The final piece of my build is the DC protection. As mentioned above, I am reusing some extra boards I designed for another project - they are the two green boards in the photo above. The schematic was lifted from an early Cambridge Audio amplifier, nothing fancy, but it works really well:
C1 C2 R2 R3 is the input LPF, Q1 Q2 is the DC detector, Q3 discharges C3 when DC is detected. The voltage at C3 is monitored by the Schmitt trigger Q4 Q5. R8 R9 set the switching threshold and provide positive feedback for hysteresis and faster switching. The relay I used is Amplimo LRZ.
Although Mauro did not use an output filter, my protection board includes one, with an air core toroidal inductor. I wound one with parasitic turn compensation, similar to that used in the Omicron, but with 16AWG wire.
C1 C2 R2 R3 is the input LPF, Q1 Q2 is the DC detector, Q3 discharges C3 when DC is detected. The voltage at C3 is monitored by the Schmitt trigger Q4 Q5. R8 R9 set the switching threshold and provide positive feedback for hysteresis and faster switching. The relay I used is Amplimo LRZ.
Although Mauro did not use an output filter, my protection board includes one, with an air core toroidal inductor. I wound one with parasitic turn compensation, similar to that used in the Omicron, but with 16AWG wire.
If there is enough interest, we can set up a group buy I guess.
There have been questions about part list aka BOM, so here it is:
Some comments:
Hope this helps.
| Names | Device | Value | Qty | Package |
|---|---|---|---|---|
| C4 | NP0/C0G ceramic capacitor | 10pF | 1 | Radial LS=5mm |
| C3 | NP0/C0G ceramic capacitor | 22pF | 1 | Radial LS=5mm |
| C6 | NP0/C0G ceramic capacitor | 150pF | 1 | Radial LS=5mm |
| C2 | NP0/C0G ceramic capacitor | 220pF | 1 | Radial LS=5mm |
| C5 | NP0/C0G ceramic capacitor | 1nF | 1 | Radial LS=5mm |
| C7, C8, C19, C20 | X7R ceramic capacitor | 2.2uF | 4 | Radial LS=5mm |
| C1 | Electrolytic non-polar capacitor | 220uF | 1 | Radial D=10mm LS=5mm |
| C21, C22 | Electrolytic capacitor | 47uF 25V | 2 | Radial D=5mm LS=2mm |
| C23 | Electrolytic capacitor | 100uF 6.3V | 1 | Radial D=5mm LS=2mm |
| C17, C18 | Electrolytic capacitor | 220uF 35V | 2 | Radial D=8mm LS=3.5mm |
| C9, C10, C11, C12, C13, C14, C15, C16 | Electrolytic low ESR capacitor | 1000uF 35V | 8 | Radial D=12.5mm LS=5mm |
| U1 | LM318 | LM318 | 1 | DIP8 |
| U2 | LM3886 | LM3886 | 1 | TF11B |
| REG+ | LM7812 or LM7815 | 12V or 15V | 1 | TO220 |
| REG- | LM7912 or LM7915 | 12V or 15V | 1 | TO220 |
| R14 | Resistor 5W | 0R47 | 1 | Radial |
| R15, R16 | Resistor 1W | 430R | 2 | Axial D=4mm L=10mm |
| R4 | Resistor 0.25W | 1R | 1 | Axial D=2.5mm L=7mm |
| R17, R18 | Resistor 0.25W | 10R | 2 | Axial D=2.5mm L=7mm |
| R12 | Resistor 0.25W | 47R | 1 | Axial D=2.5mm L=7mm |
| R2 | Resistor 0.25W | 390R | 1 | Axial D=2.5mm L=7mm |
| R3, R9 | Resistor 0.25W | 3k3 | 2 | Axial D=2.5mm L=7mm |
| R13 | Resistor 0.25W | 12k | 1 | Axial D=2.5mm L=7mm |
| R19 | Resistor 0.25W | 15k | 1 | Axial D=2.5mm L=7mm |
| R6 | Resistor 0.25W | 22k | 1 | Axial D=2.5mm L=7mm |
| R10, R11 | Resistor 0.25W | 22k 0.1% | 2 | Axial D=2.5mm L=7mm |
| R7, R8 | Resistor 0.25W | 47k 0.1% | 2 | Axial D=2.5mm L=7mm |
| R1 | Resistor 0.25W | 100k | 1 | Axial D=2.5mm L=7mm |
| R5 | Resistor 0.25W | 470k | 1 | Axial D=2.5mm L=7mm |
Some comments:
- I only tested the LM318 with Rev.C compensation. You can try Rev.A or Rev.B. Rev.A allows other opamps to be used. I believe the Fremen Edition thread has additional information on rolling opamps. You'd need to fit an extra compensation capacitor for that. Use a ceramic NP0/C0G capacitor.
- Voltage regulators are three-pin fixed regulators in TO-220. You may be able to get away with 78L/79L variety packaged in TO-92, but I did not test that.
- C1 is the large cap in the feedback network that drops the DC gain to unity. I used Nichicon ES 220uF 16V. Feel free to experiment.
- C2-C6 are NP0 (C0G) ceramic capacitors, radial package, 5mm/0.2inch lead spacing. MLCC of disc capacitors can be used. Use what's available or the cheapest you can get, just make sure they're NP0 rated for at least 50V.
- C7, C8, C19, C20 are X7R ceramic capacitors, radial package, 5mm/0.2inch lead spacing. Make sure they're rated for 100V. They are not going to see that voltage, but X7R capacitance drops when a DC voltage is applied, so you want these to be rated at 2.5-3 times your actual rail voltage.
- C9-C16 are low ESR electrolytic capacitors. I used Panasonic FC. You can also use general purpose caps (I also tested with Samwha RD) and/or higher voltage/lower capacitance (e.g. 470uF 50V) with slightly reduced performance. Taller (longer) capacitors are better.
- C21, C22 are the decoupling caps for the opamp. Use audio or low ESR type. I used Panasonic FC. 22uF can be substituted.
- C23 is the muting capacitor. It doesn't see much voltage at all. Lower values and/or higher voltage can be substituted.
- Do not populate C24 if using the LM318 with Rev.C compensation. Also, see the first bullet point above.
- Most resistors are axial metal film 1% 0.25W. Four resistors (R7 R8 R10 R11) are 0.1%.
- R14 is a radial 5W or 7W 0.47 ohm resistor. I used Fukushima MPC, but these are difficult to find.
- R15 and R16 are 1W 430 ohm or 510 ohm resistors. Metal oxide or indeed anyhting that fits is fine.
- +/-28V supply voltage is recommended. If using +/-35V, you'd probably want C9-C18 rated for 50V. See bullet point 6 above.
Hope this helps.
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