[Radio Wrangler]

Take a look at the circuitry around all switches. What sets the DC voltage on the circitry for each position?.... even when that position isn't selected? Because if the path is only AC coupled and relies on being switched in circuit to have a resistive path to ground, then when switched out the DC voltage can drift through stray leakage, capacitors can charge, a little (compared to the HT voltage, but an awful lot compared to the signal) and so when you next throw the switch you get a loud 'crack'. A while back, someone used the phrase 'tone stack' which is mostly used in guitar amp circles. This is the sort of thing where the amplifier tears the cones out of speakers.

So look particularly at all amplifying device inputs. What maintains precisely the same DC bias on it when switches are thrown? Even if all positions of the switch are OK, then what about when the switch is transitioning from one contact to the next? It's easy to have a valve grid or transistor base floating for a moment, and you can get a pop.

Often you have to add high value resistors around the place to bleed off any leaked charge, or to keep networks out of use charged to the same potential they run at when switched in.

Sometimes you have to add extra AC coupling and leak resistors just to keep things tethered close to ground.

Similarly, try not to let any DC run down potentiometer tracks. This is the most popular way to make them go noisy. Do not use the sliders of pots to provide ALL the DC setting of a grid. Add a fixed resistor grid leak. Pot sliders skip out of contact as they are scraped along the track, and floating grids quickly drift at DC, and this too makes for noisy pots.

These are things not taught on electronics courses. These are things picked up by designers who have been around the block a few times and have learned from their mistakes. These are things missing from a fair few hobbyist and magazine designs, and plenty of professional designs. But doing this is 'good housekeeping' and makes for reliable equipment which just works smoothly. Get this wrong, and though your amp might sound great when working normally, but if anyone switches the input or a tone bypass etc, the front row of the audience is blown back out through the foyer.

Even with a simple home hifi setup, you don't want even small clicks and pops, or scratchy pots.

Good housekeeping, attention to detail, call it what you will, it makes a big difference to the end user.

In ye olden days (tm) valves and transistors were expensive. So hifi designs did everything possible to economise on them. Where multiple inputs needed a mixture of different gains and even RIAA shaping, the usual ploy was to have the first device in the preamp have different feedback networks switched around them. These systems were a right devil to try to make pop-free and many designers didn't even know to try. Eventually as circuitry became cheaper, separate input stages were used for the different types of input, and the outputs of these stages were switched. Feedback paths were left carefully alone.

Switched feedback could be done fairly well, but it took a lot of care. Have a look at the arrangements in the Quad 33 preamp.

David