Did I blow the amp?

[Ken B.]

I have a little cheapy 20 watt stereo amp I ripped out of some old Harman Kardon desktop speakers. I hooked up a pair of cheapy JVC speakers to it, but the left channel makes the speaker cone stick out and stay out while it plays music. If I reverse the polarity, the cone gets sucked in and stays in.

I took the right side speaker and hooked it up to the left channel on the amp, and same problem.

Someone suggested hooking up a 1.5V battery and seeing if the suspect speaker pushed the cone out a little, and it did, so the speakers themselves appear fine.

Did I somehow manage to blow a channel on the amp? I don't remember doing anything that would have caused it.

[jamikkim]

Re: Did I blow the amp?

If it stays out/in but plays music the same loudness and no excess distortion then there might be a capacitor that is blown/dried out that would reduce the DC signal.

Did you notice this as soon as you hooked up the new speakers or did it happen after a bit of playing? If the problem already existed then you may not have noticed it before with the smaller H/K drivers.

[Sydney]

Re: Did I blow the amp?

... but the left channel makes the speaker cone stick out and stay out while it plays music. If I reverse the polarity, the cone gets sucked in and stays in...

This typically indicates that the amp has a significant DC component in it's output. Often it is the raw DC from the power supply that causes this ( perhaps being passed from a blown output device ).
Use a volt meter to determine if DC is present.

[PWR RYD]

Re: Did I blow the amp?

The little amp is probably just fine. What I've seen them do in those HK desk top speakers is use the DC bias to power a small LED in the "remote" speaker (the one that doesn't have the amp inside). Instead of having the blocking cap on the amp board they stick it in the "remote" speaker. That way they can block the DC offset to the driver but still have the small DC voltage to power the LED. If you still have the enclosures bust apart the remote one and I bet you will find the cap and probably a small resistor to current limit the LED. It will be a polarized electrolytic around 1000 uF.

[PWR RYD]

Re: Did I blow the amp?

Forgot to mention that the blocking cap for the "master" speaker IS mounted on the amp board.

I only know this because our IT manager gave me 5 pair of those harmon/kardon PC speakers when he upgraded several users to a sound bar under their monitor. I only wanted the amps and threw the plastic boxes and pathetic drivers away. Hope this helps.

[tyger23]

Re: Did I blow the amp?

I'd be more worried about you having damaged your speaker....

Speakers can't take much DC voltage across the leads for very long.

[Ken B.]

Re: Did I blow the amp?

I did notice that the capacitor highlighted (not sure if it it was the electro or the ceramic) was pretty warm when I was testing everything. Could one of those be the problem?

PWR, there was no cap in the other speaker. This amp worked fine last time I tried it (a few weeks ago).

How do I test "if DC is present", and what do I do about it?

Thanks guys, you are the best!

[envisionelec]

Re: Did I blow the amp?

Considering the speaker output comes directly off the IC, I think the "series capacitor" idea doesn't apply here. It's probably blown. A new amplifier IC might cost more than another set of speakers...

[JRT]

Re: Did I blow the amp?

Considering the speaker output comes directly off the IC, I think the "series capacitor" idea doesn't apply here. It's probably blown. A new amplifier IC might cost more than another set of speakers...

Why?

It is probably operating off of single supply instead of a split supply. For example, see the circuit depicted below (lifted from analog.com), where the output series coupling capacitor "Cout" is blocking a DC offset roughly equivalent to Vs/2.

[JRT]

Re: Did I blow the amp?

Forgot to mention that the blocking cap for the "master" speaker IS mounted on the amp board.

I only know this because our IT manager gave me 5 pair of those harmon/kardon PC speakers when he upgraded several users to a sound bar under their monitor. I only wanted the amps and threw the plastic boxes and pathetic drivers away. Hope this helps.

Can't beat the price. If you use those to biamp 2way loudspeakers, you are likely adding a film and foil capacitor in series with the tweeter, which would block that DC offset. I expect you already well know that, but others might not.

Andy Graddon would say, "No worries Mate."

Where is Andy BTW? Haven't seen him post here in a very long while.


.

[Wolf]

Re: Did I blow the amp?

I've been meaning to shoot G some mail.

He had to move awhile back and has not had time, money, or room for more speakers.

I'll let him know he's being asked for...
Wolf

[Andy_G]

Re: Did I blow the amp?

Hello all.. Thanks for asking.. Been sorta busy with other stuff. trying to finish this darn PhD, while lecturing/tutoring at uni.

Also had a lot on eustacian tube problems affecting my hearing a bit, and nowhere to put any new speakers anyway.

So I've sort of backed away from speaker building for a while. Still have some good drivers on the shelf, so something might get built at some stage.

I've dropped ina couple of times, but never seem to have the time to read through threads etc.

I will try not be a strangler... I mean... stranger :-)

Thanks Wolfy, for reminding me that you guys exist.

[PWR RYD]

Re: Did I blow the amp?

I did notice that the capacitor highlighted (not sure if it it was the electro or the ceramic) was pretty warm when I was testing everything. Could one of those be the problem?

PWR, there was no cap in the other speaker. This amp worked fine last time I tried it (a few weeks ago).

How do I test "if DC is present", and what do I do about it?

Thanks guys, you are the best!

[ATTACH=CONFIG]29728[/ATTACH][ATTACH=CONFIG]29729[/ATTACH][ATTACH=CONFIG]29730[/ATTACH][ATTACH=CONFIG]29731[/ATTACH]

Yeah what you have pictured is quite different than the ones I have. Mine have a single ended (12VDC wall wart) power supply and use the TDA2007A chip. Probably only 5-7 wpc.

Best of luck with your amp.

[Wolf]

Re: Did I blow the amp?

You're welcome, G!
Wolf

[lhwidget]

Re: Did I blow the amp?

How do I test "if DC is present", and what do I do about it?
[ATTACH=CONFIG]29728[/ATTACH][ATTACH=CONFIG]29729[/ATTACH][ATTACH=CONFIG]29730[/ATTACH][ATTACH=CONFIG]29731[/ATTACH]

You can be certain DC is present because the driver's cone is pushed out or in depending on the polarity of the speaker connection. You are literally seeing the driver respond to a DC signal (same as hooking a small battery up to a driver to test its polarity). The cone's position is offset physically in proportion to the amp's DC offset.

Also, what was mentioned is true, drivers can't survive long when DC is applied to them, esp tweeters.

You could try adding the 1000 µF electrolytic cap to stop the DC, but it sounds like the amp is failing.