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Old 28th December 2006, 05:27 PM   #1
okapi is offline okapi  United States
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Default LM4780 oscillation occurs when input is removed

one of four LM4780's i have built up goes into oscillation when the input is removed.

i am using the chips in parallel mode.

with an input the amp behaves reasonably. it differs from the other three amps in that it has a slightly lounder hum and it's dc offset with the inputs shorted is 12 mV as opposed to 4 mV or 5 mV. it also runs five degrees celcius warmer (40 vs 35).

however, when he input is removed (or input short is removed) the amp goes into oscillation.

I already attempted removing the input from the board but this did not prevent the amp from oscillating.

has anyone experienced a similar problem? i have already purchased replacement parts but before i attempt to rebuild i wanted to see if anyone had any suggestions.

thanks.
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Old 30th December 2006, 03:47 AM   #2
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post schematics?

gain?

bypassing?
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if only it could be used for good, not evil...
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Old 30th December 2006, 05:44 AM   #3
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if it is running warmer it might be already oscillating at a higher frequency. or the 2 halves are fighting more than normal because one has something wrong with the circuit.

can you separate the 2 halves by disconnecting the output resistors and see if they both working normally?
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Old 30th December 2006, 03:41 PM   #4
okapi is offline okapi  United States
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Default thanks

i rebuilt last night with the same parts and a new chip and it is working fine. don't know what the problem was.

neutron 7, i did attempt to check for oscillations with a scope and did not see any (with the input connected) but i can't recall the sampling frequency of the scope (it could have been relatively low) and i may have missed it. i would have liked to try your suggestion but i rebuilt before i saw your post.

thanks to everyone for viewing my post and especially for the replies.
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Old 31st December 2006, 04:57 PM   #5
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I have a very similar problem with my LM4780 Gainclone. I am a real newbie so bear with me please. I am using the excellent Audiosector kit and followed the detailed instructions included (kidding! there are no instructions!). I want to thank everyone on the forum for educating me enough to attempt to make an amp.

When I turn the amp on with no input connected there is a terrible noise (not hum) coming from both channels. Also, the chips get hot. When I connect the RCA cable the sound completely disappears and the amp is mostly quiet except a slight hum. The chips get warm but not crazy hot. I am using a small 1/8 inch adapter for my MP3 player and if I touch it then the amp hums from whichever part of the tip touched. The hum is improved to being almost completely quiet when I connect a ground loop isolator device (needed that in my car). The mp3 player is run by a battery. The earth ground is connected to the chassis ground on each amp pcb. My input ground and output grounds are connected to the appropriate places on the pcb board.

Can someone diagnose the problem?
Thanks
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Old 31st December 2006, 05:10 PM   #6
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Default oscillation

Lift your signal ground from system ground via a 1ohm 1/2 watt resistor, also is your case at earth ground or system ground? if it is the case should be earthed and not connected to system ground.
What your seeing are ground impeadance differances causes oscillation, and noise
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Old 3rd January 2007, 05:43 PM   #7
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Thanks for the input. I tried your suggestion and placed a 1 ohm resistor between my RCA jack ground and the input ground. That made no difference at all, so I tried a 3 ohm resistor and still no difference.

There must be a grounding issue. When I have the RCA cable connected it is nearly completely quiet. When I plug in the CD placer or MP3 player (both are battery operated) then there is a distinct humming. When I use the ground loop isolator then the hum goes away almost completely. My earth ground is only connected to the chassis ground on the amp board. The star ground of each amplifier pcb is isolated from the other (which I read somewhere is correct) Should I connect the star grounds of each amp board?
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Old 4th January 2007, 08:02 AM   #8
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Default Question for Moamps

I am still too new to email Moamps directly, but he cured another buzzing gainclone in another post. The post was called "Gainclone trouble" dated 4-16-2003. The cure was to create star grounding of signal and output grounds on the PSU central ground. Here is my question: Does this mean connecting PG + and PG - on my PSU and sending all my grounds there? I am a newbie so I am too timid to just try this without checking. I am using the Audiosector PCBs. I am also using a wood box as a chassis. Also, I welcome any help, but would be especially interested in Moamps reply. Thanks!
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Old 16th January 2007, 04:28 AM   #9
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Just wanted to follow up if anyone cares to hear. Learn from my mistake! I designed my amp chassis with aluminum plate for the sides with the plan to mount each LM4780 to the side plate for optimal heatsinking. Pretty smart eh? No! THis was a huge mistake. THis made it necessary to snake wires all over the place, the toroid was in the middle of the case and wires came too close, and whatever I did with rewiring and "creative grounding" it buzzed. I though it was a grounding issue since it hummed most loudly when I connected the input, and little otherwise.

I was about to chuck it all and was researching buying a professionally made amp javascript:smilie('')
frown when I realized more experienced builders mount the chips quite close to one another. Just look at any of Peter Daniels designs. Then the wires are short, and no interference.

I wonder how many grounding issues in this forum are really due to poor layout. My final grounding scheme is exactly as the PCBs are labelled, and now I hear zero hum unless I apply my ear to the speaker.

Sorry NAD, Rotel, I'm Gaincloning now! javascript:smilie('')
Big Grin
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Old 16th January 2007, 03:50 PM   #10
CarlosT is offline CarlosT  United States
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Happy ending then? cool!

One thing I noticed and I emulated was to twist all wires...especially any carrying input signals and alternating currents.

One of the handiest tools when building my first GC came out of my garage for the motorcycles...a safety wire plier. This tool can twist the bejesus outta solid core cable and worked out great.

http://www.aircraftspruce.com/catalo...retwisters.php

I know that many DIYers tend to make pretty symmetrical GCs with a channel on each side of the chassis (this also makes good sense to stick huge heatsinks on each side) and the tranny in the middle. Even some retail amps have the huge toroid in the middle. Nothing wrong with that but I guess you cannot maximize the distance between the toroid and the amp PCBs that way.

In my very crammed mono GC, I strived to separate the signal from power as best as I could. Notice all signal stuff is on the right (front) and the power stuff is on the left (rear) of the chassis.
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