Clone Marantz M7 Line Amp

[kmtang]

Hi,

I have just completed building a clone M7 line amp with 12AU7 as buffer.

I measured the max Vout without clipping was 28Vp-p. I am wondering if it normal. My guess is that there is a 2Vp-p appears at the first 12AX7's cathode through the feedback resistor which would cause it to saturate.

Wonder if any one could correct me if I am wrong.

Anyway, the M7 line amp sounds good to me.


Thank you,
Johnny

 

[kmtang]

Corrections

Oppsss. I am very sorry. There's a mistake in my previous measurement.

I double check the measurements again this evening.

The max o/p swing should be 80Vp-p or 28Vrms. The output impedance is 1.1K-ohm (or 1100-ohm).

The -3dB points is from 2Hz to 600kHz.

73
Johnny

 

[cogsncogs]

@ kmtang

With or without the tone controls i.e. streamlined? Mine is very, very similar, except I'm currently using a 12AZ7 (12AT7/ECC81) for the CF, a VTL Ultimate clone.
Your measurements sound about right. I too like the sound of mine, well I'm used to it. One thing I don't like about it tho is the subsonic hump, peaking at just a little below 1Hz. I can change, move it around a bit by changing the value of the coupling caps and the tube used for the CF, but it can not be eliminated because of indirect coupling i.e. capacitor in the feedback path, which can lead to motorboating due to inadequate PS decoupling and/or bad layout.
One way around this problem is by use of direct coupling in the feedback path. Downside; an input coupling capacitor is required and it is a more complicated circuit.

Here you can find Norman Koren's Modified "Spiced" Dynaco PAS pre-amp.
www.normankoren.com/Audio/Spice_preamp_2.html#Purist

I have yet to try this, been planning on it for a couple of years. I've simmed it, but that's about as far as I've got.
A little food for thought.

Wayne

 

[kmtang]

Hi Wayne,

Thanks for your info. My audio gen doesn't go that far down to 0.1Hz. Therefore, I really can't test it yet.

I have built the Spiced purist line amp but it employs only the last two stages because I don't need that much gain. It has rather nice sound too. Of course, you can't compare it with the M7.


Johnny

 

[cogsncogs]

Ok oops THE AudioNote M7! I was thinking of the Marantz M7! Now we are on the same page, correct? If we are talking about the AudioNote (and which one?) the is no global NFB loop, just current NFB from the cathode resistors. So the low frequency resonance I was blabbering about doesn't really apply in the case of the AudioNote M7!

See this thread:
www.diyaudio.com/forums/showthread.php?s=&threadid=65835

Wayne

Edit: I was right the first time by the title of the thread! But your description sounds like a a run of the mill grounded cathode stage coupled into a cathode follower when said:

I have built the Spiced purist line amp but it employs only the last two stages because I don't need that much gain. It has rather nice sound too. Of course, you can't compare it with the M7.

That kind of threw me for a loop! I need to brush up on how to read again...

 

[kmtang]

Hi Wayne,

Pls excuse for my english.

First of all, I built the clone Marantz 7 line amp (not AudioNote).


I also built the Purist Line Amp with two 6SN7 but I omitted the 1st stage. My friend laughed at me doing such experiment but I did it just for fun.


Are you clear now?



Johnny

 

[cogsncogs]

Are you clear now?

Yup, thanks.

I also built the Purist Line Amp with two 6SN7 but I omitted the 1st stage. My friend laughed at me doing such experiment but I did it just for fun.

I see nothing FUNNY about that, at all...

Wayne