Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 25th November 2009, 09:14 PM   #1
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
Default Power supply filters - sim results

Folks,

I've been playing with power supply filters in my 6LU8-based Spud lately. I'm using a 5AR4 rectifier and an Antek AN-1T250 transformer to deliver a B+ of 300-ish V. Actual B+ is 290 V for an AC line voltage of 115-120 V. The reservoir cap is 47 uF.

I would prefer not to drop too much voltage across the supply filter as that will limit the available output power. But I would like to have decent ripple rejection.

With an RC filter, I had quite audible 120 Hz hum on the output of the amp even with my 87 dB efficient speakers. It was audible from the listening position a good 2~2.5 meters away. I measured 28 mV RMS @ 120 Hz on the output.
I figured a choke would be a better option, so I got myself a 1.5 H Hammond choke from AES and rigged that instead of the resistor. That knocked the output hum down to 4~5 mV - which, amazingly, was still audible from the speakers.
So, inspired by Pete Millett, I started looking at capacitance multiplier circuits. With a BJT, the base current sets the upper limit on the base resistor as it determines the base voltage, hence, the voltage drop across the cap multiplier. As I wanted to limit the capacitance on the base to 100 uF (keeping an eye on cost and parts availability here) that determined the lower end of the cutoff frequency for the ripple filter. Also, beta (ratio between collector and base current) tends to vary quite a bit with temperature, collector current, and - more importantly - part to part. For high-voltage BJT's beta also tends to be quite low. MOSFETs become really attractive at this point... So that's what I ended up with. With a voltage divider consisting of a 33 kOhm in series with a 1.35 MOhm (2M7 || 2M7), a 100 uF cap, and an 800 V NMOS, I get about 500 uV of hum on the output. I need to rig a spectrum analyzer to it, but it sounds like it has some 3rd order modulation products in it. I'm running the heaters on AC, so I'm guessing that I'm starting to see whatever ripple is left being modulated by the 60 Hz from the heater.

Sound quality... Aside from the hum, I also listened to some music at moderate volume. The amp with RC and LC filters sounded about the same. As the volume was turned up, OPT distortion came into play (I'm using Edcor XSE15-8-5K) and the bass became very muddy. But at more reasonable listening volumes (which is actually fairly loud) I found the amp quite laid back and melodic. It wasn't super accurate. The bass was firm but not tight. The highs were a bit fuzzy, which was a bit tiring. But aside from this, it's actually quite nice.
I got the MOS filter installed last night. With the MOS filter, the highs really open up! I was shocked! The bass was a bit firmer, but the main difference was the precision in the highs. It's not harsh and sterile like my sand amp (125 Wpc based on an LME49810), though, the final verdict on the sand amp is pending a little debugging to remove some hiss. The added imaging in the high end was certainly welcome. It's reasonably precise and laid back at the same time. The sound stage has opened up quite a bit as well.

Anyway... To get to the point and subject of the thread. I ran some simulations of the different ripple filters to satisfy my curiosity. Specifically, I was wondering if there was any significant difference between the BJT and MOS cap multipliers. I used a 60 Hz 1 V peak sine wave to emulate the ripple. Of course, actual supply ripple is 120 Hz and isn't sine shaped at all, but I was looking for worst case and really just the differences between the different filter types. I suspect the differences in the sound quality has more to do with the output impedance of the supply, though. Well... Aside from the obvious attenuation of the 120 Hz hum that I was hunting in the first place.

I thought I'd share the results...

~Tom
Attached Images
File Type: png SupplyFilters.PNG (18.5 KB, 276 views)
File Type: png SupplyFilters_Results.PNG (25.8 KB, 260 views)
File Type: png SupplyFilters_Results_MOS_BJT.PNG (25.3 KB, 252 views)
  Reply With Quote
Old 25th November 2009, 11:11 PM   #2
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
...and output impedance plot. The fact that the BJT and RC both have 100R Zout at LF is purely incidental. The NPN beta happens to be just shy of 100, hence, the output impedance becomes 10k/100 = 100R. It is interesting to note that the output impedance of the MOS follower is constant with frequency (for the simplistic SPICE model anyway). Another benefit of no base current, I guess... I'm not normally a fan of MOS, but in this case I guess I have to accept its superiority.

~Tom
Attached Images
File Type: png SupplyFilters_Results_OutputImpedance.PNG (18.5 KB, 220 views)
  Reply With Quote
Old 25th November 2009, 11:12 PM   #3
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
Units on the Y axis are Ohm not Volt. Details, details...
  Reply With Quote
Old 14th August 2012, 07:58 AM   #4
diyAudio Member
 
Vincent77's Avatar
 
Join Date: Jan 2011
Location: Bruxelles
I know it's been 3 years, but... thanks for sharing that, Tom!
I was meaning to do the same sims, then I found yours...

So, it appears that a MOS-FET filter is indeed a valid choice for smoothing high voltage...
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
DIY power filters? ak_47_boy Everything Else 2 18th November 2006 11:33 PM
PCB for active filters - supply question GrzMiL Parts 7 4th April 2006 06:03 PM
Hooking up Charlize to the PC power supply, with some weird results eVITAERC Class D 24 18th November 2005 05:09 PM
Batteries as power-supply filters? Dartagnan Solid State 3 6th October 2004 06:08 PM


New To Site? Need Help?

All times are GMT. The time now is 11:49 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2