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Old 6th March 2015, 11:20 AM   #1
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Join Date: Mar 2004
Location: Krakow
Default LM1875 transconductance amp

Hi!

Based on Esa Meriläinen’s project Transconductance amplifier project | Current-Drive - The Natural Way of Loudspeaker Operation I built similar easy transconductance amp on LM1875s.

V1,2 are just the working supply voltages from Philips F4220 dead amp.

For the simplicity on the schematic bellow there is no right channel which is identical as left.

The chips and mosfets are mounted on original heatsink from the amp.
I used volume pot, input circuitry, output sockets from the amp too.

The sound is absolutely fantastic, reminds good sounding tube amps, especially in detailed and silk trebles and mids but without heat losses and massive transformers. I made several tube SE and PP amps before and I know what I am writing.

As transconductance amp, it has very high output impedance, some speakers might sound boomy due to not existing damping factor. I tried it with Jamo Compact 90, Saba Acoustic Monitor 80, Sansui SP3000A with excellent results.

Please read carfully Joe’s excellent thread to prevent any damages.

The project is simply but is worth the best parts like caps, resistors, power supply, etc. due to extraordinary sound.

Power indicators from Philips F4220 at party volume levels are showing only ~5W

Have fun!
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File Type: png lm1875inPhilips.png (17.1 KB, 182 views)
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regards, Pawel
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Old 6th March 2015, 11:42 AM   #2
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Join Date: Jun 2002
Location: Where work is
Nice and simple circuit to verify the concept.
Kindly share your views (even if it will be subjective) on the following:
1. Lo, mid and hi while listening to music
2. Dialogues, music and effects while listening to movies

This will help me/ us visualize where a current drive makes a difference.

Also, are you planning to use this with a full range in a sealed enclosures?

Thanks!!
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Old 6th March 2015, 12:22 PM   #3
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Join Date: Mar 2004
Location: Krakow
Quote:
Originally Posted by bimbla View Post
1. Lo, mid and hi while listening to music
I would say, the highest quality low, mid and hi with the remark that the resonant frequency of the speakers might be boosted

Quote:
Originally Posted by bimbla View Post
2. Dialogues, music and effects while listening to movies
I did not check, but I started to think about rebuilding sound amps in my TV and 2.1 Creative pc amp to current drive...

Quote:
Originally Posted by bimbla View Post
Also, are you planning to use this with a full range in a sealed enclosures?
the names of the speakers from the first post are fully enclosured full range speakers
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regards, Pawel
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Old 6th March 2015, 12:37 PM   #4
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Join Date: Jun 2002
Location: Where work is
Thank-you for your quick reply.
Kindly do find some time to check it out with TV and post an update here.

Thanks again!
This is going to be inspirational!!
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Old Yesterday, 07:03 AM   #5
kp93300 is online now kp93300  Malaysia
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Join Date: Mar 2007
Hello Pawel,
Thanks for sharing. I will build this soon. few questions
1) You used the IRF 9540 as cap multiplier ? Or to drop the V ?
2) What size transformer ?
3) What is the expected power output with 24 V power supply ?
4) What is the pre amp used?
5) What is the function of 5.4 and 68 resistor ?
6) Can i used +/- 24 V dc power supply ?
7) Is the input cap in the signal necessary ? Can this be substituted for 10 UF ?
My speaker is 7 R Markaudio 12 P
Thanks

kp93300

Last edited by kp93300; Yesterday at 07:04 AM. Reason: spelling
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