I recently finished a pair of AJ monoblocks and find myself in audio heaven, but....I seem to be having trouble quitting there. I built these with a great deal of help from a friend and would now like to venture out on my own and build something all by my lonesome. Well, i have already failed because here i am asking for help. My next adventure is based on the F3, or more specifically, the LU Power Jfets. I would like to experiment with them first since they are cheaper than their SS counterparts. I have read the ZEN 8 and 9 articles, Vladimirk's power F3 thread, and EUVL's "another source follower thread". Based on what i have read, I would like to build a standard F3 and then increase power by paralleling devices. THis is where my question comes in. IN Vladimirks thread, he just paralleled more of the cascoded mosfets, but in EUVL's thread there is a passing comment about creating a power amp paralleling both Jfet and mosfet. What advantages are there to paralleling the jfets as well as the mosfets.
I have done some reading and perhaps found the answer. In paralleling devices, gain is multiplied, but capacitance is multiplied as well. THe LU Jfets already have high Crss (Miller Effect) and multiples would exxagerate this problem even more. Vladimirk added mosfets to handle heat and at the same time, avoided the capacitance multiplier. EUVL seems to suggest that his Taylor CS cuts the capacitance in half and this is how he would suggestdealing with the increased capacitance from paralleld devices. Hope i am close.
Your thinking is absolutely right, big parasitic capacitances of LU1014 is the largest evil preventing us from getting in to sound nirvana. Cascoding the LU in Zen9 schematics helps a lot to diminish the negative effect of parasitic capacitances, but the need of connecting resistor to its gate moves us away from what we have achieved by cascoding. Therefore, if your preamp can work with 3-5kOhms load, better to decrease resistors values in the voltage NFB chain. Anyway, we must drive the F3 and Zen9 with low Zout preamplifier. Also pay attention to output caps. If you plan to increase output power, make 25-30mF output cap, preferrably consisting of a cluster of 1000uF Elna Silmics.
I have not tried x-ing the Zen9, but I do not think this could lead to substantial change of sound.
In this schematics, you could try experimenting with NFB resistors, output capacitors, another caps, also with disconnecting of the Aleph Current Source modulation. These are the most sensitive points of the schematics.
Actually, total effect of paralleling devices is quite different. Please read this:
http://www.diyaudio.com/forums/pass-labs/129040-mosfet-output-stage-capacitance.html#post1599135
Thanks Juma. That is very helpful, although I haven't quite got my head around it yet. It seems to suggest that while under certain conditions, capacitance is an issue when paralleling devices, when used in amplifiers where the gate sees higher voltages, the gM ofthe paralleled devices helps cancel out some of these issues. I will have to think on it more. Unfortunately I do not have the quality equipment needed to experiment first hand. I also am having trouble with the concept of capacitance and cascoded devices. Thanks again.
So I have done some reading while traveling. Cascoding the fets not only protects the Jfet's from excess heat, it also helps in reducing capacitance. Capacitance is reduced further by adding a source resistor, taking advantage of degeneration, which throws away gain in exchange for more linear operation and also higher input impedance. From what I have read, the addition of the source resistor can lead to increase noise. Is this only at radio frequency and not a concern for amplifiers? Suggestion for keeping degeneration and reducing noise include the use of inductors or even transistors. If relevant to audio frequencies, could his be a place of potential improvement or is his just unecessary, added complexity? This is probably obvious to most, but I figure hashing it out not he forum might help other newbs gain some understanding. Thanks!
Thanks for the explanation Juma. This does indeed simplify the concept. I am not schooled in the physics of semiconductors. I wrote my last post before realizing you had responded. I need to read more about cascaded devices and try to figure out the whether or not t is best to parallel both devices or just one. It would seem that by paralleling just the upper fets, Uou have reduced the drive requirements and a single power jfet is sufficient as in vladimirks amp, but I am still curious as to the suggestion by EUVl to parallel both. Oh well, time for more studying.
Perhaps a helpful link on cascode modulation(tm)!
http://www.diyaudio.com/forums/pass-labs/156081-impact-cascoded-transistor-zenamp.html
Interesting comment on affect of cascode on dynamics. Sadly we audiophiles are more like the fashion industry than we would like to be
http://www.diyaudio.com/forums/pass-labs/156081-impact-cascoded-transistor-zenamp.html
Interesting comment on affect of cascode on dynamics. Sadly we audiophiles are more like the fashion industry than we would like to be
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Cascoding is a simple matter too - the point is that JFET's Vds stays constant in spite of input and output signal variations. This constant Vds stops unwanted variations of parasitic capacitance (know as Miller effect) and the composite device (JFET + cascoding MOSFET) performs better than single JFET.
Off course there's more to it, but this is basic concept, easy to understand.
Off course there's more to it, but this is basic concept, easy to understand.
Thank you Mr Mod, I have seen the thread, but hope to stumble through on my own in the hopes of learning something. I know his stuff is old news to you guys. The LU jfets are old news now that Pass' new SIT's are on the scene. Thanks for the help, I am sure I will need more.
One thing I remeber reading in the above thread was a very contentious argument between yourself and Magura about the benefit of a negative rail at lower voltage. His argument was that it was an unnecessary complication, whereas you eluded to the fact that it helped with noise. How does the addition of this rail help. CMRR is already high due to balanced inputs, how does the negative rail change or add to this benefit?
well - I can't remember that I said anything about benefits regarding noise , when including negative PSU ;
maybe I did ...... but what I know for sure that major benefit is deleting those input caps .
in any case - I didn't conduct any measuring comparisons between these two variants 0 wth/wo negative leg .
but - as you said - you're building your own haystack ......
maybe I did ...... but what I know for sure that major benefit is deleting those input caps .
in any case - I didn't conduct any measuring comparisons between these two variants 0 wth/wo negative leg .
but - as you said - you're building your own haystack ......
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