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BPA300 mono block finished and measured
BPA300 mono block finished and measured
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Old 8th February 2018, 07:06 PM   #281
ChristianThomas is offline ChristianThomas  United Kingdom
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Hang on, I'm wrong on the supply question. He has a split supply and you don't need to do anything about DC. It'll have negligible DC offset. Sorry, I had assumed he was doing a single supply and bridging it. I did mine off a single supply. Actually I have done both but the workhorse was a single supply with some clever stuff included to make it independent of load.
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Old 10th February 2018, 06:34 PM   #282
tomchr is online now tomchr  Canada
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Quote:
Originally Posted by ChristianThomas View Post
The advantage of running things differentially is huge.
That depends. A fully differential amp should in theory cancel all the even order harmonics. In theory... In reality, such cancellation schemes don't work all that well and some cancellation schemes actually degrade performance. Even if the cancellation scheme does result in a lowering of the even order harmonics, you're left with the odd order harmonics, which many report to sound harsh.

If you want to improve the sound quality of the LM3886, you are far better off using some kind of error correction on it. That's what I do in my Modulus amps. My Modulus-686 is basically a BPA300 with error correction and differential input.

One area where differential signalling is a clear benefit is at line level, especially with longer interconnects. The benefit comes from the common-mode rejection of the differential input, and results in a reduction of hum by several orders of magnitude. It also moves the ground connection out of the signal path, greatly reducing the ground loop issues that seem to plague many DIY builds.

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Apart from output impedance, which is again unambitious here with 0R2 resistors balancing the current, there is little to be gained by having 3 LM3886s in parallel except spreading the thermal burden - which is valuable and makes for some of the worst parts of the failings. But a single 3886 can deliver 11A, which is far more than any average speaker can manage.
0.2/3 = 67 mΩ output impedance. While improvements can be made, it's really not bad. I do question whether the 0.2 Ω resistors will survive constant operation with a sine wave, though. Significant power is dissipated in those resistors if the amp is driven to clipping.

Running multiple LM3886es in parallel does reduce the THD as each LM3886 sees a fraction of the load impedance. The improvement is relatively small but definitely measurable.

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If you can send the amp a proper differential signal, ie. one the inverse of the other (not that it matters from the point of CMRR) you will find it is a class above what you get from a single ended signal and all the trouble around finding a true ground. When I did that for the first time about 7 years ago, it was the biggest single improvement I had ever heard on any amplifier.
That I definitely agree with. That's why all my amps have differential inputs.

Tom
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Old 11th February 2018, 12:51 PM   #283
ChristianThomas is offline ChristianThomas  United Kingdom
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I don't think we disagree on much there, Tom. Error correction or some form of additional gain and feedback is a must. I do mine a little bit differently in that I drive each 3886 differentially. This stems from doing a single supply version some years ago, unbridged, and it lent itself to getting the DC blocking capacitor out of the direct signal path.

Going back earlier in the amps the same differential topology is used on the inputs. That goes to an active volume control (with a parallel fixed resistor to save on crackle a year down the line), and thence to a LP filter. The output of that LPF is then inverted and those two op amps drive the inputs of the 3886. You can then drop your gain to x12 and get a bit more bandwidth from the amp, while not having lost overall gain.

For some reason the amp went about two years with the socket for that inverter left empty, with the other input being picked up from the local ground, but with the 3886 still in the differential topology. Once that side was powered it was night and day. I don't think I ever bothered to measure it after doing that, but the distortion figures were already about half of the datasheet.
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Old 11th February 2018, 01:09 PM   #284
ChristianThomas is offline ChristianThomas  United Kingdom
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Regarding those 0R2 resistors (which I would anyway try and get down in value precisely because of the power dissipated) I think a proper amp should really parallel them up. I used to like the little Dale resistors for this purpose. Low TCR and low inductance. 3 of the 3W ones or two of the 5W.

BTW, I assume you are taking the feedback after these resistors so they shouldn't contribute to your output impedance. And if you are error correcting then you output impedance should be zero, or thereabouts.
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