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LM1875 Gainclone Build

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currentkills91

Thu Jul 18 2013, 02:09AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

To begin, this was originally going to be the amp for the speakers in my other thread, but I have decided to go ahead and build a new set of 3 way monitors to replace those. I gave them to my girlfriend, and will still building an amp for them, but I will just a post a pic of the finished amp for those in that thread later on.

Earlier this week I picked a Phillips MCD702 Mini Theater system, and have chosen to use the case from the amp.. well.. for my new amp. The specs on it were terrible, 10% distortion at 20 watts into 8 ohms, and terrible crosstalk figures between the two channels.

I have chosen the LM1875 for a few reasons:

1 - Space restrictions, inside dimensions are only 200mm x 170mm x 45mm and the transformer is 90mm in diameter..
2 - It puts out 20 watts into 8 ohms with only about .02% distortion. (Using the exact voltage the transformer provides)
3 - It only dissipates about 18 watts so heatsink requirements are manageable in the provided space.
4 - It is a mono amp, so besides the transformer and a common ground channel separation will be at a maximum.

The amplifier will have the following specs:

20 watts per channel into 8 ohms
3 switchable inputs, 2 in the back, 1 in the front
Stereo subwoofer output jack
Preamplifier with adjustable gain of 3-9dB
It will also eventually have an optical input

I still haven't worked out the stuff for the monitors yet, but I have plenty of time for that. Recommendations on good quality drivers would be helpful. I imagine going to an audio specific forum will be my best bet though.

Here is a video of the first amp playing, rest of the stuff will be built tonight and tomorrow :)

I don't know how to make the pictures I add from the ones I posted in the attachments folder small enough to be allowed, can anybody help me with that?

I added the photos to my Flickr for now:

The Finished Amp
Both Amps In The Chassis
Preamplifier Power Supply

I have chosen to add a stereo output so I can run 2 subwoofers. I received the wrong parts from Digi-Key so the adjustable gain will added later when I receive the correct parts.

Sulaiman

Fri Jul 19 2013, 01:41AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Designing/building the crossovers for a 3-way system is non-trivial,
I prefer the approach of one power amplifier per driver unit
and low level active crossovers.

The power of each amplifier should be chosen to (almost) match the sensitivity of the drivers,
e.g. 87dB spl 'woofer' and 90 dB spl 'tweeter' have 3dB difference ...
so for this two-way example a 20W 'bass' PA and a 10W 'treble' PA

The direct connection of PA to driver gives good 'damping'
The total PA power required is less than with one PA with passive crossovers
e.g. a 20W 'bass' note (17.88 V peak into 8 Ohms)
plus a 10W 'treble' note (12.65 V peak into 8 Ohms)
may require 17.88 + 12.65 = 30.53 V peak which is equivalent to one 58 W amplifier !
The situation is even more pronounced with a 3-way system.

Low level active crossovers/filters using op-amps, resistors and capacitors are a lot cheaper and more accurate than a high power passive crossover.

So you will get better, louder, cheaper sound !

If you do go this route I reccommend building the crossovers, amplifiers and power supply into each loudspeaker cabinet, so that each cabinet is an 'active' loudspeaker requiring line-in levels ... otherwise you will end up with a lot of speaker cables !
(I built a pair of loudspeakers with 2x 10" 'bass', 2x 4" 'mid' and 1x 1" 'treble' drivers with a separate PA box ..... what a lot of speaker cables .. big mistake, may one day fix it) !

Using this approach you will have two loudspeakers that can be driven directly by 'audio out' from almost any device, without a central control box required.
A 33 Ohm resistor can be switched in across the input to give a good load for 'earphone/headphone' sources etc.

and of course there will be negligible crosstalk.

Steve Conner

Fri Jul 19 2013, 01:11PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The active approach has never been easier, if you have a couple of hundred bucks to drop on a Behringer DCX2496 digital crossover.

It might not be the cheapest solution in terms of up-front cost, but you can reprogram it to drive any other active speaker setups you might build in the future.

It won't please an analog purist though. I've never tried the DCX2496, but I swear by the DEQ2496 for equalising bass room modes. I've measured the performance of mine and it is pretty damn good for the price.

currentkills91

Fri Jul 19 2013, 08:50PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

I would have built different amps for each driver, but I wanted to keep the cost to a minimum. Parts were supposed to be here today, but that means I will have them tomorrow for sure. I still have to find a decent potentiometer though..

currentkills91

Sun Jul 21 2013, 08:15PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Top post updated with links to pictures.

Steve Conner

Sun Jul 21 2013, 08:31PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Nice build! I'm a fan of the Alps Blue Velvet pots for audio volume controls. They are a bit pricey, but extremely high quality.

currentkills91

Mon Jul 22 2013, 09:27PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Thanks Steve! hey are indeed a bit on the pricey side, I had forgot about those. I suppose I could invest in one since everything else is decent quality. I just need to order some RCA jacks, banana plugs/jacks, a switch for the gain control, and a switch for the inputs.

currentkills91

Sun Jul 28 2013, 05:45PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

I have made a lot of progress on this over the past few days, including rebuilding the amps to provide better noise figures. I get almost no noise on the output, except a slight hum which I will try to fix by snubberizing the power supply. By that I mean adding a 1 ohm resistor between the bridge rectifier and the smoothing caps, 2.2K ohm resistors across each capacitor, and a pair of .47 ohm and 1.5nF caps across each cap.

I removed the thermal pads from each amp (the heatsinks were originally gonna be mounted to the chassis) and the amps seem to run a lot cooler now.

I have chosen not to use the preamplifier I first built, and replace that board with a buffer, volume control, input selector, and a subwoofer control.

Volume control is still a mystery at this point, and I am kinda leaning toward a rotary encoder.

Here is a picture of the amp the way it sits now.

I used the schematic from Elliot's project page and substituted values to suit my needs.

Things left to do:
Build the board described above, route the wires as best I can to prevent noise pickup, and once thaose are finished run tests using a dummy load to get the amp specs.

currentkills91

Wed Aug 21 2013, 12:48AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Got this little thing all finished, for the most part anyway, the setup has become nothing more than computer speakers O.o

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