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Old 14th December 2008, 08:50 PM   #1
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Default P3A Bootstrap and NFB Capacitors

Hello all,
I built a stereo ESP P3A amplifier with my own boards and am very satisfied with its sound.

I am issuing a 2.0 board with more room for (physically) bigger resistors. At this point I am asking if it could make sense to place a footprint near C4 and C5 to place a small (or not so small) MKT capacitor to bypass them.

I even thought to use, instead of the usual 0.1/0.22uF bypass value, a Wima MKS2 6.8uF/50V (only 5mm pitch) in parallel with a 220uF/35V Elna Cerafine.

In the FB leg for example, 6.8uF with 1k (R4) would make an LF -3dB frequency of about 25Hz, bringing the contribute of the MKT down to medium/low frequencies and leaving the electrolytic alone to pass only very low frequency signals. What do you think about this?

Thank you and cheers from italy!
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Old 15th December 2008, 06:26 AM   #2
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Click the image to open in full size.

You are talking about C3 (100uF) and C5 (100uF) in this schematic.

I am not sure much to improve by using smaller film caps to parallel those
... I am not the one to parallell capacitors

C3, FB blocking, has got a good value: 100uF
which matches C1 x R2 = 4.7x22

To me, the Bootstrap C5 100uF electrolyt seems a bit high in value
I would like to experiment with 10, 22, 47uF here
For example 10uF (or 6.8uF) MKT, MKS, MKP film cap might work well as Bootstrap.
But high quality electrolytics might do.
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Old 15th December 2008, 02:26 PM   #3
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yes, sorry, they were C3 and C5
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Old 15th December 2008, 06:01 PM   #4
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Big bootstrap is good, decreasing it will give increased distortion at low frequencies.
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Old 15th December 2008, 07:01 PM   #5
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My first question would be why you want to do this. Do you feel you are missing something in the sound?

I have by-passed big caps in the NFB path with lower value polypropylene before but I cannot say that it made any audible difference.

I am sure if Rod felt the need for a low value by-pass cap he would have included one.

If you want to do this, there is no harm.

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Old 15th December 2008, 09:34 PM   #6
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Default i have a question also

somewhere you mention about fysicly biger resistors ....would you like to tell us more about that ?????

( i keep in mind something i read somewhere that many exotic designs that use exotic huge caps have some trouble since big exotic caps are working more like antenas compaired to normal capacitors due to size ......eventhough might sound better but also might be more sensitive to anything ....)

as about bypassing in this areas i would go with Nico Ras ......

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Old 15th December 2008, 11:14 PM   #7
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Thank you for your replies guys.

I really like the sound of this amp but when I'm in the phase of PCB design, I always feel nervous about missing something that I'd regret later. So I asked about those bypass caps.

I understand bypassing electrolytic capacitors would not bring a great improvement.

About bigger resistors, nothing exotic. I wanted to try some Holco H4 0.5W resistors and I'm laying down a PCB with 12.7mm pitch against 10mm of standard 0.5W metal layer resistors.

I understand bypassing electrolytic capacitors would not improve the sound.

Being also in the market for new components, if you don't mind I'd also like to know what are your choices about some other things about this amp and the like.

1 - I am using silvered mica caps for C2-C4-C6.
Do you think some other types would be worth trying? What about something like Wima FKP2 (polypropylene film and foil)? I read polystyrene is a very common choice for C4 and C6.

2 - What about reducing C2 and reducing/removing R1/R3?

3 - What kind of resistors do you use for R13/R14? I'm using 4W MOX now.

4 - I'm planning to use a 2SA1406/2SA1306/2SA1209 (depending on what I find) as Q4 since their low Cob. Is it also worth trying other transistors than BC546 for the input differential pair?

5 - Has anyone ever tried modding this particular amp to use lead compensation? I am very curious about this. Would 10 pF for both C4 and the new capacitor between collector of Q4 and base of Q2 be two good starting values? Simulations tell they're ok but I wonder if anyone already tried this in real life.

6 - Now something hardcore. Using a tube preamp and having another power amplifier (a SE DHT with lower gain than the P3A) I'd really find useful to have less gain from this amp. Would bringing R4 to 1.5K be too harmful for stability? Spice tells everything is ok with 1.5K for R4 but only using lead compensation. With standard 100pF Miller compensation I get an awful dip in magnitude response at about 1MHz.

Thank you for your help!

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Old 15th December 2008, 11:14 PM   #8
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Default errata corrige

At 1MHz I get a peak, not a dip.
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Old 16th December 2008, 01:49 AM   #9
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Peak at 1MHz?

That suggests you have too low phase margin

Maybe the circuit depends on the internal miller capacitance of the VAS to be stable? What transistor did you simulate with?

If the current through LTP is increased, the circuit gets more unstable too because of increasing transconductance of the input stage.

Slow output stage (wrong bias current?) or capacitive loading of the whole amp will also make that happen.
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Old 16th December 2008, 10:25 AM   #10
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Please note that I get this peak only in simulations using 1.5K for R4, that means increasing the global feedback value by about 3.5 dB. I'm simulating using MJE15035 or MJE350 as VAS transistor (I can't find 2SA1406-2SA1306-2SA1209 spice models!), BC546 input pair, MJE15034/5 drivers and MJL4281A/MJL4302A output.
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