Great thread, I have been reading along and find it very educational. I have one question that I can't seem to find the answer to. What function does D2 (from the last schematic) have in parallel with the feedback capacitor?
It prevents reverse voltage across the electrolytic capacitor.What function does D2 (from the last schematic) have in parallel with the feedback capacitor?
That's the wattage I got too and it lines up with SPICE simulation. However, it's a bit too close for comfort, so I whacked in some BD139/140 instead. Those babies should handle 1W without a heat sink, I believe.I calculated something like 488 mW, the voltage drop across the VBE multiplier helps a bit. That is below the rating up to 52.4 degrees Celsius, but taking no margin for signal-related temperature fluctuations. I also didn't take into account that the current drops somewhat at high ambient temperatures, and I guessed the base-emitter voltage.
It's not nearly as bad as what I have seen in a commercial amplifier design from Philips, but it would certainly be good for reliability to reduce the current a bit or improve cooling.
This makes me happy! As you have noticed, this is a learning experience for me too. Hopefully there's a few of us who can learn together.Great thread, I have been reading along and find it very educational
There is another issue. Potentially the LTP BE reverse voltage could exceed ~7V resulting is a DC latch-up. A well-placed diode guarantees the DC feedback does not exceed the LTP BE breakdown.It prevents reverse voltage across the electrolytic capacitor.
Why not protect the input differential pair with antiparallel diodes between the bases? That also protects against excessive input signals (preventing base-emitter avalanching that could reduce current gain and increase 1/f current noise).
By the way, a unipolar aluminium electrolytic capacitor also works as a diode, it starts conducting when the reverse voltage is more than a volt or two
By the way, a unipolar aluminium electrolytic capacitor also works as a diode, it starts conducting when the reverse voltage is more than a volt or two
We're getting close. Hopefully I can finish up the PCB this weekend so we can get a first impression of what the first prototype sounds like.
(I guess I should change the name to Crimson Excalibur 3000 or something if I'm ever to encourage people to actually build this... 🙂 )
I like the name Bog Standard👍
Just make sure to mention its rich, tight, & never dry bass, warm and clear mids and crisp, silky but never tiring highs.
Just make sure to mention its rich, tight, & never dry bass, warm and clear mids and crisp, silky but never tiring highs.
No “multidimensional soundstage”?Just make sure to mention its rich, tight, & never dry bass, warm and clear mids and crisp, silky but never tiring highs.
People seem to like acronyms for DIY amps - just make BOG stand for something. "Built, Ordinary, Good". Something like that.(I guess I should change the name to Crimson Excalibur 3000 or something if I'm ever to encourage people to actually build this... 🙂 )
Yup. That worked even better, especially when I fed way too much amplitude on the input. It adds a tiny amount of THD in the simulation but that will likely drown in the other distortion when this meets the real world. I’m assuming it’s due to tiny leakage across the diodes.Why not protect the input differential pair with antiparallel diodes between the bases? That also protects against excessive input signals (preventing base-emitter avalanching that could reduce current gain and increase 1/f current noise).
By the way, a unipolar aluminium electrolytic capacitor also works as a diode, it starts conducting when the reverse voltage is more than a volt or two
Time for some ground routing. Yes, the board is a bit crammed. I'd like to be able to fit it in a compact chassis. Anyway, back to the grounding. It's a best effort star ground. Understanding that power ground is a high power signal carrier, I've tried to avoid having any low-power tracks in parallel. The only thing I don't like are the long traces to the filter caps, but I can't think of a better way. The whole idea of local filter caps on the amplifier board is to have a nearby energy reserve, so I want them as close to the output devices as possible. Any comments? Just because it's Bog Standard doesn't mean we shouldn't pay attention to the PCB design.
Also give some thought as to how two channels would be connected together. You don’t want ground return currents in the reservoir caps going through any place where a minuscule voltage drop can show up as an INPUT signal. On a single board with a single channel that may not be too hard, but when putting a second board in unexpected things can happen.
So you're suggesting I lose the local caps? I have built two amplifiers with caps like that and they work beautifully. But you seem to know what you're talking about and it would definitely make the design simpler without that ground bar in the middle.
The local capacitors are only for RF decoupling. They form a capacitive current divider with the main filter capacitors. You don't want 120Hz ripple currents in the local capacitors.
Ed
Ed
Hmmm... I'm pretty sure I've seen lots of amplifier boards with local reservoir capacitors. I know... It doesn't mean that it's a good idea.
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