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Old 6th February 2007, 05:53 PM   #1
Arc is offline Arc  United States
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Default LM3886 Questions and Suggestions

Hi I am new to the whole DIY Chipamp thing, and I have a couple questions. Mainly about Toroidal transformers and what are recommended. I know there are a lot of these threads, and I just didn't see any that answered my questions directly. If there are some please direct me and I will look again. Thank you for bearing with me here.

I am planning on making the Dual Mono kit from chipamp.com. I am planning on keeping the left/right/power all completely separate. And the chassis will be completely aluminum, with copper plates for the amp boards to sit on.

The speakers I will run off of this will most likely be around 4ohms with slight spikes to 8ish. I know this comes to a factor of what V I need on the transformer. After looking at Vikash's designs that he made I would like to get a 300va 20/20 because this seems to be the best option for me.

The question I have is would it be best to get one transformer or two smaller ones? I have searched and seen many designs with two. The only benefit I can see from this is complete separation of channels. Any others? If I go with two should I just divide the 300 by two to 150Va for each toroid?

Also, any good (reasonably priced) sources to get these custom wound?

Thank you for all the help, any other comments or suggestions on this?

-Alex
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Old 6th February 2007, 06:20 PM   #2
sek is offline sek  Germany
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Hello Alex,


Quote:
The question I have is would it be best to get one transformer or two smaller ones?
This has been discussed many times before, the consensus is that true monoblocks benefit from better load regulation behaviour of the transformers.

But using a shared transformer and completely independent rectifier pairs, capacitor banks and distribution wiring is almost as good. The higher load influence on the transformer can be compensated for by higher transformer part quality!


Quote:
If I go with two should I just divide the 300 by two to 150Va for each toroid?
In theory, yes. But practical transformers aren't ideal, the behaviour under load will be a little better if you keep the power rating higher. But all in all it depends on your needs. You won't use a GC for sound reinforcement in a movie theater, so what?

OTOH, I use a 120VA transformer to power two LM3886 and I power my main speakers (8Ohm load) in my living room with them (four LM3886 in active speakers total)! You see, such discussions can quickly get a matter of taste.

I can't help out with custom winders, sorry.

Cheers,
Sebastian.
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Old 6th February 2007, 06:30 PM   #3
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If Your speakers are 4 ohm, I would defenetly go for paralling the 3886 chip - it looses a few % in the mid/high region, but gains 100% in the bottom end, mostly in low bass definition. What you do is you tie the in and outputs of two channels together, inserting a 0,1ohm powerresistor in series with each output - se app note an1192 from national - very, very good reading, even for more "experienced" diy'ers ;-)

Power in parallel @24 volt trafo(34/35volt after rectrification) is 50watt in 8 ohm, 100 in 4 and 140 in 2 if I recall. because of the parallel configurations better abilities to handle current in low impedances, 24 volts are a good choice here, but read the app note, and consider to build the amp on "labboard" i.e. hardwire it - it can be done even with a small experience on diy building.

Regarding the single/double transformer question, I would say that the most important thing is seperate retrifiers for each channel - under load the bridge can drop a few milivolts, and I think that can be heard - double trannies is the best, but I would use the money on good retrifier diodes and good caps, in my opinion this gives best performance for the $$$ - regarding size - take the poweroutput @ half the nominel speaker load, and multiply it with two, and multiply it with the number of channels. Ex. - a stereo PA100 amp delivers 140 watt in 2 ohm(assuming 4 ohm speakers), x2 x2 = 560 watt - so I use a single 500 VA trannie.

I have build single 3875, 3886, zen, aleph and more, but for everyday use I use a parallel 3886, also called PA100 - its really good if build properly.

Good luck !
Hans
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Old 6th February 2007, 06:31 PM   #4
Arc is offline Arc  United States
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Sebastian,

Thank you for your reply. Although I am a little fuzzy on the terminology used with making amps I will learn before I continue much further.

It appears (to my untrained mind) that for a "true" dual monoblock, I would use two transformers. But with the dual power supplies, it is almost as good to use just one. So this seems to be a matter of cost and preference?

If cost permits I would like to do a dual transformer, might as well do it right the first time, but with this I wouldn't need dual secondaries correct? So if I went with two, would 250Va be a good fit. And then I would only need 20V instead of 20/20, which should help cost..a little.

This will only be used for periodic music sessions when alone. And to brag and show off the craftsmanship.

Thanks agian,
Alex
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Old 6th February 2007, 06:34 PM   #5
Arc is offline Arc  United States
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Quote:
Originally posted by Buhl
If Your speakers are 4 ohm, I would defenetly go for paralling the 3886 chip - it looses a few % in the mid/high region, but gains 100% in the bottom end, mostly in low bass definition. What you do is you tie the in and outputs of two channels together, inserting a 0,1ohm powerresistor in series with each output - se app note an1192 from national - very, very good reading, even for more "experienced" diy'ers ;-)

Power in parallel @24 volt trafo(34/35volt after rectrification) is 50watt in 8 ohm, 100 in 4 and 140 in 2 if I recall. because of the parallel configurations better abilities to handle current in low impedances, 24 volts are a good choice here, but read the app note, and consider to build the amp on "labboard" i.e. hardwire it - it can be done even with a small experience on diy building.

I have build single 3875, 3886, zen, aleph and more, but for everyday use I use a parallel 3886, also called PA100 - its really good if build properly.

Good luck !
Hans

Thank you for your reply. If I am understanding you correctly, you are saying to use the LM3886 dual mono kit to drive one channel? In other terms taking two chips to one channel? Sorry new to this.

If this is your intention, this was a plan that I did have for the future. But for now I just would like to get my feet wet, and since the only thing I really need to figure out with this kit is the Transformer, I feel it a good kiddie pool for me.

I intend to read through the spec sheet form National when I get some more free time. I will also do some research into the "hardwired" technique you speak of.

Thanks,
Alex
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Old 6th February 2007, 06:59 PM   #6
Buhl is offline Buhl  Denmark
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You are rigth about the dual mono channel - two chips in parallel pulling each speaker. A way to wet your feet slowly, is to make sure you can build two identical stereospeakers - build each with one transformer and one set of bridges - keeping in mind that you are gonne use each stereo amp as a monoblock in the future - make a switch that couples the inputs together, and add a 0R1 resistor in series with each output - this way all you have to do is to flip the switch and put a piece of wire between the amp positive outputs - now its a paralle amp.
Read this: http://www.national.com/an/AN/AN-1192.pdf

Absolutly "must read" that i'm sure will teach you alot (it did for me...)

Cheers !
Hans
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Old 6th February 2007, 07:15 PM   #7
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Hi again,


Quote:
If cost permits I would like to do a dual transformer, might as well do it right the first time, but with this I wouldn't need dual secondaries correct?
If cost permits, You should probably do it.

Let's jump into terminology right here: Power amplifiers of the kind we discuss (e.g. chipamps) need a positive and a negative supply voltage, as they have to resemble both the positive and negative component of the eletrical music signal. You can't get around supplying two voltages of equal magnitude and opposite sign (e.g. +/- 20V).

Those two voltages can be derived out of one single secondary winding, but it has to have a so called center tap, a reference point exactly in the middle of the winding - a third wire. Technically, this would make it a center-tapped 40V secondary winding (denoted as 40Vct or +/- 20Vct).

But as windings on transformers are nonideal, the ct is never exactly in the middle - neither physically, nor electrically. For that reason, a dual secondary transformer is preferable. It can resemble a center-tabbed transformer by connecting the windings together in series (which is why it is always at least as good as a ct one).

The improved performance shows when the two secodary windings get rectified and smoothed independently. The absolute voltage levels might vary under load (as with any real transformer), but the secondaries never 'fight' each other, shifting the amplifiers ground level actually up and down with varying loads. The ground reference point is derived by joining the split supply halves in series at their outputs (instead of at their inputs, so to speak).

This very advantage is not so important with only one single amplifier connected (one could compensate for the drawbacks elsewhere). But two amplifier modules inside the same enclosure actually require a stable ground, as one channel should never be influenced by the signal and load situation of the other.

So the alternatives are: Either two regular ct (or dual secondary, doesn't really matter) transformers for true monoblocks - or one big, quality, dual secondary transformer for a common transformer supply.

Regarding parralleling channels: this doubles the current capability of the resulting amp channel, and thus increases it's power capability into lower load impedances (Ohm's law applies ). E.g. twice the power into 4Ohm, but not into 8Ohm, let alone 16Ohm. Why not (and how to get it done anyway) can be read in National's application note AN-1192.

Hope this helps,
Sebastian.
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Old 6th February 2007, 08:21 PM   #8
Arc is offline Arc  United States
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Quote:
Originally posted by sek

...

So the alternatives are: Either two regular ct (or dual secondary, doesn't really matter) transformers for true monoblocks - or one big, quality, dual secondary transformer for a common transformer supply.

Hope this helps,
Sebastian.
Thank you again. I think I am going to go with dual trans. I can understand why to a point. I will start looking for high quality toroids. I think the only thing I need now is the Va rating for each. I am thinking about going to 200-250, with 18/18v or 22/22v dual secondaries because I seem to see a lot of those, maybe get a pair of 250va 20/20's made.

I am not going to parallel them, if this turns out like I want, I will think about making another one and just bi-amp the components. This why I can stick to 4ohm mids, and then build the other to match the ohm's of the tweeter. Maybe find a way to be able to exchange the trans. out for others depending on the resistance of the drivers.
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Old 6th February 2007, 08:51 PM   #9
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Just to help seal the deal, it is worth looking at my thread on "Got Chipamp Hum? Possible Solution".

I have sound that using the dual mono kits with a single transformer results in hum and the solutions are to either use a single rectifier board for both channels (which is a compromise) or the best solution is to go with two transformers.

All the best!

Ian
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Old 6th February 2007, 11:15 PM   #10
sek is offline sek  Germany
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Ian has a point about hum and his findings, as those are the boards you intend to use.


Quote:
maybe get a pair of 250va 20/20's made
Don't bother. 20V vs. 22V won't make any difference, but getting them custom made will heavily increase their price.

A good toroid has some properties as: low output voltage sagging under load (a good manufacturer specifies that), shield winding (preferably copper foil) to help safety and mains noise suppression, a good vacuum resin cast (to highly damp the copper winding from vibrating with line voltage fluctuations), and no thermal protection inside (as this works destructive and can't be reset).

If you want a quick set of properties to look for, choose these two: power rating (for the load stability) and resin cast (for the noise damping). The rest can be skimped upon. Good transformers don't neccessarily have to be expensive while bad ones don't need to be cheap, either.

As for power: the datasheet states that for your around 30V DC (from around 20-22V AC) the maximum power - even with extremely ugly sound due to a variety of protection circuits at work - will be no more than 80W per chip. That's why people tend to recommend 150VA transformer ratings in such a setup. Anything above 200VA per channel will be oversized by a factor of 4, unneccessary in my opinion.

Sebastian.
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