RMI-FC100, a single stage audio power amplifier - Page 112 - diyAudio
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Old 18th April 2011, 07:09 AM   #1111
roender is offline roender  Romania
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Quote:
Originally Posted by AAK View Post
Like this?
Yes ... something like that.
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Old 30th April 2011, 08:47 AM   #1112
jormajj is offline jormajj  Finland
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Any PCBs available , anyone ? I need two
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Old 17th July 2011, 10:45 AM   #1113
koldby is offline koldby  Denmark
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Unhappy Please help!!

About 1 mounth ago I finished building a version of Mihai's amplifier, using the original PCB's.
I was amazed to see how the stability of the dc was , even though ther were no trimmers or servoes.
Worked fine and I listened to it for a week, though without direct comparing it with my other reference amps.
I wanted to give the FC100 the best conditions before direct comaring it to the others, especially my current favorite , a 6C33 based OTL amp.
So I mounted in the amp. chassis 4 big 47000 uF capacitors just after the rectifiers and tested it again.
Worked fine..
I had made som changes in the loudspeaker crossover, that made them even better sounding with the 6C33 amps, so now the test could start.

The fc100 was connected and turned on..Strange noise for a while and the fuses popped in one of the supply lines and it puts out DC..

All of this is just to tell you the backgroud for my problems.

Down on my test bench with new fuses and slowly on with a Varaic, the output was ok, but with a strange noise (like instability) and it drew too much idle current.

I disconnected the output/driver stage supply and connected R7 and R37 via a 150 Ohms to the output to establish the feedback loop.
Inputstage works perfectly. No DC no stability probs. and a clean signal.

All 4 driver transistors was replaced. No change. Stiil instability and too much idle.

Lifted all the 2,2 Ohms basis resistors and connected 2 of them with 150 Ohm in series to the output (again to establish the feedback loop). No DC . No noise. Clean signal!!


Tried to short circuit some of the body diodes in the bias string, and that decreases the bias.

The current through the bias diode string (incl. the base currents of the drivers) measured over R17 is 2,47 mA.

Tried to use only one pair of output devices (lifted 4 of the 2,2 Ohm base resistors). No change. No matter wich pair was in the circuit - no change.

The base currents of the output transistors (measured over the 2,2 Ohm resistors) are almost equal.

The output devices draws each around 1 Amp.
Voltage over R 5 ca. 1,25V
Voltage acroos the bias diode string 2,35V.

Have to do the measurements quite quickly, as the output runs hot fast.

I have only done these things with one channel, but I believe the other has excatly the same problems.
HELP PLEASE.
I have run out of good ideas.

Koldby
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Old 17th July 2011, 11:16 AM   #1114
Sheldon is offline Sheldon  United States
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How is it earth referenced? Do you have a cap (1uF or so) from common to earth)?

Sheldon

edit: Earth in this context includes the heatsink.

Last edited by Sheldon; 17th July 2011 at 11:22 AM.
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Old 17th July 2011, 01:00 PM   #1115
koldby is offline koldby  Denmark
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Makes no difference.
I have, when the hole thing is assambled, a 22 Ohm resistor to chassis from star ground. And I have the input ground connected to star ground as well( no connecteion on PCB)
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Old 17th July 2011, 06:01 PM   #1116
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by koldby View Post
And I have the input ground connected to star ground as well( no connecteion on PCB)
Why?

Sheldon
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Old 17th July 2011, 11:37 PM   #1117
Sheldon is offline Sheldon  United States
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Or more specifically, I wired according to Mihai's recommendations. RMI-FC100, a single stage audio power amplifier Input and input common to the specified locations on the PCB. A 1uf film cap from the main common large copper buss (where it connects to the input circuit), to the chassis (which is connected to the heatsink). Chassis earthed, but no DC earth connection to the circuit. I had problems blowing fuses when I had a DC connection, none when I followed the recommendations. Not sure why you would do otherwise. My experience is described in the thread, starting around page 75.

Sheldon

Last edited by Sheldon; 17th July 2011 at 11:54 PM.
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Old 18th July 2011, 08:51 AM   #1118
koldby is offline koldby  Denmark
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Hi Sheldon
Thanks!

You were actually on the right track.
The ground connection from input to star ground was NOT there in the first place, but during my search for the problem , I discovered that there was no ground connection at the input and when I looked at the schematic, I misread it and thought I had miised this ground connection from the start, and that could have caused the instability that blew up the amp. I thought that the ground connection had been established outside the amp, when I first listened to it without problems for a week.

The problem was that the 10 Ohm resistor from input ground to star ground on the PCB was blown!

After replacing that the amp worked again - in that channel at least. I believe the problem is the same in the other. Now I will be using a capacitor instead of a resistor from star groun to chassis ground. Why the 10 Ohm blew I have't figured out yet, but it could be a defective source (DC)?

But thanks a million.

Koldby
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Old 18th July 2011, 11:43 AM   #1119
Sheldon is offline Sheldon  United States
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Excellent. You should be able to enjoy your amp now. BTW, I've fried that same common lift resistor in another amp (not FC100). When the return paths aren't set up correctly, an offset at the output can be returned through that 10 Ohm resistor. It doesn't take much voltage differential to cook it. You probably didn't fry anything else. One way to guard against a transient issue (mistake with a meter probe, etc.,) is to use a pair of antiparallel diodes across the resistor - at least while you are doing the testing and check out.


Sheldon
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Old 18th July 2011, 01:20 PM   #1120
koldby is offline koldby  Denmark
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Hi Sheldon

Nothing else fryed and both channel is up and running now - with antiparallel diodes!

I will report back on humble thougts about the sound later.

Koldby
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