Will no NFB hurt my amp

Albert

Albert

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Feb 21, 2014
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My Ray Lumley tube monoblocks have adjustable negative feedback (0 to 20 dB I think) so I decided to try it with the feedback turned to zero. There is a small improvement in detail and dynamics as well as better imaging. It sounds very good but I was wondering if there was a danger of the amp becoming unstable without negative feedback. I haven't heard any increase in noise or distortion, the background is dead quiet. I have the amps plugged into a Consonance power filter so the power is nice and clean and stable.

Has anyone heard of any problems running tube power amps with no negative feedback?
 
In general, actually amps have more of a chance of instability with feedback then without, atleast in the design stage. The amp IMO was designed originally with no feedback and then feedback was added, so no, there should not be any problems with no feedback.
 
That is good to know. I played five lp's in a row today without any problems and the amp sounds clearer and more dynamic with the setting at zero. I think that I will leave it there and enjoy the tunes.
 
That was an interesting read. It is good to know that it is too much NFB that can cause oscillation and not too little so I guess I shouldn't worry. In the past I had the NFB set at under 10% which is like 2 dB so the difference is subtle.
 
The Aikenamps NFB article was a good read. Thanks for the link. I have two comments.

First, the article did not mention a point that has been frequently discussed on Audiogon by some of the tech experts, such as Ralph Karsten of Atmasphere. Ralph has extensively written that while NFB can, if properly employed, achieve the positive effects mentioned in the Aikenamps article, there is a "cost" not mentioned in the article: namely, TIM distortion. TIM distortion is increased levels of odd-ordered harmonic distortion which the human ear detects as harsh loudness. That is why Atmasphere and some other amp manufacturers do not use NFB in their amps. That said, as the folks at ARC once told me, ARC uses some NFB in its amps as part of its overall "balanced" design goal of achieving certain electronic performance results. But compromises are made. There is no perfect answer.

Second, if the OP has a question about using his amp's NFB control, I suggest that his question be directed to the manufacturer's customer service folks. They know their product best and are in the best position to advise the OP how to adjust the amp's NFB to achieve "optimal" operating results. It may be that the answer is as simple as set NFB to the level that sounds subjectively best to the OP's own ears.

Cheers,

Bruce
 
The original manufacturer went out of business in the 90's as far as I know. I suspect that it is just as simple as setting the nfb to what sounds the best. I have adjusted this before and have heard slight audible differences but no other ill effects. I was just wondering if anyone had more experience with this. I have heard that some guitar amps have adjustable nfb but I haven't heard of too many stereo amps that do.
 
Hi Albert,

I did a quick web search of you amp. Many hits came up on Audiogon. Seems like your amp attracted a lot of favorable comments in the day.

As you said, it looks like the company is out of business. Do you have an operating manual. I gotta believe your question is addressed there. If you don't have a manual, check Audiogon for folks who still have their Lumley amps and manuals. They might be able to fax or mail a copy of the manual to you.

If you really love the amp, you might be able to track down a schematic too. I mention this because if your amp is getting long in the tooth, it might be a good idea to have a tech look it over. Old caps may need replacing and so forth.

If all else fails, my guess is that you can adjust the NFB control to suit your tastes. But it would be better to see that in writing.

Regards,

Bruce
 
I have a pdf copy of the owners manual and no mention is given of the adjustable NFB. I have a second pair of these amps (purchased to power a pair of future Acoustat model 8's which needs two pairs) and they do not have this feature. I suspect this was an option or was a customized version. There has been no problems so far running them at the zero setting so I am not going to worry about it. I like how the amps sound that way.

The amps have upgraded caps (from the previous owner) and I have had the amps checked out a couple of years ago and all was well. I really do love these amps and they will probably be the last power amps I buy (though I wouldn't mind trying a pair of Atma-Sphere MA-2's if I can scrape enough money together).
 
Albert said:
I have a pdf copy of the owners manual and no mention is given of the adjustable NFB. I have a second pair of these amps (purchased to power a pair of future Acoustat model 8's which needs two pairs) and they do not have this feature. I suspect this was an option or was a customized version. There has been no problems so far running them at the zero setting so I am not going to worry about it. I like how the amps sound that way.

The amps have upgraded caps (from the previous owner) and I have had the amps checked out a couple of years ago and all was well. I really do love these amps and they will probably be the last power amps I buy (though I wouldn't mind trying a pair of Atma-Sphere MA-2's if I can scrape enough money together).
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Very, very nice amp the Ray Lumley m100. Maybe I thinks so cause I have em myself!
What version of the Ray lumleys m100 do You have? I think they made 4 different versions. Mostly cosmetic changes betwen them.
Ive been looking all over the net trying to get hold of the manual for the Ray Lumleys m100. Ill be most grateful if You wanna email it too me.
Hope my english aint to bad and You understand what i write. Long time since I study it in school.
 
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