Latest experiment... Inverting-NOT

[Russ White]

Ok, this one is a little hard to follow at first. Borrowing from some ideas I have gotten from Mauro and others I have created a design that uses a LM3886 and a OPA2227 (dual low noise opamp) to create a non inverting amp, but the LM3886 is operated as inverting. Basically a double inversion with a buffer input.

The first part of the OPA2227 is simply a high input impedance buffer, which feeds the signal through the second gate in inverting mode, that signal is input to the LM3886 in inverting mode, so the final output is not inverted(or completely flipped, which ever you prefer).

Here is the schematic.
Sorry, its a mess.

[Russ White]

Here is the PCB.....

[Russ White]

It is all still very preliminary. I am just looking for some ideas on values and such. You will noticethe PCB is not finished (no mounting holes even).

[VT67]

You really honour your signature, aren't you

Regards

[Russ White]

Originally posted by GeWa
You really honour your signature, aren't you

Regards

Hey, its better than psychotherapy.

[Jocko Homo]

Why put a op-amp in the middle just to serve as an inverter??? They almost never sound good. I would rather leave it inverting, and flip the speaker leads.

OK.........you do need some sort of buffer to run it in the inverting mode, but I would not use an op-amp with unity gain.

Jocko

[Russ White]

Originally posted by Jocko Homo
Why put a op-amp in the middle just to serve as an inverter??? They almost never sound good. I would rather leave it inverting, and flip the speaker leads.

OK.........you do need some sort of buffer to run it in the inverting mode, but I would not use an op-amp with unity gain.

Jocko

You make some pretty aweful assumptions...

A good opamp makes an excellent buffer, and the purpose of the opamp is not "only" to invert the signal.

The worst assumption is about the sound. I have built an amp like this (but it was Maruo's design which is slightly different) and it sounds superb.

One of the reasons this works well is the very low output impedance of the opamp, as well as slew rate and other virtues.

Cheers!
Russ

[theChris]

question1.
what are R2, C7 for? i can only assume it is DC-offset related.
q2:
why do all inputs have resistors?
q3:
what reason did you have for the choice of resistors?
q4:
why not run the feedback path to the first opamp?

[Russ White]

Originally posted by theChris
question1.
what are R2, C7 for? i can only assume it is DC-offset related.
q2:
why do all inputs have resistors?
q3:
what reason did you have for the choice of resistors?
q4:
why not run the feedback path to the first opamp?

q1

Yes R2,C7 act to nullify DC.

q2 Well, partly to help set gain for some stages, but on the first stage it is to form a low pass filter to filter out RF.

q3 I was just using values I had used in the past for other applications. Some values I used just because they got me the gain I was looking for.

q4 The reason is the the first opamp gate is purely a buffer to aid the inverting opamp stage.

I am using techniques similar to the ones found here:
http://www.diyaudio.com/forums/showt...225#post622225

Looking over that thread may answer a lot of your questions.

[Russ White]

I have changed the regulators do to some great input from carlmart and others.

I also added the rest of the missing pieces. I think its ready to go.

The resistor at output can either be 10R 5W with a .7uH inductor wound around it, or .3R 7W.

Here is the current schematic (still a mess sorry).