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Chip Amps Amplifiers based on integrated circuits

eBay mono LM1875 kit
eBay mono LM1875 kit
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Old 18th November 2019, 09:01 PM   #311
chermann is offline chermann  Austria
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hello fdenys!


fine that you get inspired by us. please keep us informed. and post pics
chris
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Old 18th November 2019, 09:13 PM   #312
FauxFrench is online now FauxFrench  France
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Join Date: Dec 2017
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Quote:
Originally Posted by fdenys View Post
This thread has inspired me to (finally) replace my legacy 30 years old Marantz amplifier.
However i will take a different route as far as the power supply is concerned.
I bought a dedicated low noise Audio Powersuply (picture attached) for about 23 EUR from AliExpress.

This supply delivers +24V, -24V (about 6A) as well as 12V (1A) for auxillary devices. (adjustable within 10%)
Bit overkill maybe - but it should be able to get about 30W of clean power out of the LM1875t.

Supply is relatively compact and i expect building a decent amplifier with this supply should simplify matters considerably.
Output voltage of course is well regulated and remaining ripple less than typical 0.1V.

Hopefully will receive the PS in about two weeks, and first will try an unmodified LM1875t Ebay kit and do some measurements on PS and amplifier before building final system with original LM1875t and audio quality capacitors.
Will keep you informed ..

Though I only found it at a somewhat higher price, I am looking forward to hear your opinion about this supply.
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Old 19th November 2019, 09:31 AM   #313
fdenys is offline fdenys  Netherlands
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Join Date: Jul 2009
Hi FF,

Yes, i took advantage of the Sale action on 11.11 (Chinese single day) and paid $19.20 plus shipping. Now i see it is $24.
Probably coming Black Friday (29-11) is another good buying opportunity.
I took it from here: LCT300 2x24V@6A

Last edited by fdenys; 19th November 2019 at 09:46 AM. Reason: link issue
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Old 19th November 2019, 11:07 AM   #314
FauxFrench is online now FauxFrench  France
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Join Date: Dec 2017
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Quote:
Originally Posted by fdenys View Post
Hi FF,

Yes, i took advantage of the Sale action on 11.11 (Chinese single day) and paid $19.20 plus shipping. Now i see it is $24.
Probably coming Black Friday (29-11) is another good buying opportunity.
I took it from here: LCT300 2x24V@6A
Many thanks, today 30.71 Eur (total). I will watch out for 29/11.
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Old 21st November 2019, 06:45 PM   #315
chermann is offline chermann  Austria
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Default amp 2 mock up ready and its powered up...

Good evening


the mock up amp 2 is finished and is running since 2,5 hours with a sweep of 70mVrms into 8R. = about 0,6W
DC offset is -1,4mV and Rchannel has -1,9mV , psu shows in idle about 26,29V each rail.
lets run this amp some hours before i psuh my speakers - KEF Q100.
chris
Attached Images
File Type: jpg mock up_amp2_6.jpg (361.2 KB, 109 views)
File Type: jpg mock up_amp2_7.jpg (348.2 KB, 104 views)
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Old 21st November 2019, 07:14 PM   #316
chermann is offline chermann  Austria
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Default ...R2 shorted....

Hi
i read something and i am not sure if i mix something.pic 1 shows the schematic of the kit. as decribed at post 273
eBay mono LM1875 kit
i shorted the R2, this is the 10R directly at the input to GND, near the 1MR at the negative input.
i found a drawing and i want be clear if the circuit C is the resistor R3 which should be added -maybe on both inputs of the LM1875?
chris
Attached Images
File Type: jpg schematic.jpg (28.4 KB, 119 views)
File Type: jpg R2 shorted_Noise.JPG (53.1 KB, 124 views)
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Old 21st November 2019, 10:45 PM   #317
rabbitz is offline rabbitz  Australia
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Isn't circuits B & C inverting so doesn't apply to this kit which is non inverting.
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Old 21st November 2019, 11:14 PM   #318
FauxFrench is online now FauxFrench  France
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Hi Chris,
My impression is that the two sketches are concerned with different matters:
The first sketch shows how you have a signal ground and a power ground only connected through 10 Ohm, thus a way to reduce coupling of noise from the power ground to signal ground.
The second sketch (version C) is dealing with a different aim outlined in particular in the second and third bullets. With a resistor connected between the inverting and non-inverting inputs, you can increase the "noise-gain" while you at the same time maintain the signal gain and with the advantage that you increase loop stability. A trick I believe to have seen for a non-inverting coupling such that the signal gain can be reduced below the minimum gain specified (for LM1875 20dB) while the amplifier remains stable. The downsides are offset and noise (can't win them all).
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Old 22nd November 2019, 05:49 AM   #319
chermann is offline chermann  Austria
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Good morning


Thanks rabbitz and thanks FF for the explanation.


so the variant C is a way to get the gain down with better stability.

chris
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Old 23rd November 2019, 10:08 AM   #320
chermann is offline chermann  Austria
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Default ringing at higher power

Hi again
After about 30 hours run in with small power = 70mVrms

with my latest mock up i want to do a power test and found a "ringing" at the top of a 1khz sinwave +V. i start with max 600mVrms input into 8R so it should be nearly 20W per channel. i get here a havey distortion on the top as you can see. i didi this with 550mVrm input , 500mvrms input-you see the detailed scoping thats about 3,7 Mhz. at the 450mvrms its gone-or lets say not visible.

pic 1 600mVRms 1khz into 8R Rchannel
pic 2 550mVRms 1khz into 8R Rchannel
pic 3 550mVRms 1khz into 8R Rchannel_detailed
pic 4 500mVRms 1khz into 8R Rchannel
pic 5 500mVRms 1khz into 8R Rchannel_detailed
any ideas? the only thing i guess is that this are these "recovered" LM1875 chips


at 450mVrms the psu voltage go done just to 24V per rail - its good/strong

chris

Last edited by chermann; 23rd November 2019 at 10:11 AM.
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