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Old 6th May 2011, 09:49 AM   #1
zyker is offline zyker  Malaysia
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Default Cheap hybrid amp

Hi, I'm new here and this is my first post

I'm thinking of building a cheap hybrid amp with a tube buffer and dual 3xLM3886 power amp to drive my 120W 4ohm speaker.

Is there power supply and transformer enough to power the amp?

eBay.com.my: 40Vdc / 10 Amp Dual Polarity Power Supply Module,Board (item 260490141852 end time Jun 04, 2011 19:21:43 MYT)

eBay.com.my: 220V 50W R-core Transformer X-Former 12V+12V 25V+25V (item 120695989797 end time May 09, 2011 11:21:30 MYT)

Any advice is much appreciated.

I'll post the proposed layout design later.

Thanks in advance
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Old 6th May 2011, 11:02 AM   #2
alayn91 is offline alayn91  France
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Hello,

If you want to drive yours speakers at 2 x 120W, you will need, at least, 300VA transformer.
50W or 50VA won't be big enought !

Regards.
Alain.
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Old 6th May 2011, 11:10 AM   #3
zyker is offline zyker  Malaysia
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Quote:
Originally Posted by alayn91 View Post
Hello,

If you want to drive yours speakers at 2 x 120W, you will need, at least, 300VA transformer.
50W or 50VA won't be big enought !

Regards.
Alain.
I see.
Is the calculation based on 2x120 = 240. Thus I need more than 240VA?
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Old 6th May 2011, 11:48 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

FWIW to drive a pair of speakers to the equivalent of 120w music programme,
(not sine wave test signals), music has lower average power at clipping levels,
the approximate statistics indicate that 150VA will be more than adequate.
(Decent quality music programme, not garbage quality super compressed.)

rgds, sreten.
.
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Last edited by sreten; 6th May 2011 at 11:50 AM.
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Old 6th May 2011, 12:09 PM   #5
zyker is offline zyker  Malaysia
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Quote:
Originally Posted by sreten View Post
Hi,

FWIW to drive a pair of speakers to the equivalent of 120w music programme,
(not sine wave test signals), music has lower average power at clipping levels,
the approximate statistics indicate that 150VA will be more than adequate.
(Decent quality music programme, not garbage quality super compressed.)

rgds, sreten.
.
I have read somewhere that 18v-0-18v is good enough to drive 4ohm speaker with LM3886. Would 24v be overkill for the speaker? And I will be using 3xLM3886 on each channel.
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Old 7th May 2011, 09:14 AM   #6
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Calling it a 120 W speaker means that it can withstand that amount of power for a specified amount of time in a specified signal form without burning the speaker coil.

It has nothing to do with the power your amplifier needs to drive it. It does not tell you how loud the speakers will be with a certain amount of power or how loud they can become at all due to their mechanical limitations. It is even possible to destroy such a speaker with less than 120 W of electrical power. At the same time the speaker may live happily ever after on a 1000 W amplifier, depending on the way you use it.

Quote:
Originally Posted by zyker View Post
I have read somewhere that 18v-0-18v is good enough to drive 4ohm speaker with LM3886. Would 24v be overkill for the speaker? And I will be using 3xLM3886 on each channel.
It is more like an 18-0-18 transformer will ensure that you can use a reasonable heatsink for a single LM3886 with a 4 Ohm load. 24-0-24 may not be overkill for the speaker, but it can become difficult to heatsink a single LM3886 when you use 4 Ohm loads. If you use 3 LM3886 in parallel, each IC will only have to cope with a third of the heat dissipation and that will make heatsinking easy again.
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Old 7th May 2011, 10:16 AM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by zyker View Post
And I will be using 3xLM3886 on each channel.
Hi, so effectively each amp will see a 12 ohm load, not 4 ohms, use 24-0-24, rgds, sreten.
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Old 7th May 2011, 12:15 PM   #8
Fenris is offline Fenris  United States
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And why do you want a tube buffer? The 3886 is very easy to drive and has more than sufficient gain. A tube would only be a fashion statement and would offer no practical value. In fact, the additional high voltage power supply, DC blocking caps, and added complexity would introduce far more opportunities for problems while solving a non-existant "problem". The "tube sound" that everyone wants is not to be gained by a tube front end, a properly done tube front end add no character to the sound. It's the tube output stages (and transformer) that are generally responsible for the "tube character" sound in hi-fi amps.
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Old 7th May 2011, 12:24 PM   #9
zyker is offline zyker  Malaysia
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Quote:
Originally Posted by Fenris View Post
And why do you want a tube buffer? The 3886 is very easy to drive and has more than sufficient gain. A tube would only be a fashion statement and would offer no practical value. In fact, the additional high voltage power supply, DC blocking caps, and added complexity would introduce far more opportunities for problems while solving a non-existant "problem". The "tube sound" that everyone wants is not to be gained by a tube front end, a properly done tube front end add no character to the sound. It's the tube output stages (and transformer) that are generally responsible for the "tube character" sound in hi-fi amps.
After considering a proper heatsink for the chips, I prefer to separate the power amplifier circuit into another chassis.

The tube buffer is intended to be use as the preamp with addition of input selector module and volume control. Do you mean that using a tube buffer in the preamp would not give any improvement to the system?
Please advice since I'm really new in this field
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Old 7th May 2011, 03:36 PM   #10
Fenris is offline Fenris  United States
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It depends on your source(s). Most modern sources put out about 2v peak to peak, which should be more than sufficient to drive the 3886. You should have a buffer of some type after your volume control, it minimizes the effects of non-ideal interconnects.

Tube buffers aren't perfect and add their own level of distortion, based on design and implementation. They also use high voltage supplies which can be dangerous for the inexperienced. If you want to learn about tubes, John Broskie's Guide to Tube Circuit Analysis & Design is a great site, but be prepared to spend weeks reading and learning.

A simple solid state buffer would be the B1 buffer from Nelson Pass (The First Watt B1 Buffer Preamp Article Written By Nelson Pass Of First Watt DIY Audio Kits Reviews) which operates on lower voltages and is probably far less expensive to build.
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