100W Ultimate Fidelity Amplifier

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#6,263
Hi Terry, Prasi,
Got my 2 FX8 up and running. They really do sound sweet. Need some more burn in to do honest compare. i used BD139/140-16's as thats all i had at the moment. Bias set at 130mv and dc offset was from -.2 to +.2mv. All very Easy to Build & set up. Running +-36V from Apex PSU.
Rick.
 
#6,264
Rick G said:
Hi Terry, Prasi,
Got my 2 FX8 up and running. They really do sound sweet. Need some more burn in to do honest compare. i used BD139/140-16's as thats all i had at the moment. Bias set at 130mv and dc offset was from -.2 to +.2mv. All very Easy to Build & set up. Running +-36V from Apex PSU.
Rick.
Click to expand...

Hi Rick,
Great News. Do share photos, if taken and also your comparison once you do it.
reg
Prasi
 
#6,265
FX8

OK. It's all kinda of rough,but it works.
 

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#6,270
Terry,
They run very comfortably with those little heatsinks. Haven't Tried BLASTING For a long time with them yet! I choose to mount the latfets that way for my future heatsink requirements.

Ammel68,
I used a Peak Atlas DCA55 to group them. Not very closely either. hfe +- 5 to 7 between them. Measure bias with dmm in Amp mode on +plus rail
 
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#6,274
Rick G said:
Terry,
They run very comfortably with those little heatsinks. Haven't Tried BLASTING For a long time with them yet! I choose to mount the latfets that way for my future heatsink requirements.

Ammel68,
I used a Peak Atlas DCA55 to group them. Not very closely either. hfe +- 5 to 7 between them. Measure bias with dmm in Amp mode on +plus rail
Click to expand...

I was wondering that. They might run cooler on their own sink. The Latfets like to run hot. I don't know if it would be better to keep them cooler. Maybe Mile can chime in and let me know.
 
#6,276
ammel68 said:
Thanks Rick. Where do you measure the bias?
Click to expand...

Another (safer ) method would be to put a 0.5-1 W resistor (say 22ohms) in series on each supply rails and measure the voltage drop across each of these resistors. You can then calculate the bias current using ohms law and adjust the pot to have 130mA. use reading on any one rail (say +ve rail).
reg
Prasi
 
#6,278
apexaudio said:
'In a non-switching class AB amplifier none of the output transistors is switched off completely at any time.

In a class A amplifier normally a current increase in one output transistor goes together with an (almost) equal decrease of current in the other.
In a class AB amplifier normally an increase in current through one output transistor (with approx the bias current) makes the other transistor non-conducting.
In a non-switching cass AB amplifier an increase in current through one output transistor lowers the current in the other transistor in a non-linear way and such that it will not go to zero. The advantage is that this transistor will not be reversed biased (as will happen in normal class AB) and this makes switching on again much faster.

Steven'
Click to expand...

There is a Doug Self's patent for new XD class amplifiers, which is another version of non-switching amplifier. I do not know the details but I suppose that it uses some kind of advanced non-switching technique. I am inclined to think of QUAD's "Current dumping" as some kind of non-switching amplifier too.
 
#6,280
Hi,


First off finally you guys noticed,Second thanks to Sonal for the suggestions.


Regarding your post #6200:
Very nice work!!!
Click to expand...

Thank you very much.

1. In my post #6150 I wrote to use 15-20R on rails, current limmiters safetys for first turn ON.
The voltage drop will be in 0,3-0,5V range, With different current limiters (40-50R), differents voltage drops also.
Click to expand...

Yes off course,that is why I mentioned that I used 50Ohm and got the voltage reading of .9 or so.

2.The V_be multiplier (Bias ADJ) needs some mA current from VAS to work properly.

The V_be voltage = 0,7V * ( 1 + R_bc / R_be)

R_bc (base to collector) = R16 = 1K
R_be (base to emitter) = 220R + 1K trimmpot (R17 + trimmpot)
for Q V_be use BD139 or any NPN 50+V, 150+mA, 5+W, h_fe 100+

Output is a double emitter follower so we need 3,5 - 4,0VDC for V_be multiplier voltage to bias the output properly.
for 4.0VDC V_be voltage with R_bc = 1K

R_be must be 220R or a little bit lower
Probably with trimmpot adjusted to 0R the V_be voltage is in the 3,5VDC range or lower and you cannot adjust bias properly.
use 1K2 - 1K5 for R16 instead of 1K
Click to expand...

Yes Sonal as well had suggested that I use 2.2K there instead of 1K. Will try that soon (have to buy some multiturn pots). Just for the test I used 10Ohm I had and put inplace of the fuse,The voltage reading is .9 to 1V on the positive side and 1-1.2V on the negative side. Now this corresponds very high bias but still the heatsink barely even warms up after multiple hours of keeping it switched on or an hour or so of playing music (moderate volume levels). So Am totally confused as to what's happening here.

I did want to push to the brim (just for testing purpose) and turn the bias preset almost full (50-60ohms),and the devices started heating within seconds and in no time was very very hot and before I could react one of the output blew (A1943)along with the 5A fuse.So I reverted back to the original setting and have left it at that.

3. For DC offset issues
IF DC offset is positive (+ 20mV) solder in parallel to R14 (220R) a 47K - 100K trimmpot
Adjust for zerro reading, desolder trimmpot, then look for trimmpot value and solder first nearest resistor value in parallel to R14.

IF DC offset is negative, same procedure with R15
Click to expand...

Yesterday I was able to get A970 (which earlier was not able to get),So replaced all the 5401 with A970 ! Also replaced the 547 generic make with Philips make,Now the offset of between -5mv to -6mv.So will let the new transistors burn a bit before trying your suggestion.

On a different note,What are the better sounding options I can try in this ?? Am aksing cause I read in the old post that 970 was better sounding (suggested by Mr.Mile) and I replaced the 5401, I Also plan to replace few caps with Silver Mica,Already have Siemens Epcos MKP 10mfd for input. So want to use the best of the best possible to squzee as much detailed sound as possible from AX14-T.

Can I use 2N2222 (Metal package ) instead of 547 ? Please do suggest all the possible upgrades.

Regards.
 
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