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LM1875 in parallel configuration and used in a composite amplifier.
LM1875 in parallel configuration and used in a composite amplifier.
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Old 14th December 2019, 10:47 AM   #11
kokoriantz is offline kokoriantz  Lebanon
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voltage or current Holloway? If voltage , how many ohms to be the joining resistor? Can you post a picture of your 1875 ?

Last edited by kokoriantz; 14th December 2019 at 10:55 AM.
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Old 14th December 2019, 12:53 PM   #12
chermann is offline chermann  Austria
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Quote:
Originally Posted by FauxFrench View Post
Hi Chris,

I i read your values i get goosebumps - this will be a power monster.
i would be happy with a 2 parallel chip power amp...

Two parallel LM1875 will be sufficient for much. It is easy to choose two, three or or even four LM1875 in parallel because each LM1875 block will be similar with the other and only the load sharing resistors may need to be adapted to the number of LM1875 in parallel.
Personally, I may implement a three LM1875 power amplifier while other can choose for their personal needs. With three LM1875 in parallel, I will have higher current margins than with only two but else the power amplifier behavior will be very similar (gain, THD etc.)

to your description 1 chapter:
look at the note 1:
We should calculate a little less power at 4 R ratings because its written:
1......maximum output power delivered to a 4Ω load may be slightly
reduced when the tab temperature exceeds 55°C.


My first reaction is that a heatsink should be less than 55 degrees warm. As also indicated in the NS/TI datasheet, the LM1875 is a bit (current-) marginal for 4 Ohm loads. The datasheet proposes to reduce the supply voltage in order to compensate for the higher currents in 4 Ohm. By putting more LM1875 in parallel, we generally said make the load impedance seen by each LM1875 chip higher. Two LM1875 in parallel make a 4 Ohm load look like an 8 Ohm load for each LM1875 chip, roughly seen. With three LM1875 in parallel, a 4 Ohm load will look like a 12 Ohm load etc. Thus, the datasheet concerns for use of 4 Ohm loads are circumvented by more LM1875 put in parallel.

additionally to that i am not really sure i understand our first discussion about the current limiter in the other thread:
eBay mono LM1875 kit

the graph looks like that we are above 20V rail in the current limiter- with 1 chip for sure...? or not?

the page 3 current limiter "formula":
Vout = Vsupply - 10V...4A (typical)--> 3A(tested limit)
Does this mean at this current the Voltage will sag down to Vsupply – 10V??


It seems that in my posting #187 I have grossly dodged your question about how to interpret the two graphs. I hate strange looking graphs in datasheets but now I had to concentrate and think.
To me, the two graphs look like two graphs describing the dynamic value of the positive and negative chip current limiters respectively. The chip is supposed to operate in-between the to curves.
I will try to explain: Had the current limiter values been for instance 3A fixed, the curves would have been two horizontal lines at +3A and -3A. The LM1875 has a (typical) dynamic characteristic such that at an output voltage of 0V, 3A can be drawn before the current limiter is activated, at 5V output voltage 4A, at 10V-15V output voltage more than 4A, at 20V output voltage 4A. Above 20V output voltage the current limit drops very rapidly. Similarly for the negative current limiter.
With a constant value load at the output, the current in the load increases steadily and linearly from zero. As long as the load current curve stays below the dynamic current limit curve, there is no problem. Above an output voltage of 20V, the load current curve is close to the dynamic current limit curve and problems start. It appears prudent to avoid operating in the area with output voltages above 20V where the current limit drops very rapidly. The supply voltage should be limited to +/-25V such as concluded from Figure 5 of the datasheet.

If we limit the supply voltage to +/-25V such that we can expect output signal clipping at 20V, the output power calculations become:
SE-coupling: 25W in 8 Ohm.
SE-coupling: 50W in 4 Ohm.
BTL-coupling: 100W in 8 Ohm.
BTL-coupling: 133W in 6 Ohm.
BTL-coupling: 200W in 4 Ohm.
Still respectable values.

Hi FF,


first i do not want to finish with 2 parallel chips - it was just an idea.

Thanks FF for explanation that is what i mean. so 25VDC = is about 18VAC?
in real i had the 18VAC and in idle i got about 26,3VDC
or should we use a 15VAC?....15VAC * 1,5 = 22,5VDC


chris
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Old 14th December 2019, 12:59 PM   #13
chermann is offline chermann  Austria
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Quote:
Originally Posted by kokoriantz View Post
Which one to your opinion to choose?

Sorry kokoriantz...I will not support a possible fake chip manufactor.
i us eht origin 1875 by TI store
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Old 14th December 2019, 01:02 PM   #14
00940 is online now 00940  Belgium
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I honestly don't see much point in 2 paralleled 1875. One lm3886 is just as powerful. With 3 or more, things get interesting as you can maybe better spread heat around.
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Old 14th December 2019, 02:25 PM   #15
anti is offline anti  Slovakia
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.. and even then, there is the the LM1876
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Old 14th December 2019, 03:07 PM   #16
kokoriantz is offline kokoriantz  Lebanon
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Quote:
Originally Posted by 00940 View Post
I honestly don't see much point in 2 paralleled 1875. One lm3886 is just as powerful. With 3 or more, things get interesting as you can maybe better spread heat around.
The problem with IC amps is the lead thickness. The American wire table which is religiously respected by transformer, capacitor manufacturers shows the maximum current the wire can be used for power transmission. The nonmagnetic leads of LM1875 as 3886 are equivalent to 0.64mm thick wire , the table shows 0.95A max can be transmitted. If 8 ohms load is used ,max power is 3.6W. This why the bass notes being higher in power gets dried out. Paralleling resolves this problem.
The LM1875 is over 20years old by this the patents are in public domain . Similarly you don't buy BD139/140 from Telefunken but a copy of it , The generic versions are ok , but what goes wrong is to get a fake un operating one or original but factory rejects.
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Old 14th December 2019, 03:10 PM   #17
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by kokoriantz View Post
voltage or current Holloway? If voltage , how many ohms to be the joining resistor? Can you post a picture of your 1875 ?

Load sharing resistors to be discussed. I guess 0.33 Ohm or 0.5 Ohm.
It seems I paid around 3 Eur for 10 pcs. Photo appended.
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File Type: jpg LM1875Cheap.jpg (884.1 KB, 652 views)
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Old 14th December 2019, 03:16 PM   #18
kokoriantz is offline kokoriantz  Lebanon
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Quote:
Originally Posted by FauxFrench View Post
Load sharing resistors to be discussed. I guess 0.33 Ohm or 0.5 Ohm.
It seems I paid around 3 Eur for 10 pcs. Photo appended.
It looks similar to this . The shipment cost is for Thailand, the same for 10 pieces.
I suppose you optioned for voltage .
Attached Images
File Type: jpg lm4.JPG (25.9 KB, 541 views)

Last edited by kokoriantz; 14th December 2019 at 03:18 PM.
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Old 14th December 2019, 03:28 PM   #19
00940 is online now 00940  Belgium
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Quote:
Originally Posted by kokoriantz View Post
The problem with IC amps is the lead thickness. The American wire table which is religiously respected by transformer, capacitor manufacturers shows the maximum current the wire can be used for power transmission. The nonmagnetic leads of LM1875 as 3886 are equivalent to 0.64mm thick wire , the table shows 0.95A max can be transmitted. If 8 ohms load is used ,max power is 3.6W. This why the bass notes being higher in power gets dried out. Paralleling resolves this problem.
Sorry but that's just nonsense. The 0.95A max is for power transmission. Rating for chassis rating is already at 7A and even that doesn't apply to such short leads in open air. TI engineers will have determined the proper sizing of the leads based on temp raise and voltage drop.

PS: Eventual problems with low notes at high power come from the SPIKE protection system with the lm3886.
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Old 14th December 2019, 03:43 PM   #20
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by 00940 View Post
I honestly don't see much point in 2 paralleled 1875. One lm3886 is just as powerful. With 3 or more, things get interesting as you can maybe better spread heat around.

You are right in that this LM1875 project is not about how to make a composite amplifier in the most rational way. Had it been, we would have used LM3886 or TDA7293. Your level of skills has been noticed and this project will hardly teach you anything.
We are a number of persons having LM1875 (more or less genuine) in stock or the option to buy LM1875 at a good price. The aim is to show how such simple but well performing chips can be arranged in parallel to compensate for their moderate current capability and allow BTL use. On top of that, use in a composite amplifier configuration will bring down the THD and allow feedback from after the load sharing resistors. I am convinced we will reach a good result but we will neither take away the interest for the Fremen design, nor put Neurochrome out of business. Due to only 5 pins, the LM1875 is easy to use in a Vero-board. Further, as you already point out, the heat losses will be spread out.
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