Larger Caps in C-Moys

[robbneu]

I've mentioned in a few threads that I'm getting ready to build some new C-Moy amps. I've already built two C-Moys (and put together a number of circuit boards awaiting OPAMPs) using Tangent's recipe, and they turned out great. I'll be using the Serpac 9V case, like I had done with my original amps, and so I have a little extra room that I'm thinking about using for a switchable simple crossfeed circuit (likely following Pink's tutorial).

My latest noob question surrounds using larger caps in the C-Moy. On Tangent's page he says the following:

Quote:

For the power caps, 220 µF is adequate, but bigger ones will provide a bigger current reserve, which can be useful in handling high instantaneous loads, like big drum hits. I've tried 470 µF caps in my CMoy amps before, and it does improve the bass handling significantly and they're not all that much bigger than the 220s I was using previously.

and...

Quote:

As for the signal coupling caps (C1), a larger cap will improve bass handling, but it's by a different mechanism than with larger power caps. See the companion article, Input Capacitors for Headphone Amps for a full discussion. Bottom line, the default 0.1 µF is a bit on the low side. Try 0.22 µF, 0.47 µF, or even 1.0 µF instead.

Does anyone have any specific recommendations? I've had a terrible time finding anything on the Digikey website, but I was curious if anyone found a particular cap they really liked. Also, these C-Moys are going to be single 9V amps. Will larger caps be benefical to amps only using one 9V or is this tweak really more appropriate to the two 9V versions?

Robb

 

[sbelyo]

my favorite combo is 0.22 uf on the input and 1000 to 2000 uf on the power rails

 

[mono]

it may depend a bit on how much space you have...

For C1 signal coupling try a Panasonic or BC Polypropylene .22-.47uf

For bulk power, Nichicon or Panasonic FM series 470uf or higher (consider size appropriate to voltage, 16V for single 9V or 25V for option to have 2 x 9V battery.

Then consider decoupling. Add at least 1 uF (typical ~ 50V)film cap (cheap polyethylene is better than nothing, buy based on cost and dimensions), with more available space choose > 1uF, though they rise steeply in price relative to total cost of a CMOY. If there is no room for a film cap I'd add at least a .1uF to 1uF ceramic, but only for the decoupling, not the signal coupling.

 

[drewd]

I, too, prefer the slight bass boost that 2.2uF input caps give. As for the power caps, remember, the total capacitance is not the sum of the two capacitors, but, rather, their value taken individually, assuming that you're building the CMOY according to Tangent's instructions. That's because the capacitance is not rail to rail.

As far as power supply cap size goes, consider that the opamp can deliver a finite amount of energy that is a function of its voltage output and maximum current capability. Just off the top of my head, 125mA is probably tops for current.

Now, current is only going to flow from the capacitors if the rail voltage drops. That can happen in a few different ways, but the most important one to us is caused by dynamic audio events. The amount of capacitance that you need to keep the rails up is a function of the time that it takes for the capacitors to discharge and the time that it takes for the battery to "catch up" to the opamp's requirements. With a maximum current delivery of 125mA, a 470uF cap can keep the voltage rails up for about 20ms, a fairly long time, certainly long enough for the battery to catch up. That's kind of simplified, because it doesn't take into account the RLC time constant that is a function of the capacitance, equivalent series resistance of the cap and the inductance of the leads and traces of the system, although those effects are, for practical purposes, pretty small. There are certainly other aspects of the CMOY design that have greater effects on the sound than the discharge rate of the cap.

So, that's a long-winded way of saying that 470uF is my choice for CMOY power caps. More than that is good for bragging rights, I suppose, but from a practical point of view, you don't gain much.

-Drew

 

[robbneu]

Thanks, guys! I appreciate everyone sharing their thoughts and opinions on this. It sounds like there's a fairly strong consenus for at least .22 uf caps for the input and then 470 uf caps for power (with Steve's suggestion being for even larger caps).

Mono's comments about decoupling is a little over my head, though. Does this have to do the use of input capacitors to reduce DC offset?

Also, out of curosity, does anyone know of a good guide for what the various series types actually mean? Mono suggests using a Panasonic FM series cap for the power side of things. From what I understand of Tangent's part list, he suggests using NHG series caps. Is there a practical difference?

Drew, I appreciate your long-winded "Capacitors for Dummies" explanation. When I was younger I built a number of electronic kits/projects, so I understand much of theory surrounding different components, but it helps to hear them again... but as they relate to audio. So, thanks! I appreciate it.

Right now, I'm looking at these caps (unless someone has a good reason why I should use something different, such as Mono's suggestion of a different series):

.22 UFD POLYPROPYLENE CAP
http://www.digikey.com/scripts/DkSea...224-ND&Site=US

CAP 470UF 35V ELECT NHG RADIAL
http://www.digikey.com/scripts/DkSea...?KeywordSearch

Thoughts?

Robb

 

[sbelyo]

I like the panasonic fm series a whole lot. I use 3 of them (470uf) for the power rails of my cmoy. I of course use a tle2426 voltage dividor instaed of the 2 resistors. It provides enough current and still fits into an altoids tin. You can also try the nichicon UHE series. Mouser sells them and they are the same size as the panasonics. In a cmoy I don't think you can tell a difference but the panasonics look nicer (black and gold like blackgates).

 

[Fresno Bob]

I know its over kill but im thinking about using 470uf 50volt caps on my moy and mints when i do build them i also have some WIMA FKP-2 0.01 63volts 1% tol not sure what i will do with these or if they could even be used on an pocket amp or any amp.

robbneu i'm using WIMA MKS2 0.22 63volt 5% tol as i said along with the cap size above in my mint project. dont know if this helps or not. you might finsh your build before me if so let us know how the larger caps work out. my voltage on my caps my be excess but they are the same size as all the others i was looking at and i have them on hand. any case good luck on your build or builds.

 

[NeoteriX]

This is just a wild suggestion, but what about ditching the input capacitors? I've noticed that a couple amps like the Govibes and M3 off the top of my head, ditch the input caps in favor of offering the least amount of "stuff" in the signal path. How serious is the threat of DC offset?

Also, what is the pink tutorial on crossfeed? I'm slowly making the plans for my first DIY amp, and I'd like to have crossfeed too.

 

[sbelyo]

Quote:

Originally Posted by Fresno Bob I know its over kill but im thinking about using 470uf 50volt caps on my moy and mints

50 Volt is kind of high. What happens is you decrease uf and increase size. In cmoys and mints you need a smaller size most of the time over anything else.

If you are making a cmoy or mint portable, it will never see above 18 volts unless want to carry a ton of batteries.

So IMO 25 Volt is moe than enough with a max of 35 in case you use a power supply instead

 

[Fresno Bob]

they are what i have at this time and they are small enough to fit on the board and in the mint tins. so they will be used. since the power souce can not give 50volts the caps in essence will only take what they get. so i have more than enough head room. on the volt side.

as a side not i do have 330 and 220 uf at 50volts they too are small.

 

[sbelyo]

In that case, by all means you're fine. The fit would be the only drawback. It took me a while to figure out that 16mm in height is the max for a cap in the altoid tin. Unless you lay them down.

 

[robbneu]

Steve... ahh, so your preference for the FM series caps is really just a cosmetic thing? That's actually good news, because its one *less* thing I need to worry about.

Fresno Bob... Well, you certainly have a headstart on me. I have a couple of jacks and maybe a switch or something, but need a pile of parts if I'm going to get these things built. I appreciate the well wishes, though. Good luck on your amp as well!

NeoteriX... the "pink tutorial" is a step-by-step guide that a member here, Pink Floyd, wrote up on his web page. It looks fairly simple but I'm still pretty undecided on whether or not I'm going to include a switchable crossover in my new C-Moy. He actually has two write-ups available at:

http://www.rock-grotto.co.uk/x-feed.htm
http://www.rock-grotto.co.uk/crossproject2.htm

I'm thinking about building one in the next week or so. If I like the sound, I'll start thinking seriously about adding the crossfeed to my new C-Moys (and figure out how to lay it all out with the 276-0150 protoboard).

Tangent also has a crossfeed circuit on his page, complete with a PCB that you can purchase, but it seems a bit overkill for a C-Moy. I've been trying to get a little more experience under my belt before I tackle the Pimeta and I've been saving the crossfeed board I bought from Tangent until I then. Depending on what you're working on, Tangent's article about the topic might be useful:

http://tangentsoft.net/audio/mlxfeed.html

Also check Chu Moy's article at:

http://headwize.com/projects/showpro...=cmoy1_prj.htm

Robb

 

[Fresno Bob]

robbneu how many parts are you missing? also if you have any old computer parts and desoldering iron you might get some good caps that way if they not blown.

 

[mono]

Quote:

Mono's comments about decoupling is a little over my head, though. Does this have to do the use of input capacitors to reduce DC offset?

No, the input caps are necessary IF there is potential to use the amp with a source that doesn't have DC coupling caps, and so there is DC offset out of it, and thus DC going into the amp. If the source has DC in and no coupling caps, the voltage gain on the opamp will result in a larger DC voltage going to the cans, potentially (a very real chance of) damaging them depending on how bad it is.


The decoupling caps clean up the power rails. They are small, fast caps, much lower impedance than the large bulk power caps. Most of the faster opamps spec sheets recommend using them but this is a generic recommendation that covers frequency ranges above that audible too.

Keep in mind that a CMOY is an introductory, basic reference circuit type amp, it's beauty is in it's simplicity (and sometimes size), not in it's accuracy or smoothness. These suggestions were made in the context of your wanting to modify, better it but still have what would be considered a "CMOY" by most.

If you can't or won't use decoupling caps then choose the bulk caps with the lowest ESR and impedance rating possible, within budget. I don't recall the specs on same/smilar NHG caps but suspect they'd do fine as well, being a comparable part from different manufacturer but you can check the specs yourself. Towards that end, type matters and so does size. For example an OSCON of same size as a wet electrolytic would be better and a larger electrolytic better than a smaller even if the larger cap had same mfd value but a higher voltage (though it makes more sense to choose the higher mfd instead of higher voltage up to a certain point). The limit here is that the decoupling caps will "supercede" the impedance and ESR values of the bulk caps, making their values less important to an extent.

On the other hand, some may try to tune the sound through their use of different bulk caps, for example Cerafines... not as common on a CMOY and pretty large for this purpose but similar role. IMO, since you're ordering from Digikey anyway I'd get the Panasonic FMs, the biggest that will fit without elaborate maneuvers to mount them.

 

[sbelyo]

Quote:

Originally Posted by robbneu Steve... ahh, so your preference for the FM series caps is really just a cosmetic thing? That's actually good news, because its one *less* thing I need to worry about.

In a cmoy, yes. I can't really hear a difference in sound quality between the two. But in a PPA or Pimeta that could be a different story.