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eBay mono LM1875 kit
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Old 29th November 2019, 10:07 PM   #351
FauxFrench is online now FauxFrench  France
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Quote:
Originally Posted by chermann View Post
Hi

the ringing starts at 500mVrms input (15,7W at 8R)
pic 1 with 2k7 resistor at the R channel (DC offset 34mV)

pic 2 original at the L channel (Dc offset -1,5mV)

- more power get worse at both versions..not really a difference

520mVrms input - 16,8Watt at 8R
pic 3 with 2k7 resistor at the R channel (DC offset 34mV)

pic 4 original at the L channel (Dc offset -1,5mV)

chris

Once more very good work, Chris.
You can for learning eventually try to raise the resistor value to 4K7 to see if that is still good enough. The lower the resistor value, the lower the effective feedback correction and thus higher THD and higher output offset. Nothing is for free. The resistor value needed may be individual for each 'fake' LM1875 specimen.

The most important for you is to have a further tool in your toolbox for when you run into such stability issues. The self-oscillation seen is perhaps a difference in stability between a 'fake' and a genuine.
If you really wanted to make a good amplifier with the 'fake' LM1875, perhaps the 1K resistor could be used and a controlling OP-AMP (composite amplifier configuration or even DC servo configuration) used to reduce the output offset (and THD if used in the composite configuration). You should try such a 'controlling OP-AMP' at a certain moment. For now, you may also continue with the genuine LM1875s instead of working on compensation of weak specimens. Then, you can spend more time on listening and draw conclusions between class AB and class D sound.

Since three very active years for you with DIY electronics, you can now realize how much you have learned about controlling and improving amplifier circuits. Today you learned one more trick. There are a few more tricks to learn but you progress very fast. With good knowledge, you can decide when to take a construction towards perfection or to stop a project when almost all learning has been gained. After all, we need only one amplifier at a time and having the basement full of optimized amplifiers is as such a waste (apart from the knowledge that was gained).

Last edited by FauxFrench; 29th November 2019 at 10:10 PM.
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Old 29th November 2019, 10:08 PM   #352
chermann is offline chermann  Austria
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Hi


be aware that a scope has the BNC and the probe is earth grounded !!!!
so any contact to a not earth grounded device could give you strange results or damage something: Scope is not a multimeter



thx to Dave...

YouTube
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Old 29th November 2019, 10:15 PM   #353
chermann is offline chermann  Austria
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Hi FF


Thanks but it is your work too - because you help a lot of noobs like me.

Thank you FF




LM1875 journey is not finished for me.


1 next Step - LM1875 kit with genuine parts -still ordered and on my bench
2 composite amp --> power is nothing without control
chris
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Old 30th November 2019, 10:58 AM   #354
chermann is offline chermann  Austria
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Default 5k at the inputs to avoid ringing

Hi


i did tthe "healing" at the other channel with 10k//10k so i get about 5k and -17mV DC at the output. load is 8R at both channels.
better at 500mV but at higher power the same.
pic 1 +2 with 5k, without (original)


pic 3+4 550mVinput with 5k, without
chris
Attached Images
File Type: png 5k 500mV in 8R_Lchannel.png (50.1 KB, 75 views)
File Type: png 500mV in 8R_Lchannel.png (50.6 KB, 78 views)
File Type: png 5k 550mV in 8R_Lchannel.png (50.6 KB, 82 views)
File Type: png 550mV in 8R_Lchannel.png (51.4 KB, 81 views)
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Old 1st December 2019, 10:09 AM   #355
fdenys is offline fdenys  Netherlands
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I believe the LM1875T delivered with the kit could very well be this Unison LM1875T.
For the datasheet look here

That does not look bad

Fred
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Old 1st December 2019, 10:43 AM   #356
FauxFrench is online now FauxFrench  France
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Quote:
Originally Posted by fdenys View Post
I believe the LM1875T delivered with the kit could very well be this Unison LM1875T.
For the datasheet look here

That does not look bad

Fred

I believe you could be right. Pretty similar to TI specs.

As I recall it, the LM1875 design dates back to around 1990. It is more than unlikely that Asia, in 2019, cannot come up with similar specs as "the West" 30 years before.
Asian chip designs are generally up to specs and on par with most of what we find among brands we know.
There seems to be a tendency of "die-schrinking", which is logical from an economical point of view but may mean that such die-schrinked items are closer to the spec. limits than we are used to.
I find it unlikely that someone in Asia let incompetent chip designers make an own new (poorly performing) wafer design for large-scale production, marked as TI chips, to be sold at very low prices. More likely, what we find at very low prices are those items that did not pass the production tests for one reason or another, remarked items of a known but less performing type (TDA2030 -> TDA2050) or die-schrinked versions of existing genuine designs.

Last edited by FauxFrench; 1st December 2019 at 10:46 AM.
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Old 2nd December 2019, 09:29 PM   #357
chermann is offline chermann  Austria
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Default square wave stability...

Good Evening


i did at my recovered chips the modifications with the 2k7 (Rchannel)or 5k (L channel)at pin 1-2. for this configuration i look at the square wave stability with 8R load and a 440nF mkt cap at the spekaers terminal.
the 100pF is in parallel with the feedback 22k resistor. at this chip i got the ocp at 400mVrms input (as always) into 8R. (about 10W) -- both channels are identical
i checked it.

pics

100mVrms in 10k, 20k, 30k

200mVrms in 10k, 20k, 30k
400mVrms in 10k, 20k, 30k - ocp hick in - i set the scope to single

so everything fine.
chris




but look at the next post
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Old 2nd December 2019, 09:36 PM   #358
chermann is offline chermann  Austria
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What i do not expect is that if i switch from square to sine wave i normally get a destroyed ringing at the top of the upper sine wave above 400-500mVrms - this is why i get the hint to use the reeistor between pin 1 -2 (2k7 or 5k)

look at post #347 or #320


now with 440nF at the output it gone.


examples:
pic 1 shows 550mVrms with 440nF at the output
pic 2 the same power without the 440nF at the output

pic 3 shows 600mVrms with 440nF at the output
pic 4 the same power without the 440nF at the output


pic 5at my input 620mVrms


??? why does the 440nF healed here???

chris
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Old 2nd December 2019, 10:03 PM   #359
willsw is offline willsw  United States
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This seems like an OK place to ask this question:

I had some boards made based on BrianGT's LM1875 boards and the datasheet schematic (very similar either way). I'm using a PS-SS from Glass-ware and a 30VA CT transformer. I was measuring about 22-23V on either side of the power supply before hooking it up to the board. When I hooked it up to the board, the chip popped!

I've made many LM1875 amps using protoboard, point-to-point, other PCBs, and many different power supplies, as well as what were likely fake chips and real chips. All of them so far have functioned without issue. These chips I ordered from Mouser and should be real. This is the first time a chip has popped. It happened almost instantly after applying power. What do you think could have caused this? There was no input or output hooked up, only power. It was mounted on a hefty heatsink.
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Old 2nd December 2019, 10:55 PM   #360
FauxFrench is online now FauxFrench  France
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Join Date: Dec 2017
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Quote:
Originally Posted by willsw View Post
This seems like an OK place to ask this question:

I had some boards made based on BrianGT's LM1875 boards and the datasheet schematic (very similar either way). I'm using a PS-SS from Glass-ware and a 30VA CT transformer. I was measuring about 22-23V on either side of the power supply before hooking it up to the board. When I hooked it up to the board, the chip popped!

I've made many LM1875 amps using protoboard, point-to-point, other PCBs, and many different power supplies, as well as what were likely fake chips and real chips. All of them so far have functioned without issue. These chips I ordered from Mouser and should be real. This is the first time a chip has popped. It happened almost instantly after applying power. What do you think could have caused this? There was no input or output hooked up, only power. It was mounted on a hefty heatsink.

I understand your surprise. Did you notice if the chips got very hot when you connected the power? If the chips for some reason started self-oscillation they heat up very fast. Did you short the inputs before you put power on? Leaving the inputs open is an invitation to oscillation.
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