If I put my notes here, I might be able to find them again later!
LTSpice filter simulation masterclass: 0 to -100dB in five easy steps!
I've never put everything into a single LTSpice worksheet like this before: I find it fascinating. You can really pull apart a circuit to see what makes it tick, before solder ever hits the iron.
Power supply ripple, frequency response, gain, and crosstalk can be established. You can look at turn on and turn off transients, inrush currents, and conductance angle, and check peak currents in the filter capacitors. It's all there if you care to peek in and poke around.
I'm such a huge fan of LTSpice...
The only problem, really, is it is too perfect: all devices are perfectly matched, every part value is exact, and the temperature is always 25 C. Ground loops, wiring inductance, and thermal runaway do not exist. So no, of course there are no guarantees - but as a tool to get you 90% of the way there with the minimum of fuss and bother it is truly indispensable.
Actually I find the more experience you have the more useful LTSpice gets, because you know much better what to look for in the simulation. For the example shown, torture-testing the shunt regulator with a pulsed current load made it clear which configurations would remain stable and which ones would fly off the rails...
*****
Actual measurements confirm LTSpice is reliable for this kind of simulation.
*****
OK, so in the present case I have the whole circuit in front of me and I need to configure the power supply filtering such that the ripple at the output is minimal.
The phono stage itself has very poor ripple rejection. The PSRR is at bass frequencies (<100 Hz) is negative 20 dB. That means 1 mV of 60 Hz hum on the supply rail feeding the phono preamp circuit will end up as 10 mV on the line output.
The goal is to have <10 microvolts of ripple on the output, at any frequency. This means the power supply filtering must manage to provide an voltage rail with under 1 microvolts ripple/noise at bass frequencies. For an input ripple of about 1 V with fundamental 120 Hz that requires 120 dB ripple rejection at 120 Hz.
By way of comparison, an LM317 with the adjust terminal bypassed manages about 64 dB.
So how to achieve 120 dB? Here's some things that won't work:
1. A shoebox sized capacitor bank. The inrush current will blow your diodes or transformer, and even if they survive the charging currents will soon destroy the capacitors.
2. Passive filter CRCRCRC etc. As the capacitor impedance increases at lower frequencies, to be efficient at 120 hz this filter network would take 10 minutes to stabilize to a constant voltage.
3. Active single regulation stage with very high feedback. It can be done. Harder than it looks to keep stable though.
I finally opted for a shunt regulator with 75 dB RR, and CRC filter stages on the input and before each stage of the amplifier.
The input CRC gives an additional 20 dB, while the amplifier CRC stages another 40 dB. 20+40+75 = 135 dB, but the ripple from the first stage is about 5-6 V subtracting 15 dB from the total, giving the desired target of 120 dB.
The largest filter caps are right next to the amplifier, an inversion of the normal scheme of things. However, this circuit requires exceptionally high filtering and putting those large 1000 uF capacitors values higher up the chain towards the diodes would have resulted in unacceptably high peak charging currents.
Generally speaking ripple rejection is hardest to achieve at low frequency. So if the desired result is obtained at the ripple fundamental of 100 or 120 Hz, performance at higher harmonics will be as good or better still without having to tweak further.
Power supply ripple, frequency response, gain, and crosstalk can be established. You can look at turn on and turn off transients, inrush currents, and conductance angle, and check peak currents in the filter capacitors. It's all there if you care to peek in and poke around.
I'm such a huge fan of LTSpice...
The only problem, really, is it is too perfect: all devices are perfectly matched, every part value is exact, and the temperature is always 25 C. Ground loops, wiring inductance, and thermal runaway do not exist. So no, of course there are no guarantees - but as a tool to get you 90% of the way there with the minimum of fuss and bother it is truly indispensable.
Actually I find the more experience you have the more useful LTSpice gets, because you know much better what to look for in the simulation. For the example shown, torture-testing the shunt regulator with a pulsed current load made it clear which configurations would remain stable and which ones would fly off the rails...
*****
Actual measurements confirm LTSpice is reliable for this kind of simulation.
*****
OK, so in the present case I have the whole circuit in front of me and I need to configure the power supply filtering such that the ripple at the output is minimal.
The phono stage itself has very poor ripple rejection. The PSRR is at bass frequencies (<100 Hz) is negative 20 dB. That means 1 mV of 60 Hz hum on the supply rail feeding the phono preamp circuit will end up as 10 mV on the line output.
The goal is to have <10 microvolts of ripple on the output, at any frequency. This means the power supply filtering must manage to provide an voltage rail with under 1 microvolts ripple/noise at bass frequencies. For an input ripple of about 1 V with fundamental 120 Hz that requires 120 dB ripple rejection at 120 Hz.
By way of comparison, an LM317 with the adjust terminal bypassed manages about 64 dB.
So how to achieve 120 dB? Here's some things that won't work:
1. A shoebox sized capacitor bank. The inrush current will blow your diodes or transformer, and even if they survive the charging currents will soon destroy the capacitors.
2. Passive filter CRCRCRC etc. As the capacitor impedance increases at lower frequencies, to be efficient at 120 hz this filter network would take 10 minutes to stabilize to a constant voltage.
3. Active single regulation stage with very high feedback. It can be done. Harder than it looks to keep stable though.
I finally opted for a shunt regulator with 75 dB RR, and CRC filter stages on the input and before each stage of the amplifier.
The input CRC gives an additional 20 dB, while the amplifier CRC stages another 40 dB. 20+40+75 = 135 dB, but the ripple from the first stage is about 5-6 V subtracting 15 dB from the total, giving the desired target of 120 dB.
The largest filter caps are right next to the amplifier, an inversion of the normal scheme of things. However, this circuit requires exceptionally high filtering and putting those large 1000 uF capacitors values higher up the chain towards the diodes would have resulted in unacceptably high peak charging currents.
Generally speaking ripple rejection is hardest to achieve at low frequency. So if the desired result is obtained at the ripple fundamental of 100 or 120 Hz, performance at higher harmonics will be as good or better still without having to tweak further.
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