Lifeless Sound at Modest Volume

[bentoronto]

I'll second AndrewT on this...a well performing system should provide the illusion that the volume control magically moves one physically closer to or more distant from the source. The brain is quite adept at interpreting the nuances of the Fletcher-Munson curves in the real world.

Can you explain or state more fully each of those two sentences, please.

Does it have something to do with what "loudness" or "distance" we perceive the sound to be originating at? If you are right, then it must.

I have also been wondering if Reubens had astigmatism? Why did he draw long people?

Goods explanations gratefully received.

 

[rabbitz]

jstan

I've built a few of Peter's LM3875 kits and found my preference was for the 1500uF caps to be used on the power supply and 100uF 50V caps of good quality (I used Elna Cerafines) at the chip. I tried a snubbered supply but preferred the arrangement as above. If you have a look at Peter's commercial amps, you'll notice that the cap arrangement is the similar to what as I have mentioned.

Check if you have built with or without the zobel as can make a difference. Peter prefers without but all mine have used zobels.

I haven't experienced poor sound at low to medium levels but that could be speaker or speaker impedance load related as ChipAmps can be fickle with different speakers. I've found the LM3875 works very well with a passive pre (pot only) and is the only amp I've ever built that can get away without an active pre stage. I also used ALPS pots and IIRC was 10k. You may want to check the output impedance of the source to ensure it's reasonably low but I don't think it's an issue for you as you said it sounds great at higher levels.

If you need an active pre amp, check out the B1..... no cap symmetrical supply version with the lower output impedance. There are a lot of good op amp based pre amps out there and I have a soft spot for the LM4562 op amp.

 

[danielwritesbac]

jstan,

I have an Gainclone with a passive attenuator based around an Audiosector LM3875 Premium kit (18-0-18v, non regulated PSU). For my passive attenuator I used an 50K alps dual log pot. Initially, I had similar problems to you with the sound quality at low level. Also, the volume got very loud, very quickly with only a small turn of the pot.

I solved the issue using a shunt resistor (metal film .25w) of 47k directly in the input line, using the Alps pot as a bypass to ground (look up volume pot shunt mod on Google).

The difference was night and day. At low level, it was like the difference between having your fingers in your ears and then taking them out.

For about 14 pence (about 10 cents), and a few minutes with a soldering iron, it is well worth doing, and much, much cheaper than an active pre.

Chris.

Thank you for providing an answer to the common and difficult problem of "potentiometer sounds good at only one spot, which is too loud." The problem is common and the cure, up till now, has been elusive.

You have set the amplifier to line level too:
47k shut // 50k pot // 22k input load = about 12k input load. So, I'm not sure which problem this fixed, but it surely helped fix impedance mismatch.

 

[bentoronto]

Thank you for providing an answer to the common and difficult problem of "potentiometer sounds good at only one spot, which is too loud."
snip

How do you know it is the pot or your ears or your taste?

 

[bentoronto]

I'm sorry, it was El Greco, not Rubens (which I also mispelled as "Reubens").

Here's explanation of the fallacy.

 

[jstan]

Thanks for all the input and ideas. I've almost totally rebuilt the amp based on Joe Rasmussen's tube buffered design and am very happy with the sound now. The only problem is that I now have a hum problem that I have to sort out, but that's a problem for another post. Thanks again!