Amplifier problem

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Albertinho
Neuling

#1 erstellt: 16. Jan 2006, 17:08

Hello!
I have a Sansui AU-217 II 2x40W integrated amp my dad bought in 1981. and 2 new Magnat Monitor 220 speakers connected to it. The amp is connected to the SBLive 5.1 soundcard in my PC.

(speakers: )

http://www.magnat.de...4622&pl=1,%204&s=125

The setup works fine on most songs I play, I can go as loud as I want, the bass is strong and everything sounds OK.

But, on certain songs, the woofer unit of the speaker connected to the left channel is travelling very much (several milimeters) even under quite low volume and with little bass that I can hear. I haven't noticed any audible distortions, but I'm afraid to go louder on those particular songs or I might damage the woofer unit because it looks as if it may pop out if I increase the volume further.

I think the problem is in the amplifier since it's only on the speaker on the left channel (yes I've tried swapping them). I don't have much experience with working on actual audio equipment but I study electronic engineering so I understand the circuit theory well. As far as I can tell, maybe there is a very low freq. AC component in the left channel, but let me not guess, that's why I came to ask the question here!

So, I hope I wll get some answers. Cheers!

Amp_Nut
Inventar

#2 erstellt: 17. Jan 2006, 03:56

Hi,

Do the large cone excursions happen consistently, Only when the same passages are played ? If yes, your amp should be OK.

If not, it indicates a DC instability in one channel.

Albertinho
Neuling

#3 erstellt: 17. Jan 2006, 09:57

Amp_Nut schrieb:

Hi,

Do the large cone excursions happen consistently, Only when the same passages are played ? If yes, your amp should be OK.

If not, it indicates a DC instability in one channel.

The large excursions happen only on the same passages of certain songs and only on the left channel.

So you say it might be the songs are bad mp3s?

Btw, i've done a spectrum analysis of a normal song and a problematic one. Maybe it can even tell you something

http://img13.imageshack.us/img13/526/normal2ca.jpg
The normal song has pretty fat bass but the peak power is around 65-70 Hz. I can turn up the volume very high here and everything is fine.

http://img18.imageshack.us/img18/1059/excursionlch7wk.jpg
The song which produces large cone excursions on the left channel has peak power on the lowest frequencies. Maybe it's the problem in the song?

Behram
Ist häufiger hier

#4 erstellt: 17. Jan 2006, 12:19

I too faced this problem with the same amplifier but with the right channel. It is due to a DC offset at the output. Connect a voltmeter at the output of the problematic channel on its lower range and switch on the amplifier, you will find the meter needle swaying. This can be due to some leaky electrolytic capacitors or due to a drift in the input differential pair. Either way I was not able to locate the exact problem.

Albertinho
Neuling

#5 erstellt: 17. Jan 2006, 14:23

Behram schrieb:

Hmm how bout putting a high pass filter with 35Hz cutoff on the left channel output? Would it kill the DC?

Behram
Ist häufiger hier

#6 erstellt: 17. Jan 2006, 15:13

The AU 217 is a direct coupled amplifier, therefore you will be "killing" the bottom end if you do so. Moreover the High pass filter that you talk about will have a definite slope depending on the design / order from 6 db/octave to 18 db/octave. It is therefore easier said than done. In my opinion have an expert technician locate and replace the faulty component and set it right. The PCB tracks are quite delicate though. I tried it myself and ended up dmaging certain tracks.

Albertinho
Neuling

#7 erstellt: 17. Jan 2006, 16:53

Thank you for your answers! I will try to find a good repairman. I hope it won't cost me too much though!

P.s. what are PCB tracks?

Amp_Nut
Inventar

#8 erstellt: 18. Jan 2006, 04:27

[quote]

"The large excursions happen only on the same passages of certain songs and only on the left channel.

So you say it might be the songs are bad mp3s?"

To me this indicates a problem in THAT song only.

One possible cause could be that the mp3 was ripped from an original off a record, and what you see ( Large cone excursions) are an artifact of the rumble / Record warp.

Get yourself another version of the song, and DONT fix your amp ( if it aint broken, dont fix it !

) unless, as Behram says, you have a DC ofset at the output (speaker) terminals.

Have you checked the Speaker DC ofset ?

Amp_Nut
Inventar

#9 erstellt: 18. Jan 2006, 04:35

#5 created: Jan 17. 2006, 14:23 quote PM Mail

Hmm how bout putting a high pass filter with 35Hz cutoff on the left channel output? Would it kill the DC?[/spoiler][/quote]

Throtle all your Low Freq for a Single Song ?

A better option would be to run That song through a High Pass filter, using software

Albertinho
Neuling

#10 erstellt: 18. Jan 2006, 11:51

I'd check it myself but I don't have any instruments so I'll bring it to a repairman. The thought that SOME songs cause such problems is now getting pretty silly to me as well

The DC offset seems like the best explanation I have been given!

I will post what happened when I get the amp checked

Albertinho
Neuling

#11 erstellt: 24. Jan 2006, 22:54

The problem is solved! And it wasn't the amp, but the mp3s!

I've applied a subsonic filter on those songs (gradual cutoff under 35Hz), and now the songs can be played loud without excessive cone excursions.

i would never have thought that the mp3s could have such frequency components which could damage the speakers... they were probably recorded or ripped by someone without any real knowledge...

Now i can finally put the speaker masks on and not bother checking the cones all the time!