A bass boosted preamp or buffer advice needed

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Hi all,

Last weekend I've tested my last LM4702 project. As mentioned its a very good and "hot" amplifier. I use that circuit;
An externally hosted image should be here but it was not working when we last tested it.

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
 
Dxvideo said:
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.
 
In fact I already have an input buffer (or may be a preamp) with LM4562. Its a bass boosting linkwitz NFB preamp circuit. It has a 6dB gain on flat curve and before 50Hz point the gain increases up to 12dB at the 20Hz point.
I use that circuit for my gainclone (sometimes) and it gives a lot of benefits to drive it with different sources and to have a satisfactory bass performance.
So I will use it in my next try.
 
the circuit does have low input impedance -- but it is also lower in noise than the other LM4702 incarnations. Your source might "poop out". As well, the transformer you are using can become less efficient as it hots up -- so you'll see a reduction in the rail voltage as the amplifier is driven harder.

with respect to the capacitors -- I use 39,000 uF -- you can do the math -- this keeps the rail voltage more or less constant under duress.

i don't like using a silver-mike across the bias generator -- but as it is in parallel with a 500 ohm pot its effect on sound should be de minimis.

if you have the nerve, you should hook the amp up to a resistive load and determine the point at which the output is down about 0.5 dB from the 1kHz output. Note that the AN1645 circuit uses only a single set of output devices so the Ciss (input capacitance) is higher in your amplifier than the single devices shown in the application note. If you use a pair of output devices you have to tweak.

The input capacitance of the lateral MOSFET's is about 600pF for the N-Channel and 900pF for the P-Channel.

A scope and distortion analyzer are helpful here -- you can play around with the compensation caps, placing caps from gate to source on the N-Channel devices, the gate stopper resistors, the zobel -- the LM4702 will really sing when it's optimized for your particular setup.
 
jackinnj said:
Note that the AN1645 circuit uses only a single set of output devices so the Ciss (input capacitance) is higher in your amplifier than the single devices shown in the application note. If you use a pair of output devices you have to tweak.

I couldnt understand what you mean? I am using two pairs of output device. However each pair is using for ONE CHANNEL! Theyre not paralelled. So I think there is no problem.

And concerning about the capacitor across the bias resistors. In fact I dont know the reason of this capacitor. But I thought "If it is exist there so there must be a reason". May be I am wrong. As you see its based from AN1645. And its a general application note. Not especially for Renesans MOSFETs. So this capacitor may there to compansate the other type of MOSFETs input capacitance, but I dont know.

Its too easy to remove it from there. But there is another problem about Rbgs.. I also dont understand why its there? Do you have any idea?

Thx.
 
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