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Old 15th August 2012, 10:19 PM   #931
mikelm is offline mikelm  England
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Based on previous spice tests I could guess that Sonny will loose a tiny bit of speed / stability with his CFP front end BUT with two stages the stability seems to be so good that tiny loss is probably not much of a big deal and I guess it improves THD somewhat.
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Old 16th August 2012, 05:05 AM   #932
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with two stages the stability seems to be so good that tiny loss is probably not much of a big deal and I guess it improves THD somewhat.
Stability with two stages only is really remarkable, I remember in the testing phase when I used two stages only, no compensation cap was needed at all. BJT as front-end follower has more gain (also more OLG) so it is more suitable as a VAS transistor or CFP input, than mosfet. Extra gain dB (CLG subtracted from OLG) obviously and expectedly reduce THD.

Now currently by CSA, even with 28 transistors present on PCB, it is still three stages amplifier only, maintaining superb stability, achieved with minimal compensation needed.
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Old 16th August 2012, 06:02 AM   #933
sonnya is offline sonnya  Denmark
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It needs compensation. That is a fact.

On the new board, i have added 47pF miller cap across KSA1381 + KSC3503.

At certain source impedances it was oscillating when i was changing it from Renesas 2SK216 + 2SJ76. The 47pF matches roughly the Cgd value.

It is a concern with CFP BJT pair as they have there own pole. It gives more gain and reduces re (emitter equalint resistance).

An easy way to get more gain (lower THD) and reducing the output impedance is to use ECW20N20-Z/ECW20P20-Z or ALF16N16W/ALF16P16W.

This way the class A current can be set higher as well as they have a lower K/W.
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Old 16th August 2012, 06:06 AM   #934
mikelm is offline mikelm  England
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It was demonstrated on the fetzilla thread that actually more OLG can be achieved with a mosfet as VAS than with a BJT.

The i/p stage and VAS are in anti phase and any current leakage between them ( that BJT's draw ) results in in a loss of OLG which dominates over any extra gain caused by a BJT have higher transconductance. Because of this the fetzilla with it's Jfet i/p needed a mosfet VAS to give enough gain for the design to sound good.

However in Sonny's design here it is not so straightforward because there we have a BJT / MF darlington pair so I'm not sure how this would impact the explanation above. But with any 3 stage design I believe a MF VAS will give higher OLG.

Last edited by mikelm; 16th August 2012 at 06:11 AM.
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Old 16th August 2012, 06:13 AM   #935
sonnya is offline sonnya  Denmark
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Due to the EARLY voltage effect the transistors will have lower hoe than a mosfet.

But high voltage types will typically have a higher EARLY voltage. that is what we try to compensate for by using hawksford cascode.
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Old 16th August 2012, 06:17 AM   #936
mj777 is offline mj777  Philippines
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Dear sonnya do you still have available KSA1381 + KSC3503 transistors?
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Old 16th August 2012, 06:26 AM   #937
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Sonny your previous amp has DB input stage and similar next stage, so the logical question would be, how do they compare regarding SQ?
Sonny, opinion please.
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Old 16th August 2012, 09:19 AM   #938
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It was demonstrated on the fetzilla thread that actually more OLG can be achieved with a mosfet as VAS than with a BJT.
Would be nice to try TSSA from the first hand.
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Old 16th August 2012, 09:43 AM   #939
mikelm is offline mikelm  England
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well, with a simple version of TSSA I am playing around with in spice changing the O/P VAS stage driver transistor from BC560 etc to ZNV3306 reduces OLG from 44.45 to 44.07 !

So no real effect in this case and because the mosfet had more voltage drop BJT seems like the best choice in this circuit.
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Old 16th August 2012, 09:46 AM   #940
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Originally Posted by mikelm View Post
It was demonstrated on the fetzilla thread that actually more OLG can be achieved with a mosfet as VAS than with a BJT.

The i/p stage and VAS are in anti phase and any current leakage between them ( that BJT's draw ) results in in a loss of OLG which dominates over any extra gain caused by a BJT have higher transconductance. Because of this the fetzilla with it's Jfet i/p needed a mosfet VAS to give enough gain for the design to sound good.

However in Sonny's design here it is not so straightforward because there we have a BJT / MF darlington pair so I'm not sure how this would impact the explanation above. But with any 3 stage design I believe a MF VAS will give higher OLG.
This is near always the case, even with any LTP circuit, mosfet will have higher gain, but that is not such a good thing as you might think. As the mosfet has much higher capacitances it will show much higher high frequency distortion (usually from 2 Khz and up). The only benefit of mosfet vas is only a very small lowering of low frequency distortion. A plain lone mosfet will never outperform a BJT as Vas, even when cascoded and only if you know some groovy tricks its can possibly outgun a BJT.

Last edited by homemodder; 16th August 2012 at 09:48 AM.
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