Roender's FC-100 prototype and builder's thread

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A

AndrewT

R.I.P.
Joined 2004
AndriyOL said:
Hi,
Can anyone help, what could be a reason of fc100 amp shorting after powering it up with 40W light bulb tester?
Used it with 30VDC main PSU and 37VDC fronted PSU. They are work fine alone.
Click to expand...
by "shorting" do you mean the bulb lights up?

One cannot check a high bias amplifier using the bulb tester method.
All ClassAB amplifiers can be checked prior to setting the bias current. Once all the wiring has been checked and the voltages at the amplifier have measured as "normal" then the bulb is removed and the bias is then set with the amplifier direct on line.

The FC is no different.
You set the bias to "OFF" by shorting each of the diodes that set the bias voltage. That turns the FC into a an unbiased ClassAB amplifier. NOW everyone can test the wiring and the voltages for "normal".

Now remove one diode short at a time and see the effect on the bias voltage.
 
Rudi_Ratlos

Rudi_Ratlos

Member
Joined 2009
Andrew,

Andriy owns a very "individual" version of Mihai's FC-100, which he calls "FC-100 MKII", and which he has layouted and etched by his own.

From the pictures of the PCB that he sent me, I can see that he has done several modifications to the frontend, he is using Toshiba 2SC5200
and 2SA1943 (?) as OP-transistors and "simulates" the TT-diodes of the NJLs by the base-emitter junctions of some TO126-case transistors
that he screwed on top each of the Toshibas.

The problem he has, is, that the backend rail-voltages drop to very low voltages (about +/-2VDC) after powering-on the backend.

I find it very hard to assist with his build, though it is interesting, because his version differs in many aspects from Mihai's design and evaluations.
I suspect a problem in the TEMPCO / BIAS adjustment.

Maybe Andriy tells in detail, what he changed compared to the original FC-100 design.

Best regards - Rudi_Ratlos
 
A

AndriyOL

Member
Joined 2008
AndrewT said:
by "shorting" do you mean the bulb lights up?

One cannot check a high bias amplifier using the bulb tester method.
All ClassAB amplifiers can be checked prior to setting the bias current. Once all the wiring has been checked and the voltages at the amplifier have measured as "normal" then the bulb is removed and the bias is then set with the amplifier direct on line.

The FC is no different.
You set the bias to "OFF" by shorting each of the diodes that set the bias voltage. That turns the FC into a an unbiased ClassAB amplifier. NOW everyone can test the wiring and the voltages for "normal".

Now remove one diode short at a time and see the effect on the bias voltage.
Click to expand...
Yes, lights up.
I shorted one of 5 junctions and noticed a little higher voltage on FC with tester (2.3VDC, with 5 transistor junctions was 1.5VDC) and some hum from speaker. Shall I try to short them all and measure voltages?

Rudi_Ratlos said:
Andriy owns a very "individual" version of Mihai's FC-100, which he calls "FC-100 MKII", and which he has layouted and etched by his own.
Maybe Andriy tells in detail, what he changed compared to the original FC-100 design.
Best regards - Rudi_Ratlos
Click to expand...

I used Toshiba TTC5200\TTA1943 devices. It's Mihai's design also, but he didn't published the layout for this one. http://www.diyaudio.com/forums/soli...gle-ended-folded-cascode-vas.html#post1621315
 
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A

AndriyOL

Member
Joined 2008
When only 3 "TT diodes" left everything seems working fine. The bulb glows once during capacitors charge. Output dc offset is not stable and rising starting from 3mV and higher. Perhaps this is bacause jfets are not maintaining the same temperature.
So, bias for this amplifier should be set not like on a schematic or I've used not fully suitable base to emitter transistor junctions?
 
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A

atupi

Member
Joined 2010
I give up in matching Q1,Q3 (15034/15035) because it seems impossible to find some overlaping hfe values.

I've asked few more fellow DIY-ers to measure their transistors in order to match with mine and the results are discouraging.

Did anyone of you managed to found mathed pairs?
What will the results of using unmatched pairs ?

Thanks,
Adrian
 
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roender

roender

Member
Joined 2006
atupi said:
I give up in matching Q1,Q3 (15034/15035) because it seems impossible to find some overlaping hfe values.

I've asked few more fellow DIY-ers to measure their transistors in order to match with mine and the results are discouraging.

Did anyone of you managed to found mathed pairs?
What will the results of using unmatched pairs ?

Thanks,
Adrian
Click to expand...

Hi Adrian,

The unmatched q1 and q3 will increase even order distortions.
Go ahead with what you got and be happy. Life is short, listen to the music and don't worry much about not perfect parts.

Regards,
Mihai
 
A

AndriyOL

Member
Joined 2008
atupi said:
I give up in matching Q1,Q3 (15034/15035) because it seems impossible to find some overlaping hfe values.

I've asked few more fellow DIY-ers to measure their transistors in order to match with mine and the results are discouraging.

Did anyone of you managed to found mathed pairs?
What will the results of using unmatched pairs ?

Thanks,
Adrian
Click to expand...

If you wish we can make a group bulk orfer for Onsemi and they will tune their manufacturing process and produce much more maching complementary devices. :)
 
A

atupi

Member
Joined 2010
Would be great but as far as i know they don't offer this kind of service.

So far i've aquired NJL's 0281/0302 and i'm happy but on the other side i'm very pissed off by PacificSemi as they shipped the 2SA/2SC transistors 5 weeks ago and they don't do nothing to find why my parcel is still in L.A since 4 weeks as i see on internet.
 
A

AndriyOL

Member
Joined 2008
A

atupi

Member
Joined 2010
Yes Rudi i know that and that's way i've ordered mine from the same but it seems there is a problem with postal services and i'm getting nervous as 5 weeks are already passed and nobody knows where are my transistors.
Are the only parts missing to build this nice project.
I will post some nice pictures to show my progress.
 
A

AndriyOL

Member
Joined 2008
When I tried to solder resistors on diode string and than measuring VRe, I'm not sure, but guess accidently shorted emiter pin and Vcc (dmm lead was close enough, but I haven't observed any spark). When I first power on with 4 diode string VRe was 10-16mV and bulb was dimmed. After second start the bulb was at full bright and glows off only with 3 diode string (when bias is almost off) and Vre is 4mV, 0.55Vbe on one side with bias off state (before was 0VRe). Other side shows 0VRe and 0.14Vbe. DC offset is normal. So, I think something was burned.
Also one strange thing, during first ten seconds after powering on that high Vbe side (0.55V) is showing low value (150-200mVbe) and starts to rise, other side shows high Vbe and starts to decrease from 0.5V to 0.14V.
 
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