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diyAB Amp  The "Honey Badger" build thread
diyAB Amp  The "Honey Badger" build thread
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Old 19th February 2019, 09:54 AM   #3381
Johno is offline Johno  Australia
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diyAB Amp  The "Honey Badger" build thread
Quote:
Originally Posted by stuartmp View Post
Hi All.

Just been doing a bit more back to basics to help me better understand some of the building blocks of the HB and had the following Question in relation to the input network.
I am assuming that R1,2,3 combined with C1,2 form some form of low pass / high pass filter?
I assume that the coupling capacitor C1 blocks any DC that might be present from the source.
I assume that R3 provides a return path for the input bias current of the amplifier’s input stage. This also keeps the non inverting input node of the amplifier near 0 V.

I assume that the capacitance of C1 against R3 forms a high-pass filter.
I assume that the resistor R2 and capacitor C2 form a first-order input low-pass filter.
I assume that the resistor R1 keeps the input terminal from floating at DC.

If so can someone please provide some more information if this is correct and tell me why active filters are not used?
Hi Stu,
Barely had a chance to play with mine, too busy dammit.
C1 is DC isolation with R1, eg a hi pass element.
The ratio R2/R3 identical with R5/R6 sets the gain of the amp (same as seen for an op amp in a differential config)
C2 with R3 is a low pass to bypass RF pickup and to reduce the gain for frequencies above the audio band to ensure the gain is less than unity where the phase inverts at the upper end of the bandwidth (to prevent oscillation).
C3/C4 are also DC decoupling, this time for the feedback leg.

Cheers
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Old 19th February 2019, 03:36 PM   #3382
ostripper is offline ostripper  United States
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The input filter is a quick easy way to ensure the HB will not amplify AM (radio) frequencies. The main miller caps at the VAS set unity gain @ 1mhz where there is still 90 degrees of phase margin. Where you hit 180 degrees (oscillator) ,there is attenuation of >-20db ( it would never oscillate). So the input filter just attenuates 100khz - 1mhz , where there still is (purpose-less) usable gain. The input filter has nothing to do with oscillation. PS - should not of said "purpose-less" , extra gain (above native gain) at 20khz + means there is plenty extra for NFB <20khz.

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Last edited by ostripper; 19th February 2019 at 03:43 PM.
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Old 20th February 2019, 09:00 PM   #3383
malibutwins is offline malibutwins  United States
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Can the Honey badger be bridged?
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Old 20th February 2019, 09:28 PM   #3384
ostripper is offline ostripper  United States
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Quote:
Originally Posted by malibutwins View Post
Can the Honey badger be bridged?

MT-200's ,40V rails and 8R , maybe ?


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Old 21st February 2019, 07:16 AM   #3385
stuartmp is offline stuartmp  Australia
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Thanks for those responses. Johno and OS.

Just one last part to my original question.

Is there a reason that an active low / high pass filter is not used?

I assume its because the voltage drop across the input network is not that great considering the input impedance of Q1.
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Old 21st February 2019, 10:12 AM   #3386
Johno is offline Johno  Australia
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diyAB Amp  The &quot;Honey Badger&quot; build thread
@Stuartmp

The simple RC roll off is all that is needed. Series C1 is simply to block DC at the input from any previous stage and should be large enough to pass anything above approx 20Hz. Shunt C2 attenuates RF signals in the many MHz range.
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Old 21st February 2019, 07:45 PM   #3387
malibutwins is offline malibutwins  United States
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I need help with my psu for my "Rabid Badger" build.
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Old 22nd February 2019, 10:09 AM   #3388
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@Malibutwins what exactly do need help with?
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Old 22nd February 2019, 01:46 PM   #3389
malibutwins is offline malibutwins  United States
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Mainly capacitance of filter caps and do in need to put bypass capacitors in the power supply. Also do i need to put bleeder resistors across caps?
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Old 22nd February 2019, 07:48 PM   #3390
ostripper is offline ostripper  United States
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Quote:
Originally Posted by malibutwins View Post
Mainly capacitance of filter caps and do in need to put bypass capacitors in the power supply. Also do i need to put bleeder resistors across caps?

Below -
C2 -14 can be much larger 8 X 10kuf/80V . C1 is a RF filter ,
I used 8 X 6800uf/80V on a far larger amp than the HB
C12 + TX is a common mode filter.
BR1 can have snubber caps 250v/.01uf in parallel
with each diode.

R2/3 , I think they should be 3.9K .... I use 5.6k on 60+V supplies.
C6/7 are the small bypass caps you mention.
RT1 is a soft start - negative temp. co-efficient inrush limiter.
12a inrush limiter Inrush Current Limiters | Mouser
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