Comment on the schematic

formula 22 · 17th September 2011, 06:43 AM

Hi !
I found this schematic and i want to create an amplifier preparing the schematic.
So seek help, and that means little comment she ate good, and so on?
thanks

djoffe · 17th September 2011, 04:30 PM

I haven't simmed it, or looked up transistors...just a quick look...there's probably 1 thing missing, and 1 or 2 things that should be modified...
1. Missing-RF ingress filter on the front end.
2. arrangement of bias transistor and pot should not be carried into the real design, though ok for sim...if the POT wiper goes open, e.g. sits on a bit of dirt, then the Vbe multiplier voltage goes up very high, and cross-conduction kills the amp.
3. Q13 will die if the amp is clipped against the negative rail for any length of time...even square wave tests are probably enough to cause mayhem..Q5 and Q14 might then die from collateral damage.

simonov · 17th September 2011, 04:59 PM

It's wise to use a current limiting resistor in Q13 collector. The author of the schematic had tried to combine two types of VAS. The emitter follower (Q13) will effectively enhance the beta of VAS adding more local negative feedback. This results in better linearity. By cascoding Q5 and Q14 this effect is further increased, resulting in even better VAS linearity. This however had forced the designer to increase the dominant pole compensation in order to maintain the stability.

CBS240 · 17th September 2011, 06:12 PM

Hi

It appears Q5 & Q14 are mislabled. IMHO Q13 may not be necessary if Q5 is a high beta device such as a 2N5089 (Hfe~500) It does not matter that its Vceo is 25V, due to cascode.

C3 seems large for Cdom. Perhaps TPC or TMC could be used? Switch C6 & C8 around. The drivers output is lower Z than VAS and the pull off capacitors should reflect that. Degeneration in the LTP might be a little light.

Mooly · 17th September 2011, 06:58 PM

The circuit appears to be little more than a copy of Doug Selfs original (1994/5 ish from memory) "Blameless Class B" with a few component value changes to make it look different and the addition of Q14.

That design worked well and did all that was claimed of it.

simonov · 17th September 2011, 07:13 PM

C3 should not be reduced in this design. The open loop phase margin will suffer, causing potential instability.

jaycee · 17th September 2011, 08:32 PM

330p for a miller capacitor is ridiculously high. If it needs to be that high to be stable, there is something fundamentally wrong.

17th September 2011, 04:30 PM	#2
djoffe diyAudio Member Join Date: Oct 2008	Looks pretty reasonable...a few things though I haven't simmed it, or looked up transistors...just a quick look...there's probably 1 thing missing, and 1 or 2 things that should be modified... 1. Missing-RF ingress filter on the front end. 2. arrangement of bias transistor and pot should not be carried into the real design, though ok for sim...if the POT wiper goes open, e.g. sits on a bit of dirt, then the Vbe multiplier voltage goes up very high, and cross-conduction kills the amp. 3. Q13 will die if the amp is clipped against the negative rail for any length of time...even square wave tests are probably enough to cause mayhem..Q5 and Q14 might then die from collateral damage.

17th September 2011, 04:59 PM	#3
simonov diyAudio Member Join Date: Aug 2005 Location: PZ	It's wise to use a current limiting resistor in Q13 collector. The author of the schematic had tried to combine two types of VAS. The emitter follower (Q13) will effectively enhance the beta of VAS adding more local negative feedback. This results in better linearity. By cascoding Q5 and Q14 this effect is further increased, resulting in even better VAS linearity. This however had forced the designer to increase the dominant pole compensation in order to maintain the stability. __________________ MY DIY AUDIO BLOG Folow me on Twitter, Google+

17th September 2011, 06:12 PM	#4
CBS240 diyAudio Member Join Date: Aug 2005 Location: K-town	Hi It appears Q5 & Q14 are mislabled. IMHO Q13 may not be necessary if Q5 is a high beta device such as a 2N5089 (Hfe~500) It does not matter that its Vceo is 25V, due to cascode. C3 seems large for Cdom. Perhaps TPC or TMC could be used? Switch C6 & C8 around. The drivers output is lower Z than VAS and the pull off capacitors should reflect that. Degeneration in the LTP might be a little light. __________________ All the trouble I've ever been in started out as fun......

17th September 2011, 06:58 PM	#5
Mooly diyAudio Moderator Join Date: Sep 2007	The circuit appears to be little more than a copy of Doug Selfs original (1994/5 ish from memory) "Blameless Class B" with a few component value changes to make it look different and the addition of Q14. That design worked well and did all that was claimed of it. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

17th September 2011, 07:13 PM	#6
simonov diyAudio Member Join Date: Aug 2005 Location: PZ	C3 should not be reduced in this design. The open loop phase margin will suffer, causing potential instability. __________________ MY DIY AUDIO BLOG Folow me on Twitter, Google+


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17th September 2011, 08:32 PM	#7
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	330p for a miller capacitor is ridiculously high. If it needs to be that high to be stable, there is something fundamentally wrong.

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