LM3886 questions

[Makk]

I decided to do active preamp..
Pre Tone Control Stereo by IC NE5532 -Audio -Electronic Circuits Diagram-Elecpod
what do you think about this? sure looks easy and NE5532 should be enough good chip. Does this circuit keep signal flat? like ____________, when pots centertapped. I'm going to use rotary switches instead of pots. Volume pot will be 10k.

[nightanole]

you need a +-12v supply. You could grab that from your lm3886 rails via some lm317 and lm337 regulators. If you swap the volume pot from 100k down to 10k you might have to change the input cap (then again its 47uf allready).

[mjf]

hello.
if you turn up the volume pot fully and the balance pot too (in direction of that channel) ..........it looks to me the input is shorted.so a protection res on the balance pot could be usefull (e.g. 2,2k from balance pot centertap to ground).
greets

[Makk]

OK. I can test that circuit then. But is it "same class" with LM3886?

Second option is using Pedja Rogic's J-fet buffer. I just don't know can I somehow include passive tone stack (baxandall) with that. Should buffer go before or after tone stack and is one buffer per channel enough.

[mjf]

QUOTE=Makk. I can test that circuit then. But is it "same class" with LM3886?

.........this is a standard tone control,used in hundred's of amplifiers........you can change the values a little bit if you like.
and you can increase the powersupply up to +-15volts.

[Makk]

Hey, I made preamp using the schematic mention above. It doesn't work like it should. The problem is that it generates output all the time. Output when volume is 0 is 2,4V and when its full its 2,7V. Bass and Tone controls affect the voltage. Bass full anticlockwise and volume 0 the voltage is 3,3V. Turning also treble to full anticlockwise its 5,3V. If I turn all three knobs fully clockwise voltage is 1,8V. And when Treble and Bass are fully clockwise and volume fully anticlockwise the voltage is 0V. Sound from my test speaker is very BZZZZZZZZ even when there is that 0V from preamp. What could be the reason for these problems? I haven't tested amplifier without preamp, but I think it works. I measured DC offset before preamp assembly and it was ok. Testing sound comes from computer.

[AndrewT]

Makk,
you say the 3886 tests properly without the pre-amp/tone control circuit.

Then post a new thread to ask how to sort your pre-amp/tone control.

[pacificblue]

Originally Posted by Makk

What could be the reason for these problems?

The inverting input is AC-coupled, while the non-inverting input is DC-coupled.

[AndrewT]

Originally Posted by pacificblue

The inverting input is AC-coupled, while the non-inverting input is DC-coupled.

this is what I refer to as mixed AC & DC coupled.

An AC coupled amplifier has a DC gain of 1. This is usually done by inserting a DC blocking cap in the NFB loop.
The AC coupled amplifier also has a DC blocking cap at the input to prevent source offset passing through the amplifier.

Both DC blocking caps must be in place for AC coupled.
Both DC blocking caps must be omitted for DC coupled amplifier.

[pacificblue]

Exactly. And since more than 90 % of all sources have an AC-coupled output, there is little choice other than to make fully AC-coupled amplifiers.