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Old 31st March 2005, 02:52 PM   #1
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Default My "audiophile" LM3886 approach

my modest contribution to this forum...

Mauro Penasa
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Old 31st March 2005, 03:20 PM   #2
Giaime is offline Giaime  Italy
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It looks good! But I'm not sure about the use of an opamp in front of the LM3886, many people there believe in a "less parts better" approach towards Gainclones. I don't know...

And what's that circuit (the first one on page 2)? Is it a speaker protection or a startup delay?

And where do you get LM3886 in Italy? How much you pay?
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Old 31st March 2005, 03:27 PM   #3
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Hi maupenas!

Very nice! Thorsten would like it..
But you have overtaken us! It's ages that my friend pushes me to realize this concept! Though he wants it a bit modified, so as to get "constant power" drive, instead of current drive.

For me, the question is: how this all react with Your speaker? [Mine is a two-way bass reflex]
Did you measure the actual Rout? Stability?

Ciao, George
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Old 31st March 2005, 03:40 PM   #4
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Hello,

I've just glanced at your design, so my question is surely silly. How does your speaker protection cope with negative DC offset?
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Old 31st March 2005, 03:55 PM   #5
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A very unusual design. The 3886 is just a power booster for the 318. I wonder how it sounds compared to the more traditional approach. Any problems with stability?
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Old 31st March 2005, 06:34 PM   #6
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No, it's not a power booster..

Doh, I was not looking close enough at the first glance - so I take it back - it IS a kind of a power booster.. though a tricky one. [it's not a unity gain buffer on the output..]

At first I was only looking at the 3886 circuit. My note was referring to that. I still would like to try one day.
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Old 31st March 2005, 06:49 PM   #7
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Ehm, what would you call it? It's got a unity gain and falls under the 318 feedback loop.
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Old 31st March 2005, 06:57 PM   #8
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sorry, analog_sa, as you see, in the meantime also I have recognized it

The opamp controls the output current..
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Old 31st March 2005, 07:05 PM   #9
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So as to be clear: the LM3886 is a voltage controlled bipolar current source in this configuration. It does not have any voltage gain; it has an input voltage/ output current transfer characteristic - which is, in this case, is 100.

And it is stable in this config.
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Old 31st March 2005, 07:07 PM   #10
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Quote:
And it is stable in this config.
I am sure it is. What about the entire amp? I guess it's fine with the compensation caps.
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Old 31st March 2005, 08:04 PM   #11
ingrast is offline ingrast  Uruguay
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Default Overall transfer

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
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Old 1st April 2005, 12:56 AM   #12
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Quote:
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.
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Old 1st April 2005, 11:25 AM   #13
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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
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Old 1st April 2005, 12:00 PM   #14
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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
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Old 1st April 2005, 12:25 PM   #15
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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
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Old 1st April 2005, 12:36 PM   #16
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and the photo of the my ugly prototype (detail):

good work...

Mauro Penasa
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Old 1st April 2005, 01:04 PM   #17
macboy is offline macboy  Canada
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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).
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Old 1st April 2005, 01:21 PM   #18
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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
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Old 1st April 2005, 01:29 PM   #19
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Thank you, macboy

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

Mauro
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Old 1st April 2005, 01:46 PM   #20
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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
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Old 1st April 2005, 01:53 PM   #21
GregGC is offline GregGC  Canada
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Quote:
Originally posted by 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).

Quite an interesting and original topology. Yes we need more of that on this forum.

On the other hand implementing something so simple as the snubbers with such a great positive effect on the overall sound (using bigger PS caps) deserves getting exited about. I think the goal should be a good quality reproduction and as far as I'm concerned, the simple the topology the better. One thing I don't want to see is unnecessary stuff thrown in an amp just to make the circuit look sophisticated (this is not the case with the amp discussed in this thread).

Bottom line, if a resistor placed in a right place improves the performance of the amp as much as adding a preamp, then I'm exited about the implementation of the resistor more than by the implementation of the preamp.


Good work Mauro! And your English is fine too. Keep on talking!

Greg
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Old 2nd April 2005, 10:35 AM   #22
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Quote:
Originally posted by maupenas
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 )
Ok. Did you try the well regarded OPA627 and similar from BB?

Quote:
I am not able to explain shortly the theme of the topology ( in English ).
I understand Italian quite well
I can help yopu with the translation if you want but I think that you are clear enough.

Quote:
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).
[/B]
I've done too much programming lately... must give a look at my electronics books...

Quote:
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 [/B]
So another project on queue....
Where do you live? I'd like to hear the baby singin'

Cheers

Andrea
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Old 2nd April 2005, 11:41 AM   #23
Giaime is offline Giaime  Italy
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Good! See.... italians just do it better...


(in Gainclones at least)


If my beliefs are confirmed, this is gonna be the most advanced and good sounding LM3886 implementation. Let's hope it sounds well... and it's not so dependant on expensive op amps!

Ciao a tutti e buona fortuna... Continuate così!
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Old 2nd April 2005, 12:08 PM   #24
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I believe that will like you so as it is. Nothing forbids to try other solutions, but my suggestion is to do it if dispose of suitable tools.
I not have tested OPA627.

I have led by departs JLH 69 and other "clones" for this, but the tastes are taste.

The theoretical treatment do it only if like you the result.


Ciao

Mauro Penasa
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Old 4th April 2005, 03:35 PM   #25
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Hello Maupenas,

Can you provide basic principles of your speaker protection circuit?

Regards,
Yoghourt
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