Newbie here, i want to ask for clarification about what
impact the rc filter to ground in the feedback loop has regarding
the low frequency cutoff of the input filter.
Does it affect the cutoff frequency at all?
The datasheet for the lm1875 (and some other opamp's) mention
a higher cutoff frequency f=1/(2*pi*R*C) than the input filter.
But here
A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics
it's mentionend that the feedback loop cutoff frequency should be 3-5 times lower than the input rc frequency, and would cause distortion otherwise.
May someone enlight me? Thanks!
impact the rc filter to ground in the feedback loop has regarding
the low frequency cutoff of the input filter.
Does it affect the cutoff frequency at all?
The datasheet for the lm1875 (and some other opamp's) mention
a higher cutoff frequency f=1/(2*pi*R*C) than the input filter.
But here
A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics
it's mentionend that the feedback loop cutoff frequency should be 3-5 times lower than the input rc frequency, and would cause distortion otherwise.
May someone enlight me? Thanks!
In my experience you should try to keep the two RC time constants different from each other. If you keep them the same or close together, you get a double roll off and thus double phase shift at that frequency which can lead to low frequency oscillations, known as 'motorboating'.
Rule of thumb that many use is to chose one at least 5 or 10 times higher than the other. Details do depend on the actual circuit.
Jan
Rule of thumb that many use is to chose one at least 5 or 10 times higher than the other. Details do depend on the actual circuit.
Jan
Thanks for you answers.
So if i want to build an lm1875 amp with flat frequency response down to 30hz, i now have two cap choices for the feedback loop filter(1k) with an input filter of 2.2uF and 27k -> 2.7Hz:
-mostly same cutoff with 56uF -> 2.8Hz
-about 5x higher cutoff with 10uF -> 15.9Hz
I don't want to weaken bass response, do these values matter in that respect?
Some circuits just use 220uF for the feedback loop and don't seem to care.
Would an mkp cap be benefitical in the feedback loop? I have some laying around up to 68uF.
Johannes
So if i want to build an lm1875 amp with flat frequency response down to 30hz, i now have two cap choices for the feedback loop filter(1k) with an input filter of 2.2uF and 27k -> 2.7Hz:
-mostly same cutoff with 56uF -> 2.8Hz
-about 5x higher cutoff with 10uF -> 15.9Hz
I don't want to weaken bass response, do these values matter in that respect?
Some circuits just use 220uF for the feedback loop and don't seem to care.
Would an mkp cap be benefitical in the feedback loop? I have some laying around up to 68uF.
Johannes
Don't bother with mkp as feedback cap.
For more info, see this post by AndrewT: https://www.diyaudio.com/forums/parts/268610-220uf-cap-gnfb-2.html#post4199826
For more info, see this post by AndrewT: https://www.diyaudio.com/forums/parts/268610-220uf-cap-gnfb-2.html#post4199826
Thanks, very helpful. So i'll go with 1k/470uF filter with the feedback loop.
@Tom: Offtopic, on Taming the LM3886 Chip Amplifier: Power Supply Decoupling – Neurochrome you use a 4.7uF X7R ceramic cap, which is hard to get in that value where i life. Which alternative cap-type do you recommend(tantal,film,elco,..)?
There AndrewT states
The widest roll off would be no cap at all at the feedback loop.
The lm3875 datasheet does consider the cap(there Ci) optional.
The gaincard circuit as statet here RJM Audio - LM3875 Non-inverting Gainclone also omits that cap.
The lm1875 datasheet does not, but sets the feedback roll off higher than the inputfilter, so maybe only therfor needs the cap?
From my lm1875 testcircuit i now removed the cap(C2 in the datasheet), seems to work and sounds great, also rich bass is present.
Is there any drawback be removing this cap or some functionality/stability missing?
Removing the cap in the feedback-ground leg makes the DC gain the same as the AC gain. That means that any DC offset of the chip gets amplified too, but you don't want DC at the output. That is the reason for that cap.
Jan
Jan
1.0 uF, X5R/X7R/X8R would all be good. The 'S' temperature coefficient would work also, so X5S, X7S, X8S.
The capacitor should be rated for 50 V minimum. Don't go lower even if your supply voltage is lower. The X?R dielectric has a pretty steep voltage coefficient. At the rated voltage it's common for the capacitor to be down to 20% of its marked value.
You can read about these codes here:
Difference between X7R, X5R, X8R, Z5U,Y5V, X7S, C0G Capacitor dielectrics - Embedded World
If good ceramic caps aren't available in your area, I'd go for a Wima MKP or similar film cap. 0.1-1.0 uF, 50 V would be fine.
Tom