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Novice's filter cap questions


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5/12/2005 10:25 AM
semitone Novice's filter cap questions
1. Early guitar amp filter cap values were commonly 16uF. Today 50u is common, even in supplying the first stage in small amp designs, and values of 160u and more appear in series in amps under 50W. What motivated the change to higher values?  
 
2. What is the advantage of placing high value caps in series in the B+1 position rather than using a single lower cap?  
 
3. Some schematics show caps in series or parallel before the standby switch. What is the reason for this?  
 
4. For 20W to 40W amps, what filter cap value is adequate (and economical)?  
 
Thanks very much for lending your expertise.
 
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5/12/2005 1:22 PM
Glenn

1) The switch from tube rectifiers to diode rectification allowed the designer to add more filtering to the PS. A tube rectifier, due to the current draw, cannot see very high capacitance or else it will burn out.  
 
2) The capacitors, in series, will produce a lower-valued capacitor ( Ctotal = 1/(1/C1 + 1/C2 + ... + 1/Cn)) but the voltage rating will increase. Thus, 2 100μF @ 500Vdc caps placed in series will effectively become one 50μF 1000Vdc cap.  
 
3) Those would be the caps mentioned above in #2. Depending upon the mfg's source for caps, PS-design reasons, or standby method, these cap arrangements look different.  
 
4) Once you choose the amplifier cct. that will get you closest to your sonic goals, find an amp that is or is close to that amp. Copy that amplifier's PS cct and you are in the ballpark (guitar amp companies make the cheapest amp they possibly can and still have a decent product. That price-cutting research has already been done for you!).
 
5/12/2005 1:53 PM
Chuck
"1. Early guitar amp filter cap values were commonly 16uF. Today 50u is common, even in supplying the first stage in small amp designs, and values of 160u and more appear in series in amps under 50W. What motivated the change to higher values?"  
 
Probably the biggest reason for the lower value filter caps in early amps is the construction techniques. A 50uf 500v cap back in 1948 probably would have been twice the size and three times the relative cost than it is today. So for space and production cost reasons the smaller values were used as long as they got the job done. Since these were values found to be effective alot of tube MFGs would reprint these values in "example" circuits and provide them in "kits". So some of the trend toward smaller filters continued even after smaller cheaper caps started to become available due to momentum. Back in the day none of these designers ever dreamed that we would intentionally induce distortion in these circuits. Theres where the drawback to less filtering are. Not enough filtering can cause ghost notes and hum due to excessive power supply ripple and a superimposed 120hz signal on the guitars signal. This problem tends to get worse as you crank the amp. Because the cost and space saving factors are not an issue anymore some designers are trying to take advantage of the benefits of more filtering. But theres always a trade off. Too much filtering can make an amp sound sterile and dry. Smaller filters seem to give a nice focus and rounder midrange which is very desireable for the guitars frequency range. So too small and you get ripple and ghost notes, too big and you get bad dry tone. The balancing act is up to each designer. And not all modern tube gear is using higher value filters. But overall I think that is the trend.  
 
"2. What is the advantage of placing high value caps in series in the B+1 position rather than using a single lower cap?"  
 
Sometimes a particular value/volt rated cap is too large or not convenient. By placing two caps in series of the same voltage rating you double the voltage rating and half the ufs. ie: You could use two 100uf 300v caps in series to create a single 50uf 600v filter. This is done because 50uf 600v caps are hard to come by.  
 
"3. Some schematics show caps in series or parallel before the standby switch. What is the reason for this?"  
 
Same answer as above. It's convenient for some reason. Series has the advantage of being able to use lower voltage caps to make a lower uf higher voltage cap. Paralelling two caps of the same value will double the uf at the rated voltage of the caps. Also, I guess two caps would also effectively half the ESR value too and that would cut down on ghost notes. As for convenience with paralell caps, I guess if you have 1000 amps to build and you are getting a cost break on 16uf 450 volt caps, but you wanted 32 uf for your first filter, there is no reason to buy 32 uf caps when you can improve performance and simply paralell two 16 uf caps.  
 
"4. For 20W to 40W amps, what filter cap value is adequate (and economical)?"  
 
This all depends on the circuit your building. Are you trying to emulate a vintage design/sound? Are you using a tube or diode rectifier? There are limitations to how much capacitance you can use with tube rectifiers. I don't actually know all the ins and outs. There are certainly formulas to figure what the adequate uf filter is for a circuit. It's also certainly more math than I'm willing to do. I usually start in the midrange values for examples that are similar to whatever I'm building and then I experiment from there. I find that a bit high for first filter, in the middle for most others and small filters on the first preamp stage seems to give me a sound I like. "I like" being the key phrase here. Some guys also find that any ghost notes whatever are unacceptable and use alot of filtering. They like that hard spanky sound. Some guys can tolerate a good bit of ghost notes and like the smoother tone of lower filter values.  
 
Hum is also a consideration. In a well designed power supply higher uf values will reduce hum better (lower impedance). For that reason you see higher uf values used most often in high gain circuits.  
 
In the end the correct value "to me" is as few ufs as needed to get acceptable performance in hum and ghost note rejection for whatever gain structure I'm working with. Every ones opinion on these matters is going to be different.  
 
I hope in my low tech way that I was able to help.  
 
Chuck
 

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