Coupling capacitors and bass response

[HobbswheresCalvin]

Was wondering what roll coupling capacitors can play on bass response. Will a larger or smaller value allow more bass to come through. Or how do they relate

[martin manning]

A larger cap means more bass response. The cap is in a series orientation, and is typically followed by a resistance to ground which might be a fixed resistor or a volume pot. This forms a high-pass RC filter which is characterized by its -3dB roll-off frequency, calculated as 1/(2*pi*R*C). C is in Farads and R is in Ohms. The C to use is obvious (the coupling cap value), but the R is the sum of the preceding and following shunt resistances. The preceding shunt resistance (to AC ground) is the parallel combination of the plate load (Ra) and plate resistance (ra) of the preceding stage.

A typical 12AX7 might have 63k ra and a 100k plate load, so that is 39k. Let's say the following shunt resistance is a 1 Meg pot, so the sum of those two is 1039k. A shortcut to getting all the units correct is to divide 159155 by the RC product with R in k-ohms and C in nF. If we have a 0.022uF coupler, that is 22 nF, so we have a -3dB frequency of 159155/(22*1039) = 7 Hz, which means pretty much all audible frequencies will pass unimpeded.

Notice if you halve the cap value, you'll double the -3dB frequency, increasing it by one octave to 14 Hz. Not much effect there, but if you make it 0.0022uF, the "corner" frequency is 70 Hz, and there will be some noticable loss of bass. Of course this is one of three principal ways to shape the frequency response of a common cathode stage, with the other two being the cathode bypass cap and R's and C's at the input grid.

[ChrisM]

A larger cap means more bass response. The cap is in a series orientation, and is typically followed by a resistance to ground which might be a fixed resistor or a volume pot. This forms a high-pass RC filter which is characterized by its -3dB roll-off frequency, calculated as 1/(2*pi*R*C). C is in Farads and R is in Ohms. The C to use is obvious (the coupling cap value), but the R is the sum of the preceding and following shunt resistances. The preceding shunt resistance (to AC ground) is the parallel combination of the plate load (Ra) and plate resistance (ra) of the preceding stage.

A typical 12AX7 might have 63k ra and a 100k plate load, so that is 39k. Let's say the following shunt resistance is a 1 Meg pot, so the sum of those two is 1039k. A shortcut to getting all the units correct is to divide 159155 by the RC product with R in k-ohms and C in nF. If we have a 0.022uF coupler, that is 22 nF, so we have a -3dB frequency of 159155/(22*1039) = 7 Hz, which means pretty much all audible frequencies will pass unimpeded.

Notice if you halve the cap value, you'll double the -3dB frequency, increasing it by one octave to 14 Hz. Not much effect there, but if you make it 0.0022uF, the "corner" frequency is 70 Hz, and there will be some noticable loss of bass. Of course this is one of three principal ways to shape the frequency response of a common cathode stage, with the other two being the cathode bypass cap and R's and C's at the input grid.

Great post Martin!

[HobbswheresCalvin]

Damn, i think my mind exploded. Haha. Thanks tho. I understand how you did the basic math of it from what you wrote. But wow.

I am sure this post should go in the trainwreck discussion. But i figured it could pertain to any amp.

I am in the process of building my first wreck and i play in b standard with my band. The amp sounds great. Until i go below E. It sounds like its farting out. Overloading it i guess somehow. And i am trying to figure out how to get it to not do that. Just open it up a little wider, i dont want to change the tone much especially in the highs or mids, i love the mid peak. But the low end is seriously lacking. Any suggestions beyond this.

Ive read about some type of feedback cap that can be used to send some signal back to the input to add some bass. But im not sure i understand that completely

[Reeltarded]

What amp? Express? What's done to it? I have a lot of ideas.

Couplers won't tighten anything up! Firstly, you are tuning to B? Drop your pickups until a clean sound on your amp doesn't go 'SPLAT' when you dig in. Trust me.

You might want to look up 'partial bypass' and think about carving out the lowend up front, and then subbing in those big couplers for the PI if you want it to growl, although it could be a simple as changing one bypass cap for another.

[surfsup]

Yes nice post MrManning. Just to clarify what you said for the OP,

A shortcut to getting all the units correct is to divide 159155 by the RC product with R in k-ohms and C in nF.

The 159155 (160k) number is basically the 1/6.28 multiplied by 1000000. This converts the formula to get one easy number to remember divided by the product of R*C. So since capacitors and resistors are never perfect, you can just take 160k and divide by the R and the C value (in nF) and get your frequency. So,

160k/RC = frequency

An argument can also be made to say you can ignore the parallel plate/plateload resistances and just use 1M, simplifying things even more.

[martin manning]

An argument can also be made to say you can ignore the parallel plate/plateload resistances and just use 1M, simplifying things even more.

Thanks, Chris and surf. In this simple case yes, the preceding shunt resistance is not very significant, but that is not always true, and I also tacitly assumed that the pot wiper was connected to a high-impedance (i.e. the grid of the next stage) and so ignored anything down-stream of it.

160k/RC = frequency

This would confuse me. I just do 159155/kR/nF on my calculator. No chance of losing decimal places.

[Structo]

I am in the process of building my first wreck and i play in b standard with my band. The amp sounds great. Until i go below E. It sounds like its farting out.

I'm not sure what you mean?
Are you playing bass?

For guitar the amp should work fine if built to the accepted schematic.

If you want to increase the bass, try raising C8 to .005-.01uF.

Raise bypass caps to 25uF.

[HobbswheresCalvin]

Thanks Guys,

Yes it is an express, its a stock kit, minus the filter caps, from RJ.
i cant seem to find anything when i do a search on partial bypass, i am kinda new to this, im a stringed instrument tech, and my experience is limited, i do have access to an amp tech though at my work, ill ask him about it when i get there, but if you could point me to the right page on it, i would greatly appreciate it.

My pickups have a good amount of distance from the strings, i learned a great while ago i get better high end clarity when i allow the string distance from those magnetic poles. Thanks

No Structo, i am not playing bass through it, i play in a metal band, you can find us on youtube, were called A Band Of Orcs, all of our material is in B standard, and Drop B. so i have a 60 gauge string tuned to B on the low side, and the amp doesnt seem to like that very much.

Ill Try the C8 Trick with a 0.005 first, and see what happens from there.

[cbass]

Excellent answer Martin.

I find this website helpful http://www.ampbooks.com/home/amplifier- ... capacitor/

But its always good to know the math.

[HobbswheresCalvin]

Thanks for the website. That really helps