Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tapsnap on November 21, 2017, 09:53:32 am
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Can someone point me to where I can find some info about how to calculate values for filter capacitors and resistors? This is quite mysterious to me. I'm working on a single ended amp which only has two stages of filtering. I have managed to reduce the hum significantly since I learnt about grounding and then re-designed my whole circuit board, but now, I would like to put in another stage of filtering. I really don't know anything about the subject. If the math is really complicated, perhaps someone can explain some general rules that can be guides to choosing the values for these components.
Thanks
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Download this software and play with it: http://www.duncanamps.com/psud2/
For the main power tube in a single-ended design it's common to have a CRC filter, instead of pulling plate voltage off the main capacitor you'd have a resistor and another cap. The resistor could be as low as 100 ohms and you'd see a significant improvement. AX84 designs use this value with 47uf caps with good results, I have done the same with good results.
If you're willing to sacrifice more voltage, something like 47u-->100ohms-->47u-->100ohms-->47u is more effective than using 1x 200ohm resistor or even doubling the caps, but there's no free lunch-- you're also clearly using more caps and resistors to accomplish this.
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If you want to learn about the inner mysteries of amp design theory, you need to read reference works: e.g. Radiotron, TUT1 (Kevin O'Connor of London Power).
If you want to build, then plagiarize from schematics of amps that you like.
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Read the sections of Merlin's website that deal with power supply design: http://valvewizard.co.uk/index.html
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Merlin's 'Designing Power Supplies' book is highly useful if you can get a copy.
But Chapter 11 of his latest (Designing Hi-Fi Tube Preamps) book is available for purchase from the link on his website, and is very useful.
Basically, a R/C filter is a lo-pass (or high-cut) frequency filter, and a amplifier power supply filter chain consists of a series of dropping resistor/filter caps that is simply a series of R/C filters connected together.
The R/C values in a typical power supply filter chain are designed to for 2 main purposes, namely:
1) To remove any possibility of ripple frequencies from the power supply affecting the DC stability of the whole power supply. This 'ripple' is comprised of residual peaks and troughs from the reservoir cap's charging cycle, and corresponds with the 2 x mains frequency in a full wave rectified power supply; and
2) To decouple any single gain stage's signal from 'pulling' on the DC power supply voltage, by shunting any variation in DC that may otherwise appear at the filter cap's charged (+ve) pole, to ground.
The bigger 'C' is in relation to 'R' in each R/C filter section, the more HF gets rolled off. I say 'HF', but this is a relative concept, because the comparative values of 'capacitance' (compared to 'resistance') in a typical R/C filter stage, is designed to roll of virtually all audible frequencies. So it is not so much a 'HF filter' per se, but rather is a total signal filter. Ideally you want the filter to be able to shunt all audible frequencies that you would encounter in an amp, so that you don't even get any subharmonics of the fundamental frequencies affecting the power supply stability. This is why, in bass amps in particular, is is common to see large capacitance values, particularly on the screen supply node, to ensure that undesirable screen current feedback from the power supply is otherwise avoided.
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> how to calculate values for filter capacitors and resistors?
Do what everybody does. (as jjasilli said) Plagiarize. Find a known-good plan, similar to yours, and steal the values.
Seriously. This is a problem that has been solved before. Same as: how big a stud do you use for a wall? Even without a building code, you can bust a few walls and learn it was usually 2x4, but now 2x6 for more insulation. This general size has been used since before Joseph the Carpenter married Mary (except stud walls were less common before power rip-saws). There's no reason to work-out the properties of wood and the structural situation for repetitive framing: use 2x4 or 2x6.
Moreso since electrolytic caps are stupidly cheap today. 10% drop and 40uFd covers almost any situation.
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Uncle Doug on YouTube has a great series of videos on capacitors in audio circuits which will help you understand this.