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Old 12th April 2005, 08:15 AM   #1
kmj is offline kmj  Sweden
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Default Low sensitivity speakers and valvs, no nfb?

Hi, i just ran in to some problem that has the potential to totally sink my valve-amp plans, or atleast put them on hold for another year or two.

Here goes. A friend of mine pointed out a thing that i had totally missed.

As i understand it valveamplifiers can become unstable when working with low sensitivity speakers and to fix this negtive feedback is used to stabilise the amp. The downside to this is that NFB reduces the feeling of "atmosphere" in the music.

So, my problem is that my darling 70W amp with 4xEL34/channel has to drive a pair of bookshelfspeakers with their senitivity in the lower 80db, around 83-85db i think it was. This without using NFB.

The speakers are 8ohms and the amps power (70w PP UL) is calculated at a load of 5ohms.

The output transformers to be used are either hammond, lundahl or the recommended piltron. The primary impedance of the original trafo is 2757ohms at a secondary load of 5 ohms.

Am i in deep **** or is it ok the start to order components?

Best regards
Kmj
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Old 12th April 2005, 08:22 AM   #2
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Default Re: Low sensitivity speakers and valvs, no nfb?

Quote:
Originally posted by kmj
As i understand it valveamplifiers can become unstable when working with low sensitivity speakers and to fix this negtive feedback is used to stabilise the amp.
No. Not as I understand it, anyway.

Using low efficiency speakers means that you will need more power to reach a given SPL. If an amplifier (valve based or otherwise) becomes unstable because of increased power requirements, it is poorly designed.
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Old 12th April 2005, 08:34 AM   #3
kmj is offline kmj  Sweden
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Ok.
So the "backlash" from the speakers, the puls so to speak, doesn't affect the preformance?

excuse the bad description, im trying to describe something that i have no ida of what it is.
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Old 12th April 2005, 08:43 AM   #4
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Quote:
Originally posted by kmj
So the "backlash" from the speakers, the puls so to speak, doesn't affect the preformance?

excuse the bad description, im trying to describe something that i have no ida of what it is.
Uhh... I'm not sure if I understand what you're trying to say. Do you mean back-EMF from the speakers or something?
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Old 12th April 2005, 09:46 AM   #5
kmj is offline kmj  Sweden
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Quote:
Do you mean back-EMF from the speakers or something?
Sorry, as i understood it the problem was that due to the high outputimpedance of the valves, compared to solidstate. Due to the difference, valves shouldn't be as good as SS when used with low sensitivity speakers and this difference in impedance could cause an instabilty in the amplifier. Also the quality of the outputtransformer had impact on the result, do I make any sense?
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Old 12th April 2005, 09:46 AM   #6
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A difficult load, like a speaker with poor efficiency will not make your amp unstable. Espescially if it has no NFB. Increasing the negative feedback will make any amp less stable! So dont worry about that.
A speaker with low efficiency is not by itself bad in terms of making an amp unstable. But usually a speaker with low efficiency is just that because it has complex crossovers and correction networks to give it flat frequency respons, which is so important on paper. (doesn't matter so much in the real world since most listening environments screws with the respnos anyways.) Also when u drive a low SPL/W speaker u must crank the volume up a lot more so the amp is more stressed, which can in some cases, where the amp is on the edge stability wise, make the amp go into oscillation. But there again, if the amp has no neg feedback, it will not have that problem.

How low in efficiency are those speakers of yours?
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Old 12th April 2005, 09:50 AM   #7
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Forgot the point about complex crossovers: It introduces phase shifts and impedances that are like rollercoasters, all up and down, which is difficult for many amps to handle.
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Old 12th April 2005, 09:53 AM   #8
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Quote:
Originally posted by kmj
Sorry, as i understood it the problem was that due to the high outputimpedance of the valves, compared to solidstate. Due to the difference, valves shouldn't be as good as SS when used with low sensitivity speakers and this difference in impedance could cause an instabilty in the amplifier. Also the quality of the outputtransformer had impact on the result, do I make any sense?
The impedance and efficiency of a speaker are two different things. Because a speaker is low efficiency does not necessarily mean that it is also low impedance. So, no; it does not make sense.
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Old 12th April 2005, 03:21 PM   #9
kmj is offline kmj  Sweden
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the sensitivity of my speakers are about 83db but sent a mail to the designer who didn't recommend using those speakers since it would make the amp work way to hard and it would be very noticable.

well, ill just hade to build a solidstate amp and save money to a pair of proacs PRIOR to the valves.

ah, all these choises

anyway, i'll get my facts straight and return with a reply so perhaps others can use the possible replys.
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Old 12th April 2005, 04:08 PM   #10
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Quote:
Originally posted by kmj
the sensitivity of my speakers are about 83db but sent a mail to the designer who didn't recommend using those speakers since it would make the amp work way to hard and it would be very noticable.
Well, any amp is going to have to work hard.

The only difference is that tubes become dramatically more expensive above the 20W range.

With some medium quality iron you can probably make it for 2USD/watt, that is to say, $200 for a 100W amp.

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