Cheap hybrid amp

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zyker

Member
Joined 2011
Hi, I'm new here and this is my first post :D

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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sreten

R.I.P.
Joined 2003
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.
.
 
Last edited:
Z

zyker

Member
Joined 2011
sreten said:
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.
.
Click to expand...

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.
 
P

pacificblue

Member
Joined 2008
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.

zyker said:
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.
Click to expand...

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.
 
F

Fenris

Member
Joined 2004
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.
 
Z

zyker

Member
Joined 2011
Fenris said:
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.
Click to expand...

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 :)
 
F

Fenris

Member
Joined 2004
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.
 
Z

zyker

Member
Joined 2011
Z

zyker

Member
Joined 2011
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Fenris

Member
Joined 2004
Volume control without a buffer or gain stage is called a "passive preamp". Basically it's just a volume control and/or input selector. As long as the input impedance of the power amp is greater than the impedance of the passive preamp, and the impedance of the passive pre is higher than the output of the source, you should be fine. The 3886 has a very high input impedance which is characteristic of op-amps. Most sources have an output impedance of between 600 and a few thousand ohms. A reasonable volume pot between 5k and 50k ohms will work fine and can easily be integrated into the power amp. The advantage is less connectors and wires external to the amp. The disadvantage is that if you change power amps, the volume control/selector can't be easily re-used.
 
Z

zyker

Member
Joined 2011
Fenris said:
Volume control without a buffer or gain stage is called a "passive preamp". Basically it's just a volume control and/or input selector. As long as the input impedance of the power amp is greater than the impedance of the passive preamp, and the impedance of the passive pre is higher than the output of the source, you should be fine. The 3886 has a very high input impedance which is characteristic of op-amps. Most sources have an output impedance of between 600 and a few thousand ohms. A reasonable volume pot between 5k and 50k ohms will work fine and can easily be integrated into the power amp. The advantage is less connectors and wires external to the amp. The disadvantage is that if you change power amps, the volume control/selector can't be easily re-used.
Click to expand...

I guess a passive preamp is the way to go.
Unless Juergen Knoop has another explanation about the 'integrated' design.
 
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