Double PCB LM1875 problem hum need help

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Hello everyone,

I have 2 purchased PCBs already assembled made by Zero Zone.

I have only one toroidal transformer of 100VA (18-0-18v), the transformer has no ground wire. If I plug in a mono PCB at a time, there is no hum, but as soon as I add the 2nd, the hum appears.

Screws on PCBs don't seem to have any grounds, so that suggests to me that PCBs aren't grounded. In addition there is a separate rectification on each PCB.

If you have any suggestions or ideas that can help me, that would be much appreciated. I also want to mention that I am quite new to electronics.

Thank you.
Denis.
 

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Use only one PCB power supply (rectifier bridge). Connect the second with three wires (+, -, zero) in parallel after the rectifier bridge of the first directly to the large supply capacitors.
Capacitors to capacitors, is that clear?
You have to experiment with signal ground. Take isolated RCA connectors. Connect everyone to their PCB.
 
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# 4 is wrong. Take a close look at the power, capacitor, and signal ground circuits. Select a grounding point to the chassis.
Shielded input wires. You have them intersect with the exit. Route the outlet along the bottom of the case.
I don't see where your diodes are? One or two bridges? Is there a podium under the torus?
 
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I also want to mention that I am quite new to electronics.

Thank you.
Denis.
I am VERY WORRIED at your mains wiring.
:worried:
Power cable black and white immediately go to a screw terminal strip where they change colours, so you lose reference, then go to various places in a not too clear path, but worst is you show a plain LED connected straight to mains voltage , the other end is lost in a cable tangle.

Connections are floating, completely insecure.

No need to repeat that MAINS VOLTAGE IS DEADLY.
 

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I am VERY WORRIED at your mains wiring.
:worried:
Power cable black and white immediately go to a screw terminal strip where they change colours, so you lose reference, then go to various places in a not too clear path, but worst is you show a plain LED connected straight to mains voltage , the other end is lost in a cable tangle.

Connections are floating, completely insecure.

No need to repeat that MAINS VOLTAGE IS DEADLY.

The led is not connected to the AC, we cannot see clearly on the photo, but it is connected to a DC transformer. This LED has worked for years without problems. Yes I agree, I put a terminal block and it is not standard and safe. It looks worse than it is. The wiring that goes from front to the switch is bent and above the wiring below. It's an old project that I got back. I've seen a lot worse in tube amps. But I will follow your advice and I will redo the wiring, remove the screw terminal block and solder the wires cleanly.
 
Two rectifier bridges connected to a single transformer is going to create hum.

I fixed the HUM issue. I simply added a 2nd transformer, because there is a diode bridge on each PCB. In short there was no hum, it's very quiet even the ear stuck on the hp.

I have a question. The 2nd transformer comes from my other LM1875 amplifier. So it's gonna take me a new transformer. Currently I have a 100va 2x18v and to save money I thought to put a 50va. An LM1875 channel in // will consume a maximum of 45-50w taking into account the efficiency. Is it a good idea where it is better to put a completely identical transformer?

Thank you all for your help.
 
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I finally bought a 100va, because it came back to me at the same price from another supplier.

The amplifier has no whistling even with the ear glued to the ribbon tweeters. It's very clean, a nice soundstage and it seems to go further than my other LM1875 in singles.

As soon as I receive the new transformer, I will remove the terminal block and redo the wiring with soldering. I will come back to put pictures.

thanks again
 
Hi Denbret


Congratulations for solving your problem. no the 50VA is with normal listening no problem. if ou remember at the ebay kit thread ...rabbit is usually using small transformers. 50 or 80VA for 2 channels. maybe your small transfomer get short stress at the beginning of power on because of the 44000µF caps.



please report if you have time. i am interested in the easy paralleling of the LM1875 without trimmer for exact gain issues. is some of the output resistor (the ceramic) hot during listening??


chris
 
Hi Chris,

Thank you,

Yes I agree, the 2nd that comes from my other LM1875 stereo is a 50va and it works well. With the two transformers and the capacity of the condos, the softstart will help start.

I will get back to you regarding the heat of ceramic resistors.
Denis
 
Hi denbret


thanks..


maybe you remember the ebay kit thread and the Thread of FF with ..lm1875-parallel-configuration-composite-amplifier
LM1875 in parallel configuration and used in a composite amplifier.


it is mandatory that if you connect 2 amps in parallel the gain ( 22k and 1k resistor) must be perfect and the feedback "reaction" too - therefore the load resistors are for. They waste the energy = the "difference" of both outputs, if they are not simultaneously. the resistor should be not hot. i am afraid that this chinese amp are exactly with the same gain.



chris
 
Hi chermann,

I ran out of space in the living room cabinet to remove and check the temperature of the resistors.

Now I have more space, as soon as possible I will crank the amplifier to check the heat of the resistors to the touch.

In addition I have a new toric transformer of 100VA, which will arrive soon for the other channel, currently it is a small transformer.

Denis
 
Hi Denis,


please do not understand me wrong. you know that in the 1875 parallel amp thread. it is not so easy to just put s+the some chips at the output in parallel and hope and pray. its important that the gain of both amplifier are very exact the same without trimming each amp its nearly not possible + plus the DC offset of both outputs must be saved with a serial resistor at the output . lets say about 0,1R to 0,33R.



this is the reason why i ask you kindly to check this parallel situation on your board. i dont believe that the zerozone people use better resistors as 1% and select the resistors from 50 ...and then use them for each board and need no trim pot.


regards
chris
 
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