Double PCB LM1875 problem hum need help

[denbret]

Originally Posted by chermann

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

Hi chris,

I was not informed when I bought it.

Yes there is a channel which has a higher gain.

So if I understand correctly, we will have to buy resistors with good precision, and measured them to have the most equal gain on the 2 channels.

Denis

[chermann]

partly yes...
because the gain must be the same over the complete frequency band, e.g. 20Hz to 20kz...ever mini deviation of voltage at amp 1 force to other amp "to be pushed". that´s why you should use a small resistor of 0,1R - 0,33 R.

[denbret]

Okay I understand a little more. The LM amp was plugged in fullrange on my speakers, I just changed the configuration, now it is plugged into the woofers cut to 900hz, the LM is superb in the low midrange, and double the bass is powerful and precise. The woofers are Wavecor.

The gain seems equal in the bass. I keep in mind about the adjustments.

Denis

[denbret]

Hi,

I installed the 2nd toric. I didn't expect him to be that big. It has 1 volt AC less on each outlet.

Then it's time for listening tests..

Denis

[denbret]

Hi everyone,

I listened several times with the LM and its 2nd toric transformer. The sound is perfect, no imbalance between the two and it seems to me on all frequencies. The soundstage is precise and does not move.

I will eventually measure with a mic and REW both channels. My new 3-way speakers that I have designed are quite linear and well balanced.

The tone of the LM1875 DE Zero Zone mono cards is really to my taste, it is a tone that I would describe as smooth, but at the same time quite dynamic, the bass is powerful and articulate, the midrange is all spread out in the soundstage, and the highs and extreme highs have more presence and transparency than my other LM1875 in singles.

I highly recommend it.

Regarding the adjustment of bias resistors (or balancing), I could not touch anything.