Audio Amp output?

[Nigel Goodwin]

Well I did some more messing around. I set my scope to DC coupling and turned the volume all the way down on the cd player. There is a 24VDC offset with my probe connected to either of the speaker terminals.
whats up with that?

That's because it's a bridged amplifier, and only uses a single supply rail, the DC value across the two terminals (where the speaker goes) will be zero.

maybe this is why the wavefrom is funny looking?

Don't worry about it!.

I'm gonna try to isolate my scope from the amp by using a small transformer.

NO!! - no need, it's working as it should!.

Also, the amp is only 1 channel, so there really isnt anything to 'bridge'.

It's a bridged amplifer, but you can't use the two halves seperately, as it's not designed to be used that way - hence the DC voltage on the outputs.

 

[audioguru]

maybe this is why the wavefrom is funny looking?

Don't worry about it!.

I would worry about it. I bet some of its P-channel output Mosfets are blown.

 

[Ankit8j]

Plz send me a diagram of 8'' woofer cabinat. 8)

 

[Nigel Goodwin]

I would worry about it. I bet some of its P-channel output Mosfets are blown.

Perhaps?.

But as both outputs have 24V on them it sounds unlikely, a P-channel MOSFET short circuit would cause the DC offset to shift to the negative rail - which in this case is prbably 0V?. If one went open circuit, it most probably wouldn't have any effect in to a 4 ohm load (only a quarter of the devices are required to handle that load).

As the DC output is 24V, this means a supply of 0-48V, the bridged configuration giving an effective +48V/0V/-48V - a potential 96V output swing, but obviously limited by the MOSFET outputs and the drop under load.

 

[audioguru]

Hi Nigel,
Since this is a class-D switching amp, I believe that the P-channel output Mosfets are common source and pull-up. They seem to be doing a lousy job compared to the excellent pull-down, so I think that a few of them are blown open.
I don't think that the remaining working P-channel Mosfets can survive attempting to drive a 1 ohm load.
The protection circuitry must be very dumb, ignoring dynamics and concentrating only on idle DC.

Someone is trying to hyjack this thread for his "8 inch woofer". He can just stick it in any old cabinet and it will make some noise!
Never mind parameters and matching the woofer to its enclosure.
Never mind discussing requirements and the trade-off between having deep extended bass but risking over-excursion or having very powerful upper frequencies but the lows cutoff at a higher frequency.
Let him port his cabinet without proper design and tuning and let it be a boom-box!

 

[Juglenaut]

It is clipping on the negative swing that all I see, positive swing looks clean enough for bass.

Resistors are dumy and stationary loads, coils move and change impedances according to current and frequency.

Most amps made in the US are 1 ohm stable, but that doen't mean that a given manufacturer will warrenty that type of load, companies that do ussually state it in the manual or somewhere.

Why would you want a class D for audio anyway it doen't really fit ther purpose to well.

 

[audioguru]

Hi Juglenaut,
Class-D amps are getting better and better. Most cell phones use them to ring the music loudly without killing their battery and burning their user.
Their efficiency is around 90% or more compared to hot 60% and less for ordinary old analog amps.
They have very low distortion for hi-fi bass and soon for all audio frequencies.
TI has a class-D amp IC that puts out 20W from a tiny surface-mount package without a heatsink!