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Old 9th June 2010, 10:58 AM   #1
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Smile Stereo Amplifier with pre Subwoofer

I would want to advise you that I don't have much experience with DIY.

I have a decent Onkyo Receiver which I use to drive my ART Monotors along with Pre-Amp to my Velodyne subwoofer.

I know I should be using a dedicated 2-channel amp,but didn't know much about audio during the purchase.

I wanted to know whether it would be possible to build a decent 2 channel amp to power my monitors which are 8 ohms 50 watts programmed power each.
I would want the amp to pre my subwoofer.
I don't want a complex setup,as this would be my first time soldering since college,but I also don't want an amp which would sound worse then my Onkyo SR502.

I would prefer not spending much on the parts,may be upgrade it later depending on what it sounds like when completed.

Could someone lead me in the right direction.
The best store for me is Jaycar and RS Components to buy my parts,I would need everything from scratch-including soldering iron.
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Old 9th June 2010, 04:14 PM   #2
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Well, for a beginner, you should probably get a bit of practice in soldering and other necessary skills before you attempt a project. An understanding of the theory would probably help a bit as well when troubleshooting the circuit.

Anyway, that aside, for the main amplifier, you're probably best sticking to a simple chipamp design using the LM1875. A suitable design can be found on Rod Elliot's site here. The LM1875 outputs ~25W max and would suit the speakers perfectly. Add to that, if built properly, it sounds great!

At the end of the page, there is a section on power supply design-but be warned, this means dealing with possibly lethal voltages, so it's probably best to get a qualified electrician to do the wiring or at least consult one.

As it happens, Rod also lives in Australia and sells well designed boards which you can use for the project. This takes a lot of stress out of the project and makes it easier for a beginner.

For the sub woofer, if it has a LPF (low pass filter) on the input already, then you're probably best just making a simple mixing circuit to combine left and right channels into mono (one channel) and then set up the LPF on the sub appropriately. You may also need a high pass filter (HPF) to stop the lower frequencies from going to the monitors (as the sub deals with these instead). Some preamps do have separate mixed (or mono) outputs just for subs which would make it slightly easier.

If your sub doesn't have a LPF, then you'd need a crossover circuit to separate the signals going to the sub and the main speakers. These can also be found on Rod's site although they would complicate the circuit quite a bit.

If you look around, you can get all the parts needed for the amp cheaply and there's plenty of room for upgrades.

One thing I would suggest first though, is to get some more experience and brush up on your soldering skills before you attempt something like this. There is a wealth of info in Rod's site and a very good soldering guide can be found here.

Last edited by jackand08; 9th June 2010 at 04:21 PM.
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Old 9th June 2010, 11:14 PM   #3
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Thanks for your reply,the subwoofer has a low pass filter on the input,also a crossover.The subwoofer is a Velodyne CHT-8R.

The Onkyo receiver which I currently use also had a crossover which i have set at 100Hz.

Thanks for the soldering guide,I feel I will get a soldering iron first along with some cheap PCB to test using it.
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Old 9th June 2010, 11:45 PM   #4
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Thanks for the suggestions and advise,my Velodyne subwoofer has a low pass filter and a crossover setting.

The Onkyo receiver also has a crossover setting.

I have e-mailed ROD for more details,but from what I read on his website,I feel the 2 channel amp desogn is a little too primitive and wouldn't better my Onkyo receiver.
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Old 10th June 2010, 04:27 PM   #5
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You're welcome .

I would not underestimate the LM1875 amplifier at all. If built properly (which is easier using Rod's boards) it can sound stunning and outperform some much more expensive amplifiers-not to mention, you have the satisfaction of building it yourself and it will be much easier to modify should you wish.

Also, if you order Rod's boards and put everything together well, not only can the amp sound great but also, it is really easy to fix should it go wrong or if it doesn't work in the first place, with the wealth of support that Rod's site provides and not to mention diyAudio itself.

In my opinion, you should start off with something simple (like a chipamp) that uses cheap parts and is easily upgradeable. Then, if it doesn't work, it will be easier to fix. Also, if you try something more complicated and it brings lots of problems or doesn't work, your confidence can take a big hit and you might be discouraged completely .

If you wanted something else, you could try Rod's LM3876/86 amp here. This would provide about twice the output power and may sound slightly better (haven't tried it myself). But be warned, this amp uses a much larger power supply and higher volages which may increase the risk if not wired properly.

Having crossovers on the receiver and subwoofer make things much easier and greatly simplifies the setup. It saves a lot of extra circuitry and not to mention having use separate power supplies (for the lower voltages used by the opamps and the associated filter circuits).

At the end of the day, it is completely up to you, but don't try something that you feel uncomfortable with.
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Old 10th June 2010, 11:21 PM   #6
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Quote:
Originally Posted by jackand08 View Post
You're welcome .

I would not underestimate the LM1875 amplifier at all. If built properly (which is easier using Rod's boards) it can sound stunning and outperform some much more expensive amplifiers-not to mention, you have the satisfaction of building it yourself and it will be much easier to modify should you wish.

Also, if you order Rod's boards and put everything together well, not only can the amp sound great but also, it is really easy to fix should it go wrong or if it doesn't work in the first place, with the wealth of support that Rod's site provides and not to mention diyAudio itself.

In my opinion, you should start off with something simple (like a chipamp) that uses cheap parts and is easily upgradeable. Then, if it doesn't work, it will be easier to fix. Also, if you try something more complicated and it brings lots of problems or doesn't work, your confidence can take a big hit and you might be discouraged completely .

If you wanted something else, you could try Rod's LM3876/86 amp here. This would provide about twice the output power and may sound slightly better (haven't tried it myself). But be warned, this amp uses a much larger power supply and higher volages which may increase the risk if not wired properly.

Having crossovers on the receiver and subwoofer make things much easier and greatly simplifies the setup. It saves a lot of extra circuitry and not to mention having use separate power supplies (for the lower voltages used by the opamps and the associated filter circuits).

At the end of the day, it is completely up to you, but don't try something that you feel uncomfortable with.

Thanks for your suggestion, I'am waiting on ROD to respond to my e-mail,anyway what would you feel will be an estimated costs to all the parts I would have to purchase for the LM3876 amp.

Also could you list the basic kit requirements I would need, soldering iron, soldering wire and multimeter,would there be anything else.
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Old 11th June 2010, 06:40 AM   #7
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I'm not too sure about cost but I would expect something in the range of around 100-150 (around 174-261AUD). Just remember though, some people say that for that money you should just go and buy an amplifier - but why? - when you have the DIY satisfaction and something that sounds much better.

Another alternative to buying all of the parts individually, could be buying Jaycar kit number KC5150. This is a 50 watt amp that uses exactly the same IC and probably a similar circuit for only 8.75 (~15.25AUD). It come with all of the parts apart from the heatsink, transformer, power supply (capacitors and rectifier) and a case. Buying the kit may save some money over buying Rod's boards and the necessary parts, and you will still be left with a very good amp.

One thing not to forget in your budget is the case hardware. Things like speaker binding posts, phono sockets, IEC mains inlets, switches, knobs and dials and mounting hardware (to secure the cicuitry inside the case) can all be easily forgotten (somehow ) when it comes to putting together a project. Something that isn't nice is not being able to finish a project because you've forgotten one small part . A good idea would be to think through the circuitry systematically and think about what parts (apart from the obvious e.g. the amp itself and the power supply) would be needed.

Jaycar also sell suitable cases that can be used with either their kits or Rod's boards. Rack mount cases in particular (because of their build quality and strength) if drilled neatly and finished nicely, give your project a much more professional appearance.

Tool wise, for a project like this, a soldering iron, some 60/40 tin/lead solder (don't get the lead free stuff-it's horrible to work with), a decent multimeter (doesn't need to be anything spectacular here ) and some hookup wire would probably do. Also, if you haven't already got any, get some mains rated cable for hooking up all of the mains connections. This can also be used to hook up the parts of the power supply that carry high current/voltage. Have a look at Rod's power supply wiring guidelines here.
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