Hi,

mine is a 50W x 2 RMS power amp with 29400uF of total electrolytic
capacitance for DC. It has twin output power boards, I'm wondering if
placing two 33uF MKT (Metallised Polyester) caps on DC very near each
output power board would be someway effective..or totally useless. The
idea is placing very FAST non-electrolytic caps. Unfortunately I
cannot find bigger MKT's (like 47uF), but is MKT the right choice?

Sorry for all the questions and thnak you in advance for your help!

Fabio

"Fabio De Robertis" wrote in message
om...
Hi,

mine is a 50W x 2 RMS power amp with 29400uF of total electrolytic
capacitance for DC. It has twin output power boards, I'm wondering if
placing two 33uF MKT (Metallised Polyester) caps on DC very near each
output power board would be someway effective..or totally useless. The
idea is placing very FAST non-electrolytic caps. Unfortunately I
cannot find bigger MKT's (like 47uF), but is MKT the right choice?

Can't do any harm. Will put a lower inductance cap across the lines -
reasonably standard practice. But the difference between 33 and 47Uf is
insignificant in this situation. a couple of uF would suffice....

Polyester should be fine. So would polyprope.

geoff

In article ,
Geoff Wood -nospam wrote:

mine is a 50W x 2 RMS power amp with 29400uF of total electrolytic
capacitance for DC. It has twin output power boards, I'm wondering if
placing two 33uF MKT (Metallised Polyester) caps on DC very near each
output power board would be someway effective..or totally useless. The
idea is placing very FAST non-electrolytic caps. Unfortunately I
cannot find bigger MKT's (like 47uF), but is MKT the right choice?

Can't do any harm. Will put a lower inductance cap across the lines -
reasonably standard practice. But the difference between 33 and 47Uf is
insignificant in this situation. a couple of uF would suffice....

Polyester should be fine. So would polyprope.

One caution, though. In some circumstances, adding high-quality
(high-Q) capacitors across a power supply line can make circuit
stability worse. The capacitance can interact with the inductance of
the power supply wiring or PC-board traces, forming a high-Q L/C
resonator circuit. It's possible for this resonator to be excited by
load transients, etc. to the point that "ringing" occurs on the power
supply rails. In really severe cases I suspect that it might even
push a marginal output stage into high-frequency oscillation.

Similar things can happen if you use high-Q bypass capacitors with
high-speed analog or digital ICs.

The way to correct this is to make sure that you've got adequate
"bulk" capacitance nearby... typically aluminum or tantalum
electrolytics. The higher equivalent series resistance of these bulk
capacitors, in series with their capacitance, reduces the Q of the
resonant circuit and reduces the risk of ringing or instability.

So, rather than using huge MKT capacitors, you might want to consider
using just a couple of uF of MKT (or other high-quality film
capacitor) and then a 47 uF high-frequency electrolytic in parallel.

Whether this will make any audible or measurable improvement in the
amp's sound is another matter... as Geoff said, it shouldn't do any
harm but I don't know if it'll help.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave and Geoff, thanks for the excellent help!

So, rather than using huge MKT capacitors, you might want to consider
using just a couple of uF of MKT (or other high-quality film
capacitor) and then a 47 uF high-frequency electrolytic in parallel.

I have a couple 0.1uF ceramic caps in the same point where I want/need
to apply the two 2uF MKT's + two 47uF tantalum electrolytics (ceramic,
polyester and tantalum would be all in parallel) : could ceramics
interact with the others?

Thanks you and sorry fot the possibly stupid question.

Fabio

In article ,
Fabio De Robertis wrote:

So, rather than using huge MKT capacitors, you might want to consider
using just a couple of uF of MKT (or other high-quality film
capacitor) and then a 47 uF high-frequency electrolytic in parallel.

I have a couple 0.1uF ceramic caps in the same point where I want/need
to apply the two 2uF MKT's + two 47uF tantalum electrolytics (ceramic,
polyester and tantalum would be all in parallel) : could ceramics
interact with the others?

I've read one author's statement (in a book on RF design practices)
that having both very small, and much larger caps in parallel at a
single location can occasionally lead to unexpected results. The
small-valued capacitor can interact with the lead inductance of the
larger cap, creating a parallel LC circuit whose resonance tends to
reduce the effective capacitance.

This was stated as a issue at RF frequencies, for various sorts of RF
filter circuit. I don't think it'd likely be a significant issue in
an audio-frequency power supply bypass application.

So, I'd recommend leaving the ceramic cap in place. Removing them
might adversely affect the output stage's stability.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article ,
(Fabio De Robertis) wrote:

Hi,

mine is a 50W x 2 RMS power amp with 29400uF of total electrolytic
capacitance for DC. It has twin output power boards, I'm wondering if
placing two 33uF MKT (Metallised Polyester) caps on DC very near each
output power board would be someway effective..or totally useless. The
idea is placing very FAST non-electrolytic caps. Unfortunately I
cannot find bigger MKT's (like 47uF), but is MKT the right choice?

Sorry for all the questions and thnak you in advance for your help!

Fabio



It won't help. Audio frequencies are much too low for electrolytic
inductance to be of concern. The amp should already have local bypass
capacitors where needed to keep the semiconductors stable. They're
there for the wiring inductance more than anything else.

As others have mentioned, a perfect capacitor is often a bad choice for
a power supply. It's like taking the shock absorbers out of your car.
The losses are important for dampening.

So, rather than using huge MKT capacitors, you might want to consider
using just a couple of uF of MKT (or other high-quality film
capacitor) and then a 47 uF high-frequency electrolytic in parallel.

I have a couple 0.1uF ceramic caps in the same point where I want/need
to apply the two 2uF MKT's + two 47uF tantalum electrolytics (ceramic,
polyester and tantalum would be all in parallel) : could ceramics
interact with the others?

I've read one author's statement (in a book on RF design practices)
that having both very small, and much larger caps in parallel at a
single location can occasionally lead to unexpected results. The
small-valued capacitor can interact with the lead inductance of the
larger cap, creating a parallel LC circuit whose resonance tends to
reduce the effective capacitance.

This was stated as a issue at RF frequencies, for various sorts of RF
filter circuit. I don't think it'd likely be a significant issue in
an audio-frequency power supply bypass application.

So, I'd recommend leaving the ceramic cap in place. Removing them
might adversely affect the output stage's stability.


Dave,

your suggestions are always clear and useful and a pleasure to follow!

However I am having a hard time to find 47uF Electrolytic Tantalum 50V
capacitors... Vishay makes them (152D series solid tantalum :
http://www.vishay.com/docs/40016/152d.pdf ) but I cannot find anyone
who sells them (they also seem to be very expensive). Could you please
recommend a dealer/online store? Should I switch to 47uF/50V aluminum
electrlytics for an easier search?

Thank you!

Fabio