ADA MicroTube Output Design

[PepeSilvia]

I'm debugging a MicroTube 100, and noticed something fishy (maybe just user un-friendly) about the output circuit, and was hoping to get a few more pairs of eyes on it to see if maybe it's just something I'm missing.

In the attached schematic portion (sorry for the crummy quality, it's all I've got), and confirmed with my DMM on the actual board, when there is no plug in B OUT and no plug in the BRIDGED OUT, then CH-B OUTPUT is shorted straight to ground through the sleeve of the BRIDGED OUT jack. Am I seeing that right, or is the sleeve on the BRIDGED OUT supposed to be isolated from the body of the amp? It already appears to me that there was some tom-foolery done on this amp (the BRIDGED OUT jack only has 3 pins rather than the 4 that the schematic shows), so is it probable that the person used a non-isolated jack when they should have used an isolated?

[teemuk]

Using isolated jacks should be a standard assumption. Yes, the jack should be one and the schematic tells this by showing that "sleeve" terminal of the bridge out jack has no connection to ground ("star end") by any form. The only connection is to CH-B Output via internal switch of the B Out jack.

BUT: The Bridged Out jack must be of correct type since it is vital to proper operation of the amp. See that "bridge sense" node going to the "ring" terminal of the Bridged Out jack? That signal feed must be conditioned properly. When no plug is inside the Bridged Out jack the internal switch of the jack shunts this signal node to ground, telling the sensing circuit that the bridge mode is not in use. When you insert a standard mono plug, the sleeve and ring will short together, but no worries, the internal switch opens and the "bridge sense" node starts to float in a DC potential, which then tells the following circuitry that bridge mode has been put to use.

It's a clever system and far from being user unfriendly in any way. But if you repair it with improper components you mess up the operation and the amp will begin to give you some serious trouble.

[PepeSilvia]

Awesome, thanks. That's what I assumed, I was just thrown off by what was currently in the amp.