The important one is the C//R2
This is a single pole low pass filter that the builder designs/chooses to suit what is inside his equipment.
The ceramic interacts with the interconnects and creates a 2pole filter, due to L in the cable and C at the input for a steeper roll off of the VHF and higher, that gets picked up by the cable and sinks it to the ground before it is transmitted to the wiring inside the box.
This ceramic is not a calculated filter due to the unknowns of the interconnect and all the impedances around the RCA termination, but it does help attenuate the VHF.
This is a single pole low pass filter that the builder designs/chooses to suit what is inside his equipment.
The ceramic interacts with the interconnects and creates a 2pole filter, due to L in the cable and C at the input for a steeper roll off of the VHF and higher, that gets picked up by the cable and sinks it to the ground before it is transmitted to the wiring inside the box.
This ceramic is not a calculated filter due to the unknowns of the interconnect and all the impedances around the RCA termination, but it does help attenuate the VHF.
OK. So I have now implemented the RF filters and the feedback capacitors. I also changed out the wiring to slightly larger gauge, and changed my volume pot from 10k to 50k.
Yesterday I had the chance to compare the amp side by side with a buddy's Rega amp. He has a whole Rega setup with CD player, amp, speakers. So I heard my amp with the Rega CD and speakers. First, I should say that my amp was not totally shamed by the much more expensive Rega. But, the chip amp had a bit more, less-defined, bass, and the highs were not as present or defined. So, I would say, my initial complaints about the amp still stand.
So... is this a limitation of the 3886 chip? Could my signal paths be too long? Is there a tweak that might help?
Again, any thoughts would be welcomed. If anyone wants to see pictures of my amp build, I could take some.
Yesterday I had the chance to compare the amp side by side with a buddy's Rega amp. He has a whole Rega setup with CD player, amp, speakers. So I heard my amp with the Rega CD and speakers. First, I should say that my amp was not totally shamed by the much more expensive Rega. But, the chip amp had a bit more, less-defined, bass, and the highs were not as present or defined. So, I would say, my initial complaints about the amp still stand.
So... is this a limitation of the 3886 chip? Could my signal paths be too long? Is there a tweak that might help?
Again, any thoughts would be welcomed. If anyone wants to see pictures of my amp build, I could take some.
I just looked at my invoice from Mouser. Definitely used pF caps that you suggested. 330 pF polypropylene across R2, and 47 pF ceramics across RCA jacks. All the other caps were the ones that came with the kit.
No, I haven't fitted any other filters to the amp. The DC offset is TINY now that I installed the NFB cap. When you say figure 3, you mean the snubberized power supply? How can you tell there is mixed AC and DC coupling? How would I go about addressing the mixed AC and DC?
I'm wondering about the signal path from the RCAs to the pot to the amps. I used a 10-inch deep chassis, and I have wires going back to front from jack to pot, and then halfway back across to the amps. So there is about 15 inches of travel for the signal there. Would that attenuate highs? I used 20g solid copper wire.
Thanks for your thoughts.
No, I haven't fitted any other filters to the amp. The DC offset is TINY now that I installed the NFB cap. When you say figure 3, you mean the snubberized power supply? How can you tell there is mixed AC and DC coupling? How would I go about addressing the mixed AC and DC?
I'm wondering about the signal path from the RCAs to the pot to the amps. I used a 10-inch deep chassis, and I have wires going back to front from jack to pot, and then halfway back across to the amps. So there is about 15 inches of travel for the signal there. Would that attenuate highs? I used 20g solid copper wire.
Thanks for your thoughts.
Even without knowing which amp model your friend has, it does not come as a surprise that a solidly engineered amp from a company like Rega works better than an incomplete copy of a datasheet application. There are examples of decent commercial amps which use the LM3886.
The IC has its limitation in the bass which is usually countered by using several of them. A decent power supply layout may also help.
The trouble in the highs may be a layout issue. The necessity for a 2 W resistor in the Zobel circuit points to oscillations.
Andrew-
Do you have a suggestion of how to add the solution for mixed AC and DC? I am thinking that a adding a high pass filter, DC blocking cap, and AC coupling are all talking about the same thing, which would be accomplished by adding a capacitor in series somewhere in the signal path before it hits the chip(?). Would one object be to attenuate very low bass?, and would this affect the clarity of sound for the better, or would we be missing something desirable? What kind of capacitor and what value would you use? Where in the diagram on page 3 would you put it?
Thanks.
Do you have a suggestion of how to add the solution for mixed AC and DC? I am thinking that a adding a high pass filter, DC blocking cap, and AC coupling are all talking about the same thing, which would be accomplished by adding a capacitor in series somewhere in the signal path before it hits the chip(?). Would one object be to attenuate very low bass?, and would this affect the clarity of sound for the better, or would we be missing something desirable? What kind of capacitor and what value would you use? Where in the diagram on page 3 would you put it?
Thanks.
Fit a capacitor between Rin and R1. It will attenuate very low bass. That can be good or bad. If you choose the value of that capacitor too small, audible bass will be attenutated and some information will be missing. If you choose it too big, amplifier and speaker may be forced to work in a range where they'd rather not. That can indeed affect the clarity of sound, especially at high levels. For loud listening the smaller cap can be preferable, while a bigger cap will improve the sound at civilised levels.
OK. So what are examples of small and large values for the cap? I know there are equations to calculate the frequency of the rolloff, but I'm not sure how to apply them. What frequency would I choose that would be a good compromise between high and low volume listening? My other question is whether putting in the new filter cap would improve definition and perceived volume at the high end of the spectrum. THAT is where my real issue with the sound is.
An RC time constant for the RF filter is usually set between 0.3us and 1.5us.
The low frequency RC time constant for a full range amplifier is usually set between 10ms and 100ms.
If you know the Rin of your amplifier (22k?) and you have chosen 70ms for the LF filter, then the capacitor is C = RC/Rin = 0.07s/22k = 0.000003F = 3uF
Using RC for filters saves a lot of arithmetic, it becomes doable in your head.
The low frequency RC time constant for a full range amplifier is usually set between 10ms and 100ms.
If you know the Rin of your amplifier (22k?) and you have chosen 70ms for the LF filter, then the capacitor is C = RC/Rin = 0.07s/22k = 0.000003F = 3uF
Using RC for filters saves a lot of arithmetic, it becomes doable in your head.
Depends on your speakers. AndrewT's suggestions are good for low level listening with any speaker and for high level listening with big woofers and powerful amplifiers. If your woofers are smaller, e.g. 6" you'd rather choose a higher roll-off. It is down to testing which you perceive as the best compromise. It also depends on the music you listen to. If your usual music has no significant LF content, the filter frequency may have little significance.
No, the high pass filter will not help the highs. You have two options, either look for help from chipamp.com or rebuild the amp with all components according to the datasheet on a different PCB with improved layout.
Rin is the complete resistance after the DC blocking cap. If you put the cap in front of the potentiometer, your Rin will vary with the volume setting. With the volume turned completely down Rin is 50k. With the volume completely up it is 50k||(1k+22k)=15k75. If you put the cap between the potentiometer and R1 as Andrew suggested, you will have a defined Rin of 23k.
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