DIY Audio Projects Forum

5 channel based on LM3875
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Author:  Losergamer04 [ 14 Mar 2010, 08:39 ]
Post subject:  5 channel based on LM3875

So, I built my little amp for my computer (works great and thanks for the help), but it got me wanting more. See, the idea was great, and it is really cool, but I think I can one up myself.

Now, I'm just asking if this could work, but I want to build my own 5 channel amp. By build, I mean no kits. For the amp, I want to use 5 seperate point-to-point diagrams that Mark Houston posted years ago. And for the power supply, I was thinking of using one BIG torid and running all 5 LMs off of it.

Now, would that work and could I just run the 2 leads from the toroid and parralell it out to all 5 boards? The toroid I was looking at is Multicomp. The Nanoo use one only rated at 160VA, so 800 should be enough for 5, right?

The first amp was so rewarding that I can't wait to try something like this. I got a lot of fufilment from it. I look forward to this one since it will be 100% hand made.

Author:  tombethe [ 14 Mar 2010, 13:41 ]
Post subject:  Re: 5 channel based on LM3875

Hi again ;-).

The transformer you've considered is not very useful for your 5 boards project. The voltage is way too high.
You'll get 2*70VDC after rectification and that is really to much. The maximum supply power for the LM IC is at +30 / -30 VDC.
I'ld rather search for an dual output transformer with 2*18VAC and about 500VA. That are 14A and i think that's more than enough.
An other option would be 2*21VAC secondary with 500 to 600VA - if you can get it somewhere. So you'll get 30VDC + and -.

For use with the LM3875, a 2*25VAC transformer at about 600VA would be perfect

I hope that was a little help ;-).

Author:  Losergamer04 [ 14 Mar 2010, 16:39 ]
Post subject:  Re: 5 channel based on LM3875

I must have linked the wrong one then, because I thought each output was 25. Oh well.

Why a dual 18V? Why dual and why 18V (Allow 4 ohm loads?)? Maybe I don't understand the dual thing. I get how a "regular" transformer works (2 inductors with a bar in the middle, more or less). Is there something special with a toroid I'm not getting?

Also, is a powersupply as simple as the one I linked ok to use? I'm a little shocked at how easy it seemed.

Author:  Gio [ 14 Mar 2010, 21:36 ]
Post subject:  Re: 5 channel based on LM3875

Pick the voltage of the chip amp based on the load it will have to drive. For 4 ohm, you want a transformer with dual secondaries at about 18-20V, for 8 ohm load about 23-25V. For size, I would say 500VA min and go higher is you will often be pushing the amps hard. The simple chipamp circuit you are considering needs a balanced supply, so you must use a transformer with dual secondaries.

Do note that once you get over 500VA you will likely need some sort of soft start circuit.


Author:  Losergamer04 [ 16 Mar 2010, 11:30 ]
Post subject:  Re: 5 channel based on LM3875

I did a little looking up and I want to make sure I am understanding this "dual secondary" thing. So you have 1 primary input (120V AC) and then you have 2 sets of outputs at the voltage (not sure if I want 4 or 8 ohms). How does one create a "balanced" power supply. My research isn't turning up a lot of educational material on it.

For the soft start circuit, where can I find one or a diagram of one?

Edit(ed a few times): I think I get the idea. I found somethign comparing it to a battery. So, if the 2 secondaries are each 20V to ground, then it's 40V total. So, the circuit needs to share the ground with the transformer in order to be "balanced," am I correct? And, for that to not blow somethign up, they need to be "inveresed." As in the ground for side A be with the source for side B, otherwise they will fight eachother.

Author:  Losergamer04 [ 16 Mar 2010, 12:11 ]
Post subject:  Re: 5 channel based on LM3875

Would this work? It's a 230 step down to 40, but I'm running 120 and desire a 20V output. I'm assuming the current capabilites would be cut in 1/2 resulting in 500VA, which is all I really need.

Author:  tombethe [ 16 Mar 2010, 14:33 ]
Post subject:  Re: 5 channel based on LM3875

Hi again, gamer ;-).

I'll give it a try and place a little explanation here...

A balanced (symmetrical) power supply will give you two voltage rails, that you need to fire up most Audio ICs (ICs in general or OPs).
This two rails are one positive rail and one negative rail. This two rails are "symmetrically" connected between
the positive and the negative potential. The connection between the "+" and "-" provides the "null" or "ground" potential.
This is the same effect like using a "centre tapped" transformer, or two batteries connected in the middle, between the + and -.
The minus side of the batteries will become the negative rail and the centre connection becomes ground. Plus is plus.

I think that's the same that you've meant in your edit.
Just give it a try. Set up two batteries and measure them with a DMM. Can't go wrong ;-).

I don't want to create some multi posts now. So, please take a look onto the supply of the TDA2050 amp (from the sig...).
The power supply schematic will show you, how the positive and negative rail is made. Just follow the symbols.
There, you also can see the connection of the two potentials, to get the ground rail.

The transformer that you've meant in your last post can be used. It will behave like you've thought.
With 60 cycles and 120VAC you'll get some more volts. Also "1 or 2" more amperes.
I'ld guess around 22 - 23VAC on each secondary and about ~550VA. The transformer will stay pretty cool so.
No problem here, but K.I.S.S. (Keep it stupid simple). Best way is using an unregulated supply!.

I hope that was a little help. Have fun ;-)

Author:  Losergamer04 [ 17 Mar 2010, 06:33 ]
Post subject:  Re: 5 channel based on LM3875

Yup, I get the idea now. I was working it out on paper and the way I saw it drawn somewhere looked a wee bit odd. But your TA2050 schematic makes total sense to me. I don't remember any of the formulas though.

Now, the reason we use a balanced PS is because it reduces the "polarity echo," which is bad for a DC chip, right?

I'm just a little suprised at how simple it is. I look at all these componants these days and ICs and what not and I have a hard time imagining that I, a person with very little training and exp, could build such a great sounding amp so easily.

Where do you guys source your parts from? LIke your caps, resistors, etc. I'm talking new, BTW. I don't have a garage full of old stuff... yet.

Oh, and thanks for the help everyone. I'm getting excited to build this. Now I need to find a use for it so I can justify the ~200 in parts I'll need, lol.

Author:  tombethe [ 17 Mar 2010, 16:27 ]
Post subject:  Re: 5 channel based on LM3875

Hi again,

now - the reason why a symmetrical supply is used on such OP circuitries, is the way how this devices work.
The ICs are powered with DC current, but the pure output signal is AC. The chips / ICs, use both DC portentials to operate
with higher efficiency (more power with less losses) and the capability of both (+/-) potentials to build a "neutral point" in the middle.
For this, i've meant to take a look on the suggested supply scheme. "The middle is neutral" :idea:.

If you take a look onto an already pre-gained output stage (OPS) you'll maybe see, what i mean.
The power devices are directly connected to both supply rails. The upper devices by the positive, and the lower device by the negative.
(That's how it is shown on most of the standard schematics for OPS).

Now what happens: The applied audio signal will open the junction (by the gate / base current), to enable a defined current
flow through the output devices. The currents now will flow through the upper and the lower device. Here they are connected
- ahh - in the middle. The now amplified audio signal also passes the output devices. Now - here comes the difference between
AC and DC and joins our party.
The DC currents are neutralized in the centre connection, but the amplified audio sinus wave can pass
through the load to the ground connection. That's because the audio signal is an alternating current and not
neutralized by the two of the DC potentials. The DC only transports the AC wave :wizard: .

The point is:
If you use a single supply (with only + and ground), you'll only be able to get a power efficiency of about 17 - 25%.
The rest are losses!. So - It would never be possible to get 56Watts of audio signal power from a small xt TO220 device ;-).
With a symmetrical / balanced supply you can get a power efficiency of about 50% and little more.
One more very fat advantage is that you don't need signal coupling / decoupling caps, because there is nearly no DC in the
signal path at the points where the AC (signal) goes to the next or OP stage.

Page two of National's LM3875 datasheet shows the basic internals (equivalent scheme).
If you follow the supply rails and the signal path, the things should appear a bit more clear.

Hope that was a little help - have fun ;-).

Author:  Losergamer04 [ 18 Mar 2010, 10:18 ]
Post subject:  Re: 5 channel based on LM3875

So, am I correct in thinking this simmilar to an AB amp? Each 1/2 of the signal is amplified by 1/2 of the circuit? And, in layman's terms, the power is DC (the signal is on top of it?), so there is no loss due to the inherant nature of AC switching polarities and the circuitry that goes along with it?

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