Bleeder Resistors

AndrewT said:
permanently connected bleeder resistor increase PSU ripple.

THANK YOOO!!! I guessed there was some sort of problem, but I just couldn't identify what it was. Doesn't the resistor there work like a wacky sort of notch filter? Oh, I don't know. Can you explain better practices for bleeder resistors?

Okay, what about bypass caps at the PSU board? Sometimes this seems to cause a problem, eventually resulting in a nasty peak in the audio output. Instead of on the side with the amplifier, I was thinking about putting just 1 per rail as the "first" caps in the power supply (instead of the last). How should I view this issue?

Thanks again!
 
danielwritesbac said:


THANK YOOO!!! I guessed there was some sort of problem, but I just couldn't identify what it was. Doesn't the resistor there work like a wacky sort of notch filter? Oh, I don't know. Can you explain better practices for bleeder resistors?

Okay, what about bypass caps at the PSU board? Sometimes this seems to cause a problem, eventually resulting in a nasty peak in the audio output. Instead of on the side with the amplifier, I was thinking about putting just 1 per rail as the "first" caps in the power supply (instead of the last). How should I view this issue?

Thanks again!

Using large value resistors that allow 1-5ma of current through them can be useful as damping resistors to help damp resonances due to the interaction of the wiring/transformer inductances with the caps. However, the value of bleeder resistors is usually quite a bit lower, and waste energy and increases ripple, as suggested. Bleeders should only be connected when amp is turned off.

Your second question is unclear. You want your bypass caps at the load, where they can reduce the impedance of the wires connecting the main power supply caps to the board. They (if you are talking about the board mounted electrolytics) can be a problem if too small. If you are asking about the small bypass caps that bypass the board mounted electrolytics, these should also be on the board, preferably the closest to the load of all the caps.
 

ptah

Member
2006-08-07 8:48 pm
The reason I ask is that the amp I am currently building is the first amp I have used bleeder resistors with the reservoir caps and I am having problems with hum. This is effecting the electronic crossover. I moved the bridge rectifiers to the rear of the chassis wich did away with most of the hum. However, if I move the crossover closer to the caps, the hum increases. If I run the amp with the cross over outside of the chassis there is no hum at all. The chassis is very cramped and I am working on eliminating the hum. It is going to be a big deal to remove the bleeders so I needed to get experienced opinion as to their possible influence.
 
Since hum changes with crossover distance, it appears that you have magnetic coupling between a crossover coil and a magnetic field in the power amp. Try adding long leads to a midrange or woofer coil and rotate or move it while music is playing to confirm magnetic coupling. Then either mount it for minimum pick up or figure out the reason there's a hum field from your power amp.

The only other obvious explanation is a ground loop, this implies you are changing connections when you move the crossover closer or further from the amp.

Why do you think bleeder resistors are involved in the problem? How much current is flowing in the bleeders? Bleeder values are usually chosen to discharge the mains capacitors in 5 or 10 seconds.
 
Have you tried without them and you got less noise?
Since you said your chassis is cramped and the hum decreased with distance, I too tend to believe that the problem is magnetic coupling. However you also noticed a reduction in hum level when you moved your bridge rectifiers, meaning that you picked up RFI form diodes switching on and off. You could try to place 4 capacitors in parallel with them (10nf would be good).
 

jnb

Member
2006-12-30 11:55 pm
Bleeders are often a much smaller load (higher value) than the load presented by the circuit. This is where peranders is correct.

The wires leading to your caps carry heavy current pulses. This is radiated. You could identify pairs of wires that carry equal and opposite currents, and twist them together.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
I agree with PA and others

I have to agree. Bleeder resistors are helpful for discharging capacitors after you turn the equipment off, and I recommend that you use them. Just don't mount them on the capacitor terminals where they will heat the capacitor and cause shortened life (depending on how hot they run).

Bleeder resistors are normal in earlier equipment. They also can ensure a minimum current draw that you need for chokes and some regulator IC's. They can be less noisy at higher currents than a few mA.

I have to second what Andrew said. Please do not make multiple threads on the same topic. It makes us cranky.

-Chris
 
AndrewT said:
permanently connected bleeder resistor increase PSU ripple.

Hey Andrew! ;) Does it bother you being right all the time?
Well, I just grabbed the desoldering iron and got those 2.2k off that power supply, and oh boy! That amp sounds much better now.

Shortly after that, I removed the bypass cap, shmubberize system, and that was okay, not better or worse, just different--not much different either.

Next up, I got into my boxfull of components that sound usually awful when carrying audio signals. From this box (labeled "shunting components"), I fetched a pair of bright green Nichicon Muse BP, 33uF, 50v (very forwards all the time). I put these as the first caps after the rectifier, intending to drive whatever details and shouts came over the power lines--right on to ground. That actually worked! And, the amp pumps out nearly twice the power--on dynamics, not average. An orchestra hit gave me a conniption. ;)
Should it have worked this way?
 
Why don't you use the circuit i use in all my stuff? It's very simple to implement.
Using a relay, bleeder resistors are always connected when the circuit is unpowered.
When power is up, relays disconnect bleed resistors. Simple, isn't it?
 

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