OT Solid state amps dc to speaker?

[sonic66]

Hi, haven't posted for awhile have bee messin around with amps.
Actually built a champstyle 4watt tube amp out of recycled parts.
may do a web page on it some time.

my question for anyone who could help is about a solid state bass amp.
Its a little samick sm 25b and has a 10 inc speaker.

i think it might be putting dc on the speaker. As when it turns on the speaker jumps out,
with a little thump and jumps back when turned off. It doe the same with other speakers too,
Thes speakers don't do this with other solid state amps including one that is reasonably new

i measured the dc voltage with no speaker attached and no input plugged in ant it was .35 volts.
So i guess the out put blocking capacitor needs replacing?

i can find a schematic, but ita appears to be a green cap of some sort, i'm not sure what value.
what would be a reasonable value to replace it with . how can i tell if its crumby or if i barking up the wrong tree.

thanks

Sonic66

[George Giblet]

Does the amp work?

It's not really DC as such but more a strong transient pulse at power up.

Firstly this does happen on a lot of amplifiers.  Particularly those less than say 120W.  Larger amps tend to put specific sections in the amplifier to power up in a softer manner - this reduces the magnitude of the pulse.  You can get away without soft start circuits on smaller amps since the pulse doesn't do damage to the speaker.

There's two reasons:
- on single supply power amplifier you get a big bang when you turn on the power because the output cap charges up through the speaker.
- on dual rail power amplifiers it happens when caps charge up or some parts of the haven't  yet powered up enough that the feedback can force the DC at the output to zero (as SS amps do).

You will find the problem is model dependent ie. all models which are the same will have the pulse.

It's probably not worth modding the amp to remove it.

[sonic66]

Yes, the amp works,

its apparently an 80watt amp and is probably similar to those practice style amps by drive or prince or gorilla etc...

I'd read some where though i can remember where, that if a speaker jumped on or moved and stood out / in with out a signal,

that is stays away from its resting position, that it has constant DC to it it would burn out the voice coil.

is that true and whats the difference as per what you describe?

thanks

[George Giblet]

> that if a speaker jumped on or moved and stood out / in with out a signal,  that is stays away from its resting position, that it has constant DC to it it would burn out the voice coil.

Yes that is bad and yes it would eventually burn out the voice coil in a *very short* space of time.  The key is the cone *stays* push in/out after power up.  (If you see that type of thing power the amp off straight away).

> is that true and whats the difference as per what you describe?

The difference is the speaker cone stays pushed in/out that there is a constant DC voltage applied to the speaker, the amp is driving DC into the voice coil.   This is an abnormal situation, a sure sign that something has blown in the power amp.

In the start-up "pop" case of your original post, the speaker cone is not pushed out indefinitely.  There is a transient at power up which *goes away* after a short period.   It just happens that the time the transient condition lasts for is long enough that you can see the speaker being pushed in/out, but you can also see it goes back to the resting condition.   The cause is simply the electronics starting up but once it starts up all is normal.

For a given amp the transient is in one direction hence the amp will consistently push the cone in or out - like DC being applied to the speaker for a short time then being removed.

The transient doesn't last long enough for the speaker coil to heat-up and fuse.  The bigger the amp the bigger the transient which is why large amps tend to have measures to prevent it.  For small amps it's not worth the additional parts and complication to the circuit.

[sonic66]

(edit) was editing this post when you replied ...... below

[George Giblet]

You got it.

> .35 volts DC

Most amps are below 0.1V. However, I wouldn't be overly concerned with 0.35V.  If a schematic was availble it's easier to judge what an acceptable range is for a given amp.

> I guessed that it might be a a blocking capacito

No need.  Also on an amplifier with a dual rail supply an electrolytic cap won't work.

> Will the small amount of voltage i measured just buzz, as the input jacks do not ground themselves with no lead?

The DC voltage won't cause any audible noise it simple displaces the speaker cone in one direction by a small amount.

The ground of the jacks should connect to the 0V on the preamp somewhere in the scheme of things.

How the preamp ground/0V connects to the power amplifier can be a little obscure.  You might find 10ohm resistors between the grounds or something weird like that.   You need to dig around your ohm meter a bit more.  I doubt there is a problem with the jacks.

[sonic66]

Tried it a couple of more times sitching it on and off again quickly.

The firstime it jumps (though it appears not too fully return to rest) and makes a ploping sound when turned off

the seond and subsequent times turned on & off in quick sucession, it does not appear to jump.

so i that would fit with your explaination right.

You've been a great help, i won't be concerned now

Thanks for your replies