Chipamp for FF85K pair

jheridan · 16th September 2009, 02:55 AM

I'm working on my first build and I decided to start with something small and cheap to go with my computer: The µFonken from planet-10. I've got the cabs in the process of being built but I want to get started on the electronics.

Which brings me to the real issue: The amps. Y'all around here know more than anyone (that I know) about DIY audio. Would y'all be willing to help look over what I've got? I'm having trouble with the amp and PSU design.

Using the design guide I've come to the following: since I'm only pushing out 2x10W RMS I was planning on going with an LM4780 driven by 16.8v (12V toroid from the PSU) giving me a total power of 12.62W/ch from 14.21 Vpeak @ 1.78A. Is this too much power for the FF85Ks? Given that I'm using a stereo configuration, that would mean I need at minimum a ~50VAC toroid which seems feasible.

However there is some parts of the design guide I'm not so sure about. I'm having trouble selecting the proper resistors for the gain as well knowing the proper cutoff circuit.

Regarding gain, wikipedia tells me that the standard consumer Vrms is 0.316V, and equation 8 from the LM3886 indicates that if I want 10W @ Ohms from a 0.316V power source, I need a gain of 28.3 (Av >= Sqrt(Pout*RL)/Vin). A gain of 28.3 would require Rf to be ~28kOhm and Ri to be 1kOhm.

Regarding cutoff, the specs from the FF85K say that the frequency response is from 122Hz -32kHz. Should I design the lower -3dB cutoff to be ~122Hz? If so, I'd need ~65pF Cin and 20kOhm Rin, but I'm completely guessing off the design guide for this.

Are there any other things I need to consider when designing an amp and power supply? While I'm familiar with electronics, I come from a computer science background and not an audio engineering background so this is pretty foreign.

planet10 · 16th September 2009, 03:05 AM

You don't have to worry about the power... a little more wouldn't hurt.

dave

preiter · 16th September 2009, 05:33 AM

Too much power is never an issue. And you might want to use that amp for some other application some day.

jheridan · 16th September 2009, 12:05 PM

With that in mind, could I just make a dual mono LM3886 amp that pushes out 60W/ch and not really worry about it, provided I keep the power down? I assume that I could actually damage my drivers if I put a way too much through them (as in 5x the rated power).

Does this seem reasonable? Chipamp.com has a sale on the dual mono PCBs right now so I'm gonna jump on this if possible. =]

16th September 2009, 02:55 AM	#1
jheridan diyAudio Member Join Date: Sep 2009	Chipamp for FF85K pair I'm working on my first build and I decided to start with something small and cheap to go with my computer: The µFonken from planet-10. I've got the cabs in the process of being built but I want to get started on the electronics. Which brings me to the real issue: The amps. Y'all around here know more than anyone (that I know) about DIY audio. Would y'all be willing to help look over what I've got? I'm having trouble with the amp and PSU design. Using the design guide I've come to the following: since I'm only pushing out 2x10W RMS I was planning on going with an LM4780 driven by 16.8v (12V toroid from the PSU) giving me a total power of 12.62W/ch from 14.21 Vpeak @ 1.78A. Is this too much power for the FF85Ks? Given that I'm using a stereo configuration, that would mean I need at minimum a ~50VAC toroid which seems feasible. However there is some parts of the design guide I'm not so sure about. I'm having trouble selecting the proper resistors for the gain as well knowing the proper cutoff circuit. Regarding gain, wikipedia tells me that the standard consumer Vrms is 0.316V, and equation 8 from the LM3886 indicates that if I want 10W @ Ohms from a 0.316V power source, I need a gain of 28.3 (Av >= Sqrt(Pout*RL)/Vin). A gain of 28.3 would require Rf to be ~28kOhm and Ri to be 1kOhm. Regarding cutoff, the specs from the FF85K say that the frequency response is from 122Hz -32kHz. Should I design the lower -3dB cutoff to be ~122Hz? If so, I'd need ~65pF Cin and 20kOhm Rin, but I'm completely guessing off the design guide for this. Are there any other things I need to consider when designing an amp and power supply? While I'm familiar with electronics, I come from a computer science background and not an audio engineering background so this is pretty foreign.

16th September 2009, 03:05 AM	#2
planet10 frugal-phile(tm) diyAudio Moderator Join Date: Oct 2001 Location: Victoria, BC, NA, Sol III Blog Entries: 4	You don't have to worry about the power... a little more wouldn't hurt. dave __________________ community sites t-linespeakers.org, frugal-horn.com ........ commercial site planet10-HiFi p10-hifi forum here at diyA

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16th September 2009, 05:33 AM	#3
preiter diyAudio Member Join Date: May 2006	Too much power is never an issue. And you might want to use that amp for some other application some day.

16th September 2009, 12:05 PM	#4
jheridan diyAudio Member Join Date: Sep 2009	With that in mind, could I just make a dual mono LM3886 amp that pushes out 60W/ch and not really worry about it, provided I keep the power down? I assume that I could actually damage my drivers if I put a way too much through them (as in 5x the rated power). Does this seem reasonable? Chipamp.com has a sale on the dual mono PCBs right now so I'm gonna jump on this if possible. =]

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