Can you have too much filter capacitance?

[Axtman]

I often wonder while recapping an amp if I should not increase or double the amount of filter capacitance. Some of the vintage amps that I have seen seem to have low value filter caps (10, 20 40 uf). Wouldn't the amp benefit from increased capacitance? Would the increase capacitance help with bass notes....especially for a bass amp? Thanks!

[defaced]

http://music-electronics-forum.com/t2855/

This is a really good thread on the topic. Basically, yes you can increase the power supply filter cap values to a limit depending on whether it has a tube rectifier or not, but you may find the amp feels different than it did with less filtering.

[daz]

I jacked mine way up from 32/32 and 22uf nodes in my marshall style number to 50/50uf and 80uf nodes, then even went to a 100/100uf can in place of the 50/50. i thought there were benefits, but soon i realized the tone and feel was just not happening anymore. Went all the way back to the original values and man o man, the tone and feel was soooo much better ! What amps might benefoit more, i don't know. It's possible some amps, maybe fender style could benefit from this. I really can't say. But i CAN say that i believe pretty much no player no matter what style could play thru my amp now and how it was with the big filters and like it better than with the high capacitance. It's just so much better with less. Richer, springier, all around better. you'll get lots of conflicting opinions, but this is what i experienced. And i AM indeed a tone fanatic to say the least, and one who believes feel trumps tone. And while it does affect tone, it was the feel that suffered most.

[Steve Conner]

It's true! With small filter capacitors, the B+ voltage pumps up and down more as you play. That gives you more feel and springiness sure enough. It's almost like having a compressor in there somewhere.

The downside is that smaller capacitors lead to more hum and ghost noting, because they're not so good at filtering out ripple from the rectifier. Different players have different tolerances for this. I personally hate hum and gladly sacrifice a little springiness to get rid of it, so I like to use somewhat bigger filter caps. I've even tried completely regulated B+, which is like infinite sized filter caps, but in hindsight I think that was going too far.

[tedmich]

It's true! With small filter capacitors, the B+ voltage pumps up and down more as you play. That gives you more feel and springiness sure enough. It's almost like having a compressor in there somewhere.

The downside is that smaller capacitors lead to more hum and ghost noting, because they're not so good at filtering out ripple from the rectifier. Different players have different tolerances for this. I personally hate hum and gladly sacrifice a little springiness to get rid of it, so I like to use somewhat bigger filter caps. I've even tried completely regulated B+, which is like infinite sized filter caps, but in hindsight I think that was going too far.

Steve, I am interested in what you mean by "too far"?

Too much bother, time, expense? Or it sounded too unspringy? The audiophiles love their filtering and it is credited with increased dynamic range and dynamic response. Modeling shows big filter caps do indeed slow down the B+ response, ie its rise time at startup, but does this really mean "slow response" once at voltage? I have heard the argument that phase delays along a single long power rail (power tubes->->->->preamp tubes) is responsible to observed "slow response" and that is exacerbated by big filter caps. This suggests that parallel rails (or even multiple PTs) with decent size filters might give "the best of both worlds".

[rontoz]

I agree with Steve on his point of power supply pumping effects. I go back to the sixties when we played with nothing but tube gear, and cranking an amp near full volume gave wild and crazy sounds [just what we were looking for].
Banging hard on the guitar really made the supply dip down, then slowly come back up, and it really gave a cool feel/sound to your playing. I attribute that cause-effect sound with the guitar solos on "No Time" by The Guess Who.
Huge caps on a solid state bass amp makes good sense, as long as the turn on surge is dealt with. Some Peavey and Soundcraft gear have a triac and/or relay circuit controlling a resistive current limit at turn-on, to avoid the surging of a large capacitance.
I built a device as such so I can power up all six of my Crown studio monitor amps without blowing a circuit breaker.

[6267]

You'll also want to watch B+ when you do this, since the first cap essentially sets the voltage in a cap-input power supply. As mentioned, large caps can make rectifier tubes blow up! Use a small cap, then a resistor, then a large cap to filter.

[Chuck H]

All of the above is pretty well correct. Consider...

Vintage guitar amps used smallish filters for two reasons. rectifier tubes can't take the big inrush of a large cap. And filter caps were limited in their values vs. expense and size. As mentioned in another post, you can use a smaller filter followed by a resistor and then a larger filter for tube rectified circuits. But this would have been more expense for amp companies. And besides, at the time many vintage amps were built a 220uf 450V cap would have been the size of a frozen orange juice can (thanks Enzo) and packaged in similar materials. And it would have cost better than double what a 16, 20, or even 40uf cap would. Today cap technology is such that large uf values can be had affordably in much smaller packages. And I think most amps benefit from larger cap values. Not just in hum and noise rejection but also in feel and dynamic response. If you take it to extremes you will get something akin to a hi fi amp. All dynamics and not enough compression. But double value of what most vintage amps used generally improves performance in a positive way IMHE.

I tend to beef up the main filter and leave the preamp caps at stock values as they aren't nearly as sensitive for hum rejection. I notice the sound of oversized preamp filters more easily than double value on the main fillter. Too big on the preamp filters and an amp can start sounding blatty and dry. But bigger main filters (within reason), at least for me, always seem to improve tone. I really think that if larger value filters were as available and affordable then as they are now that many more vintage amps would have used larger filters.

Chuck

[daz]

I'm using 32/32 for the main plate/screen and 22 for the preamp filters and i don't notice any ghost noting and i have zero hum. I don't don't why, but could it have to do with the 20H choke?
Chuck....if you are running the values i am and you jack the main dual can up to 50/50 or 100/100, how would you describe the results? I ask because i have both 50/50 and 100/100 cans i could use, but i changed back to the original 32/32 at the same time i changed my preamp filters back to 22 from 80. So maybe i would have been better off leaving the 50/50 or 100/100 in there, eh? I'm really happy as is, but you have me wondering. (not necassarily a good thing )

[Gaz]

FWIW, I switched from 32uf to 100uf in my 50 watter. I'm always using it in a loud band, and i really needed a 'stiffer' power supply keep the amp sounding 'controlled' at it's limits (volume wise). To me, any sort of compression and/or sag at such loud volumes sounds like the amp is breaking down in a bad way. For bluesy or more dynamic styles (opposed to balls to the wall punk, metal, hardcore, etc.) i could envision 32uf sounding beautiful. Surely my amp doesn't exhibit the qualities of an 'over-filtered' amp, but it can't achieve any of the qualites of an ostensibly 'under-filtered' amp either.

[Steve Conner]

Steve, I am interested in what you mean by "too far"?

Too much bother, time, expense? Or it sounded too unspringy?

Probably mostly the bother. I'm not that bothered by the springiness, an amp with a stiff power supply has a more aggressive, snappy tone that I find just as good in other respects (see Gaz's post above)

Modeling shows big filter caps do indeed slow down the B+ response, ie its rise time at startup, but does this really mean "slow response" once at voltage?

Circuits made from resistors and capacitors are linear. That means that their time constant is always the same. It doesn't depend on the B+ voltage.

However, one crucial difference: A big capacitor will take longer to charge at power-up, and longer to recharge after it's been drained by an audio peak. But being larger, the peak will drain it less. So increasing capacitance makes the sagging/pumping slower, but also subjectively less noticeable. And, it will make the attacks of notes harder and snappier, because there's more energy to dump into the attack before the voltage sags down. So in that sense, the response doesn't sound "slow" at all.

When you look at the R-C-R-C bypassing scheme for the preamp, the time constant of that lot is in the tens of seconds. You can crank the amp to the ragged edge, scope the preamp B+ voltages, and they just slowly wander around.

[tedmich]

I often wonder while recapping an amp if I should not increase or double the amount of filter capacitance. Some of the vintage amps that I have seen seem to have low value filter caps (10, 20 40 uf). Wouldn't the amp benefit from increased capacitance? Would the increase capacitance help with bass notes....especially for a bass amp? Thanks!

too much:
86,400uF @ 450VDC