testing for stability

maxpou · 25th April 2007, 02:16 AM

Hi everybody,
i want test my poweramp with capacitive charge but i would like to know what value of cap and if i have a precaution to take, which? i don't want burn my output ( hexfet ) thank you! Maxpou

fab · 25th April 2007, 04:17 AM

Hi Maxpou

I suppose you have a scope otherwise...

Always put a resistor load in parallel with the cap ( 8 ohms is the standard one used and ususally represents the nominal impedance of lots of speakers).

Use an input 10 KHz Square wave and adjust the level to have an ouput of about 4V peak. Do not exceed 10 V otherwise you may have your cap to explode!!

and it smells like HELL !

1) Try first with no cap in parallel
2) then try with the small value of 1 nF then increase the value with 2.2nf, 4.7nf, 10 nf, 22 nf, 47 nf, 100nf, 220nf, 470 nf, 1uf, 2.2uf.

Normally you should see different output shapes going from a perfect square wave for very low cap values (1 nf, 10nf, 22nf ...).

After that you will reach a point in caps value where sometimes the square shape may start to deteriorate a bit on the edges. It is still OK.
After that possible roughness on the edges, you will start to see overshoot (maybe 220 nF or more) but it should be damped.

Note: If you begin to see oscillation on the square wave for a specific cap value just stop the test at this value since it means your amp is not stable and depending on the oscillation it could eventually damage the amp...

But you can test the other cap values to see the extent of the stability problem if any.

Just send us some pictures if you can (2 uf, 100 nf, 4.7nf for example).
Good luck!

maxpou · 26th April 2007, 12:06 AM

HI Fab,
thank's for your answer. but i don't have 8 ohms ressistance but 6.8 ohms, it's ok? thank you! maxpou

fab · 26th April 2007, 02:24 AM

6.8 ohms is OK. Anyway the speaker impedance varies a lot depending of frequency (sometimes from 3 to 18 ohms at specific frequency...). This 6.8 value should give you reliable results even to compare with other amps where the test was done with 8 ohms...

maxpou · 27th April 2007, 07:23 PM

Hi everybody,
i send you some pictures of test stability with my poweramp. please comment on this. thank you! Maxpou

maxpou · 27th April 2007, 07:25 PM

excuse me first picture is 10Khz, 10vp-p, 0.033uf
seconde picture: 10khz, 10vp-p, 0.068uf

maxpou · 27th April 2007, 07:26 PM

third picture: 10khz, 10vp-p, 0.1uf

maxpou · 27th April 2007, 07:28 PM

and last 10Khz, 10Vp-p, 0.22uF load for all 6.8 ohms

AndrewT · 27th April 2007, 07:45 PM

Hi Max,
that looks like a damped 200kHz (220nF) ripple fed from a Thiele network into your R//C load.
Are you using an inductor//R in the series feed to the load?

The overshoot and ripple is looking a bit big for inductor/load interaction.

maxpou · 27th April 2007, 08:25 PM

Hi Andrew,
i don't have a inductor//R in serie to the output because inductor decreased dynamic that somebody said to me. Maxpou

25th April 2007, 02:16 AM	#1
maxpou diyAudio Member Join Date: Apr 2006 Location: Québec	testing for stability Hi everybody, i want test my poweramp with capacitive charge but i would like to know what value of cap and if i have a precaution to take, which? i don't want burn my output ( hexfet ) thank you! Maxpou

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25th April 2007, 04:17 AM	#2
fab diyAudio Member Join Date: Mar 2004 Location: Canada	Hi Maxpou I suppose you have a scope otherwise... Always put a resistor load in parallel with the cap ( 8 ohms is the standard one used and ususally represents the nominal impedance of lots of speakers). Use an input 10 KHz Square wave and adjust the level to have an ouput of about 4V peak. Do not exceed 10 V otherwise you may have your cap to explode!! and it smells like HELL ! 1) Try first with no cap in parallel 2) then try with the small value of 1 nF then increase the value with 2.2nf, 4.7nf, 10 nf, 22 nf, 47 nf, 100nf, 220nf, 470 nf, 1uf, 2.2uf. Normally you should see different output shapes going from a perfect square wave for very low cap values (1 nf, 10nf, 22nf ...). After that you will reach a point in caps value where sometimes the square shape may start to deteriorate a bit on the edges. It is still OK. After that possible roughness on the edges, you will start to see overshoot (maybe 220 nF or more) but it should be damped. Note: If you begin to see oscillation on the square wave for a specific cap value just stop the test at this value since it means your amp is not stable and depending on the oscillation it could eventually damage the amp... But you can test the other cap values to see the extent of the stability problem if any. Just send us some pictures if you can (2 uf, 100 nf, 4.7nf for example). Good luck!

26th April 2007, 12:06 AM	#3
maxpou diyAudio Member Join Date: Apr 2006 Location: Québec	HI Fab, thank's for your answer. but i don't have 8 ohms ressistance but 6.8 ohms, it's ok? thank you! maxpou

26th April 2007, 02:24 AM	#4
fab diyAudio Member Join Date: Mar 2004 Location: Canada	6.8 ohms is OK. Anyway the speaker impedance varies a lot depending of frequency (sometimes from 3 to 18 ohms at specific frequency...). This 6.8 value should give you reliable results even to compare with other amps where the test was done with 8 ohms...

27th April 2007, 07:45 PM	#9
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi Max, that looks like a damped 200kHz (220nF) ripple fed from a Thiele network into your R//C load. Are you using an inductor//R in the series feed to the load? The overshoot and ripple is looking a bit big for inductor/load interaction.

27th April 2007, 08:25 PM	#10
maxpou diyAudio Member Join Date: Apr 2006 Location: Québec	Hi Andrew, i don't have a inductor//R in serie to the output because inductor decreased dynamic that somebody said to me. Maxpou