Hi Anatech,
I have noticed your presence after what seemed a long holiday.
Nice to see you back.
the "full PM" puts off requests from numerous Newbies who request private advice.
I much prefer "public" advice that is open to contrary replies.
There is little point in choosing to hear ONE BIASED viewpoint if you have not yet learned how to "sort the wheat from the chaff".
Balance the pros and cons on a public Forum.
post936 & 938 are such an example.
"I sleep good knowing I'm safe with a TVC!"
"No preamp. A tranformer volume control = no dc "
Those comments in a private conversation could lead to a Newbie buying a very expensive TVC and not realising he/she would damage the TVC on first use.
I have noticed your presence after what seemed a long holiday.
Nice to see you back.
the "full PM" puts off requests from numerous Newbies who request private advice.
I much prefer "public" advice that is open to contrary replies.
There is little point in choosing to hear ONE BIASED viewpoint if you have not yet learned how to "sort the wheat from the chaff".
Balance the pros and cons on a public Forum.
post936 & 938 are such an example.
"I sleep good knowing I'm safe with a TVC!"
"No preamp. A tranformer volume control = no dc "
Those comments in a private conversation could lead to a Newbie buying a very expensive TVC and not realising he/she would damage the TVC on first use.
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The noise of a power amp is usually not a problem and not sufficient of a problem to make doubling up the input devices necessary.
If more open loop gain is required from a jFET input stage, then increase the input stage bias current. But do check that the increased gm does not make the stability margins too low.
If more open loop gain is required from a jFET input stage, then increase the input stage bias current. But do check that the increased gm does not make the stability margins too low.
Hi Andrew,
thx for your reply. Indeed, the idea was to decrease the noise like shown in some low noise amp topologies. As I'm new in this fields, why is the Dan D'agostino amp using so much parallel transistors in the Input stage? Is this to increase the openloop gain?
Krell KAV300i uses parallel transistors too. But not in the input stage as far as I know.
thx for your reply. Indeed, the idea was to decrease the noise like shown in some low noise amp topologies. As I'm new in this fields, why is the Dan D'agostino amp using so much parallel transistors in the Input stage? Is this to increase the openloop gain?
Krell KAV300i uses parallel transistors too. But not in the input stage as far as I know.
Better late than never!
Finally finshing my FC100 that has been sitting on the shelf for the last year or so.
The first amp board went well with all measurement looking good so with renewed enthuisasm I went straight to the second amp board.
That had a couple of problems that I (hopefully) have resolved - the first was a flickering indicator LED at the backend PS that I replaced - now good.
The second problem occurred while checking the the quiescent current. The first test with two diodes bypassed/jumpered was 0 volts with the potentiometer preset to 0 ohms and adjusting it bought it up to the required 36 mv.
But with only the the one diode bypassed I couldn't adjust it, it stayed at around 10 mV. I put back the second jumper again and same result - I couldn't adjust that either even though it worked the first time.
Finally worked out that R22 was somehow broken and was stuck or shorted at 0 ohms - how this happened I don't know - could over enthusistic winding on my part had damaged it?
Anyway, luckily I had a spare and managed to replace it without destoying the board and could now adjust the quiescent current
.
On the home straight now I just had to finish my second shunt supply (used the one supply for testing both boards) but now may have a problem with this supply?
With a 2K resistor connected on each leg I get different readings. One side is 36.7V but the other is -36V - a difference of 0.7V. Is this difference acceptable or should I try changing the zener diodes?
Thanks
Finally finshing my FC100 that has been sitting on the shelf for the last year or so.
The first amp board went well with all measurement looking good so with renewed enthuisasm I went straight to the second amp board.
That had a couple of problems that I (hopefully) have resolved - the first was a flickering indicator LED at the backend PS that I replaced - now good.
The second problem occurred while checking the the quiescent current. The first test with two diodes bypassed/jumpered was 0 volts with the potentiometer preset to 0 ohms and adjusting it bought it up to the required 36 mv.
But with only the the one diode bypassed I couldn't adjust it, it stayed at around 10 mV. I put back the second jumper again and same result - I couldn't adjust that either even though it worked the first time.
Finally worked out that R22 was somehow broken and was stuck or shorted at 0 ohms - how this happened I don't know - could over enthusistic winding on my part had damaged it?
Anyway, luckily I had a spare and managed to replace it without destoying the board and could now adjust the quiescent current
.On the home straight now I just had to finish my second shunt supply (used the one supply for testing both boards) but now may have a problem with this supply?
With a 2K resistor connected on each leg I get different readings. One side is 36.7V but the other is -36V - a difference of 0.7V. Is this difference acceptable or should I try changing the zener diodes?
Thanks
the difference between the +ve and -ve supplies does not affect front end operation.
Untill you start clipping the signal. The slightly lower supply rail side will clip very slightly earlier.
But you should not be listening to clipped audio.
That's why our average listening levels are roughly -20dB ref maximum unclipped signal.
Untill you start clipping the signal. The slightly lower supply rail side will clip very slightly earlier.
But you should not be listening to clipped audio.
That's why our average listening levels are roughly -20dB ref maximum unclipped signal.
I've been listening to the FC-100 for a few months now and am still amazed that I am listening to one of the best amps I've heard and that I own it!
Took me quite a while to finish it and thought it about time to thanks those who helped me build it - in particular (of course) to Rudi and friends for their time and effort in making it all possible and to Mihai who created it.
Thanks guys.
Took me quite a while to finish it and thought it about time to thanks those who helped me build it - in particular (of course) to Rudi and friends for their time and effort in making it all possible and to Mihai who created it.
Thanks guys.
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Hi everyone!
I know i am very, very late ... But i finally found the time, yeah!
But i was not as successful as i hoped.
I've soldered and (visually) checked everything, the Shunt and the Backend PSU are working perfectly! So i went on to the quiescent current (just like Rudi suggested in his "Builder's Manual") and put all together.
Board 1:
The Bulb Tester lights up for just one second and i've got 0.00 mV between the two measurement points. But nothing changes when i turn potentiometer ...
Board 2:
The Bulb Tester is glowing (pretty bright) and i've got 86 mV between the two measurement points. But nothing changes when i turn potentiometer ...
(and the two diodes from the Backend PSU are now not glowing anymore).
And now i hope here is still someone reading and can help me
I know i am very, very late ... But i finally found the time, yeah!
But i was not as successful as i hoped.
I've soldered and (visually) checked everything, the Shunt and the Backend PSU are working perfectly! So i went on to the quiescent current (just like Rudi suggested in his "Builder's Manual") and put all together.
Board 1:
The Bulb Tester lights up for just one second and i've got 0.00 mV between the two measurement points. But nothing changes when i turn potentiometer ...
Board 2:
The Bulb Tester is glowing (pretty bright) and i've got 86 mV between the two measurement points. But nothing changes when i turn potentiometer ...
(and the two diodes from the Backend PSU are now not glowing anymore).
And now i hope here is still someone reading and can help me
Hi Rudi,
nice to hear from you again!
I've followed your manual pretty close. First I've tested the Shunt with the Bulb Tester separately and got +/- 38 V.
Then I've tested the backend PSU with the Bulb Tester (again separately and without the fuses) and got +/- 37,5 V.
Then I went on and plugged all together: I shorted the input, inserted the fuses, connected the transformer from the frontend directly to mains and connected the transformer from the backend via the Bulb Tester. I also disabled a second ThermalTrak diode.
And now I've tested the Shunt without the Bulb Tester on its own. Still +/- 38 V.
nice to hear from you again!
I've followed your manual pretty close. First I've tested the Shunt with the Bulb Tester separately and got +/- 38 V.
Then I've tested the backend PSU with the Bulb Tester (again separately and without the fuses) and got +/- 37,5 V.
Then I went on and plugged all together: I shorted the input, inserted the fuses, connected the transformer from the frontend directly to mains and connected the transformer from the backend via the Bulb Tester. I also disabled a second ThermalTrak diode.
And now I've tested the Shunt without the Bulb Tester on its own. Still +/- 38 V.
Hi atupi,
I'll shoot you a short answer.
Most amplifiers have a feedback loop to control the gain and frequency response, as well as to reduce noise and distortion. There is a feedback resistor, and normally a small capacitance to roll off the HF response before the phase change gets near the 180° mark - where the amplifier becomes a power oscillator. You don't want this to happen. Some loads have a very low impedance at high frequency, and external HF noise can enter the amplifier via this capacitor. The choke isolates the feedback loop from the outside world at high frequencies.
Contrary to what is now a popular myth, the output choke is not a disaster for extreme high fidelity audio, or normal audio. By the same token, feedback is a very good thing to use in an amplifier. The only amplifiers that don't tend to use an output inductor (choke) have designs that use feedback that isn't returned to the front end, or have no feedback loops at all. These designs are not necessarily better than ones that use global negative feedback, nor are they worse than normal designs. I have an amplifier that doesn't use negative feedback, and the rest do. I also have one where the feedback is taken from a point before the output stage. They are perform well, and I couldn't really rank them according to design. My favorite two amplifiers use normal negative feedback loops, but the others are not far behind.
-Chris
I'll shoot you a short answer.
Most amplifiers have a feedback loop to control the gain and frequency response, as well as to reduce noise and distortion. There is a feedback resistor, and normally a small capacitance to roll off the HF response before the phase change gets near the 180° mark - where the amplifier becomes a power oscillator. You don't want this to happen. Some loads have a very low impedance at high frequency, and external HF noise can enter the amplifier via this capacitor. The choke isolates the feedback loop from the outside world at high frequencies.
Contrary to what is now a popular myth, the output choke is not a disaster for extreme high fidelity audio, or normal audio. By the same token, feedback is a very good thing to use in an amplifier. The only amplifiers that don't tend to use an output inductor (choke) have designs that use feedback that isn't returned to the front end, or have no feedback loops at all. These designs are not necessarily better than ones that use global negative feedback, nor are they worse than normal designs. I have an amplifier that doesn't use negative feedback, and the rest do. I also have one where the feedback is taken from a point before the output stage. They are perform well, and I couldn't really rank them according to design. My favorite two amplifiers use normal negative feedback loops, but the others are not far behind.
-Chris
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