How to raise impedance for an LM3886 guitar amp?

[tedsorvino1]

Hi there. I'm trying to find some old layout on high to raise the impedance for a DIY LM3886 based guitar amp.
There is a trick mentioned on an old thread but the layouts and other images have been erased since the thread is way too old.
Could you please help me with a lay out about the trick or a new schematic-layout with an amp and the trick?
I ve got this board:

HiFi LM3886TF Mono 68W 4? Audio Power Amplifier Board AMP 50W/38W 8? Assemb S3O8 191466724989 | eBay


Another question. What is the best amount of capacitance on the power filter capacitors for a single LM3886 guitar amp? I guess a transformer 25-0-25 is sufficient.
But...Do I need to use all for 10000uF/ 63v caps? Will it be too filtered and chocked? Can I go with just two of them or even lower value ones?
I will use this rectifier board:

Dual Power Rectifier Filter Board Audio PSU For TDA8920 LM3886 TDA7293 Amplifier 699927490782 | eBay

Thanks in advance

 

[Bone]

What do you mean by raise the impedance? The input impedance - then you need a preamplifier. The output impedance then you need current feedback like here Lm3886 current feedback . If you mean the load impedance, Then the LM 3886 will already drive higher impedance loads.

 

[tedsorvino1]

Yes. Thank you Bone. I was meaning the output impedance. And I was searching for this link.
Variable Amplifier Impedance (new link)
I think the first part of the question is answered.

Now for my other question. What about the capacitors for the rectifier?

 

[Bone]

You probably don't need 4 10000uF capacitors - a bit overkill. 2x 3300 will probably be enough, one on each channel and easier to obtain. Make sure your transformer has sufficient VA rating.I would use twice the total output wattage.

 

[tedsorvino1]

Thanks a lot Bone

 

[Printer2]

You are talking about a circuit that simulates the low dampening factor of a tube amp. The page you gave the link to goes through what you need to do. Basically have current NFB, developed from a resistor in between the speaker and ground. You need to return the NFB to a circuit before the amplifier board.

 

[Gnobuddy]

You need to return the NFB to a circuit before the amplifier board.

This might be a very bad idea - I've never seen a chip amp with so much stability margin that it would tolerate additional semiconductors being included into a global negative feedback loop.

Nutshell version: returning NFB to a separate circuit before the amplifier is quite likely to cause violent, full-power oscillations that cannot be tamed except by removing the NFB. The oscillations might occur within the audio band, or even at radio frequencies, which you cannot hear.

-Gnobuddy

 

[JMFahey]

You apply mixed feedback (part voltage part current) to the LM3886 - input.

You do not need to go to an earlier stage.

I guess you are treating the LM3886 as if it were a pair of power tubes, where feedback is returned to an earlier stage, normally the Phase Inverter.

It is not so, the LM3886 is already a full amplifier, with all the necessary gain, + and -inputs, etc. and is used on its own.

As of mixed feedback circuits, check a couple used by popular Guitar amps.

Rod Elliot knows *a lot* about it, please read his pages:
Guitar Amplifiers