Idiot-proofing circuits

[BuckarooBanzai]

Hello everyone,

I'm continuing to learn tons here on the forums. Mistakes (and smoke ) have been made, components destroyed, but as yet it has all been in the name of progress. A few bucks worth of parts and a bad smell are small prices to pay for the fun I've had.

At the moment I'm working on a very simple preamplifier and a phono stage. Assuming nothing melts on powerup, when the time comes to actually connect them to other equipment, what should I double check beforehand?

I check voltages as I go along, and I know how to look for DC offset. What else should I scrutinize before wiring up to other expensive equipment ?

 

[AndrewT]

Don't test with speaker attached.
Don't swap leads with power switched ON.
Power on EVERY new project and EVERY modified project with a Mains Bulb Tester, no matter how simple the "modification" seems and how many times you checked it.

 

[Ken Newton]

If you don't already have one, obtain a device known as a Variac. This is a benchtop variable power mains transformer. The Variac plugs into a power outlet while the component under test plugs into the Variac. The Variac's control knob can be used to slowly increase the mains voltage applied to the component under test so that an otherwise catastrophic fault can be detected before it produces an actual catastrophic failure under full circuit voltages.

Depending on what test eqipmemt you have access to, also test that the output does not produce excessive noise or other strong A.C. output with the input grounded, in adddition to checking D.C. offset.

High frequency oscillation will require a high bandwidth oscilloscope to positively detect, however, such is often indicated by gain stages that seem excessively warm to the touch.

Lastly, if possible, listen to the unit via connection to very inexpensive equipment before connecting it to your main listening system.

 

[DF96]

Unless you have a very good reason, always use input and output coupling caps with ground leak resistors. An LF response down to DC is unnecessary when everything works, and a liability when things don't work.

 

[doors666]

I use an ultra cheap woofer (without any box) to test any modifications before connecting expensive speakers.

 

[mt490]

As far as idiot proofing is concerned, my regulator of choice for line level apps always has a dropper that inherently current limits the possible short circuited supply. The smoking enamel of the dropper is a sure sign that something went wrong, and because I have one of these regulators for each board it usually prevents significant damage.

A pair of glasses is recommended on first power up. Small exploding electrolytics will fling their hot aluminium shells off like a little fireworks shell, and without a pressure relief their failure is not necessarily forewarning.

I always do a quick check each stage for oscillation using an oscilloscope and a signal check before connecting it through to other equipment.

 

[BuckarooBanzai]

Thank you very much for the advice everyone !

Power on EVERY new project and EVERY modified project with a Mains Bulb Tester

I've just finished making one of these, it's a great idea.

obtain a device known as a Variac. This is a benchtop variable power mains transformer.

These are a little out of my budget, but I'm going to keep an eye out for a second-hand unit.

Lastly, if possible, listen to the unit via connection to very inexpensive equipment before connecting it to your main listening system.

I use an ultra cheap woofer (without any box)

I have some junky earphones and a crusty ol' amp and speaker for just this purpose

my regulator of choice for line level apps always has a dropper that inherently current limits the possible short circuited supply.

From what I've read, I assume the dropper to which you refer is simply a resistor in series? If so, what would a good value be?

A pair of glasses is recommended on first power up.

I learned this lesson the hard way in chemistry lectures

I always do a quick check each stage for oscillation using an oscilloscope

Also out of budget at present, but it's on my Christmas list...