F5 power amplifier

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#9,422
Input filter calculation

Seems to me, the resistor of the source is not part of the calculation for a high pass input filter. The source resistance should always be lower than the amps input resistance or so I thought.

.....from wikipedia
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First-order continuous-time implementation

Figure 1: A passive, analog, first-order high-pass filter, realized by an RC circuit


The simple first-order electronic high-pass filter shown in Figure 1 is implemented by placing an input voltage across the series combination of a capacitor and a resistor and using the voltage across the resistor as an output. The product of the resistance and capacitance (R×C) is the time constant (τ); it is inversely proportional to the cutoff frequency fc, at which the output power is half the input power. That is,
7d87bc118166402407c6194a521f4eac.png
where fc is in hertz, τ is in seconds, R is in ohms, and C is in farads.
 
#9,424
ichiban said:
Seems to me, the resistor of the source is not part of the calculation for a high pass input filter. The source resistance should always be lower than the amps input resistance or so I thought.

.....from wikipedia
Click to expand...
Insert a low value r between the Capacitor and the tapping point where the big R picks up.

The smallr =1k0 and the bigR =100k in the F5.
The impedance seen into the jFETs is very high at audio frequencies.

For an F5 where r:R is 1:100 the effect of r is inaudible.
If the ratio r:R moves towards 1:1 (say 10k for both) the effect will be very audible. It is certainly not a zero effect.

Now add some RF filtering to the input by paralleling the 100k with a pF cap (c).
The audible effect of smallr becomes noticeable as the rc value increases.
 
Last edited:
#9,428
AndrewT said:
Rin = R9+R10 = 101k

With the DAC 50K you have 1uF & 33k4
The F-1dB ~ 9Hz.

Try changing the 50k to something much higher.
Click to expand...
What is the purpose of having a resistor to ground after the output cap of the preamp/dac? when you connect that capacitor output to power amp input, you are already providing that resistance to ground.

Could you explain if there are any differences? Thanks Andrew 😉
 
#9,435
Well I just wanted the "free" "typical food from my land..." or maybe I would find out that I didn't, but...

Anyhow reading is not all it is cracked up to be...

And, some people post and don't build, so there seems to be a fair yin/yang effect at work... at the present, I don't seem to be doing much of either posting or building, so I am safe in that regard... 😀

Also, I don't actually know anything, despite any appearances to the contrary.

_-_-bear
 
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