The Mighty AU-X1

It was, Mr Kuehne said leaving them in made his example less stable, on reading that, I had no hesitation taking them out of mine. ;)

An extract from his article:
"Obviously, ceramic capacitors between one of the operating voltages and the cooling angles had been retrofitted on the MM modules. After their removal, the continuous oscillation of the final stages was gone with the cover sheet attached."

Why where those ceramic caps added. What was the benefit?

Good stuff - cheers John
Mr. Keuhne removed them with after adding his flat amp ground modification and his DC drift modification. Will you be using these mods?
 
Why where those ceramic caps added. What was the benefit?
Fitted at the factory I think, (also possibly retro-fitted at agent level?), in an attempt to quench the causes of oscillation that initially bedevilled this amplifier. These capacitors were for supply rail decoupling, two per MM Head Amp, and ceramic were used because of their superior high frequency characteristics.

The Flat Amp ground modification is a 'must', and should be implemented as well as removing those ceramic capacitors. However, the DC drift modification is just 'fine tuning' and not particularly dependant on having any of the other mods included or not.
 
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What board or section is actually acting as the preamp in the AU-X1?
Well, it would have to be the Flat Amp, however, the amplifier can work with it switched out of the audio path. All inputs Tuner, Aux, MM, etc, (except the MC input) can drive the power amp directly. The sensitive MM input has enough gain to drive the power amp directly, however, the very sensitive MC input needs the assistance of the MM Head Amp to bring its output up to the level required to drive the power amp. The Flat Amp can be switched into the audio path for a bit more gain if wanted.
 
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Well, it would have to be the Flat Amp, however, the amplifier can work with it switched out of the audio path. All inputs Tuner, Aux, etc, can drive the power amp directly. The MM & MC inputs are entire of themselves and these sensitive inputs provide enough amplification and output to drive the power amp directly, or can be switched through the Flat Amp for a bit more gain if wanted.

That's what it looked like to me too based on the block diagram but it has me scratching me head.

I always thought a separate amp always needs a preamp to function, is that not the case?
 
I always thought a separate amp always needs a preamp to function, is that not the case?
Your quote (#266) of my post (#264) unfortunately doesn't reflect the just updated and corrected text of my post. (text captured before edit completed). ;)

However, just like the AU-919 and probably many other integrated or other amplifiers, the AU-X1 has sufficient power amp input sensitivity (1V/47KΩ) to be driven directly by a tuner, or a CD player and the like, providing they have sufficient output which most of them do.

One reason for using a preamp (separate or integrated) is for tone controls, the AU-X1 doesn't have any, but the AU-919 does - these can be used on the AU-919 whether bypassing the Flat Amp or not, but not when using the 'Power Amp In' sockets, again 1V/50KΩ sensitivity.
 
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Your quote (#266) of my post (#264) unfortunately doesn't reflect the just updated and corrected text of my post. (text captured before edit started). ;)

However, just like the AU-919 and probably many other integrated or other amplifiers, the AU-X1 has sufficient power amp input sensitivity (1V/47KΩ) to be driven directly by a tuner, or a CD player and the like, providing they have sufficient output which most of them do.

One reason for using a preamp (separate or integrated) is for tone controls, the AU-X1 doesn't have any, but the AU-919 does - these can be used on the AU-919 whether bypassing the Flat Amp or not, but not when using the 'Power Amp In' sockets, again 1V/50KΩ sensitivity.

So in essence, the design of AU-X1 allows one to use the volume knob as a passive control to the amp section.

Would that be a correct assumption?
 
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Yes, with that one word addition (above) for clarity, you have it correct. ;)

Thanks for confirming. IMO that's just another feature that makes the X1 (and 919) so special. Killer amp sections and the ability to go straight to a passive volume control. Nice!

This leads to the next obvious question, how good is the volume pot used in the AU-X1?
 
There are two of them as you know, how good? - they aren't 'Alps' pots as far as I know, they're OK, but that's about all I could say.

Ahh that's right but I was referring to the primary Volume pot (assuming the two independent LR amp volume controls are set at a fixed point and working properly (not introducing noise, etc.).

What do we know about the main volume control pot?
 
What do we know about the main volume control pot?
Ah right - yes that's an Alps I think, pretty much the best generally available pot, certainly back then. They can show their age (oxidisation) and are difficult to service (push nuts!) as it likely wasn't expected that they would be still in use this many years down the line. :)
 
Ah right - yes that's an Alps I think, pretty much the best generally available pot, certainly back then. They can show their age (oxidisation) and are difficult to service (push nuts!) as it likely wasn't expected that they would be still in use this many years down the line. :)

Thanks. I assume that the Alps master volume pot is logarithmic and not linear, is that correct?

Does anyone know what the benefits/drawbacks of each approach is? Which is most commonly used in high-end audio today?
 
Mr. Keuhne removed them with after adding his flat amp ground modification and his DC drift modification. Will you be using these mods?
Hi Ron - you have listed these mods elsewhere havent you - I think in the post about installing the new phono motherboard.

I am most concerned about oscillation with the phono section in this amp - so yes I will be doing this also.

Just to confirm on - nothing on the new phono motherboard that you supplied takes these Keuhne stability mods into account? So I would need to do both the mods you have described in the motherboard section as well as dropping in your new phono motherboard?

Just confirming, as I am a bit simple. thanks!
 
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Thanks. I assume that the Alps master volume pot is logarithmic and not linear, is that correct?
Yes.
Does anyone know what the benefits/drawbacks of each approach is? Which is most commonly used in high-end audio today?
Logarithmic is by far the most commonly used, Logarithmic potentiometers are used as volume controls in audio equipment because the response of the human ear to the loudness of sound is also logarithmic. Using a log pot therefore gives the effect that a setting of full volume on the control sounds twice as loud as a setting of half volume. A linear pot used as a volume control would give large apparent changes in loudness at low volume settings, with little apparent change over the rest of the control´s range.
 
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... to confirm on - nothing on the new phono motherboard that you supplied takes these Keuhne stability mods into account?.

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Yes. The Kuehne flat amp mod was considered. That is why on my 2772 there are, near the star ground point, two solder points added to the star ground area.
One is for the black wire that goes from the center of the phono board to the star ground.
The other is for the flat amp ground modification wire.

The DC drift modifications are restricted to the EQ boards, so the 2772 cannot be made to address this modification. The DC drift modification is restricted to the cards themselves.

I will add the relevant illustrations when I get home.
 
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