Refurbishing old amplifiers, I sometimes come across power supply bypass capacitors, 10 to 100 nF ceramics I presume for HF bypass, and almost always small electrolytics like 10 or 100 microfarad, LOCAL on (all) circuit cards.
Example: Luxman L309, SQ507 and alike main amplifiers have a 1 microfarad power bypass electrolytic and a 40 nanofarad ceramic one to each power rail on each power amp module.
However, such seem to be lacking on early amplifiers (now doing a Sansui AU555 from around 1970, but I have seen it on others, too).
What would be a good consensus about this, just leave it, or maybe just "add something thinking it does not hurt" ? Comments welcome.
Example: Luxman L309, SQ507 and alike main amplifiers have a 1 microfarad power bypass electrolytic and a 40 nanofarad ceramic one to each power rail on each power amp module.
However, such seem to be lacking on early amplifiers (now doing a Sansui AU555 from around 1970, but I have seen it on others, too).
What would be a good consensus about this, just leave it, or maybe just "add something thinking it does not hurt" ? Comments welcome.
It is correct that the small ceramics are for HF. Electrolytes have a bad HF behavior because of increased inductance. It is interesting to know why they were not applied in certain application. Because there were no problems anyway? There was no HF to suppress? The knowledge wasn't there yet? Cost and they got away with it?
AFAIK adding a ceramic parallel to an electrolyte never can do harm. Even if it doesn't do anything.
I am happy you did not start your post with "I just finished recapping this and this equipment...."
AFAIK adding a ceramic parallel to an electrolyte never can do harm. Even if it doesn't do anything.
I am happy you did not start your post with "I just finished recapping this and this equipment...."
Yamaha CA-2010 (and CA-1010?) have a 1uf/80V electrolytic cap on both un/regulated power amp rails, didn't see any ceramics
Actually there are some circumstances where it does do harm (due to very low ESR), but I don't think that's relevant here.
If you over do it, it does harm.
If the cap is too big, it will shunt too much power supply ripple current to the ground rail, which cause elevated noise on the ground.
My rule of thumb is those caps should not bigger than 1% size of your power supply filter caps.
Last edited:
@jxdking IMHO the goal of filter capacitors IS to shunt ripply current to the ground rail. The goal IS to shunt the power line with a capacitor as large as feasible and with an inductance as low as possible to the ground rail.
Can you please elaborate on that statement that and how it is harmful to shunt the ripple current to the ground?
Can you please elaborate on that statement that and how it is harmful to shunt the ripple current to the ground?
Indeed my "concern" is shorting (possible) RF voltage only, in case it would be present on the power.
Also on inputs, many years ago I cobbled together a magazine preamp, it appeared receiving some radio station, very softly but audible....
This old sansui amp now, thas nothing on the inputs, power, nor output, to really keep out RF.
There is also no grounding scheme, other than "maybe this is a good spot on the chassis to ground this to keep wires short".
Having excess parts anyway, maybe it is a good idea to have a 100nF ceramic on each circuit cards power line, hoping it would dissipate any RF present.
Being no good at metalwork (nor painting), deciding the rust penetrated too much, I painted this mid section frame white, and the brown transformer is black again, too
a little bit of OCD..
Also on inputs, many years ago I cobbled together a magazine preamp, it appeared receiving some radio station, very softly but audible....
This old sansui amp now, thas nothing on the inputs, power, nor output, to really keep out RF.
There is also no grounding scheme, other than "maybe this is a good spot on the chassis to ground this to keep wires short".
Having excess parts anyway, maybe it is a good idea to have a 100nF ceramic on each circuit cards power line, hoping it would dissipate any RF present.
Not finished yet, although I successfully recapped the mid section ☕
Being no good at metalwork (nor painting), deciding the rust penetrated too much, I painted this mid section frame white, and the brown transformer is black again, too
a little bit of OCD..
So it is better to keep the RF on the power lines so it ends up in the amplifier circuits?
Ground (or expressed more accurately, the amplifier circuit's common) should be exactly that. The common is a potential which does not carry any voltage anywhere. So if you would conduct a HF current from the power line to the circuit common, exactly nothing happens with any voltage on the common level.
The effect it would have is that any HF voltage at the other end of the capacitor (on the power line side) is strongly attenuated. Which is exactly the effect you want to achieve.
If it were true what you state about inserting current in the ground, it would also imply that it is better not to use too large electrolytic capacitors in the power supply. Large capacitors result in large currents due to the periodic charging of the capacitors. And that would result in 100Hz ripply in the ground circuit.
And indeed it does. In a badly designed ground layout. As anyone knows who ever built an electronic circuit. And it does not matter whether it is 50Hz or 500kHz. Ground must be a common voltage as perfect as can be realized. And any AC component (any!) on supply lines should be transferred to the common. And the common should, by design, remain a common.
I am not a full-educated engineer.
Maybe I used the wrong english words, but I never intended to keep possible RF on the power lines, nor stated anything about inserting current into the ground.
I am just looking for consensus, what other people do if they encounter the situation I do.
It is about old circuit cards connected through wiring (which are never an ideal common anyway, so yes I do understand why @jlinkels mentions things).
Maybe I used the wrong english words, but I never intended to keep possible RF on the power lines, nor stated anything about inserting current into the ground.
I am just looking for consensus, what other people do if they encounter the situation I do.
It is about old circuit cards connected through wiring (which are never an ideal common anyway, so yes I do understand why @jlinkels mentions things).
More designers know about correct grounding to keep PS ripple currents out of the signal ground system than in decades past. More, not all. I've seen various "grounding 101" errors in many amps, receivers, preamps and pieces of test equipment of all vintages. I've found if I wanted to add local power filtering to some PCBs that had only a signal ground, I had to add another ground, lest the caps modulate the signal ground with power supply ripple. I didn't use the word "bypass" because that can imply some small ceramic or film cap that can't pass much ripple current, though in the grand scheme of things they're connected in the same place as my "local filtering" caps. This wasn't the most elegant fix, but you can get some idea from my de-humming of a Sansui 881- https://www.conradhoffman.com/Sansui_881_mods.htm
