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Old 6th March 2015, 11:20 AM   #1
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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, 405 views)
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Old 6th March 2015, 11:42 AM   #2
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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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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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Old 6th March 2015, 12:37 PM   #4
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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 30th March 2015, 07:03 AM   #5
kp93300 is offline kp93300  Malaysia
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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; 30th March 2015 at 07:04 AM. Reason: spelling
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Old 31st March 2015, 08:29 AM   #6
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Quote:
Originally Posted by kp93300 View Post
1) You used the IRF 9540 as cap multiplier ? Or to drop the V ?
zener type regulator but you can use LM7824/7924 or use no stabilisation if the voltage of power supply will be lower than +/- 30V
Quote:
Originally Posted by kp93300 View Post
2) What size transformer ?
attached picture of the schematic of power supply of Philips F4200 inside which I built amp. As you can see the transformer is not too big, with ct 50Vac which gives +/- 33VDC after rectifying and filtration and this was one of the reasons I decieded to use zener type regulators with IRFs
Quote:
Originally Posted by kp93300 View Post
3) What is the expected power output with 24 V power supply ?
single or double? circa 20W @+/- 24VDC
Quote:
Originally Posted by kp93300 View Post
4) What is the pre amp used?
no preamp, just volume pot from F4220
Quote:
Originally Posted by kp93300 View Post
5) What is the function of 5.4 and 68 resistor ?
5.4 is speaker resistance (measured), 68 is for protection when no load at the amp
Quote:
Originally Posted by kp93300 View Post
6) Can i used +/- 24 V dc power supply ?
max for 1875 is +/-30VDC, so yes!
Quote:
Originally Posted by kp93300 View Post
7) Is the input cap in the signal necessary ? Can this be substituted for 10 UF ?
in general yes, necessary, and you can use 10uF
Quote:
Originally Posted by kp93300 View Post
My speaker is 7 R Markaudio 12 P
there should be not the problem with driving them.

BTW loading input resistor with the value of 22k gived the lowest offset at the output (a few mV)
Attached Images
File Type: png schf4220.png (198.6 KB, 223 views)
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Old 22nd April 2015, 03:12 PM   #7
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Default current drive based on TDA7293

Hello again,

I finally built a current drive based on TDA7293. Big amp really for my needs...
A non-inverting configuration with DC blocking RC input filter and the loudspeaker with the sense resistor forming the gain components. No R-C across the output.
I added a DC servo loop to keep the output DC in check.
I tested it with as many full-range loudspeakers, in sealed enclosures, that I had.
The results are simply fantastic. Each one of them sounded as if it was 4 times as big and 4 times as expensive.
I confidently shorted the output much to the amusement of the watchers.

Bass, Mid-range and treble associated with dialogues, music and effects, whether listening to music tracks or watching a movie all get placed correctly.

It seems like a per-equalized system.

Without the current drive, the same content becomes unbearable, dull and lifeless, so beware.

All audio DIYers must do this at least once.
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Old 23rd April 2015, 05:59 AM   #8
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regards, Pawel
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Old 24th April 2015, 09:35 AM   #9
kp93300 is offline kp93300  Malaysia
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Default heatsink advice

I intend to build the circuit in the first post.
Is this heatsink big enough?
Power supply is+/_ 24 v dc.
speaker is markaudio 12P.
Can i expect about 20 W from this amp ?
thks
kp93300
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Old 24th April 2015, 10:16 AM   #10
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Quote:
Originally Posted by kp93300 View Post
I intend to build the circuit in the first post.
Is this heatsink big enough?
Power supply is+/_ 24 v dc.
speaker is markaudio 12P.
Can i expect about 20 W from this amp ?
thks
kp93300
Heatsink - I do not know, I guess so
power s is OK
speaker resistance is?
slightly less than 20W, I would say 18W, but this is plenty
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