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Chip Amps Amplifiers based on integrated circuits

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Old Yesterday, 12:05 PM   #3491
soongsc is offline soongsc  Taiwan
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I remember this, but I am not sure if there was a measured difference. Pure Howland Current Pump probably would have.
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Old Yesterday, 12:37 PM   #3492
ClaveFremen is offline ClaveFremen  Italy
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Originally Posted by AndrewT View Post
Yes, I am serious. Go back and look at the Howland Current Pump notes.
I attempted to describe this years ago in a Rev C Thread. Few took any notice.
I remember your posts about it and since them I've always found your approach at least extreme and not needed in this application of the HCP, 0.1% resistors are enough to ensure stability and, as calculated by Joseph K, at worst the change from a different R3 is 0.001% way under the specified tolerance.
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Old Yesterday, 01:24 PM   #3493
Joseph K is offline Joseph K  Hungary
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Andrew, I think there are loads of errors in your logic here..
I would quote the article linked at by You:

Most applications notes just indicate the circuit and the ratio, that R1/R2 must be equal to R3/R4. However
they do not indicate how important it is to have precise matched or trimmed resistors. If all 4 resistors
were 10 k ohms with a 1% tolerance, the worst-case output impedance might be as bad as 250 k ohms --
and it might be plus 250k, or it might be minus 250k! For some applications, this might be acceptable, but
for full precision, you might want to use precision resistors such as 0.1% or even 0.01%. These are not
inexpensive! But it may be preferable to use precision resistors rather than to use a trim pot, which has to
be trimmed (and which may get mis-trimmed).
What we can see here that they are principally suggesting to use 0.1% resistors (and this is what we are doing here!) and maybe, "even" 0.01%... So even the author knows the impossibility of that mission.
Then, what you miss completely is the output impedance of the bridge. The levels of precision are calculated for getting at least 250kohm output impedance.

Mauro instead very clearly had defined his target for output impedance: 500ohm.

So, instead of irrelevant studies not satisfying for our application at hand, what if You would go back and re-read Mauro Penasa?

Initially, I’ve used a bridge with LM3886, configured with an Rout quite small, at about 4-10 ohms
(see My_ampli), because my main target was to exploit the good “ NFB neutrality” charactheristics of
this configuration. The result was quite OK right away, because the final sound reflected almost
exclusively the tonal characteristics of the input stage (lm318). One basic problem remained, namely
that in this configuration the resulting gm of the bridge was quite high, which in consequence raised
both the level of the output damping factor and also it’s frequency dependency.
After having conducted a series of studies on the problems connected to the damping Factor (DF; the
relation between Zint of the amp and Zload) and after having re-considerated the way how global NFB
works (shown in the theorical circuit description),
I have found it beneficial to raise the Zout of the bridge to about 500 ohm, which, in consequence, also
reduced it’s gm to gm=1.
My choice of the LM3886 is based on it’s low cost, high reliability, and good power output level
(68Wrms) even at 4 ohms load level. A general efficiency problem remained, though: it’s caused by the
value of the resistor in the output, R5=.47ohm.
This value was needed to get the high Zout and low gm levels described above. Under heavy load
conditions the dissipated power in R5 gets quite high, but I consider it acceptable in the light of the
obtained acoustical results, especially if we consider that the losses provoked in this way are much
smaller than in case of class A circuits – which would be the direct competitors here..
And, finally, I have to say I really feel sorry for being polemic with You but I feel an extremely stubborn resistance from your part.
Stubbornly being in the wrong.

Ciao, George

Last edited by Joseph K; Yesterday at 01:32 PM.
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Old Yesterday, 02:06 PM   #3494
Spartacus is offline Spartacus  United Kingdom
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Originally Posted by madisonears View Post
I do not object to leaving C9 in the circuit. I realize that is the ultimate goal of all this modification, but I have no problem listening to music with C9 in the circuit.
Just to clarify this point, the aim of this modification is to replace the LM318 with a modern, higher performance part. Doing this results in a clear step forward in the sound. Linking out C9 is a side benefit that's worthwhile, but not as large an improvement as the main opamp substitution..
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Old Yesterday, 03:21 PM   #3495
UnixMan is offline UnixMan  Italy
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Originally Posted by madisonears View Post
[...]The last time I messed around with C10 and C34 values according to Dario's recommendations, I did not like the results at all. I don't want to go back through all that or start another debate about comp values. Does simply removing those comp parts make the amp sound better?
Tom, if we were talking about technical aspects, it would be easy(*) to show the results here, as George have already done.

But! How could anybody else tell you whether you will or will not like the results, subjectively? Clearly, there's no other option than to try it and judge yourself.

FWIW, I have listened to many successive "evolutions" of George's FE boards and -IMHO- these mods are definitely well worth it. A lot of quite sensible improvements, on all the "parameters" of perceived SQ.

(*) it's gettin' such results which is not easy at all... but that's another story. Fortunately we have George for that.

Originally Posted by madisonears View Post
Then why were they there to begin with? Because of the LM318?
in short, yes. Though there are many ways to skin a cat. Mauro himself did change those details several times, while keeping the LM318. The "Evo Rev.A" have been introduced by him here:


Originally Posted by ClaveFremen View Post
After My_Evo Rev. A mod is applied, to change opamp you will need to remove C34 and change C32 (OPA827 only from what I've understood, Joseph K could help here)
yes, the change of the main compensation cap (C32) value is required only for some OpAmps, among which OPA827.

The ADA4627 (one of the recommended types) will work just fine with exactly the same ("Evo Rev.A") compensation originally designed for the LM318.

(OTOH, yet some other - currently non recommended - OpAmp types may require even deeper changes to the compensation scheme to work in the circuit).

Originally Posted by AndrewT View Post
Yes, I am serious. Go back and look at the Howland Current Pump notes. These show the use of tighter tolerance resistors for the ratio setting AND they put in a low value trimmer to allow precise tuning of the RATIO. This is very important. [...]
yes, this would be quite important... should your goal be that of producing a true voltage controlled costant current source. That is, to obtain an output impedance as high as possible. But that is not the goal in this application!

Originally Posted by Spartacus View Post
Just to clarify this point, the aim of this modification is to replace the LM318 with a modern, higher performance part. Doing this results in a clear step forward in the sound. Linking out C9 is a side benefit that's worthwhile, but not as large an improvement as the main opamp substitution..
I would (partially) disagree here. Indeed, the OpAmp replacement was a goal in itself, and it constitutes a quite sensible improvement (both WRT measurements and perceived SQ).

Yet, IMO/IME, gettin' rid of C9 also provides a "huge" improvement in terms of SQ. YMMV.
"We should no more let numbers define audio quality than we would let chemical analysis be the arbiter of fine wines." N.P.

Last edited by UnixMan; Yesterday at 03:34 PM.
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Old Yesterday, 03:56 PM   #3496
madisonears is offline madisonears  United States
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Thanks to most of you who responded. I'm getting the picture. I appreciate your patience and detailed explanations, especially Dario, as usual.

I'll begin with the basic mods. You know I won't hesitate to share my impressions.

Tom E
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