Juma's Easy-Peasy Capacitance Multiplier
I have built many amps and have been working on improving my power supply ripple with the usual CRC or CRCLC circuits. My best effort was actually an Abletec SMPS followed by a CRC where I got 1mV ripple under quisescent bias current. With linear toroidal transformers, I could never get below maybe 5mV and that was with a lot of effort. However, in class A, start pulling a couple of amps and that rises to 20mV sometimes 100mV depending on your trafo and CRC filter setup.
Novi amp sa 2SK2013/2SJ313 - Solid State - diyAudio.rs
Juma's cap multiplier:
Here is nice schematic of the basic core cap Mx for a single rail use by Jwksana:
I recently built a small class A designed by Juma based on the Pass F5 but using the little 2SK2013/2SJ313 MOSFETs in parallel.
F5 with 2SK2013/2SJ313
The amp works well and sounds very nice. It draws about 1.5amps continuously and I am feeding it with my usual class A power supply: a 400VA 18VAC Antek with a 33mF/0.12R/33mF CRC. I was measuring 20mV ripple per rail and audible hum was coming through when no music was playing.
Juma's plans for the little F5 also included a simple capacitance multiplier circuit. It was so simple I had to try it. I have tried others in the past but they were always more complex and typically used BJT's and Darlingtons that I did not have on hand. This one however uses a single IRFP240/9240 per rail, a few resistors and easy to find caps, and a diode. It was so simple I could P2P on the legs of the MOSFET sticking up in the air:
I powered it up, now fed from my usual CRC that had 20mV ripple under load. There is a 2v voltage drop so you will lose some headroom. However, if you plan ahead, just get a slightly higher voltage secondary rating transformer. Then I hooked it up to the amp and adjusted bias current to usual 1.5amps and now measured the ripple...
Cap multiplier under test with F5 amp and 1.5amps flowing:
I was floored to see the Fluke AC volts drop to 0.000V AC (across +/- rails at 40v total). Sometimes flickered to 1mV then back to zero again (Fluke 101 has 1% accuracy of measured value + 3, the number added to the least significant digit, data valid over 40Hz to 500Hz range). I would love to have an o-scope to see what it really is doing - but there was a significant improvement in the sound quality. The hum was gone. I will try in my class AB amps next but thought folks might like to try this. It is dead simple and so easy. Anyhow, I think I will be making many more of these in the future and a little PCB would be handy. Just wanted folks to know about what a great circuit this is and to give it a thumbs up! Thanks to Juma for designing it.
If you have any experience with cap multipliers, please share. I have seen the ESP one and the one by Keantoken. This one with a single MOSFET is the easiest one I have seen though.
Edit Nov 13, 2016: stereo cap multiplier on one heatsink.
Here is a really nice one by Olafk:
Nice 4 cap Mx on 100mm square PCB by Prasi:
Gerbers on post 245
Here is another nice one by Prasi for use with external bridge block:
Small SMT/TO-220 Cap Mx (Dual rail) by Prasi:
Works very well.
Maybe these other MOSFETs would work even better in Juma's circuit design. They are rated for much greater current and much greater power dissipation.
I don't think there is an issue with current yet - 12amps with IRP240/9240 is quite high already. I don't know too many amps that pull that much current.
Main reason for me is that I literally have a drawer full of them already. My favorite MOSFET. :)
These are very effective ripple filters, the only downside is the loss of headroom. This shows a load current of around 650ma. You can see how small the ripple is, but also the loss in headroom.
Thanks for the simulation! Wow - that is indeed very effective and confirms my experience and data from the Fluke. I was wondering if it was just missing bandwidth but this appears to be a very sound topology. Loss of headroom probably not an issue for someone wishing to listen to music as lower volumes where hum is audible.
Most of Juma's amps have PSU with cap mutiplier. You can see attached some examples.
The last one is with BJT if you need less voltage drop (2.1V compared to 4V drop with MOSFETs).
Mooly's expression "loss in headroom" can be translated "enormous voltage drop from Vin to Vout; notice that Vout is 4 volts below Vin!!"
This is a result of using "standard" gate threshold MOSFETs whose Vth is about 4 volts. "Logic Level" gate threshold MOSFETs would reduce the voltage drop to about 2 volts, and of course bipolar power transistors would lower it to 0.7 volts. Keantoken's Complementary Feedback Pair ("Sziklai") capacitance multiplier has a Vin-to-Vout drop of about 1.4 volts, as does a two transistor darlington-connection.
This is like an F5 with the venerable 2SK1058 and 2SJ162 lateral FET. Will have to try it one day.
Thanks for brining up lower dropout voltage options. I may have try them but for most cases I am not at a loss for head room for home use at moderate listening levels. IRFP's have a very low Rdson and perhaps have an advantage in instantaneous transient current response for more bass impact?
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