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Is R15 bridged by VCC?

6 years 6 months ago - 6 years 6 months ago #1143 by Ray
Is R15 bridged by VCC? was created by Ray
Hi Josh!

According to the schematic, the terminals around R15 both go to VCC, meaning it should be bridged. But in the board layout file, one end of R15 goes to VCC (via C9, which also goes to the LM386 power), and the other end goes to the rest of the powered components.

Is one of the VCC points in the power section of the schematic a mistake, and maybe should have been TP5 - which is otherwise missing from the schematic?

You are right, the schematic notation is a bit misleading; If you have a look at the schematic ( www.electrosmash.com/images/tech/1wamp/1...atic-bias-points.jpg ) the VCCs at each side of the R15 resistor should have a different name, i.e VCC1 at the left and VCC2 at the right:
  • VCC1: should power only the LM386
  • VCC2: should power the preamp stage (the rest of the circuit)
  • .
R15 creates a low pass filter together with C9 and C10, this part is crucial to remove noise and make the circuit stable, otherwise, noise from the LM386 will go into the preamp and all will oscillate (R15 will stop this LM386 noise to go into the first stages).

The 1Wamp layout is critical, have a look at the layout images you can find in the article and also read the notes about that.

All the best for your project!, upload some pics when you finish it ;)

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6 years 6 months ago #1146 by Ray
Replied by Ray on topic Is R15 bridged by VCC?

Ok so then in the schematic - R14+LED should be moved to the right side (VCC2)?

Yes, it should be at "VCC2" side, although it is not crucial in the design (it does not disturb the audio signal).

Pardon my ignorance - isn't the voltage going to VCC2 through R15 lowered because of R15? Does that not matter?

Yes, the voltage is lower, but just a bit lower because the preamp stage takes just a few milliamps, so the drop is slow but the benefit of having a great low pass filter overcomes that factor.

If you check the bias points of the circuit:
you will see that the voltage drops from approx 8.75 to 8.00V.

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