Bravissimo Lars, now that's what I would like to see playing music. 
Yes.
The one that matters is capacitance refering to ground which unlinearly loading VAS as complex impedance, shifts phase of the current and therefore tends the amp to oscilate. In our case are parallel MOS-FET input capacitance plus sum of all parasitic capacitances from electronic parts to ground. Exact value you can only get with measurement of the populated board mounted in the case.
It will still be A class, because VAS bias current will still be enormously higer than MOS-FET input current. Your VAS will be in perfect A class.
No. Output stage has +/-50V, minus few volts losses on Vsat, minus reserve to clip and you get app. 100W/8ohm rms.
Lazy Cat said:
Thanks, Lazy Cat . Thus it is approx the sum of Ciss of N and P plus
something more ?
Excuse me , my question was referred to the power stage
I do not understand. Is it not sufficent 28V RMS to make 100W/8ohm?
Why I need 38,9 V RMS ? 😕
Thanks for help me to understand
🙂
Cheers,
Paolo
Lazy Cat said:Hi MIKET
This design I like very much
We can all do the math and take it to play. 😀
Hi Lazy Cat,
very flattering, thank you. I am doing a PCB layout for Le Gecko with the Lazy Cat output mod. I think it will be a very nice little amp.
Kind regards
Nico
Lazy Cat said:... This design I like very much
An externally hosted image should be here but it was not working when we last tested it.
We can all do the math and take it to play. 😀
Me too 🙂
inertial said:
Yes, plus all artificial little capacitors from separate electronic parts to ground plane (ground islands on PCB to parts, output transistors to heatsink which is at ground potential, etc). At the end when you install amp in metal case the value can easily reach nano Farrad or more from MOS-FET gates (VAS output point) refering to ground.
inertial said:
OK, your VAS output is at A class in any case, but the A or B class rank of the amp output stage is depending on the level of quiescent current through output transistors (biasing). When the level of output (load) current exceed quiescent current of the output transistors, your amp cease to be in A class.
inertial said:
Listen very carefully I will tell this only once:We have +/- 50V output stage supply, so theoretically we can squeeze out 100Vpp of output swing and that is exactly 156,25Wrms/8ohm. And that is only theoretically without any losses.
In real conditions you have to consider saturation Uce of the output transistors, app. 2V, U-Re, app. 0.5V, 3% of PSU voltage collapse, app. 1,5V and at least 2V reserve before clipping. So now you have max. 88Vpp real output swing and that is exactly 121Wrms/8 ohm. For me that is 100W/8ohm amplifier
Seems to be a problem as dependending on standing current the PNP and NPN draw different current in relation to each other. Are there really any that can be matched?
No problem at all, because quiescent current through MOS-FET is stable (sum of parallel Ibe currents from one half: NPN or PNP) the only difference you can measure is dissimilar quiescent currents Ice through each output transistor depending on relevant Hfe. You don't have to find 100% Hfe equivalents but let say in 90-95% range of similarity.
From my measurements and observations on ready made units there is no sign of any unstability or unreliability and I have (also many of my customers) different amps with BIGBT output for more than two years. One very strong 400Wrms/8ohm is my workhorse on daily basis 😉
From my measurements and observations on ready made units there is no sign of any unstability or unreliability and I have (also many of my customers) different amps with BIGBT output for more than two years. One very strong 400Wrms/8ohm is my workhorse on daily basis 😉
revintage said:
Matching is going to be near impossible because you have to match four BJTs with its mate and its compliment in different configurations.
The degeneration resistor does its job and although currents are not identical (far from it), they do add together.
In the mock up Gecko I had to adjust off-set several times before stabilizing at about 5 mV off-set and 63 degrees centigrade on the heat sink.
At cold turn-on the off-set can be as high as 70 mV it stabilizes very rapidly and off-set drops.
Lazy Cat's output stage sounds very different from any other I have ever heard. I am not one to explain sounds with words as it is even more misleading and then debates are never ending.
I have not made any real measurements and only listened in mono.
In the posted circuit I did not decouple the 18 ohm NFB resistor and did so with 1000uF in parallel with 1 uF polycarb.
I am guessing that the different and very appealing sound character could be attributed to the geometric summation of all the individual currents at the output terminal.
Kind regards
Just a question on biasing. Lazy Cat in your proposed output devices, should the Mosfets be mounted on the OP heatsink or on its own smaller heatsink?
http://www.diyaudio.com/forums/attachment.php?postid=1690232&stamp=1229728575
If we take Dr.Bora's design linked above, do Q25 and Q26 track, the OP stage heatsink, or the Mosfet driver heatsink (if mounted separately on their own heatsink) or do they track the OP devices by being mounted on top of the device itself?
Similarly on which heatsink are Q18 and Q21 mounted?
Thanks.
http://www.diyaudio.com/forums/attachment.php?postid=1690232&stamp=1229728575
If we take Dr.Bora's design linked above, do Q25 and Q26 track, the OP stage heatsink, or the Mosfet driver heatsink (if mounted separately on their own heatsink) or do they track the OP devices by being mounted on top of the device itself?
Similarly on which heatsink are Q18 and Q21 mounted?
Thanks.
MOS-FETs are mounted on the same heatsink like output NPN/PNPs. The same is valid for thermal track transistors.
In the case of Mr. Bora's amplifier Q25, Q26 are on the main output heatsink, otherwise Q18, Q21 are on separate heatsink.
In the case of Mr. Bora's amplifier Q25, Q26 are on the main output heatsink, otherwise Q18, Q21 are on separate heatsink.
Thanks Lazy Cat, very appreciate explanation.
I'll continue to follow your thread and in the same time better I start to study basic and easy electronic books.
Cheers,
Paolo
I'll continue to follow your thread and in the same time better I start to study basic and easy electronic books.
Cheers,
Paolo
Just had to play around a little more😀.
With a very simple thermal compensation it seems to behave perfect, slightly negative up to over 150degrees C.
Also changed the input stage to SE as the overtone structure was with to much even overtones.
Even in Class- AB with 200mA Iq/transistor it looks good.
Anyway it sims beautiful, maybe one should have a go.........
With a very simple thermal compensation it seems to behave perfect, slightly negative up to over 150degrees C.
Also changed the input stage to SE as the overtone structure was with to much even overtones.
Even in Class- AB with 200mA Iq/transistor it looks good.
Anyway it sims beautiful, maybe one should have a go.........
Attachments
Nico Ras said:
SOUNDs good 😎
Hi LazyCat, have you tried the LME49810 approach (very easy to do)? ... how did it sound ...
🙂
Samuel, mount MOSFET-s on the same (main) heatsink together with outputs and Q25&Q26. Also, mount Q21&Q22 on separate small heatsink, Q9 on it's own separate small heatsink as well as Q18 on it's own! Q9, Q18 &Q21 should never be on the same heatsink! In actual unit, it might prove necessary to add small cap in the range of 10pF...33pF between collector of Q21 and base of Q20 (emitter of Q27). R31 is preset-trimer of 1k for BIAS adjustment.Samuel Jayaraj said:
KLe said:
Well I have to say that my discrete VAS is certainly more realistic for my ears and long term listening more bearable. LME driver is not bad at all, it gives you more "instant" solution, easy approach to high power, very dynamic, fast and a little bit edgey sound. But certainly better than any D class amp, bacause there is no loss of fine detail and imaging.
I would recommend LME for a good studio monitor driver, strong PA amp and of course home Hi-Fi. This is completely subjective opinion of mine and it depends on what you've heard so far.
revintage said:
SE input stage is fine and I like hybrid amps design with output BJTs, especially compact ones as yours.
It can be even build in point to point technicque or on a very nice looking PCB with a little nostalgia look.
I'm a bit anxious about it, can we expect something likely to see in near future?
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