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Old 10th November 2013, 11:20 PM   #11
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Bigun, I'd be really interested in putting this up against my P3A that I recently built. Hopefully someone produces some boards (either SMT, TH, or a mixture - doesn't bother me).

Is there much difference between the SC1943/5200 and Motorola silicon? Reason being I have a small stockpile of MJL4281/4302/3281/1302 devices.

I prefer the simplicity of the first version without the MOSFETS in the OPS. 50wpc is a useful amount of power.
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Old 11th November 2013, 12:12 AM   #12
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I notice you have added R-C filters for the VAS and OPS, but omitted the filter caps and 100nF decoupling caps on the OPS rails. Was that intentional?

Another request:

When you produce the boards can you make it a stereo layout with bulk caps onboard? This circuit has a very low parts count so should be possible, whilst keeping the circuit board fairly small, particularly if SMT components are utilised :-)
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Old 11th November 2013, 12:22 AM   #13
Bigun is offline Bigun  Canada
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Hi All,

PCB - well I'm still deciding on the approach. It's primarily for my project rather than public use so I can't promise it will please all.

The schematic I posted is a simplified layout - there will be additional parts. I haven't looked at the VAS yet and if I stay with a simple VAS I will include footprint on the PCB for two-pole compensation and phase lead compensation along with bypass caps on the rails.

Hugh - I will give some proper thought to your suggestions for sure.

One thing to note, the pin-out for the MOSFET and the power BJT are almost inter-changeable (gate-drain-source usually maps onto base-collector-emitter) allowing a 'universal' layout and experiments to follow.
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Old 11th November 2013, 02:15 AM   #14
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by Ranchu32 View Post
Is there much difference between the SC1943/5200 and Motorola silicon? Reason being I have a small stockpile of MJL4281/4302/3281/1302 devices.
I believe these devices of yours are very good and should work just as well although I haven't used the MJL4281 myself.

Quote:
Originally Posted by Ranchu32 View Post
When you produce the boards can you make it a stereo layout with bulk caps onboard? This circuit has a very low parts count so should be possible, whilst keeping the circuit board fairly small, particularly if SMT components are utilised :-)
That's an interesting idea, I know Hugh likes this approach. I'll give it some thought to see if it will suit what I have in mind but the freeware version of Eagle layout software I use may not allow a large enough PCB for it all. I was thinking of a compact mono board (that could be used in a multi-channel amplifier) with solid-state dc-protection on the same board as an integrated unit - would this be appealing ?
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Last edited by Bigun; 11th November 2013 at 02:18 AM.
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Old 11th November 2013, 02:26 AM   #15
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Quote:
Originally Posted by Bigun View Post
I was thinking of a compact mono board (that could be used in a multi-channel amplifier) with solid-state dc-protection on the same board as an integrated unit - would this be appealing ?
Sure would be - and I notice there's a few other Aussies interested in this thread, happy to assist in coordinating the dispersal of some PCBs downunder if the project gets under way...
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Old 11th November 2013, 02:45 AM   #16
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Onboard DC protection would be appealing, as would anything else that results in greater integration. I prefer to use large 30A relays that can reliably handle the breaking current. PCB types that carry this rating are quite bulky so you could keep them off-board and integrate just the trigger logic. These components are compact and wouldn't add much to the overall board size, and could easily bypassed if the constructor didn't want the protections.

If it has to be a mono design, could you please also consider incorporating the filter caps also. If space constraints are an issue, a small amount of capacitance could be accomodated, which constructors could augment with off-board caps as desired.
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Old 11th November 2013, 04:03 AM   #17
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1206 SMD resistors seem big enough for older diy-ers to work with.

I also think that the circuit will work fine with higher rails voltage, two pairs of output BJTs and single drivers (if high power is really needed). The proof is LJM L12 amp that Ian Finch built.

Also, the circuit may be tried with only one pair of MOSFET outputs, but CFP output configuration should remain.

Last edited by ivanlukic; 11th November 2013 at 04:18 AM.
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Old 11th November 2013, 03:34 PM   #18
Bigun is offline Bigun  Canada
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What about the VAS ? - is it important to leave this alone so as to retain the character of the P3a. We could allow for an option on the PCB for an enhanced VAS so that we don't have to change it, but we could still try an enhanced VAS and keep it only if we wanted. The penalty is space on the pcb.

What I had in mind, in the full spirit of the P3a, was a CFP based VAS. Why might we consider this ?
- It should be more linear, especially at lower frequencies and large signals. At higher frequencies the simpler single VAS device is partly linearized by Cdom, which takes signal from the collector and feeds it back to the base.
- The P3a does not have a class A emitter follower buffer driving the output devices, the drivers also switch off with the outputs which puts some switching noise right back at the VAS. A CFP VAS will provide a buffer.
- We could get fancy and use a CFP VAS to bootstrap the input, this would get us another 10dB of open loop gain (I did this in my TGM6 amplifier - thread around here somewhere) and it works well.

Thoughts ?
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Old 11th November 2013, 06:39 PM   #19
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CFP based VAS? Why not? It is good idea if there is space on the pcb.
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Old 11th November 2013, 10:00 PM   #20
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I've tried some of the ideas suggested here on the P3a. One I spent some time on, was increasing the number of output devices. I was still using MJ15003/4 at the time and these are quite different to modern triple-diffused and LAPT transistors that are more flexible in application, due to their higher Hfe, Ft and lower capacitance.

It seems that with a compact layout, you can preserve CFP stabilty without needing lots of base stoppers, ferrite beads or bypassing to squish signs of oscillation. However, my version that used MJ15003/4 in virtually just a paralleled output stage with 4.7R stoppers, sounded less like a P3a and more like other old DIY designs from the 1970s. I think Sakis was right in his conclusion about adding more output devices but I regret not having access to an analyser to see what had actually changed to spoil it. That would be an interesting study, whether drivers, load, matching etc. with implications for many other designs too. Given that when we add more devices for more power but really, still use the amplifier at a tiny 1-5W in typical domestic use, we do change the operating conditions and usually, the bias too.

I suspect that when there is noticeable crossover distortion, adding more devices adds more distortion products - given the same device operating conditions. Of course, if you only simulate results using identical models, you may not notice what more likely happens in real life, where individual device physics will all differ to some degree, as will their distortion products - which become massive in quantity with IMD, the higher in frequency you look.

I don't think, when based on bootstrapped capacitor VAS design, that linearity is the order of the day with P3a. I'd say that even-order non-linearity was the aim, without hesitation. I have my doubts about the significance of a CFP VAS to the spirit of the P3a or a benefit but then, its your project - only my
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