Lifeless Sound at Modest Volume

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I built a LM3875 amp based on the premium kit from audio sector which I'm using to drive a pair of Tannoy R1's. At higher volumes the system sounds beautiful, but at modest volumes the music has no life: other ways to describe it would be to say that the sound lacks energy or isn't dynamic. The Tannoy R1's are relatively inefficient speakers (87db), which I understand can be a problem for LM3875 based amps. I'm considering a couple possible solutions, but being pretty inexperienced in the diy hifi world, I wanted to get some opinions. I've considered building a preamp to increase gain and I'm considering modifying the amp by increasing the ps capacitance and adding a snubber. Would either of these help or do you guys have other ideas? Thanks.
-Jeff
 
I built a LM3875 amp based on the premium kit from audio sector which I'm using to drive a pair of Tannoy R1's. At higher volumes the system sounds beautiful, but at modest volumes the music has no life: other ways to describe it would be to say that the sound lacks energy or isn't dynamic.

As Audio Sector are kind enough to show a PCB layout on their website I'd say the problem you've got is that their layout is poor. They're using ground fill rather than star earthing. Whenever I find a design with such the sound tends to be harsh and lifeless, just as you're describing.

One very obvious problem is that the high current ground paths to the reservoir caps aren't separated from the audio signal ground. If you're handy with a craft knife you can probably fix this but its not much fun hacking grooves in the copper. The results will be worth it though:D
 
I too suspect the problem is the inefficient speakers. Unfortunately, the only other speakers I have are also inefficient (90bd Tannoy Saturns), so I have no "compatible" with which to test the amp. Do you guys think that a 3886 based amp would do better?

Abraxalito, I don't think that the problem is the ground, because the setup doesn't sound harsh at all, there are just no dynamics at lower volumes (it's almost the opposite of harsh).

Adason, what do you mean by a loudness volume conrol?

Thanks again for all the input.
 
Right now, there's 1500uf per rail (actually on the voltage pins of the chip, only 10uf per rail in the ps) which seems very low to me. I was thinking of adding 10,000uf per rail plus a snubber to the ps, but don't have the parts on hand and don't want to spend the money if people who know more than me think it won't solve the problem. Tyler, I take it that you think this would help?
 
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It could be due to your speakers lacking the finer resolution
Usually seems more apparent at low levels
But it aint so easy to achieve
And has nothing to do with money, or power
Its about balance

Some say big speakers cant play well at low levels
And it would kind of make sense, somehow
If it wasnt for the fact that I use 15" woofers, in a 3way dipole
And they are just fine at low level(SPL)
 
Tinitus, I think the speakers are, themselves, very good as I don't have this problem when they are driven by my big NAD amp; also, they are definitely not too big as they are only small 2-way bookshelf speakers (6" woofer and 1" tweeter).

Greg, the volume control is an ALPS 50k stereo pot (blue 50kax2) wired with pin 1 to input signal, pin 2 to the input on the pcb, and pin to ground.
 
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Greg, the volume control is an ALPS 50k stereo pot (blue 50kax2) wired with pin 1 to input signal, pin 2 to the input on the pcb, and pin to ground.

Which means you are using passive "pre/att" ?

Maybe you only need a preamp/buffer
When you turn down level on a passive, you turn down the signal
Thus it may loose ability to drive interconnect cable
A preamp/buffer is usually after att/pot, thus it will still have power to drive your cables when signal level is turned down

Or, if you really is using passive attenuation, then 50k ohm pot might simply be way too high, thus it rolls off highs too much
 
Abraxalito, I don't think that the problem is the ground, because the setup doesn't sound harsh at all, there are just no dynamics at lower volumes (it's almost the opposite of harsh).

I made a mistake in my first impression of the grounding - its not as poor as I originally thought as I hadn't taken account of the blue tracks on the underside. So it was unfair of me to categorize it with the other ground fill designs I've seen. This one does have a single point ground (marked OG) where the signal ground (SG) is connected. It also has two power supplies so the charging current between the capacitors won't be significant.

However the ground fill is not identical to a star earth - HF current through the zobel network will create a voltage difference between output ground and signal ground, as will RF currents from the power supplies. The short length of track between OG and SG acts as an inductor and this means RF coming from your source will be superimposed on your signal. This latter effect could well be your problem - you're using a 50k pot. Its output impedance will vary with volume setting - if the low volume setting gives the amp a higher source impedance that might explain it. Which ground point are you connecting the pot to? I'd be inclined to try with a lower impedance pot myself, say 5k.
 
Yes, simple passive attenuation -- no preamp at all. This is one of the potential problems that I've been considering. Lets say I eliminate the volume control from the amp and build a preamp for volume control (and presumably some gain as well), any recommendations for a preamp design?
 
HF current through the zobel network will create a voltage difference between output ground and signal ground

The zobel network is an optional element of this kit and I didn't include it in my amp

you're using a 50k pot. Its output impedance will vary with volume setting - if the low volume setting gives the amp a higher source impedance that might explain it. Which ground point are you connecting the pot to? I'd be inclined to try with a lower impedance pot myself, say 5k.

This is an interesting point. The pot is connected to the signal ground. What you're saying agrees with tinitus' point here. This makes me wonder if I'm better off trying a 5k pot or building a preamp. What do you all think would be a better solution?
 
What you're saying agrees with tinitus' point here. This makes me wonder if I'm better off trying a 5k pot or building a preamp. What do you all think would be a better solution?

Depends what your source is. A CD player for example wouldn't be bothered by a 5k load and they generally have 2VRMS maximum output so would be totally fine (and probably sound best) with just a pot. But some sources might not like such a low impedance. Also some sources (FM tuners spring to mind) have lowish output levels so you might not have enough gain in the system to play loud.
 
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If it plays fine above moderate SPL, then I dont see why it should be a bigger problem at lower SPL
One could think of noise maybe getting more audible

Anyway, even if you may have a point with the small supply caps, I think he should solve his preamp/att problem first

And he did ask fore suggestions how to solve the preamp/buffer issue
 
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