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Old 16th January 2008, 06:47 AM   #1
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Default which chip/design for mids/tweets amp?

I'm getting the itch to build a pair of monoblock amps for the mids and tweeters of my bi-amped homemade speakers, but I am bewildered by the chip choices and circuit variations posted by so many DIY'ers. As I have absolutely no interest right now in the low end (below, say, 300 Hz), can someone state their preference for a design that provides the best mid and high-end response? I don't need a lot of power (30 or so watts), but I need an amp stable into low impedance because two 8 ohm midrange drivers are paralleled in each enclosure. I don't want ultimate detail, but I would like natural timbre and instrument separation, along with three-dimensional spaciousness and lots of air. This is a tired cliche, but tube sound is my goal. I would even consider tri-amping if there is an ideal tweeter amp and an ideal midrange amp of lower power output. Cost is important, but I am not seeking the cheapest available amp. I plan to add an attenuator to enable driver matching. Can a variable shunt type be used?

The people here seem to have a wide range of experience, and I really would appreciate anything you care to share. For reference, right now I'm using an Audio Research 100.2 for the full range. Excellent amp.

Regards,
Tom E
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Old 16th January 2008, 08:35 AM   #2
Nuuk is offline Nuuk  United Kingdom
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For a little extra cost, building separate amps for the mids and tweeters makes sense, and in my experience works well.

I currently use a four-channel LM3875 with a Pedja Rogic regulated supply but I have a nominal 6 ohm load rather than 4 ohm. However, I am confident that as you don't need too much power, this design would work for you too!
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Old 16th January 2008, 09:28 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
to save cost I would build a three channel amplifier with one transformer. Three rectifiers and three smoothing banks (or for a little extra 6 rectifiers feeding 3 smoothing banks).
Drive each 8ohm speaker with it's own amp channel and select the smoothing/decoupling on each amp to best suit the frequency range required. Locate each 3channel amp right next to it's speaker with the shortest wires and fewest connectors.
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Old 16th January 2008, 12:56 PM   #4
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isn't an lm1875 enough for the tweeter?

So then it's a lm3875/lm3886 for the mid and a lm1875 for the tweeter.

Choose one supply voltage that suits the lm1875 and you will probably have enough for your mid with the lm3875.
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Old 16th January 2008, 03:52 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

The power requirements above and below 300Hz / 400Hz are
roughly equal for an active set up, for rock you need more power
below, for classical more power above.
This is assuming equal bass and mid/treble section sensitivities.
I'm talking about clipping levels, not average power levels.

Some would regard using the AR 100.2 below 300hz with chipamps
above as an absolute travesty, have to say I agree somewhat.

I'd say check out the JLH class A design or the Death of Zen design,
given your description double the device pairs (current and power)
for the mid section, use a single device pair for treble and run them
all from a very juicy supply, capacitance multiplied or regulated.

see

http://www.tcaas.btinternet.co.uk/index-1.htm

and

http://sound.westhost.com/project36.htm

There are other class A designs ......

/sreten.
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Old 17th January 2008, 12:59 AM   #6
Leolabs is offline Leolabs  Malaysia
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Simple answer,it's a LM1875.
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Old 17th January 2008, 06:38 AM   #7
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Seems that the LM1875 is a favored choice for tweets, but no one has bothered to state why, exactly, or offered any circuit specifications. I've read that it's susceptible to hum.

How about sources for parts/kits? I'm not a novice at electronics, but I admit some of the technical discussions here leave me baffled. I've searched for and read everything on a lot of sites regarding these amps, but I'm still somewhat confused about PS size, input/output caps, that sort of thing. A basic kit would help me get started. Any advice regarding sources of knowledge would also help me. C'mon, you guys, I need your help.

To clarify, I will NOT be using the AR 100.2 to run low end with these amps on top; that would be a tragic waste of beautiful amplifier. I simply gave that info as a point of reference of my current set-up. I will find some big old SS amp to run the low end. Almost anything with a decent PS should be able to handle the lows.

Peace,
Tom E
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Old 17th January 2008, 08:43 AM   #8
Nuuk is offline Nuuk  United Kingdom
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Quote:
A basic kit would help me get started.
Yes but making one yourself would teach you more IMHO!

I haven't used the LM1875 but I don't think it is more prone to hum than the LM3875 and I don't have problems with that.

You appear to be using the GC for exactly the same job that I am using mine, ie to drive mids and tweeters. I use a 300VA traffo with dual rectifier bridges feeding the regulator circuit for each side, ie left and right channel. To be honest a 160VA per side would be big enough but I had the 300's here so I used them.

I appreciate that if you are new to all this, building the regulated supply may be a bit daunting but there is no reason not to build the four amps yourself, perhaps starting with one of the basic inverted, or non-inverted designs, of which there are many tried and tested (all the ones on my site are proven to work well).
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Old 17th January 2008, 10:00 AM   #9
sreten is offline sreten  United Kingdom
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Default Re: which chip/design for mids/tweets amp?

Quote:
Originally posted by madisonears

I don't want ultimate detail, but I would like natural timbre and
instrument separation, along with three-dimensional spaciousness
and lots of air. This is a tired cliche, but tube sound is my goal.
Tom E
Hi,

Whilst I will not argue amount the sound quality merits of chip-amps
in terms of nothing much being wrong with them I've never read a
description of one that implied the above.

/sreten.
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Old 18th January 2008, 01:40 AM   #10
Leolabs is offline Leolabs  Malaysia
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Why LM1875??Well,once u have listen to it,sure u will understand.
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