Finally made a LM1875 amp

[jemraid]

Good for you Steve!

You won't be disappointed :-)

Jim

[formula 22]

Hello all!
Is there a difference between the feedback in this amplifier made ​​with resistors: [(200k/10k) +1, and [(20k/1k) +1], if there is in what?
If measurements can?
thank you

[pacificblue]

The higher values will lead to more Johnson noise. Whether that becomes audible depends on the rest of your system, especially on the sensitivity of your speakers.

You will notice a difference in DC offset.

Depending on the parasitic capacitance of the PCB, higher values resistors can lead to different amp behaviour at high frequencies and can make it necessary to use a compensation capacitor, when you wouldn't need one with lower resistor values for the same gain.

[danielwritesbac]

You wouldn't want the feedback resistor higher than 115k. I doubt if you'd actually want it that high, but 200k is not especially valid nor desirable for LM1875 non-inverting amplifier.

[AndrewT]

High values of NFB resistors allow lower values of DC blocking capacitors.
This can be a very good compromise if you want to adopt AC coupled amplifier with wide audio bandwidth and do not want to use any polarised nor electrolytic capacitors.

If the Power amp gain is relatively low then 200k for both Rin and NFB upper leg resistor will not create much additional noise.
If you use Cherry's suggested NFB, the Rin resistor can be 3 times the NFB upper leg resistor and still maintain DC balance of LTP currents/voltages.

[tnmallen]

From BrianGT in 11th post -"Wire one of the GND connections on the PSU board to the chassis ground."
Would any of the 4 holes marked pg+ or pg- be the ones? And share it with a wire that goes to the amp board? There is no chassis yet. Where would the chassis gound be made?

[danielwritesbac]

Originally Posted by AndrewT

High values of NFB resistors allow lower values of DC blocking capacitors.
This can be a very good compromise if you want to adopt AC coupled amplifier with wide audio bandwidth and do not want to use any polarised nor electrolytic capacitors.

If the Power amp gain is relatively low then 200k for both Rin and NFB upper leg resistor will not create much additional noise.
If you use Cherry's suggested NFB, the Rin resistor can be 3 times the NFB upper leg resistor and still maintain DC balance of LTP currents/voltages.

I can't get that to work right. The soundfield collapses down to a tiny size and the output is somewhat raspy/analytical. This sound happens when its run out of speed. The highest figures I can get you is 115k with 3.9k divider for gain. That works okay. You could go higher if lower gain, but a computer source or mp3 player might start internal clipping if amp gain is too low.

But the good news is that this 3.9k is within range to use a 100uF cap for NFB cap, and the great news is that it is very easy to find excellent quality 100uF (and smaller) caps. This can be paralleled with a really tiny value polyester dip cap for excellent results since the frequency response will stay level. At this point, NFB cap selection gets extremely easy.

Do you mind posting a sketch involving the Rin resistor value relating to your comment about Dr. Cherry's NFB tapping? If I remember correctly, the speaker zobel resistor has to be a 3w or better model so as to avoid accidentally releasing the feedback loop upon resistor failure. Probably a less expensive resistor could be used if speaker zobel cap is 22nF. Its still enough to block some RF, but the smaller value cap removes some risk, and nobody would ever have an excuse to remove the zobel for audio purposes since that is so very far above the audio band.

[FromageTheDog]

I've sent BrianGT this question via email along with some other kit-related questions, but this should be super-simple to answer here. For the kits purchased from chipamp.com -- are the LM1875T chips meant to be included in the kit, or not? The kit I received included everything but these chips and I was trying to figure out if the omission was intentional or not.

[FromageTheDog]

Originally Posted by FromageTheDog

I've sent BrianGT this question via email along with some other kit-related questions, but this should be super-simple to answer here. For the kits purchased from chipamp.com -- are the LM1875T chips meant to be included in the kit, or not? The kit I received included everything but these chips and I was trying to figure out if the omission was intentional or not.

Holy smokes, talk about customer service. Posted here just wanting a quick answer before I sent in my DigiKey order, but seems even that was unwarranted; just got tracking info via email informing me that the missing bits are on their way! Thanks Brian! Can't wait to start building this thing.

[JohnnyP00]

Would it be possible to replace the 22k resistors on BrianGT's amp boards with 1/4 watt instead of 1/2 watt. Having a hard time sourcing 1/2 watts that small in Canada. Thanks for your help.. JohnnyP