Wrong pot?

[sturmj]

Hi I just finished a lm1875 amp to give for Xmas. Looks good, it's working, only one problem. The sound is a little thin, and gets a little distorted at higher volumes. I noticed that the pot I used was 10k not 100k, which is strange because I ordered 2 and the other is 100k (being used in a second unfinished amp). Since the input section of the amp looks like it is forming a filter (pot-cap-resistor to ground, before going to pin one) could this be the problem? Or should I look elsewhere, since control of the volume seems reasonable.
thanks in advance!

[pacificblue]

The filter is formed by the cap and resistor to ground. The pot size has no influence on that. It does however have an influence on the output filter of the source. The source also has a series cap followed by a resistor to ground. The pot is in parallel to that resistor and the resulting resistance will be lower than the smaller of the two.

That could indeed lead to a thin sound, if the high-pass frequency turns out too high. It also leads to a lower load on the amp, because more low frequency content is filtered out. That should rather decrease clipping distortion than increase it. On the other hand the thin sound may tempt you to turn up the volume more than you would with another filter which results in clipping.

[sturmj]

So, it sounds like the incorrect pot may be causing issues, but not for the reason I thought. Time to contact my supplier. Thanks!

[AndrewT]

post (or link to) an accurate "as built" schematic.

[sturmj]

http://makezine.com/images/23/MAKE23...pSchematic.pdf
But I am doing it on 2 boards; PS and amp x2.
I finished the second amp ( with 100k pot), but haven't tried it out yet.

[AndrewT]

Hi,
the schematic is wrong.

The high pass (DC blocking) filter on the input is set a little high but acceptable at 22ms (22k * 1uF giving F-3dB @ ~7Hz).

The DC blocking cap in the Negative FeedBack loop is shown as 0.22uF.
This should be larger than
sqrt(2) * 1uF * 22k / 1k >=31uF
Use electrolytic between 33uF and 150uF. NOT 0u22F

There is no RF filter fitted ( to attenuate Radio Frequencies).
Try adding a 150pF across the 22k input resistor.

The volume pot could be any value from 10k to 100k. I would go towards the lower end, 10k or 20k if the sources can drive that low.

[Slipstreem]

Originally Posted by AndrewT

The DC blocking cap in the Negative FeedBack loop is shown as 0.22uF.
This should be larger than
sqrt(2) * 1uF * 22k / 1k >=31uF
Use electrolytic between 33uF and 150uF. NOT 0u22F

Indeed. I make the lower -3dB point with the 0.22uF shown in the NFB loop roughly 700Hz. No wonder it sounds "a little thin"!

Maybe the designer actually meant 22uF? That would put it down by roughly 3dB at 7Hz, 1dB at 14Hz, and 0.5dB at 20Hz. That's what I'd be aiming for anyway.

[AndrewT]

22uF for the NFB cap is still a bit low.

Go higher in value if you have the space and the money.

[sturmj]

cool I'll try that!

[sturmj]

I took all your recommendations, and bass response is much better now. I did not change the pot (still 10K). But, there is still distortion during play, especially during peaks in the music (bass peaks are even worse). Its a fuzzy-buzzy kind of distortion in both channels, however the amp is dead quiet when the source is not playing. With my limited knowledge I see 2 possibilities, help me sort it out.
1. is the 10K pot causing it via impedance mismatch? I have tried different sources: Ipod, Dac, and Direct out from Cd players (differnt mixes of cables as well) and problem is still present.
2. The 100uf caps I used are Mundorf electrolytics Bipolar (M e-caps) they should be correct (by my limited knowledge). Also there is less than 10 hours on the amp, do the need to burn in more? ( the other caps are Mundorf M series film, Ive used these before they never caused this kind of problem)

Keep in mind that, the problem is in both channels, function is otherwise normal ( no excessive heat, DC voltage at the speaker terminals is too low for my meter to detect) Any advice you can give me would be appreciated.
Thanks again!