Hi all,
I want to use Bob Cordell's formula to check the Thermal Stability of Roender's output stage.
Can anyone tell me how to calculate transconductance of the 3pairs of NJL3281/1302 when I don't know the effective internal emitter resistance value.
Is there a way to measure RE (internal)?
Bob's formula is
Beta = Theta_JS * Vds * TCvgs * gm <=0.5
where Theta_JS = [ Rth j-c + Rth c-s]
Vds = supply voltage
TCvgs =0.0022 (2.2mV/K)
gm is what I'm looking for.
Will changing the bias from 170mA/Tr to 200mA/Tr change gm?
I want to use Bob Cordell's formula to check the Thermal Stability of Roender's output stage.
Can anyone tell me how to calculate transconductance of the 3pairs of NJL3281/1302 when I don't know the effective internal emitter resistance value.
Is there a way to measure RE (internal)?
Bob's formula is
Beta = Theta_JS * Vds * TCvgs * gm <=0.5
where Theta_JS = [ Rth j-c + Rth c-s]
Vds = supply voltage
TCvgs =0.0022 (2.2mV/K)
gm is what I'm looking for.
Will changing the bias from 170mA/Tr to 200mA/Tr change gm?
sticking some numbers in gives gm = 40 * 0.17 = 6.8s. for 3pair that's ~20s.syn08 said:
Now presumably this is the intrinsic transconductance if both internal and external emitter resistor are zero.
I have an external Re=0r1.
What is the internal, RE = Rb / [beta+1]?
Does Rb need to be adjusted for a base resistor or an effective source impedance from the circuit that drives the base?
What does the gm formula become when correcting for emitter resistor value?
AndrewT said:
Equivalent gm when a small emitter resistor is inserted is:
gm=gm0/(1+B*Re)
Where gm0=40*Ic
B is the transistor beta
Re is the small emitter resistor. You may consider Re as the sum of the intrinsic emitter resistor (a conservative estimate for a modern audio power device is about 0.005ohm) and the external Re (0.1ohm as you said), to a total of 0.105ohm.
Andrew,
Just build the f... thing and you'll be surprised how stable is from thermal POV.
I've spend grate time figuring how to match all the OPS TC junctions in order to have a very stable current stage, as an example, I heated up the heatsink over 80grdC when the amp is running at higher levels of signal, and after reaching thermal stability I just cut the input signal.
The idle current was 2-3% under cold bias level, but recovery took only 2 seconds!
It is on the safe side, very slightly underbiased.
Regards,
M
Just build the f... thing and you'll be surprised how stable is from thermal POV.
I've spend grate time figuring how to match all the OPS TC junctions in order to have a very stable current stage, as an example, I heated up the heatsink over 80grdC when the amp is running at higher levels of signal, and after reaching thermal stability I just cut the input signal.
The idle current was 2-3% under cold bias level, but recovery took only 2 seconds!
It is on the safe side, very slightly underbiased.
Regards,
M
roender said:
I think Bob intended the formula to apply to BJTs and to FETs, although maybe not in precisely the same form.
Around the time this was the topic, TT was also being mentioned. I cannot recall him saying that form of bias control would make any difference.
He was considering the short term stability of a worst case output device assuming the heatsink did not change temperature. so longer term compensation was deliberately excluded from his argument (he refers to that).
BTW,
I am not in any way criticising your design, but looking at how far I can stretch it's power output capabilities using a higher voltage supply. I would like to retain your low value of Re = 0r1 and thus the 600mA of total bias current. That is exactly what Bob's formula allows one to investigate.
PS,
it's almost built but the tiny pot stopped me finishing. 0.2inch pin pitch!!!!
for those guys searching for parts.
I found my small Toshiba transistors at MCM (www.mcmelectronics.com/).
The 2sk170 don't have the required suffix but with a batch of 20 i found 2 pairs with required specs.
About fake parts, from the information i have read and see on forums , they seems to be originals parts from Toshiba.
Good luck
P.S. This amp is really good, give it a try, you will be surprised. I'm presently looking to upgrade my already good speakers for better ones.
Denis
I found my small Toshiba transistors at MCM (www.mcmelectronics.com/).
The 2sk170 don't have the required suffix but with a batch of 20 i found 2 pairs with required specs.
About fake parts, from the information i have read and see on forums , they seems to be originals parts from Toshiba.
Good luck
P.S. This amp is really good, give it a try, you will be surprised. I'm presently looking to upgrade my already good speakers for better ones.
Denis
UK source for 2SK170BL
Anyone in the UK looking for the 2SK170BL JFET, or maybe further a field with the value of the GB pound dropping like a stone could try Nikko Electronics.
http://www.dalbani.co.uk/index.php
I bought a batch of this obsolete device a couple of weeks ago. They are listed as Toshiba devices, and they assured me that they were genuine
Best wishes
Dave
Anyone in the UK looking for the 2SK170BL JFET, or maybe further a field with the value of the GB pound dropping like a stone could try Nikko Electronics.
http://www.dalbani.co.uk/index.php
I bought a batch of this obsolete device a couple of weeks ago. They are listed as Toshiba devices, and they assured me that they were genuine
Best wishes
Dave
Thanks Mihai,
I don't know where I got that from, it's been a long week!
I'm glad to hear I'm wrong about it being obsolete though, since a lot of the amp designs I've looked at recently are using them for input devices.
Erno borbely also recommends using matched pairs as a substitute for the 2SK389 which he uses extensively in his kits.
Best wishes
Dave
I don't know where I got that from, it's been a long week!
I'm glad to hear I'm wrong about it being obsolete though, since a lot of the amp designs I've looked at recently are using them for input devices.
Erno borbely also recommends using matched pairs as a substitute for the 2SK389 which he uses extensively in his kits.
Best wishes
Dave
stolen 900uA
I have lost 900uA in the input stage.
Something odd around the jFETs & cascode.
D1 drops ~420mV
R29 drops ~430mV
Vds of both jFETs ~175mV
Vbe of both Q18 & Q21 ~670mV
R20 drops 1261mV indicating that ~960uA is passing to D1
R19 drops 1230mV indicating that ~13.5mA is coming from the input stage.
Looking at the mirrored source stage.
9.4mA passes both Q11 & Q13.
2.7mA passes both Q6 & Q7.
This leaves a net 6.7mA+6.7mA going into the collectors of Q19 & Q21.
The extra ~0.1mA passing R19 (13.4+0.1=13.5) confirms that a tiny current is passing R29.
I have only applied power to the front end and not the pre-drivers & drivers. The outputs are not soldered in.
I did not have R24 connected to the output initially, with power applied and D2 & D7 did not light.
I connected R24 to C1 and now D2 & D7 light correctly.
Where is that 960uA passing R20 going?
Is it the cascode that has gone faulty, or the jFETs that are gone? or something else?
The whole front end is taking ~21.7mA
I have lost 900uA in the input stage.
Something odd around the jFETs & cascode.
D1 drops ~420mV
R29 drops ~430mV
Vds of both jFETs ~175mV
Vbe of both Q18 & Q21 ~670mV
R20 drops 1261mV indicating that ~960uA is passing to D1
R19 drops 1230mV indicating that ~13.5mA is coming from the input stage.
Looking at the mirrored source stage.
9.4mA passes both Q11 & Q13.
2.7mA passes both Q6 & Q7.
This leaves a net 6.7mA+6.7mA going into the collectors of Q19 & Q21.
The extra ~0.1mA passing R19 (13.4+0.1=13.5) confirms that a tiny current is passing R29.
I have only applied power to the front end and not the pre-drivers & drivers. The outputs are not soldered in.
I did not have R24 connected to the output initially, with power applied and D2 & D7 did not light.
I connected R24 to C1 and now D2 & D7 light correctly.
Where is that 960uA passing R20 going?
Is it the cascode that has gone faulty, or the jFETs that are gone? or something else?
The whole front end is taking ~21.7mA
I can do that but it will not affect the current passing D1 & R29.
There is something else going on.
I have a temporary 1k2 jumper across the empty NJL diode locations. It has ~1400mV, indicating that ~1.2mA is passing through the jumper. I guess the other 1.5mA is passing through the pre-drivers.
There is something else going on.
I have a temporary 1k2 jumper across the empty NJL diode locations. It has ~1400mV, indicating that ~1.2mA is passing through the jumper. I guess the other 1.5mA is passing through the pre-drivers.