What is the desired lower -3db cutoff and input impedance?

[CJ900RR]

As the topic says, what is the desired lower -3 db cutoff (in Hz) and input impedance for a amp serving as a power amplifier?

Im playing around in Nationals Overture Design Guide but im not sure about witch values I should be aiming at...

A detailed explination would be much appreciated. The amp I (try to) design will be used with a preamp, and the gain setting of the amp will be around 30.

Best regards...

[Keruskerfuerst]

Imput impedance: usually 47kOhm
Lower cutoff frequency: 0,159Hz

It is better to build an DC coupled power amp. Better dynamic damping.

[CJ900RR]

Originally posted by Keruskerfuerst
Imput impedance: usually 47kOhm
Lower cutoff frequency: 0,159Hz

Ok thank you. But why those values?

[AndrewT]

10k to 100k works for most power amp inputs.
The NFB resistor values influence the Zin choice.

The high pass filter due to the DC blocking cap should be set lower than 4Hz to allow the bass to get through with minimum distortion. I suggest F-3db between 1 & 2Hz.

The low pass radio frequency filter should be set high enough to allow the highest audio frequencies to pass. F-3db of between 150kHz and 400kHz seem to work.

The filters built into the amp should be wider than the input filters. i.e. the input filters determine the amp bandwidth.

[craigg4c]

Originally posted by AndrewT

The high pass filter due to the DC blocking cap should be set lower than 4Hz to allow the bass to get through with minimum distortion. I suggest F-3db between 1 & 2Hz.

The low pass radio frequency filter should be set high enough to allow the highest audio frequencies to pass. F-3db of between 150kHz and 400kHz seem to work.

The filters built into the amp should be wider than the input filters. i.e. the input filters determine the amp bandwidth.

Using the National spreadsheet, set Rf anywhere from 22k to 47k and Ri to give you the gain you want (then set Rin and Rb to the same values). Use a film cap for Cin; anything above about .82uF will be fine. Then set Cf to a value that gives you a -3dB point between 5 and 10Hz. If you can find a sufficiently small and cheap film cap, use that value; otherwise use an NP electrolytic of at least twice (preferably 4 times) that value.

(Note that few speakers have much useful output below around 30Hz, and even fewer sources -- CD, tuner, DVD, what have you -- provide clean signals in this range. The low B on a 5-string bass guitar is around 31 Hz. Setting your -3dB at 10Hz loses only around one dB at 20Hz.)

If your PCB (or point-to-point) allows it, a 100-200pF cap between the + and - inputs will suppress all but the most overwhelming RFI.

Do not under any circumstances either omit Ci or use the inverting design. The web is full of newbies who believed the nonsense going around the ring and are getting humongous DC offsets from their unreliably-designed amps, and occasionally blowing up expensive speakers when their "good-sounding" DC-coupled inverted gainclone amplified the cat jumping on the turntable.

[danielwritesbac]

Originally posted by craigg4c
Do not under any circumstances either omit Ci or use the inverting design. The web is full of newbies who believed the nonsense going around the ring and are getting humongous DC offsets from their unreliably-designed amps, and occasionally blowing up expensive speakers when their "good-sounding" DC-coupled inverted gainclone amplified the cat jumping on the turntable.

That is surprisingly disturbing at 45 RPM, with terribly panicked housecats at top speed, and smoke coming from loudspeakers.