What could I do to get a better input protection for a QA403? Think about transients or touching a high voltage lead in a tube amplifier.
Gas Discharge tubes have a low capacitance, but the sparking voltage is too high (70 V) and TVS diodes may have a lot of capacitance.
Has anyone seen a tested and proven solution for this?
Hi ! You can have a look at low leakage diodes (see Linear Systems’) or ultra-low-capacitance TVS like those used for USB high speed transmission. Note that you will also need to limit current, so you’ll certainly have to use a resistor or a fast fuse as well. You can also use an isolation audio transformer but performance will be worsen at low frequencies and you may not get the full audio bandwidth if you want a small circuit !
or a fast fuse as well.
Yes, first I’ll note the expertise required to be poking around in circuits with more than 60V present is considerable. But if one feels they have that expertise…
There are a few different types of overvoltage events to consider. ESD (single digit nanosecond), Burst (10’s of nanoseconds), Surge (microseconds) and then effectively “steady state” (milliseconds).
Touching the QA403 input to a B+ of, say, 300V, would be considered more of a steady state. B+ supplies usually have a pretty stiff cap, and the energy involved is considerable.
If the attenuator is off and the QA403 input connected to 300V, we’d expect roughly 600mA to flow (300/470 = 638mA, where 470 is the input limiting R). That would probably be effectively shunted momentarily by the steering diodes (100’s of microseconds). That 600mA would mean 170W power in the in the input 470 ohm, which means it would probably fail open in 100 uS. You may or may not damage the input caps on the QA403 given the short duration of the fault.
If the attenuator is on, the fault power will be P=300^2/100k=900mW, which will probably result in the input resistor failing in a second or two. I’d guess the input caps would be damaged by that.
There are 1206 fuses that are rated for 125VAC/63VDC, with rated currents as low as 50mA. These will open when they see 500mA in anywhere from 100uS to 1mS. But still the input cap will probably suffer.
TI has some parts called “flat clamp TVS” devices that are very interesting. These have a working voltage of 33V, they are bidirectional, and extremely low capacitance and leakage (unlike conventional TVS) and they won’t hurt THD (unlike a conventional TVS). So, a series fuse rated for 50mA combined with one of these TVS devices can offer a lot of protection for catastrophic events.
Input Z of the QA403 is 470 ohms??? Where did this figure come from?
I was under the impression it was more like 200k.