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Old 26th June 2007, 08:23 AM   #1
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Question A bass boosted preamp or buffer advice needed

Hi all,

Last weekend I've tested my last LM4702 project. As mentioned its a very good and "hot" amplifier. I use that circuit;
Click the image to open in full size.
With Renesans lateral MOS pairs (2SK1058/2SJ162). And set the bias current at 110mA.
I have -4mV DC offset on each channel. No switch on/off pop, hiss noise (even inputz open) or any hearable distortion at listening levels. There is also no input cap as recommended by National guys.
Anyway,
I am happy with the circuit with that configuration. However I feel the deep basses some weak. My ears accustomed to LM4780 gainclones sound. And this amp has more detailed mid and treble tones. I dont know the reason, but as I heard somewhere when power supply goes to the upper voltages then basses become more weaky. Is that true?
And a request,
Does anybody may advice a good working preamp or buffer circuit with bass boost option for my amp pls?
Best regards,
Ozgur
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Old 26th June 2007, 11:51 AM   #2
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Hi Ozgur,

Have you tried connecting some much larger capacitors in parallel with Cs10 and Cs11?

By the way: What are your power supply voltages? And what is the output driving?

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Old 26th June 2007, 11:58 AM   #3
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Yes, in application there are 100uF in parallel to 0,1s..
And in PSU unit there are two 10,000 ESR, 470nF and (2R2+100n for snubberising as Carlos advices) for each rail.
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Old 26th June 2007, 11:59 AM   #4
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My suppply voltages are 40+40 VAC and 55V per rail on DC. The output MOSFETs are 2SK1058 and 2SJ162... If you mean it for "output driving".
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Old 26th June 2007, 12:05 PM   #5
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And this is my PCB design which using for that project.
Click the image to open in full size.
As you can see, there are 100uF s on each rail input.
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Old 26th June 2007, 12:06 PM   #6
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You might want to test it with caps that are more than 100 uF, there, if you haven't already tried that, and make sure that they are very close to the transistors.

How long are the wires (or PCB traces) from the PSU to the transistors?
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Old 26th June 2007, 12:11 PM   #7
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Hmmmmm
Do you think this design reduces the bass response?
And should I put some larger value caps near to MOSs source pins?
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Old 26th June 2007, 12:14 PM   #8
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Quote:
Originally posted by Dxvideo
Hmmmmm
Do you think this design reduces the bass response?
And should I put some larger value caps near to MOSs source pins?
Just a guess. But if it's easy to try it, it would be interesting to see if it has an audible effect.
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Old 26th June 2007, 12:18 PM   #9
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I will try that, but not early than this weekend. I will write for the results on Monday.
But what about the input buffer? Do I really need an input buffer? Does it affect the sound quality? Because I read that in many soruces. The voltage gain is enough for (in fact more than enough) CD players output.
Thx.
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Old 26th June 2007, 12:30 PM   #10
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Quote:
Originally posted by Dxvideo
I will try that, but not early than this weekend. I will write for the results on Monday.
But what about the input buffer? Do I really need an input buffer? Does it affect the sound quality? Because I read that in many soruces. The voltage gain is enough for (in fact more than enough) CD players output.
Thx.
I don't know. An input buffer might make some difference, especially since it looks like your circuit has a relatively low input impedance.

Maybe you could first try a "quick and dirty" input buffer, with a simple opamp circuit on a breadboard or perfboard, just to see if it has any good audible effect.
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