priced accordingly. Now, how big & expensive do you think a 10H inductor that runs at 200V & 10A would be?? Seek out PSUD from Duncan's Amp Pages for their FREE PS SIM program,...try it out.

__________________________________________________ _____Rick.........

[metalsculptor]

Inductors for low frequency smoothing duty are large, heavy and very dependant on load current. They are still popular in 3 phase power supplies where the ripple frequency is 300Hz (360Hz) and it is important to reduce harmonic content in the supply and improve power factor. In most audio applications the load current varies widely and the amplifiers have good power supply rejection meaning an inductor will not work well all the time and the amplifier does not need such smooth DC any way. For example if you have 70V rails with 0.5V ripple at full load with an amplifier which has 60dB ripple rejection the ripple on the output will be almost -100dB at full load unless the amplifier clips. Single ended amplifiers present a constant load and have poor power supply rejection making inductors historically attractive in this application but with these efficiency is hardly a consideration.

Also modern capacitors are much more compact for a given value but inductors have hardly shrunk.

SMPS's cheat by raising the frequency so inductors become quite small and PWM takes care of a lot of the ripple.

[Robert F]

Originally Posted by Richard Ellis

I am familiar with one reason right off the bat why inductors are not used in SS designs. Note that tube amp PS' still use inductors for linear supplies. The current draw is so very low for tubes....low current, high voltages. SS designs low voltages, high currents. A typical 10H inductor used in a tube PS will "hold" say 800V, but only run at 300mA. This inductor is about fist sized & priced accordingly. Now, how big & expensive do you think a 10H inductor that runs at 200V & 10A would be?? Seek out PSUD from Duncan's Amp Pages for their FREE PS SIM program,...try it out.

__________________________________________________ _____Rick.........

Except that you don't need a 10H input choke for a typical SS amp because higher current lower voltage means critical inductance is lower. The main reason, cost and weight aside, is that most SS amps are not Class A and therefore designing a choke input power supply is difficult. The current drawn from the supply varies and therefore the value needed for critical inductance changes.

Rob.

[CLS]

How about Ayre? I remember they had an amp with chokes in the PSU, model ?3 or what? ... I also remember it received quite a lot of good reviews.

Anyone knows its PSU?

[metalsculptor]

Ayre Anyone knows its PSU?

PSU's are designed by formula, you can design one to get whatever performance you want. Many electronics texts from the 50's and 60's have examples which go into great detail about the design of the power supply.

I have had several amplifiers with choke filters, they were all single ended and they all had audible hum but in the 30's to 50s when they were built hum was normal. Nowadays a semiconductor series pass regulator will cost less and outperform an LC filter by orders of magnitude both in terms of impedance and ripple.

[Robert F]

Originally Posted by CLS

How about Ayre? I remember they had an amp with chokes in the PSU, model ?3 or what? ... I also remember it received quite a lot of good reviews.

Anyone knows its PSU?

No but the smaill Class A Cello amp was choke input.

[CLS]

I remember Nelson Pass wrote in the article of Zen (?) - he prefer the sound of CLC filter, compared to regulator. Interesting. Oh, that's a SE amp (and of course class A).

[metalsculptor]

Choke input filters are rarely used with loads which a exhibit a wide variation in load current like class AB and B audio amplifiers because the choke only provides good filtering over a narrow range of load current. They are well suited to class A and class C (CW) loads.

As for the sound a PSU shouldn't have a sound, DC has no sound, you can mix in hum and noise early in the signal chain if that is what you desire and turn it off again if the desire passes, it makes no sense to design a power supply with a sound. When I listened to valve amplifiers on a daily basis I got used to the hum and noise and it sounded odd without it If I still owned the nice 60's DIY Williamson ultralinear amplifier I used in my teens bigger filter capacitors and a series pass regulator would have been installed because I am used to silent amplifiers now.

[CLS]

Hmmmm...

I have an impression that I've read somewhere stating that "LC in PSU provides better regulation than only C or RC.... ", or I read it wrong, or bad memory

As to the term 'DC', I think it's not that simple. The voltage is (or should be) constant, but the current is changing. Current is changing! Ah ha! that's AC Even in SE class A circuit, as long as there's signal, there'd be current change. I think that's the main reason we hear the PSU

The only circuit with constant current is perfectly balanced class A PP, or SE with shunt regulator.