My "audiophile" LM3886 approach

Just checked in simulation (with all the earlier caveats about the LM318 and an LM1875 model being used instead of the LM3886).

Great! :)

100 pF and lower (47 pF) for C32 seems to be unstable in the transient analysis.
It's possible that 100pF is just marginally stable on a real board.

Actually it seems so, I'm using that value (100pF) since yesterday and, btw, sounds great.

150 pF is stable, 220 pF gives ~2 dB improvement and 330pF gives ~4 dB improvement in THD20 over 150 pF (not that THD20 matters anyway - I'll look at the phase of Zo next).

Improvement? I thought that THD20 was increased by bigger value C32 :confused:

Related stuff: A local DIYer has found the TL071 to be stable in the Rev E, but claims that the mids are a bit veiled. I plan to check this out with an LF01/TL071 Class-A biased version. I also checked the LM1641 in simulation, and it is unfortunately unstable - maybe some compensation mods are required.

Sure, it should be possible to use any opamp but it should be compensated like in Rev B, with a cap going from the output to the inverting input of the opamp.

C10 should be maintained to preserve the feed-forward compensation (according to Mauro is the LM3886 that is been compensated by C10) and its improved DF coherence.

The only real drawback should be that with this compensation, like Rev A and B, the My_Ref needs a preamplifier.

Obviously stability should be checked, anyway.
 
I have not learned how to do this stability thing yet. It's down to a lack of knowledge on the AC performance of circuits and parasitics and what versions of compensations suit different circumstances.

What I have found is that the Audio Frequency performance must be exemplary into reactive as well as resistive loads. That Audio Frequency appears not to stop at 20kHz.

The first sign that something is amiss with the audio performance is an exaggeration of sibilance and an exaggeration of HF detail.
I have found that, passing a 2kHz to 5kHz square wave through the amplifier reveals an overshoot, or worse, on amps that suffer those two problem characteristics.
It is my view that this overshoot is the early sign that the compensation is not properly applied. i.e. it can not meet the Audio Frequency performance into reactive loads.
 
I have one channel MyRef C TP, and the other with the new compensation values except for C32 kept at the original value and R3 changed to the Caddock 30W resistor. I feel MyRef has a better bass presentation, it seems well controlled to me, nice and deep, stops when it should and does not seem to have tendency to mess up the performance of the other instruments. I should still let the Caddock resistor run in some more since the sound changes continuously.
 
I feel MyRef has a better bass presentation, it seems well controlled to me, nice and deep, stops when it should and does not seem to have tendency to mess up the performance of the other instruments. I should still let the Caddock resistor run in some more since the sound changes continuously.

It's not so clear to me...

Which is better for you: Rev C or alternate compensation?
 
Although directed to another Member, this explanation is well worth reading.
http://www.diyaudio.com/forums/chip-amps/122793-finally-made-lm1875-amp-3.html#post2811677

The message in there applies to the folk that are guessing at what to do for various compensation suggestions in the various MyRef's

Thanks for the link, Andrew, instructive post. :)

In fact my alternate compensation is, more or less, an experiment and, as I've already wrote, it's not the official one for my derivate My_Ref (Fremen Edition).

I'm also starting to think, after phase measurements made by Soongsc, that the wider and deeper soundstage obtained by 330pF in C32 is probably some sort of phase coloration, nice and sweet but a coloration...so undesirable, IMHO.
 
It's not so clear to me...

Which is better for you: Rev C or alternate compensation?
The bass of the Rev C is better for me, but could use a little improvements at around 80Hz just from listening impression. The alternate compensation does not seem to go low, and it seems not so well matched with the other parts of the sound from a same instrument. The other aspects, I should wait a while for the Caddock resistor to run in a bit more. After than, I switch the MyRef C R3 to Caddock and continue. So currently, no clear result yet. So listening impressions may change as components run in.

My own feed forward was unstable, sharp resonance around 30KHz and some other harmonics. Time to look deeper into the two chips. Really would like a high voltage opamp to replace the LM318.
 
Last edited:
I'm also starting to think, after phase measurements made by Soongsc, that the wider and deeper soundstage obtained by 330pF in C32 is probably some sort of phase coloration, nice and sweet but a coloration...so undesirable, IMHO.

I'm not so sure - C32 at 330pF also gives more stability and ~4 dB improvement in THD20, over 150 pF. The immersive soundstage is the key feature of the Rev E/FE (alt.) compensation, coloration or otherwise. Anyway, to each his own - let everybody hear it for themselves and decide.
 
I'm not so sure - C32 at 330pF also gives more stability and ~4 dB improvement in THD20, over 150 pF

Hi Siva,

so, I'll ask you again, with 330pF is distortion lower?

Anyway, to each his own - let everybody hear it for themselves and decide.

Sure, I'm not against it. :)

If people want to try the alternate compensation and report:

C10 33pF
C34 47pF
C32 330pF

Simply I'm no more so sure about C32 new value, at least in my system.

I've just listened again and 150pF have more impact and a fuller sound with a marginally narrower sound stange.

It' pretty different from what I've heard when I've tried the alternate compensation for the first time...

With 150pF bass was muddy in comparison, highs less refined and soundstage way smaller.

But something has changed in my system since that first time...

I think the new LM318's PS has a big part on it, since it brings, IMHO, a big improvement on impact, refinement and soundstage.
 
Power supply has a great deal of impact on sound in a power amplifier, especially the low frequency. Once, while I was trying out a New Class D amplifier, I ordered two transformers of the same rating, one was an R-core another was an O-core. Both delivered from the same factory. The O-core transformer delivered lots of power, while the R-core sounded flat and dead. Once somebody wanted to try a Siltec power cord on it, the combination was terrible.
 
Power supply has a great deal of impact on sound in a power amplifier, especially the low frequency.

I do agree :)

The O-core transformer delivered lots of power, while the R-core sounded flat and dead. Once somebody wanted to try a Siltec power cord on it, the combination was terrible.

I've had a different experience with R-Cores...

The indian Shilcar R-Cores I'm using are deadly silient, fuller and more refined than a Noratel (same factory where NAD buy trasnformes) toroidal.

Both sound great and dynamic but the Shilcar is a bit better.
 
Below is the results of listening through a few thousand CDs in regards to polarity, which was published in a Hong Kong audio magazine in 2000.

Phase is not an absolute value, but a shift in its value referenced to another signal. In this list, what is the reference? Are you meaning a 180º phase shift between left and right channel?