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Old 12th September 2009, 06:45 PM   #1
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Smile Lm 1875? 3886?

I have a little time now to decide what amps to go for to drive the OB's I am building. I thought I was set on the Tripath boards from Sure electronics, but now I have heard a LM1875 built by John Blackburn (cheers John) I am not so certain. I need two stereo amps as I am using an active crossover, and the drivers are 1 X B200 and 2 X alpha 15's each side. Would I be better going for say an LM1875 for the mid /treble and an LM3886 for the bass, or would two 1875's be sufficient? Also, if I built them, would this transformer be man enough?

http://uk.farnell.com/multicomp/mcfe...18v/dp/9531831

Lots of questions from a n00b sorry, all help welcomed with open arms!

Steve
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Old 12th September 2009, 09:14 PM   #2
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Hi Steve.

The LM1875s are sharing a 160va 2 x 18v http://www.rapidonline.com/Electrica...sformers/35506 and the LM3875 you heard at first has a 300va 2 x 22v from Airlink http://www.airlinktransformers.com/t...ansformers.asp I cant remember what voltage is called for with the 3886, it's here somewhere though.

The Airlink transformers are very nicely made, and one of the very few things still made in the UK.

You may be better to go monoblock and have the amps at the speakers. Two 150 va transformers cost a bit more than one 300va mindst. I've seen the wise suggest that it's better to have long interconnects and short speaker wires but the science of it was beyond me.

John
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Old 12th September 2009, 09:21 PM   #3
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Hi John, cheers for that. check your email btw...

I still can't decide what to do, I am hoping for a bit of advice, but I could just order the BrianGT boards (1875 and 3886) and start from there.
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Old 12th September 2009, 09:35 PM   #4
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Hopefully someone far wiser than I will chip in with an opinion. My 3875s could struggle with the 15 inchers in parallel. The chips have big heatsinks mindst and the nights are getting colder so we could be allright.

John
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Old 13th September 2009, 06:31 PM   #5
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How loud do you want to listen? If you want or need it really loud, that could sift out the LM1875.

Will you use equalizing against the OB roll-off? If you use equalizing or Linkwitz transform, you will definitely need a lot of power. That will rule out chipams for that task. Graham Maynard's T-Bass circuit, which needs an amplifier that can drive very low impedance, also pretty much rules out chipamps.

Even without equalizing, a powerful solid-state amplifier is recommendable for the Alpha 15. If a chipamp at all, you should use one LM3886 per woofer, if only because it has the highest current rating.

The B200 could get along with the output power of any of the chipamps. Another listening session would be the best approach to decide, which you like better. And it will become more meaningful, if you can check them together with the B200.
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Old 13th September 2009, 07:07 PM   #6
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Quote:
Originally Posted by pacificblue View Post
How loud do you want to listen? If you want or need it really loud, that could sift out the LM1875.

Will you use equalizing against the OB roll-off? If you use equalizing or Linkwitz transform, you will definitely need a lot of power. That will rule out chipams for that task. Graham Maynard's T-Bass circuit, which needs an amplifier that can drive very low impedance, also pretty much rules out chipamps.

Even without equalizing, a powerful solid-state amplifier is recommendable for the Alpha 15. If a chipamp at all, you should use one LM3886 per woofer, if only because it has the highest current rating.

The B200 could get along with the output power of any of the chipamps. Another listening session would be the best approach to decide, which you like better. And it will become more meaningful, if you can check them together with the B200.
Hi. I do not listen too loud, so I do not need a load of power. I am going to give the Sure 4 X 100W a go, should be good for around 35-40W with the speakers I will be using. If it is not enough, I will use each channel of that amp to drive each of the Alpha's and use another for the B200's. I am currently constructing the OB's and have ordered the amp.

Steve
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Old 13th September 2009, 07:28 PM   #7
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I am using 2 lm3875s per channel at the moment with my Mission 753 speakers and I would say that they are just about right. However the dual drivers per amp on the missions are nothing like 15' in size. I would think that the 1875 would be a little weak for this application.

The lm3886 would be more the thing. I have made a test amp with this chip and it is more solid sounding than the 3875. However my test amp was not made with any great care, it also uses a toroid versus the smps setup on my 3875. So I would not describe my comparison as like for like.

Over the Sure boards, I prefer the 2x100watt sure board over the chipamps. Even the ta2024 boards are an improvement on the lm series amps I have built.

I tried to listen to the 4x100w amps today, but I have encountered a problem, waiting for advice from sure as to how to proceed. I would think though that either it or using 2 of the 2x100w boards would fit your bill nicely. I am going for a mixture of 100w boards and 15w for the tweeter hopefully.
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Old 13th September 2009, 07:38 PM   #8
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Quote:
Originally Posted by justblair View Post
I am using 2 lm3875s per channel at the moment with my Mission 753 speakers and I would say that they are just about right. However the dual drivers per amp on the missions are nothing like 15' in size. I would think that the 1875 would be a little weak for this application.

The lm3886 would be more the thing. I have made a test amp with this chip and it is more solid sounding than the 3875. However my test amp was not made with any great care, it also uses a toroid versus the smps setup on my 3875. So I would not describe my comparison as like for like.

Over the Sure boards, I prefer the 2x100watt sure board over the chipamps. Even the ta2024 boards are an improvement on the lm series amps I have built.

I tried to listen to the 4x100w amps today, but I have encountered a problem, waiting for advice from sure as to how to proceed. I would think though that either it or using 2 of the 2x100w boards would fit your bill nicely. I am going for a mixture of 100w boards and 15w for the tweeter hopefully.
Blair, what is the problem you are having with the Sure board? I hope it isn't too serious. Despite the Alpha's being 15" in physical size. they are relativly sensitive and a benign load that doesn't dip much below 8Ω and I feel sure (!) that the board from Sure will be fine.

Steve
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Old 13th September 2009, 07:55 PM   #9
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Quote:
Originally Posted by aquapiranha View Post
Blair, what is the problem you are having with the Sure board? I hope it isn't too serious. Despite the Alpha's being 15" in physical size. they are relativly sensitive and a benign load that doesn't dip much below 8Ω and I feel sure (!) that the board from Sure will be fine.

Steve
Board faulty, but please bear in mind the one they sent me was pre-production I would not say that this is indicative of Sures normal quality standards. I'll say a bit more once I have spoken to Robert at Sure. I don't think it is a design problem.
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Old 13th September 2009, 07:57 PM   #10
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OK thanks Blair, keep us posted!

Steve
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