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Audio Note M7 line stage preaamp!

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misok

Member
2012-10-09 7:00 pm
true, but since you've build M7, i guess you want to use it .. maybe examining internal gain structure of your power amp (maybe even your source) could it make possible to integrate your M7. ..or just rebuild it with tube that has lower gain.

in fact i was in the same situation. my M7 sounded great but what bothered me was that my SS power amp simply didn't need it. so i ended up adding a power section to the M7 and practically making a SET amp.
 
Taming the Pre

Specifically - get the schematic listed, and I'm sure I (along with another dozen) DIYaudio members will make very specific recommendations as to how to "tame" the output.

The most trivial, of course, is just to use a resistive divider. It has the added benefit of synthetically lowering output impedance. And, its darn cheap. And it doesn't change the operating characteristic of the amplifier very much (if at all). And did I mention, its cheap? OK. Its cheap. And safe, and easy, and cheap. Cheap, cheap, cheap!

Other posters have identified "just get it out of the system" ... but you might like the colorizing it does to the signal. I would NOT however put an input attenuator on it - that just begs to increase noise overall.

Anyway, there you are.

GoatGuy
 

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You could get rid of the upper triode in the input cascode, running that stage as a simple common cathode stage and then use the discarded triode section as the upper valve in a beta-follower before the final cathode follower output stage. If you are using 12ay7 as per the schematic then I don't suppose that the miller capacitance of the input stage would be too much of a problem.
 

misok

Member
2012-10-09 7:00 pm
You could get rid of the upper triode in the input cascode, running that stage as a simple common cathode stage and then use the discarded triode section as the upper valve in a beta-follower before the final cathode follower output stage. If you are using 12ay7 as per the schematic then I don't suppose that the miller capacitance of the input stage would be too much of a problem.

i don't know about the variations but i had one with a single 5687.
 
The original Japanese M7 was a 4 stage phono preamp using 12ay7(or, more accurately 6072WA) only. The stages were cascode followed by RIAA network, common cathode followed by selector, balance and volume controls, then common cathode direct coupled to output cathode follower. It therefore has potentially enormously high gain, somewhat lowered by unbypassed cathode resistors in stages 2 and 3.

I am sorry, I have just noticed that this is a line amp only. Was the schematic part of an Audionote UK kit?
 
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What input cascode? I don't see no stinkin' cascode, but a simple input triode followed by a impedance reducer (AKA "cathode follower").

And... what's up with running these tubes substantially below their nominal current range? Extra gain? More exquisite sound stage and detailed separation of challenging mids? Brilliance on the cymbal-range, and power bass? [i.e. audiophile weaselwords...]

Run 'em where they're supposed to be, and they'll reward you with great response, and remarkably long life. The "problem" with the M7 circuit, all its reputation (and silliness with doubled-up resistors) notwithstanding, is that it also is a signal inverter. The phase of its output is reversed from the input.

While it "shouldn't" affect the acoustics of its result (because the Fourier Transform Theorem says that the power-spectrum of the frequencies of a signal A(t) are exactly the same as its inverse, -A(t) ...) in actual listening, it does. Entirely at the low-end of the frequency spectrum, where our ears are sensitive to phase. Mids? Maybe some ears can discern phase inversion from 200 Hz to 2000 Hz ... but not many. Me, no. Above 2000 Hz, no one apparently has particular phase sensitivity. Below 200 Hz, many people do.

Well... that's why there are those lovely reversible speaker cables with banana-plug jacks. Wait, what? We don't use 'em any more? Well that's sad!

Myself, I'd use a pair of tubes for each channel. An input amplifier through a cascode constant-voltage section (running about 100 V plate), and another identical section to amplify the signal again, cascode on the plate running 100V. Of course the 2nd stage couldn't be exactly identical 'cuz there would be too much voltage swing for its cathode. So... use the resistor-resistor lifter to tame things. Still, gain would be decent, and the output waveforms would be nice and symmetric, with semi-nulled triode transconductance being the amplifying technology.

Stick that on your sound stage and smoke it!

GoatGuy
 
Badges...

Goat guy, as I said in the edit to my second post, I mistakenly thought it was the original M7 phono-which is the only one I have seen- that was being referred to and which has, to use your eloquent phrase, a "stinkin' cascode" at the input.

I don't see no stinking... was a reference to the now nearly infamous movie line, "Badges? We don't need no stinking badges!"

Here's your wikipedia reference:
In Mel Brooks' 1974 Western Blazing Saddles, the line was delivered as "Badges? We don't need no stinking badges. Stinking badges - Wikipedia, the free encyclopedia"

... with special liberty taken with "badges" and "cascode". I rather like cascode all things considered, but consider it to also be both overused, and over-tamed with most negative-feedback distortion limiting schemes.

GoatGuy
 
Forgive me for butting in, but the line "Badges? We don't need no stinkin' badges" was originally from the John Huston film "Treasure of the Sierra Madre" (1948). It's a classic, well worth watching! Mel Brooks paid homage to that movie in "Blazing Saddles" (another movie well worth watching!)

And movie that old should ONLY be listened to using tube audio amplifiers, IMHO :D

While I'm here, and to keep my post on topic, I'm in agreement with GG that the simplest (and probably lowest cost) way around the problem is to simply put an attenuator or volume control on the output side of the M7.

Turn the volume control on the M7 up as high as possible (reducing attenuation ahead of the first amplifier) and reduce the "balance" control (to reduce the series resistor on the input).

Get the M7 output as high as possible (avoiding overdriving the M7 and introducing distortion, of course!), then adjust the attenuator on the output to keep from overdriving the power amplifier.

You don't want to reduce the input signal any more than necessary to preserve the signal-to-noise ratio while keeping the M7 distortion under control. You will have to play with the ratios (input level, output level) and listen to how it sounds.

Doesn't matter what flavor (there seem to be many) of M7 you have - too much gain (or too large input signal) in the pre-amp needs to be tamed with attenuation IF you don't want to dig into the innards of the amp and tinker.

Of course, if you want to be wild and " get under the hood" to play then lots of good suggestions have been posted. :cool:

~ Sam
 
Another possibility, if you are thinking about building something. This is a buffered volume control, slightly less than unity-gain, that I stumbled across. There are a lot of similar examples out there, if you want to search them out.

a 12AZ7 buffered volume control

The 12AZ7 was chosen, in part, due to it's higher Vhk rating. The cathode on this design is at around 200V (per the author - I've not built one of these myself). Seems if you wanted to use a similar equivalent tube (such as a 12AT7) you should be able to bias the heater off of the B+ to keep the Vhk within ratings.
 
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Hi, seem that I have solved the problem, I have changed a new potentiometer and it works perfectly now. Before this, the potentiometer, when just a slight turn, the volume already very high, now, I have turn the volume knob over 9 o' clock without problem. And actually I have tested a lot of potentiometers, including those quite expensive Alps, but at last I still have to use the custom made potentiometer, it gives a very accurate volume output, anyway, thanks all your helps and comments.
 
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