LM1875 transconductance amp

[shaan]

 

[arcgotic]

Hello all,
i tried but it oscillates.
Some moments it is ok, sound comes ok, then DC rises at some volts on output and sound gets distorted.
I use 0.44ohm instead of 0.33 and 10uF input cap. Also tried different input resistors.
The amp is done almost peer to peer, all components close to each other.

Could this be the problem? What should i try?
Thank you!

 

[arcgotic]

The problem is from the input cap/resistor.I don't use a pot. When jack is in air, all is OK. When i plug the jack in my iphone, DC gets to some volts on output and sound is distorted.

 

[stocktrader200]

I built a discreet current feedback mosfet amp that is unity gain stable for this test.
the results are outstanding

 

[arcgotic]

I am going to rebuild it tomorrow, maybe being to cramped on the board, it oscillates so bad..

Stocktrader, what speakers do you drive with your amp?

 

[kp93300]

I built a discreet current feedback mosfet amp that is unity gain stable for this test.
the results are outstanding

Care to share the schematics and a picture of the built?

 

[arcgotic]

I rebuilt the first schematic in the thread it but it still oscillates.
Do you have some best practices for construction?
I followed Joe Rasmunsen's wiring diagram. Tried different values for input cap and resistor. Added a 220pF also across the input. I hear radio with input cable unplugged.

 

[Pato666]

Looks like a great project to build.

 

[padamiecki]

the oscillations may be caused because the gain of 1875 is too low, it is not unity stable chipamp. Try to lower current sensing resistors to 0.3-0.25 Ohm.
Another thing is that rc zobel circuit should be close to the output of the chip.
I got the oscillations too when zobel was mounted far from the chip (on speakers terminal)

 

[e_fortier]

Sorry kp93300 as I was away from the forums.
Try the circuit below, It is tested by me and is the simple version of what I had originally built. A non-polar 47uF cap replaces the DC Servo to make things simple.
1. Adjust 4R15 per your personal taste.
2. 4R9 controls the gain of the circuit
3. 4C5 controls the bass response with 4R6 and needs to be bi-polar.

All the best!

Hi

Revisiting an old thread since I always wanted to try a transconductance amp. since I have full range speakers and the article from NP convinced me to one day give it a try.

Thanks to the good as built schematic of Bimbla and the great help from Prasi a PCB layout has been created. I will try to fire it up very soon and will keep you posted.

I attached some pics of the empty and stuffed PCB (need to find my LM1875

Once tested I'll have some PCB to sell at cost + shipping (cheap)

BR
Eric

 

[kp93300]

Eric
I am interested in your test results
Kp93300

 

[jwicaksana]

I am tuning in as well.

 

[e_fortier]

Would this heatsink be ok for stereo ?

Thanks

 

[padamiecki]

what power supply voltage did you use?
anyway should be ok enough...

 

[scott17]

Hello Eric,

I would be interested in a couple of the boards if possible once you get everything worked out. I've got a couple new National Semiconductor LM1875 that I have had since the 1990s.

I built a stereo amp using them. That amp sounded great and was pretty much bullet proof. Lost in a move some time ago.

 

[jwicaksana]

Hi Eric, any listening impression yet? Or maybe some Gerbers so we can try ourselves? Thank you very much.

 

[e_fortier]

Hi guys

The PCB is working but I've got a noisy smps powering the amp. Let me try with a linear supply. I then have to build a sealed enclosure for the full range speakers. Lots on my plate lately. If really interested, I can send some pcb (free), just pay shipping.

Eric

 

[Lohengrimas]

Cool stuff, Eric, subscribed for when the PCB's become available. I was thinking of building the "two resistor" minimalist version of the LM1875 but I think this is a much more fun and rewarding project, given my sets of FR speakers.

 

[padamiecki]

I built a discreet current feedback mosfet amp that is unity gain stable for this test.
the results are outstanding

Hi!
please share your schematic and setup.

 

[kking85743]

Hi guys

The PCB is working but I've got a noisy smps powering the amp. Let me try with a linear supply. I then have to build a sealed enclosure for the full range speakers. Lots on my plate lately. If really interested, I can send some pcb (free), just pay shipping.

Eric

Where do things stand? I'd like to try a simple, straight forward amp like this to see if it worked well with the speakers I have. I think it will, but I'm not sufficiently convinced to do design & breadboard on my own.