I think there is a trade-off here. You really want both high Hfe AND good matching. But what would be better: Hfe of 500 with 5% match, of Hfe of 100 with 1% match? I dunno. Can't you move the B-C connection of Q1 to Q8? Then a matched pair for Q1 and Q2 would be usefull, independent of their Hfe.
Jan Didden
Jan Didden
P-A,
Those transistors are duals, but practically speaking as badly matched as two singles, look here:
diyAudio Forums Archive - Current match on BC857BS,BC847BS:
http://www.diyaudio.com/forums/showthread/t-1003.html
Since you contributed to that thread, I guess you knew they were badly matched? Tsk, tsk... On the other hand, another good opportunity to show off your boards without contributing to the thread. Well done.
Jan Didden
Those transistors are duals, but practically speaking as badly matched as two singles, look here:
diyAudio Forums Archive - Current match on BC857BS,BC847BS:
http://www.diyaudio.com/forums/showthread/t-1003.html
Since you contributed to that thread, I guess you knew they were badly matched? Tsk, tsk... On the other hand, another good opportunity to show off your boards without contributing to the thread. Well done.
Jan Didden
Don't try current feedback!
I've told you guys dozens of times.
I did that.......my dealers hated it, and I almost lost a lot of them.
That is why I came up with the I/V stage that I did.
(You know that, don't you, P-A? Of course, you do.)
Input impedance:
Since it is pretty much a function of Ie/26 type of relationship, I really doubt that you get 0.6 ohms looking into a single small-signal transistor. I don't care what your fancy simulator says. Real world says something else.
And try harder to get below -80 dB? Why.......any measurements I make below that are suspect. Unless I want to make a better source, and a notch filter, and.......well, -80 is good enough.
Especially when you look at how linear the DACs are.
No, I'll stick with what I have, and put my time and efforts to more productive activities.
Jocko
I've told you guys dozens of times.
I did that.......my dealers hated it, and I almost lost a lot of them.
That is why I came up with the I/V stage that I did.
(You know that, don't you, P-A? Of course, you do.)
Input impedance:
Since it is pretty much a function of Ie/26 type of relationship, I really doubt that you get 0.6 ohms looking into a single small-signal transistor. I don't care what your fancy simulator says. Real world says something else.
And try harder to get below -80 dB? Why.......any measurements I make below that are suspect. Unless I want to make a better source, and a notch filter, and.......well, -80 is good enough.
Especially when you look at how linear the DACs are.
No, I'll stick with what I have, and put my time and efforts to more productive activities.
Jocko
hjelm said:
Hjelm,
I was thinking, if you can get rid of the C-B connection of Q1, then Q1 & Q2 would both have the same Ic/Ie ratio (assuming a match). But I realise it isn't possible with the current topology. Maybe I get a brainwave tonight...
In the meantime, check out THAT corporation, NPN/PNP etc arrays type THAT300, 320, 340. 5% Hfe match, 500uV Vbe match. Not cheap, about 10 $ each at profusionplc.com. But, imho, worth it.
Jan Didden
Don't bang my head jocko 
i know it is a theorethical value and will not be acheived in real life. I cannot understand why the mirrored current so you get the same Ie and Vbe drop is a problem. It should at least lower the input impedace compared to the common base input impedance.
Do not worry the quest for more than 80dB is only theorethical since i think the real life results will be worse. If the sim said -40dB i would start to worry.
I noticed that the circuit i have shown is wrong. The idraw should be connected to 7 and the resistor between ground and 7.
i know it is a theorethical value and will not be acheived in real life. I cannot understand why the mirrored current so you get the same Ie and Vbe drop is a problem. It should at least lower the input impedace compared to the common base input impedance.Do not worry the quest for more than 80dB is only theorethical since i think the real life results will be worse. If the sim said -40dB i would start to worry.
I noticed that the circuit i have shown is wrong. The idraw should be connected to 7 and the resistor between ground and 7.
You assumed that I listened to it.......
You assume that if I did, I would have been able to tell a difference.
C'mon......I'm an engineer.....all we do is measure.
Actually, the problems that they kveteched about were realted to dynamics. I have Magnepans, with open-loop amps. Dynamics were not the main design criteria.
I have feedback from several sources about a line of current feedback amps, made by a buddy of mine.
Same criticism. I was approached to talk him out of it.
Wasn't my company, or theirs. He already knew my thoughts, and only his opinion counted anyway. He liked it, he built it, he sold it.
For a while.
Anyway.......me...bang on your head..........?.....not at all. I'm a nice guy.
Snicker.
Just as long as I know that you don't believe distortion measurements on your simulator.
And we know that you don't.........
Jocko
You assume that if I did, I would have been able to tell a difference.
C'mon......I'm an engineer.....all we do is measure.
Actually, the problems that they kveteched about were realted to dynamics. I have Magnepans, with open-loop amps. Dynamics were not the main design criteria.
I have feedback from several sources about a line of current feedback amps, made by a buddy of mine.
Same criticism. I was approached to talk him out of it.
Wasn't my company, or theirs. He already knew my thoughts, and only his opinion counted anyway. He liked it, he built it, he sold it.
For a while.
Anyway.......me...bang on your head..........?.....not at all. I'm a nice guy.
Snicker.
Just as long as I know that you don't believe distortion measurements on your simulator.
And we know that you don't.........
Jocko
Oftenly lower distortion numbers shown by SPICE are probably caused by too linear characteristics of the used models. There was here one constructive thread on this topic. Once you learn to use the SPICE, you realize what a great tool it is. Of course, it will not tell you how the thing will sound. As the real world measurement won’t tell you that either.
I agree not only -80dB is low enough, -60dB can be good to me. Just wanted to say that it can be made for better specs. If you want them.
Hjelm, I had the problems with files you uploaded but I drew similar circuit and it shows very low input impedance, something like 0.02 Ohm, though it starts to rise after few tens of kHz. I’d be really surprised if this won’t be confirmed in reality.
Pedja
I agree not only -80dB is low enough, -60dB can be good to me. Just wanted to say that it can be made for better specs. If you want them.
Hjelm, I had the problems with files you uploaded but I drew similar circuit and it shows very low input impedance, something like 0.02 Ohm, though it starts to rise after few tens of kHz. I’d be really surprised if this won’t be confirmed in reality.
Pedja
Once the libraries/models were set adequately, there were problems with .control and .destroy commands. They are, depending on the used program, either not recognized or I got the message “missing cathode node for destroy”. Once I remove them and ran the analysis I have higher distortion values than those you mentioned, but I did not inspect the files in fact... cut the story short and drew the circuit.
Not the advice for this case, but generally I think in such situations it is better working option to upload LT SwCAD or MicroCap or some other’s software file that has free/demo version, and whose files include or can include used models.
Pedja
Not the advice for this case, but generally I think in such situations it is better working option to upload LT SwCAD or MicroCap or some other’s software file that has free/demo version, and whose files include or can include used models.
Pedja
Pedja:
Which file did you run and what distortion did you get?
I tried running the circuit with the ideal models but then it was just silly i got -116dB THD.
I am using Winspice but i have a feeling that the BC850C BC860C models from Siemens, actually it should be Infineon today, seems to be better than most, at least it is hard to get good values with them
. These models were posted here in a thread somewhere.
Hjelm
Which file did you run and what distortion did you get?
I tried running the circuit with the ideal models but then it was just silly i got -116dB THD.
I am using Winspice but i have a feeling that the BC850C BC860C models from Siemens, actually it should be Infineon today, seems to be better than most, at least it is hard to get good values with them
. These models were posted here in a thread somewhere.Hjelm
hjelm said:Pedja:
Which file did you run and what distortion did you get?
I tried running the circuit with the ideal models but then it was just silly i got -116dB THD.
I am using Winspice but i have a feeling that the BC850C BC860C models from Siemens, actually it should be Infineon today, seems to be better than most, at least it is hard to get good values with them
Hjelm
Please note that distortion measurements - THD - in Spice are *small signal* measurements, assuming linearization of the elements around the operating point. Fourier measurements are large-signal measurements, and as such much more realistic. So, even with perfect models, THD tends to be always too optimistic anyway.
Jan Didden
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