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Old 11th September 2013, 07:40 AM   #1201
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Hi Terry,
Do you match the transistor?
I only use hFE tester from my DMM to match them,
result not so close match
but it is better than not.
The hFE test also tell us that they are still in good condition.

Cause I play too much with my PeeCeeBee, I also burn some 10 ohm resistor & BD139-140 also dead...
Next I want to try other way how to matching transistor
& the offset will be better/easier to set.

Also it is good idea to match 2k2 // 100 ohm resistor
I did match all the resistors, only few resistor to match won't hurt you

When first time I also have dificulty to measure offset, I got oscillation there.
I put 33pF capacitor on each base to collector VAS transistor & they do the job.

Regards
John
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Old 11th September 2013, 08:08 AM   #1202
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Quote:
Originally Posted by JOHN BALI View Post
Hi Terry,
Do you match the transistor?
I only use hFE tester from my DMM to match them,
result not so close match
but it is better than not.
The hFE test also tell us that they are still in good condition.

Cause I play too much with my PeeCeeBee, I also burn some 10 ohm resistor & BD139-140 also dead...
Next I want to try other way how to matching transistor
& the offset will be better/easier to set.

Also it is good idea to match 2k2 // 100 ohm resistor
I did match all the resistors, only few resistor to match won't hurt you

When first time I also have dificulty to measure offset, I got oscillation there.
I put 33pF capacitor on each base to collector VAS transistor & they do the job.

Regards
John
Hi john,
I ask LC about match on TSSA topic. He answer me that DMM matching is sufficient as error will affect same all mesures since we don't use mesure to futher calculation just to compare transistors. With hi speed VAS transistors cap compensation is really important i think as many source can give osscillations...

Marc
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Old 12th September 2013, 12:22 AM   #1203
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Quote:
Originally Posted by Idefixes View Post
Hi john,
I ask LC about match on TSSA topic. He answer me that DMM matching is sufficient as error will affect same all mesures...
This is true, but not the whole story.

The hfe curves for PNP and NPN devices are different. So, although it is true that if both measurements are off by the same percentage, the result is still a match, but ONLY at the single point where the DMM has set the current.

So, if the DMM measures hfe at some arbitrarily low current, a complementary pair will NOT be matched if the actual (in-circuit) current is much higher (tested).

So, although using a DMM is better than nothing at all, one should not assume that a single point match at a very low current means you have a true complementary pair.

Unfortunately, at least in this thread, we do not really know what kind of distortion results from hfe differences in the case of this circuit. A simulation might show that, if the models were adjusted carefully (not sure though).

Example:

KSC3503E/KSA1381E , heatsinked, tested as random pairs
(The KSC3503E is old stock, the KSA1381E is new, from Mouser)

(pair 1, good match)
12mA:
hfe: 190/172 (10%)
Vbe: .645/.652
Vce: ~27V
18mA:
hfe: 205/193(6%)
Vbe: .620/.623
Vce: ~37V

(pair 2, fair-to-poor match)
12mA:
hfe: 192/160 (20%)
Vbe: .637/.645
18mA:
hfe: 200/172 (16%)
Vbe: .629/.634

You can see from the underlined numbers that as the current goes up, the difference in hfe gets smaller. Eventually, the hfe curves will intersect, and the devices will have the same gain, but this will happen well above the intended bias current.

By contrast, if one were to measure at a very low current, the difference in gain would seem very large...
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Old 16th September 2013, 06:16 AM   #1204
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I think there is no simple solution to this hFE problem of the active devices, other than buying a bunch and taking Vbe vs Ic vs Vce curves of each and then compare graphically... super-tech-fool-proof-0%THD.

That said, is there any audibly tremendous advantage of matching them to that degree? IMO, perfect matching won't contribute significantly to the Slew Rate of this amp, one of the main characteristic that affects perception almost equally to everyone. It's already close to perfect(audibly) and I have given up on thinking how to reduce THD "even" further. Some simple and well-known tricks can be applied to achieve desired THD while still not demanding a scope or supermatch pairs and/or five DMMs.
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Old 16th September 2013, 02:21 PM   #1205
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I think there is no simple solution to this hFE problem of the active devices, other than buying a bunch and taking Vbe vs Ic vs Vce curves of each and then compare graphically... super-tech-fool-proof-0%THD. ...
Thanks. I am definitely NOT advocating some kind of super-elaborate (and expensive) matching. Not at all. Just trying to say that a pair matched using a DMM will probably not match to the same degree in-circuit. My post was intended to show that, but I was not clear in that regard, sorry.
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Old 17th September 2013, 01:14 AM   #1206
shaan is offline shaan  India
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No No NO!

I am not saying you are doing that! Your message is clear to me.

I was just saying that because true-matching is near-impossible, so it should not be taken extremely seriously, that's all.

I'm sorry if my words seemed offensive. I didn't have any such intention.

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Old 17th September 2013, 01:17 AM   #1207
shaan is offline shaan  India
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Hey guys, PeeCeeBee got 100 thousand views.

Thanks for all the action!

Keep building, happy listening.

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Old 17th September 2013, 03:32 AM   #1208
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Originally Posted by shaan View Post
No No NO!

I am not saying you are doing that! Your message is clear to me.

I was just saying that because true-matching is near-impossible, so it should not be taken extremely seriously, that's all.

I'm sorry if my words seemed offensive. I didn't have any such intention.

Not at all Shaan, thanks for the good thoughts,
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Old 22nd September 2013, 07:31 PM   #1209
shaan is offline shaan  India
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Hi guys.

My third board is now shakin' the sub... As smooth as ever...

Bias is very low than the two satellite drivers, about 4mA in the VAS and 70mA in the FETs, PS is +/-35V, load is my old faithful 4ohm spherical sub.

Will show the pix tomorrow...
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Old 22nd September 2013, 08:49 PM   #1210
Cuda is offline Cuda  France
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Hi
Try 100mA for thermal indifference
http://www.jaycar.com.au/images_uploaded/2SK10568.PDF. page 4

Do you finish the double push ?

What si the bias for the double push ?

Thanks

Last edited by Cuda; 22nd September 2013 at 09:12 PM.
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