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Old 13th February 2011, 04:13 PM   #1
txg is offline txg  Germany
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Default improvements for my lm3886 amp

hi there,

some weeks ago i built an om3886 amp. previously, i used a sonic impact t-amp which is now replaced by the gainclone. bass response is way better, and the overall sound ist more clear.

what i liked better about the t-amp is that its sound was more natural, the gainclone seems to be a little bit sterile, and i want to improve that. my source is a computer with an m-audio delta audiophile, speakers are markaudio alpair 10 full range drivers in a bass reflex enclosure.

please have a look at my layout (there is no star ground because i use ground layers, which can't be seen in the screenshot) and my schematic (i forgot the value of c5 and c6, which is 100nF).

The amp is absolutely quiet, no audible hum is present. the 22000ĶF capacitors are relatively large, but i used a snubber circuit. i wouldn't like to change these to smaller values because i don't want to loose the tight bass.

so my question is, how can i get a more natural sound?

what do you think about a tube preamp, any suggestions for a cheap and easy circuit?

is there anything i should change in the circuit? all parts i used are cheap standard grade, nothing audiophile. the transformer used is a 2x18V 160VA toroid. diodes are mur860.

should i replace the cheap open volume pot for an alps?

thanks for your suggestions!
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Old 13th February 2011, 04:55 PM   #2
osscar is offline osscar  Latvia
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Hi,
One option would be a placing of triode before the LM -it would add a good Harmonics
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Old 13th February 2011, 06:27 PM   #3
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How do you define more natural? What is actually missing?
More harmonics are one way to create a different impression, but certainly not a more natural one.
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Old 13th February 2011, 06:37 PM   #4
txg is offline txg  Germany
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it's hard to say, perhaps natural wasn't exactly the right word, let's just say the gainclone sounds more sterile. I had the impression that music seemed to be more 'alive' with the t-amp.

i've never heard a hifi tube amp, but i know tube sound from my guitar player years, and i am interested in learning more about diy hifi and trying out some things, so my first guess was a tube preamp.

then i thought about optimising my gainclone circuit at first, so i decided to start a thread over here and ask for improvements.
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Old 13th February 2011, 10:58 PM   #5
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Quote:
Originally Posted by txg View Post
so my question is, how can i get a more natural sound?
Adopt star earthing rigorously, disconnect your groundplane from all grounds except the star point.
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Old 26th February 2011, 12:26 AM   #6
xslavic is offline xslavic  Moldova
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use a preamp,it will "colour" your sound
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Old 26th February 2011, 01:09 AM   #7
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You should remember that any tube stage other than a cathode follower will have gain. Too much in fact . Like a 6922 with resistor anode load and in a grounded cathode mode with unbypassed cathode resistor could have a gain of about 20dB ( x 10 ). That will make the overall circuit too sensitive for typical DAC outputs ( maybe about 0.1 Vin for full output !). Dropping gain with resistors isn't the best solution. Maybe a tube with less gain?

But if you can sort out the gain issue then a tube preamp would be nice. I'd stay away from a cathode follower if possible.

The 3886 appears to sound best within a certain 'gain' setting. You could experiment with that ( lower it ) !

"....use a preamp,it will "colour" your sound ...."

Yes of course, any input stage will. However either that 'colouring' shouldn't be too 'obvious or nasty' or it should sound 'nice' .
AND you should use such a stage only if you are trying to solve some problem.
Always remember that 'good sound' is what YOUR ear tells you , not necessarily what someone else tells you !
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Last edited by ashok; 26th February 2011 at 01:19 AM.
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Old 26th February 2011, 06:36 AM   #8
Piggy is offline Piggy  Hong Kong
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Use those optional components and try it again. Use star grounding. Keep the resistance of the feedback path to around 20K. Check your filter setting.
You seems asking for a more dynamic sounding amp....
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Old 26th February 2011, 09:39 AM   #9
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You first have to find out what produced the effect of liveliness you perceived in that class D amp you used before. The main suspects are frequency response and distortion.

Changes in the frequency response of a class D amp are likely to occur when the speaker impedance is not linearised and adjusted to the load the output filter was designed for. In that case the ampís output filter probably produced that impression of liveliness by boosting high frequencies. Add the same output filter your T-Amp has to the output of the LM3886 and see, if the sound becomes similar to test, whether that was the case.

Distortion in a class D amp is a different issue. Just like in class A amps there is usually no crossover distortion. But often you see that THD peaks at high frequencies which could be another explanation for lively sound.

Tube amps create liveliness by high odd harmonics. They also boost certain frequencies, usually the extreme highs, if the speaker impedance is not linearised, and always the range around the woofer resonance, because it is too expensive to linearise.

This may not improve the impression of liveliness, but you should consider increasing the Ci values. And you should absolutely add the Zobel circuit at the output.
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Old 26th February 2011, 10:01 AM   #10
AndrewT is offline AndrewT  Scotland
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I see no RF filter, no output Zobel, no supply rail decoupling, no DC block on input.
The supply rail decoupling does not go at the PSU. It is attached as close to the chipamps' Vcc & Vee pins as possible.

The gain is ~20x (100k/5.2 + 1). Do not make it any lower. Higher might sound better.

I see that there is no attempt to control output offset. What is the output offset?
Does it vary when you move the volume control?
Does it vary when you plug in and remove the Source interconnect?
Does it vary when you turn your Source OFF to ON?
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