The impedance of the battery varies according to the strength of the electrolyte and electrolyte level. If your battery has a low or dry (!) cell, your battery becomes a rather poor, leaky capacitor. There are various inductors in the charge circuit, and they create a pretty efficient LC filter. If your battery is dying, it changes capacitance and the characteristics of the "filter", so any ripple on the AC lines goes straight out on your 12V DC feed lines, switched or permanent.

One of the few remaining Mk1 owners... #00015

So exactly which power filter did you buy? Sounds like I might have to head down to my local MediaMarkt too, 'cause the other things have all drawn a blank on my unit.

Well, they have three different models, all from Freitag Elektronik. I think they were rated 5, 10 and 20 A, respectively. I was not sure how much the Empeg needed, so I bought the 10A version to be sure. It turned out the smaller model would have been sufficient too, as the Empeg has a 3A fuse.

The 10A version costs fl. 19,95 which is about 9 Euro. It appears the Euro sign was lost in my previous post (although it did show up in preview).

I have also seen a Pioneer 4A power filter at a local small Halfords store. I guess this one should be sufficient too. And it is also a lot smaller than the Freitag one.

And yes, only the head unit needed filtering. I assumed this, because - like I described in a previous post - the alternator whining was also gone when I connect the head unit to the AC adapter.

Slightly strange, though, as the empeg (mk2 I assume you have) has a filter - there's one built into the fuseholder!

Hugo

Is it similar to this device from Crutchfield?

No, the Crutchfield device is connected in between the alternator output lead.

The Freitag device is connected close to the device that needs to be filtered (i. the head unit). It has three wires; one is 12V in, one is 12V out and the third one is ground. The ground wire needs to be connected to the same ground as the device that needs filtering.

If there's any demand I'll dig the filter up and post a picture of it.

Slightly strange, though, as the empeg (mk2 I assume you have) has a filter - there's one built into the fuseholder!

Well, there's no way the filter in the fuseholder can do the same as the Freitag filter.
The Freitag filter has three wires (12V in, 12V out and ground) and the fuseholder only two.

(Yes I have a Mk2)

It can - the empeg choke is unshielded, and hence has no shield ground wire.

One of the few remaining Mk1 owners... #00015

It can - the empeg choke is unshielded, and hence has no shield ground wire.

One of the few remaining Mk1 owners...

The Mk2 fuse holder is external. It definitely does not touch ground anywhere.

Excellent information, as usual, Rob. That's going into the FAQ.

___________
Tony Fabris

It has three wires; one is 12V in, one is 12V out and the third one is ground.

More like the device on the bottom of this page.

Dragi "Bonzi" Raos
Zagreb, Croatia
Q#5196, MkII#80000376, 18GB green

On my Mk 1, the fuse holder and choke are combined in one unit. If you do not have hte same external choke unit, then I would agree - if the holder touched earth anywhere, it would make it pretty useless

Think we are talking at crossed purposes here.

One of the few remaining Mk1 owners... #00015

More like the device on the bottom of this page.

Right, this is how the filter looks. Oddly enough, that URL also has a link to Crutchfield's
How-to Tips that I used as a lead for making diagnostics.

If I follow that diagram, it leads to buying this Crutchfield filter, which, in my case, indeed was the solution.