THAT 1246

[Dave Plowman (News)]

I've just replaced an SSM 2143 with a THAT 1246 in a home made power amp.
With it connected to a peripheral, it produces a much larger switch on
'thump' to the speaker than the 2143 did. Pin for pin replacement -
nothing else changed. The device connected to the input has a muting
circuit which un-mutes a second or so after power up, and the thump occurs
before that happens.

With no input, the 'thump' is about as before - quite reasonable. The
thump isn't anywhere near enough to damage the speaker - but I'm just
curious why it is worse than with the 2143. Otherwise everything just fine.

--
*I pretend to work. - they pretend to pay me.

Dave Plowman London SW
To e-mail, change noise into sound.

[Scott Dorsey]

Dave Plowman (News) > wrote:
>I've just replaced an SSM 2143 with a THAT 1246 in a home made power amp.
>With it connected to a peripheral, it produces a much larger switch on
>'thump' to the speaker than the 2143 did. Pin for pin replacement -
>nothing else changed. The device connected to the input has a muting
>circuit which un-mutes a second or so after power up, and the thump occurs
>before that happens.

That thump is DC offset somewhere.

Measure the outputs of the thing you have plugged into the amp and see
what kind of offset there is.

If there are coupling capacitors getting charged up, larger caps will
extend the thump and bring it down to a lower frequency.

>With no input, the 'thump' is about as before - quite reasonable. The
>thump isn't anywhere near enough to damage the speaker - but I'm just
>curious why it is worse than with the 2143. Otherwise everything just fine.

Could be the THAT chip has better response at very low frequencies. Could be
the thump is common mode DC and the common mode rejection of the THAT chip
isn't as good at low frequencies. Could be the THAT chip has lower input
impedance so the thump is at a higher frequency, could be it has higher
input impedance so the thump is at a lower one.

A scope will tell you for sure!
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Jay Ts[_4_]]

On Mon, 16 Jun 2014, Dave Plowman (News) wrote:
> I've just replaced an SSM 2143 with a THAT 1246 in a home made power
> amp. With it connected to a peripheral, it produces a much larger switch
> on 'thump' to the speaker than the 2143 did. Pin for pin replacement -
> nothing else changed. The device connected to the input has a muting
> circuit which un-mutes a second or so after power up, and the thump
> occurs before that happens.
>
> With no input, the 'thump' is about as before - quite reasonable. The
> thump isn't anywhere near enough to damage the speaker - but I'm just
> curious why it is worse than with the 2143. Otherwise everything just
> fine.

That's not much information to go on -- anyone would need to at least see
a schematic of the circuit to do better than a guess. (Just a disclaimer,
not a criticism.)

I looked at the datasheets for both parts, and noticed that the SSM2143
datasheet lists input offset voltage, but the datasheet for the THAT1246
does not.

When you have two competing products and one omits a spec, it can be
because it isn't very good, and nothing to be proud of. So there is a
possibility that the THAT1246's input offset voltage is not nearly as
good as that of the SSM2143.

If that is true, it could explain your thump. Input offset voltage is the
voltage required at the input to zero the output, and this needs to be
done in cases where a DC offset at the voltage is to be avoided. For
example, to avoid a start-up transient.

Some circuits have a trim for the input offset voltage, which varies from
part to part. If your circuit has that, you might be able to trim out
some or all of the thump.