Thanks for reply.
Isn't it possible just to replace the diode string with NPN only base to emitter junctions and make it like this: http://www.diyaudio.com/forums/soli...gle-ended-folded-cascode-vas.html#post1621315 regarding pot?
Alas BD135, BD139 have TO225 Style1 package (emitter-collector-base). I think better will fit TO225 Style3 (base-collector-emitter), but I only found MJE13003. Do they need to be thermally matched?
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You could use most any diode, if you want to do it that way. Ideally, they will match the temco of the thermaltrak diode. Perfect match is not strictly necessary, but should be reasonably close. You could use the holes in the PCB for that purpose, but I don't see how you are going to get them fastened to the heat sink. Easier to mount them on top of the output devices, and just run wires to the transistor/diode terminals, in which case it doesn't matter what the pin orientation is.
There is no pot in the link you show. But I assume that the bias was set with a pot, the value determined, and a resistor substituted for the pot. You can do that with the standard version if you like, and if you don't intend to change the bias point.
Sheldon
I've remade a layout to fit TO225 Style 3 on the heatsink. But if it's not necessary, can I use mur120 or any other equivalent diode is the spare holes of the PCB without fastening them? Are you refer that I can change the pin orientation by wires or it doesn't matter what the diode string orientation is in the schematic?
Regards.
I was assuming that you are using Mihai's layout, which would make it mechanically difficult to solder transistors or diodes to the board and also attach them to the heatsink. But if you have to change the layout to account for the pinouts of the new output devices, then it should be possible to make the necessary changes to accommodate a diode.
The diode electronic orientation, of course, matters. It must be as shown on the schematic. But if you are not soldering the diode directly onto the board, and instead connect them with wires, then the mechanical orientation is flexible and you can use any suitable form of diode. If you are redoing the layout, then I would suggest that you arrange it so that the diodes sit directly on top of the output devices (except for one, as you need only 5 diodes).
Sheldon
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Hi guys!
There is no reason to use an external diode string for the Vbe thermal compensation. It is complicated to mount and there is no advantage from thermal tracking point of view. Instead, use a classical transistor based Vbe multiplier mounted on top of one of the power transistors, like in the AKSA design.
How is life now, here in DIY community?
Regards,
Mihai
There is no reason to use an external diode string for the Vbe thermal compensation. It is complicated to mount and there is no advantage from thermal tracking point of view. Instead, use a classical transistor based Vbe multiplier mounted on top of one of the power transistors, like in the AKSA design.
How is life now, here in DIY community?
Regards,
Mihai
That's what I did. I made the layout to attach diodes to the heatsink.
What is preferable, attach diodes to the heatsink or place them on top of the output devices?
Hi Mihai. As you see this thread of RMI-FC100 amp cannot sustain without it's designer.
No advantage even if I can attach diodes (base to emitter junctions) to the heatsink (I made it as MKII layout)?
Vbe multiplier might work hot.
Regards.
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The advantage of the Thermaltrak devices, is that the diode is on the same die as the transistor. Therefore it closely tracks the transistor temperature. The further away it is, the more it lags behind the transistor temperature. So if it can't be on the same die, then mounting it (or a Vbe multiplier) on the output transistor is next best.
Isn't that exactly what the designer just said, or is there some misunderstanding?
Why would it work hot? It's not dissipating any significant power. It's simply working against temperature related bias change.
Sheldon
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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
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
Or more specifically, I wired according to Mihai's recommendations. http://www.diyaudio.com/forums/soli...age-audio-power-amplifier-79.html#post1805690 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
Sheldon
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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
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
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
Sheldon