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Manley Euro Stereo 100 purchase and first impressions.


bracurrie

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OK, Adcom's 970 mV in for 60 W out vs. ML's 750/800 mV in for 100 W out. Doesn't seem to be a huge difference. Thanks.

I agree. My amp now is running AU7s instead of AT7s so the gain factor is down from 45 to 15. Not sure how to calculate the change to power output but its less than 100 with the tube change and the small amount of increased negative feedback from an added resistor.

Also, just to be clear, I am not an audio engineer or electrical engineer. I am only parroting out what I have heard and read and I probably am getting some of this technical stuff wrong. I apologize to the audience members that really do know how amp circuits work. Its amazing how circular the cause and effects appear to be when someone tries to explain how all this works to me.

With the transformer based HF attenuator I am seriously thinking of returning the amp to its designed configuration.

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because I am pretty sure the noise you're hearing is from the preamp.

What you have to remember is that in my case the preamp is driving the miniDSP not the amps. The noise is coming from the miniDSP most probably. When I disconnect the preamp from the miniDSP there is no change in noise. When I turn the gain down on the miniDSP there is no change in noise. Also there is no difference in using the phono stage in either switch position or the other inputs on noise.
That's a lot of stuff hooked together, makes it hard to diagnose and treat.

From what I can tell, the noise is coming from the DSP circuitry after its volume control, since its volume control won't decrease the noise. That suggests to me that the mini DSP should be plugged into the preamp, not the other way around. It's probably supposed to work that way. It sounds like the DSP has too much residual noise to be driving an amplifier directly.

Then, especially if your preamp's gain switch is turned to Low, I would hope there isn't a noise problem. You might have to turn your DSP's VC up fairly far.

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From what I can tell, the noise is coming from the DSP circuitry after its volume control, since its volume control won't decrease the noise. That suggests to me that the mini DSP should be plugged into the preamp, not the other way around. It's probably supposed to work that way. It sounds like the DSP has too much residual noise to be driving an amplifier directly.

I agree that the DSP is a source of some noise. But the amp has a low 80's SNR so with ultra high sensitivity speakers you are going to hear both. The miniDSP is connected to the preamp outputs to do its crossover work before outputting tot he low frequency amp and the high frequency amp. There is no other way to do it. By the way just for grins I connected the preamp to the amp directly and the result was quieter which further illustrates that the miniDSP is a noise source. If I had 90-100 sensitivity high frequency horns I wouldn't be hearing noise.

Now that I have the high frequency horns attenuated properly the noise is no longer a problem. I subtracted 12db which pushes the noise below audible at my listening position. There is a way to do the math to illustrate this but I do not know how to do that.

You might have to turn your DSP's VC up fairly far.

After reviewing the specs of the SP6B and the miniDSP my tech determined that I should change the input sensitivity to the 2mV input option on the miniDSP. This had the effect of my now having to turn the volume knob past 12 o'clock to get 85-90db of sound and it seems to have given the high end more air which could indicate that the change reversed some high end rolloff. I am now in a very happy place with my Manley Labs.[:D]

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12aY7's are quieter than 12AT7's or 12AU7's. The 12AY7's have an amplification factor of 44, which is about in the middle of the T's and U's.

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12aY7's are quieter than 12AT7's or 12AU7's. The 12AY7's have an amplification factor of 44, which is about in the middle of the T's and U's.

The following was my source for confirming what the Manley Labs people had said. http://www.300guitars.com/articles/preamp-tube-gain-factors-and-substitution-chart/

12AX7 Family Tube Substitution

The 12AX7 family of dual-triode preamp tubes consists of the 12AX7, 5751, 12AT7, 12AY7, 12AV7 & 12AU7. These are all pin compatible with one another, the only differences being the gain factor of each tube. A common substitution is to replace a 12AX7 with a 5751 or a 12AY7 to tame a preamp that tends to overdrive too easily, allowing you to get a better ‘clean’ sound out of your amp.

Tube Gain Acceptable Substitutes
12AX7 100 5751 12AT7 12AY7
5751 70 12AX7 12AT7 12AY7
12AT7 60 5751 12AY7
12AY7 45 5751 12AT7 12AV7
12AV7 41 12AY7 12AU7
12AU7 19 12AV7

It is important to note that the above is not carved in stone and any of these tubes can be substituted for any other. For example in the 4×10 5F6-A Bassman the stock first gain preamp tube is designated as a 12AY7 which has a gain factor of 45. Replacing it with a 12AX7 that has a gain factor of 100 dramatically changes the tone and feel of the amp. Experiment and see what works best for you.

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12aY7's are quieter than 12AT7's or 12AU7's. The 12AY7's have an amplification factor of 44, which is about in the middle of the T's and U's.

The following was my source for confirming what the Manley Labs people had said. http://www.300guitars.com/articles/preamp-tube-gain-factors-and-substitution-chart/

12AX7 Family Tube Substitution

The 12AX7 family of dual-triode preamp tubes consists of the 12AX7, 5751, 12AT7, 12AY7, 12AV7 & 12AU7. These are all pin compatible with one another, the only differences being the gain factor of each tube. A common substitution is to replace a 12AX7 with a 5751 or a 12AY7 to tame a preamp that tends to overdrive too easily, allowing you to get a better ‘clean’ sound out of your amp.

Tube Gain Acceptable Substitutes
12AX7 100 5751 12AT7 12AY7
5751 70 12AX7 12AT7 12AY7
12AT7 60 5751 12AY7
12AY7 45 5751 12AT7 12AV7
12AV7 41 12AY7 12AU7
12AU7 19 12AV7

It is important to note that the above is not carved in stone and any of these tubes can be substituted for any other. For example in the 4×10 5F6-A Bassman the stock first gain preamp tube is designated as a 12AY7 which has a gain factor of 45. Replacing it with a 12AX7 that has a gain factor of 100 dramatically changes the tone and feel of the amp. Experiment and see what works best for you.

Guitars amplifiers are an entirely different goal oriented device. Guitar players in many cases switch out pin compatible tubes to mold just how the Guitar overdrives "distorts" the sound. With Hi Fidelity amplifiers the goal is very different. Changing tube types from what a circuit was designed for without changing Plate, Cathode and Supply voltages will 100% of the time increase distortion. Increasing negative feedback has its negative effect on sonics also. No way exists to cheat these facts.

The above blanket statement quoted above stating any of the above tubes can be safely swapped for another is dangerous.... IT IS ABSOLUTELY NOT TRUE IN HIGH FIDELITY AMPLIFIERS and probably not 100% true in all Guitar amps. Over heated plate and cathode resitors can be the result. In many cases it may work but I'd be checking with the manufacturer before doing so if they are still around. At the very least find plenty of instances where the swap has been successful.

Watch what you read on the internet folks just because someone wrote it does not make it true.

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Guitars amplifiers are an entirely different goal oriented device. Guitar players in many cases switch out pin compatible tubes to mold just how the Guitar overdrives "distorts" the sound. With Hi Fidelity amplifiers the goal is very different. Changing tube types from what a circuit was designed for without changing Plate, Cathode and Supply voltages will 100% of the time increase distortion. Increasing negative feedback has its negative effect on sonics also. No way exists to cheat these facts.

The above blanket statement quoted above stating any of the above tubes can be safely swapped for another is dangerous.... IT IS ABSOLUTELY NOT TRUE IN HIGH FIDELITY AMPLIFIERS and probably not 100% true in all Guitar amps. Over heated plate and cathode resitors can be the result. In many cases it may work but I'd be checking with the manufacturer before doing so if they are still around. At the very least find plenty of instances where the swap has been successful.

Thanks Craig. Point taken. Manley Labs were the ones that recommended the small added negative feedback and swapping the tubes to reduce gain. The plate resistor was already on the low side for the AT7 as was David's habit to do. In any case I probably will return the amp to original spec because the transformer based attenuator is doing its job nicely.
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