Dear Gurus,
A while ago we built a 4 channel dual LM3886 amp to power speakers direct to driver - fed by MiniDSP 2x4HD.
It works brilliantly apart from one issue which is white/pink noise like hiss, audible at the listening position & most potent from the tweeters...it's present from the moment it's switched on & is too loud to be acceptable.
Looking for any help/suggestions for a cure.
Thanks!
Steve
A while ago we built a 4 channel dual LM3886 amp to power speakers direct to driver - fed by MiniDSP 2x4HD.
It works brilliantly apart from one issue which is white/pink noise like hiss, audible at the listening position & most potent from the tweeters...it's present from the moment it's switched on & is too loud to be acceptable.
Looking for any help/suggestions for a cure.
Thanks!
Steve
Is the DSP do-dah feeding into the LM3886 directly ?
Do you get the hiss with the LM3886 input correctly terminated to ground (in other words short circuit the audio input to the chip) ?
Do you get the hiss with the LM3886 input correctly terminated to ground (in other words short circuit the audio input to the chip) ?
Metal film resistors are no more or less noisy than other types, carbon composite resistors excluded. All resistors exhibit thermal noise to the same extent (en = 4*k*T*R). Carbon composite resistors (can you even get those anymore?) exhibit 1/f noise (flicker or popcorn noise) as well. That would show up in the woofers, not the tweeters though.
My suggestion: Lower the gain of the LM3886 to no more than 26 dB (20x). 20 dB (10x) would be even better.
Try this: Short the input to the LM3886. If this cures the hiss, what you're hearing is the output noise of the MiniDSP amplified by the LM3886. In that case you have two options: 1) use a less noisy DSP (the 4x10HD is better) and 2) lower the gain of the LM3886.
Tom
My suggestion: Lower the gain of the LM3886 to no more than 26 dB (20x). 20 dB (10x) would be even better.
Try this: Short the input to the LM3886. If this cures the hiss, what you're hearing is the output noise of the MiniDSP amplified by the LM3886. In that case you have two options: 1) use a less noisy DSP (the 4x10HD is better) and 2) lower the gain of the LM3886.
Tom
Do a search on gain structure with DSP processing. This is a problem especially with very high efficiency speakers. You can set things up with a little careful work. You need to force the DSP to operate as close to full scale as possible.
Thank you everyone for help! I made a set of shorting phonos & viola no hiss- super silent ...so next step is to reduce gain as advised. Any pointers on the best way to go about this?
Thanks again,
Steve
Thanks again,
Steve
Lowering the gain of the LM3886 (by altering the feedback resistor network) needs to be done with caution to avoid instability.
A far simpler approach would be to use a simple resistive divider on the input to the LM3886 such as two 4k7 resistors and take the output from the junction of the two. That would cut the signal by half. Or use a preset pot to initially find suitable attenuation values and then substitute with fixed resistors.
Gain Structure 101
A far simpler approach would be to use a simple resistive divider on the input to the LM3886 such as two 4k7 resistors and take the output from the junction of the two. That would cut the signal by half. Or use a preset pot to initially find suitable attenuation values and then substitute with fixed resistors.
Gain Structure 101
Thanks Mooly, that did the trick! Now listening in nearfield against a lovely black background...sounds spectacular..!!
Thank You Everyone for your help, the DIYaudio community is awesome :-D
Steve
Thank You Everyone for your help, the DIYaudio community is awesome :-D
Steve
that's good.- Status
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