Thanks, for some reason missed those.
Much better performance compared to the LM3886DR. If I compare at 5 kHz (ignoring 10/20 kHz due to the "missing harmonics") there's a significant lead at 0.0045% for this PCB vs 0.014% for the LM3886DR (eyeballing), albeit the LM3886 is at 45W and yours is at 40W. Any idea why that might be?
Ok, I think it might all go down to bandwidth limiting. If you go to the specs page of the lm3886DR, there is a graph with THD+N vs frequency at 100hz, 1khz, 6.67khz. You'll see that at 40W, THD+N for 6.67khz is at 0.007%, in line with the datasheet. But it's a 20khz bandwidth measurement, while the graph above is done with a 60khz bandwidth.
A limitation of using ARTA and soundcards is that you can't really define the bandwidth of your measurements, it depends on the soundcard ADC input filter.
So my take away is that, due to the use of proper instrumentation tools, you know for sure that the LM3886DR matches the datasheet specs. Otoh, due to the limitations of my test setup, we cannot be as certain wrt my board. We know there isn't some crass problem and I'm pretty confident we're probably in the same ballpark. But I wouldn't want conclusions to be drawn by comparing apples and oranges under 0.01% thd+n.
PS: Another possibility (in conjunction with the first) is that I possibly left a weighting filter checked in ARTA options.
PS2: I really should rebuild and remeasure a module anyway.
A limitation of using ARTA and soundcards is that you can't really define the bandwidth of your measurements, it depends on the soundcard ADC input filter.
So my take away is that, due to the use of proper instrumentation tools, you know for sure that the LM3886DR matches the datasheet specs. Otoh, due to the limitations of my test setup, we cannot be as certain wrt my board. We know there isn't some crass problem and I'm pretty confident we're probably in the same ballpark. But I wouldn't want conclusions to be drawn by comparing apples and oranges under 0.01% thd+n.
PS: Another possibility (in conjunction with the first) is that I possibly left a weighting filter checked in ARTA options.
PS2: I really should rebuild and remeasure a module anyway.
Please do, and it would be great if you include the following in your measurements:
- noise floor when using smps in series and/or psrr vs frequency if you have access to a network analyzer
- common mode rejection of this amp wrt signal ground
- SINAD figure, ie, measured signal to total noise and distortion level in db at 5W into a 4 ohm load
- also some measurements in bridge mode would be nice
It would be interesting to see measurements with different grounding schemes, that is with and without hum resistors...
Reporting back after I powered the modules (one at a time) with a linear PSU:
The hiss is still there. Has the situation improved over SMPS? Yes, it did, the hiss level with a linear PSU is lower than with SMPS. Is it still annoying? Yes, it is.. at least on a 91dB speaker I tested. The hiss is always there no matter the scenario (input shorted, signal on input etc.). I wonder If I am the only one who hit into this... As i said... 6 modules, same issue (2 modules with different PN's and different IC batch)...Hmm
The hiss is still there. Has the situation improved over SMPS? Yes, it did, the hiss level with a linear PSU is lower than with SMPS. Is it still annoying? Yes, it is.. at least on a 91dB speaker I tested. The hiss is always there no matter the scenario (input shorted, signal on input etc.). I wonder If I am the only one who hit into this... As i said... 6 modules, same issue (2 modules with different PN's and different IC batch)...Hmm
G'day Mate,
I had no such issues with these boards, they worked flawlessly for me right from the get go.
I would hypothesise that it will be a grounding issue or an external noise issue.
Is the amp inside a metal box? or simply an open testing rig?
If not then you might find that putting them inside a (preferably) mild steel box gets rid of the noise.
The more likley cause is a grounding problem.
Are you able to draw us a block diagram of your wiring scheme. MS paint is fine.
Please include all grounds, signals and power supply connections.
Here is an example from my most recent amp which in fact used these boards.
I had no such issues with these boards, they worked flawlessly for me right from the get go.
I would hypothesise that it will be a grounding issue or an external noise issue.
Is the amp inside a metal box? or simply an open testing rig?
If not then you might find that putting them inside a (preferably) mild steel box gets rid of the noise.
The more likley cause is a grounding problem.
Are you able to draw us a block diagram of your wiring scheme. MS paint is fine.
Please include all grounds, signals and power supply connections.
Here is an example from my most recent amp which in fact used these boards.
Hello,
Thanks for intervention..I think there is no need to draw..it's easy to picture the test rig (not in a box)
I have the psu output +0- connevted to LM board, the output connected from the LM board to a speaker, and an input via jack from telephone (or you could consider no input cable attached). One thing I could possible test is the speaker gnd connected to the star ground, not to the LM board output gnd.
Anyaway, doesn't ground loops cause hum, not hiss?
Thanks for intervention..I think there is no need to draw..it's easy to picture the test rig (not in a box)
I have the psu output +0- connevted to LM board, the output connected from the LM board to a speaker, and an input via jack from telephone (or you could consider no input cable attached). One thing I could possible test is the speaker gnd connected to the star ground, not to the LM board output gnd.
Anyaway, doesn't ground loops cause hum, not hiss?
The speaker grounds should be connected to the 0v output of the PSU rather than to the star. Only 1 connection should go to the star from the PSU 0V.
Ground loops definitely cause hum. However as far as I understand, if your ground scheme is not quite right, one can couple in other types of noise.
Definately try moving the speaker grounds as above however I would suggest you might find that shielding everything inside a steel box gets rid of the noise. If it is possible to try quick and dirty then give it a go and see what happens.
Maybe a photo might bring another obvious issue to light.
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Hi,
I just literally fired up the first 2 of 4 channels.
It´s been playing for a couple of days and I was so concentrated on the new DIY speakers I didn´t hear the hiss at first.
I can hear it from the listening position with no music but when I disconnect the input from the amp though the hiss very low in volume and I have to put my ear next to the speaker. (no volume pot)
I´d guess it´s just grounding/cabling or similar but I´ll definitely have a look at the amp on the scope sometime.
Let you know if I shall find something.
And at this point and big thanks on my behalf to everybody involved realizing this design. I came late to the party but scored 4 PCBs at the "swap meet" from Terry. Thank you 00940 in particular!
I just literally fired up the first 2 of 4 channels.
It´s been playing for a couple of days and I was so concentrated on the new DIY speakers I didn´t hear the hiss at first.
I can hear it from the listening position with no music but when I disconnect the input from the amp though the hiss very low in volume and I have to put my ear next to the speaker. (no volume pot)
I´d guess it´s just grounding/cabling or similar but I´ll definitely have a look at the amp on the scope sometime.
Let you know if I shall find something.
And at this point and big thanks on my behalf to everybody involved realizing this design. I came late to the party but scored 4 PCBs at the "swap meet" from Terry. Thank you 00940 in particular!
Hey, that's my question from post 253
. Yes, I always use shielded cables for line level signals, especially in amps.I haven't looked up the problem, just wanted to say that I've some hiss too. I think it's grounding and should be solved easily. Don't think there is a problem with the PCB.
I understand what you're getting at but ideally the star is located between the ps caps at 0V so I don't know whats different between your star and 0V
Long battles have been fought over the grounding of amps... My own amps have been silent with speaker ground return to the PCB. There is a wealth of info on hifisonix 's website.
Anyway, wrt hiss:
- joensd: if the amp is mostly silent with no input or shorted input, it points out towards a slightly noisy source combined with the relatively high gain of the lm3886. Maybe an attenuator at the input would help (if there is volume to spare).
-selwe: what did you use as value for the mute resistor and on which voltages do you run the amp ? Do the chips get hot ?
Anyway, wrt hiss:
- joensd: if the amp is mostly silent with no input or shorted input, it points out towards a slightly noisy source combined with the relatively high gain of the lm3886. Maybe an attenuator at the input would help (if there is volume to spare).
-selwe: what did you use as value for the mute resistor and on which voltages do you run the amp ? Do the chips get hot ?