LM3886 problems

[Rnickl]

I bought some of the BrianGT boards and stuffed 2 mono blocks. At first listen the highs were piercing and the lower end was bloated and overwhelming, very bad! After a bit of thought I figured I might have an impedance mismatch because I'm not using a volume pot so I put a 25k R on the input to ground.

Results in the high/mid frequencies were better but still not good, can't seem to get much "imaging" from them. An oddity, they drive my old Mirage 280i's (test speakers) with sloppy, bloated bass. When driving my Ariel's there are no lower frequencies to speak of, no measurements but I'm guessing the Ariel's are rolling off at 50-60Hz.

It's has been near 20 years since I've used my electronics training but, this problem is still making me feel stupid! I'm guessing I'm overlooking something obvious.

Quick specs:
BrianGT boards stuffed to specs.
Triad 48VCT 100VA per channel.
Temporary test chassis with no signal input filtering.
Tried several source components, CD, TT(VSPS preamp) and an MP3 player

Any ideas what might be going on here or how to troubleshoot with minimal equipment?

[Michael Bean]

Where did you buy the chips? There are problems with fake chips being sold on flea-bay and such. Also, a schematic of what you built would be helpful.

Mike

[Rnickl]

I hadn't considered the chips might be fake. I did buy through a US supplier but they are a discount/second hand type place.

Here's a link to the assembly manual for the amp. The schematic is on the 3rd page
http://chipamp.com/docs/lm3886-manual.pdf

Thanks for the reply,
Rick

[BWRX]

What kind of amp were you previously using with your speakers?

[Rnickl]

Originally Posted by BWRX

What kind of amp were you previously using with your speakers?

To put things in perspective, my $30 off the shelf Labtec PC amp (using LM1875's) sounds much better.

Rick

[AndrewT]

Originally Posted by Rnickl

................. with no signal input filtering...............

That's your first mistake.
Fit both a Low Pass Filter and a High Pass Filter to turn your amplifier into an Audio Signal Amplifier.

Once these passive filters are in place, you can alter component values to change the turn over frequency/ies of these filters (independently) to hear the effects and you may be able to come to some conclusions that tell you something about your room/ears/speaker interactions.

Have you fitted all the stability component options?
A badly behaving amplifier can/will sound horrible !!!!!!!!!!!!!

The p3 schematic shows 6 stability components, Cs has 4 caps, Rz & Cz on the output. There is R//L missing on the output. Are Cs in the correct location? In the wrong location they can be effectively useless.
As noted earlier the High Pass Filter will also give you a DC blocking cap on the input. That cap must be in the correct location, after the volume pot.

[bcmbob]

Rnickl,

Please take a look at this thread. The RR is a little gem.

New "Kit" on the Bolck - BrianGT /LM3886 Upgrade

[Rnickl]

Thanks for the input Andrew. Yes, all the stability components are in place but I did omit RL. Perhaps that is my problem here. How many watts does RL need to be? I'm assuming it needs to be fairly high power, is that correct?

"Fit both a Low Pass Filter and a High Pass Filter to turn your amplifier into an Audio Signal Amplifier."
I'm a bit rusty, OK way rusty, on this stuff. Can someone push me in the proper direction on how to calculate the values for this?


Thanks to you also Bob. I ran into that thread a few days ago and it might be something I play with down the road a bit. I need to get something that sounds reasonable first.

Rick

[AndrewT]

the R of the R//L passes significant signal only when the L impedance approaches as high as the R value. That will not happen in the Audio passband. R should never heat up unless you test with high frequency signal, say around 300kHz to 1MHz.
I use a parallel pair of 10r 600mW for an effective 5r, 1.2W

Similarly the R of the R+C should only get hot on HF test signals. I use a quad set of 10r for an effective 10r, 2.4W
Finally I add a R+C at the speaker terminals. This R is just a pr of 10r for a total of 5r. The amp output sees all these R values at HF as a single 10r load, when the speaker cables are not fitted.

The input filter can be
Low Pass RC: 1.5us to 0.3us
High Pass RC: 10ms to 200ms

RC time constant = R value times C value, i.e 1ko times 47uF = 47ms
F-3dB turn over (roll-off) frequency = 1 / Pi / 2 / RC

[Rnickl]

Thanks much for the input Andrew. I didn't have proper values but I did fit a couple caps that seem to have put me on the proper path. I'll need to pick up some components for fine tuning.

Rick