treble peaking caps, and RK/CK cut off. help my math

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4/23/2006 2:14 PM

Joe

treble peaking caps, and RK/CK cut off. help my math I just wanted to make sure I have this correct concerning treble peaking caps, and cathode bypass caps.     Say a marshall 470K/470p combo gives a cut-off freq. of 720hz, and a soldano combo of 470K/.002 gives 170 hz cut off point. 720hz to me is low-mid - mid-mid, frequency, and 170 is a bass freq. would the 170hz cut off make the amp sound woofy since everything above that freq. passes?, but it really sounds middy, and the 470 sounds bright. What am I missing?     Also, say you have a gain stage set up like a common cathode stage in an amplifier where the Rp=100K, the Rk=1.8K, the CK=1uf, and the input resistance of the next stage 1M. also say the 12ax7 has a MU of 100, and an internal plate resistance of 60K. Now would the cut-off freq of the rk/ck combo of that stage be roughly 190hz?   say we change the RP to 220K, then would the RK/CK cutoff freq be 758hz? just changing 1 component have that much off an affect? again 190hz seems low to me. what am I missing.

 

4/24/2006 12:03 AM

Aletheian

170Hz is not that low. Anything below 120-150Hz starts to get woofy. Also, parallel "treble peaking" filters are -3dB shelving filters... there is no cutoff there, the are just down a 3dB shelf... same with RC cathode resistor cap circuits and plate snubbers.     470k/470p peaks the freq above the midrange of a guitar... making it sound brighter and crunchier... too much in my opinion.     In a Marshall, that circuit is to peak the highs, in a soldano, it is to sculpt the lower mids and let the chunky freq through and roll off a bit of the flubby ones.     How do you get 758Hz from 220k/1.8k1u? The formula is [1/(2*pi*Ck*RaRk)], so the 220k makes for a knee at 150-160Hz or so.

 

4/24/2006 1:11 PM
joe	I thought the formula was: 1/(2*pi*CK*RK'RK). Thats how I got 758HZ. If I'm wronk thank you for clearing it up for me. and thanks for explaining it as well.

 

4/24/2006 2:47 PM
Aletheian	Gotcha... yeah there is that extra step in there.     www.aikenamps.com has most of the formulas that you will ever need

 

4/24/2006 10:26 PM
joe	but is my math correct? is 758hz right.

 

4/25/2006 10:06 AM
scott	Nope... The knee changes to approx. 155hz with the 220k plate. I compute 193hz with the 100k plate. Raising the plate lowers the knee a little. Even if your next stage impedence was 1k and not 1M, the knee would only go to 342hz so you definately made a bad error in your computation.

 

4/25/2006 12:34 PM
joe	Show me an example of a solved one so I can see what I did wrong if you don't mind. I really want to know

 

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