My "audiophile" LM3886 approach

[ingrast]

While it is true the output stage is transconductance, the overall input / output is voltage amplifier with a nominal gain of 30.

Have someone actually tested this circuit?

The LM318 may be unstable in integrator configuration as presented.

Frequency response and distortion performance at high frequencies worries me a little, with gain dumping capacitors C31 (C32) as shown (but depends on actual values).

Rodolfo

[carlosfm]

Originally posted by analog_sa
Ehm, what would you call it? It's got a unity gain and falls under the 318 feedback loop.

To make it stable at unity gain the value of C31 and C32 is critical.
Value not specified on the schematic.

[mauropenasa]

sorry, my English is not good.

you have enough centered the operation of this circuit.

The LM3886 it's the power "bilateral current source" (National Semi app...), but the Low value of R3 end R20 (0,01 ohm or 20mm jump 05mm wire) create an "modest" Output Impedence of the bridge.

The LM318 controlling all over close loop.
This architecture it's very complex for tuning, but I the have chosen on account of the fantastic integration with the loudspeakers, end the modest "load invariant" propriety.

The close loop stability It's controlled on dominant pole (C33 end C34) end C31 C32 (0-3p3) not used for this configurations.

In the schematic 2 view a simple Power ON speaker delay (2sec) plus -+ DC protections, and power supply. The transformer It's 225 VA 240/25+25Vac, and Diode (Bridge D1, D5) 5A 250V C5000/3300

to attach the preliminary test of my (1 years old) prototype.

Ciao

[mauropenasa]

I add other technical explanations:

The zobel network doesn't exist intentionally.
The phase shift for RCL network create additional problem of stability,
and not have a positive influence on complex speaker load (I drive loudspeaker, not industrial motors).

This OP-amp "nesting" structure not it's a news.

I have testing various configuration, to optimize the "sound"(and tec. performance).

The circuit to has been tuning with long listenings, not only on laboratory tools.
(various preamp,B&W CM2, DYnaudio Audience52, KEF102 Ref. speaker, ecc...)

to attach the second part of testing (high freq. dissipated perplexity)

ciao

Mauro Penasa

[Andypairo]

Hi Mauro,
did you choose the LM318 by purpose or it was just what you had in stock?

What are the advantages of such a topoogy over the "regular" one?

You mention a intrinsic tuning difficulty of this amp, any suggestion on how to implement it in case someone wants to try it?

Cheers

andrea

[mauropenasa]

and the photo of the my ugly prototype (detail):

good work...

Mauro Penasa

[macboy]

This is the most original design that I have ever seen on this forum. This forum needs more innovation like this; the last thing that we got excited about was putting a freakin' snubber in the power supply (like nobody has ever done that before).

[mauropenasa]

Hi Andypairo,

The LM318 choice it's extremely determinant (for my). I have tested various OP-amp (NE5534, OP27, TL081, TL071 ecc...), but the audio performance (compareted a class A amplifier) are superior.
An other motivation is that the slew-rate of this OPAMP is suited to this application (or all final amplifier device )

I am not able to explain shortly the theme of the topology ( in English ).
eventually if find the time, I will do a little relationship and will translate you.
In brief, say that this type of amplifiers "feelses" the load that in minimum departs comes to be part of the feedback network, on account of the presence of the bridge in exit (or an other high impedance current driver).

No tuning problem (for you, and this schematic).
The recommendation it's a good bypass and short length of
a input network on LM3886 . I have built all the circuit a 100*160mm card, (as in the photo), and abundant stagnated all power copper track.
If have a oscillations problem, increase C33,C34 on 110 or 120 pF.
If to is not enough, added C31,c32 sperimentaly, (1- 3,3 pF).

ciao

Mauro Penasa

[mauropenasa]

Thank you, macboy

this is the beautiful thing to behaviour this things for "audiophile" passion and not for money.

Mauro

[mauropenasa]

oopss,
I have seen that *.zip files lose the extension during the download. Enough renames the files with .zip final and all works.

Good weekend

Mauro