P3A Comparison table ( long .... )

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#981
Ian Finch said:
Polyester caps are only just ok in quality audio circuits - seldom specified now. The function of those 100n caps in parallel with "C+" and "C-" is to augment the filter performance of those electrolytic bypass caps at high frequencies, where electrolytics can be less effective at filtering noise. So it figures that unless the film caps have much better high frequency performance, they probably don't help at all. In fact, they are seldom specified nowadays and you don't see them on schematics here at DIYaudio.

In most cases, just fitting modern, good quality electrolytic caps alone means the 100n film caps are redundant. There would be little point to fitting the film caps unless they were high quality types such as polypropylene film, not cheap polyester or ceramic caps since they there to improve performance, not just come along for the ride. Otherwise, they are not necessary.

The 100p capacitor is an RF filter cap. so it needs to be effective at frequencies much higher than audio, so cheap ceramics and polyester caps are out. Most audio amplifiers used polystyrene fim caps in this role but it should also be OK to use genuine NPO or COG grade ceramic caps there. I doubt 100pF value polyester or polypropylene film types time will even be be available anyway.

As always, beware of fakes and low grade components unless you use reliable, commercial parts distributors for your components.
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Thanks Ian Finch, I have four 90pf polystyrene capacitors at home, I will install them in parallel (2 + 2). Under the plate I will put one more capacitor 100pf np0, I want to leave the input filter with approximately 380pf, I believe it will highlight the low frequencies. I already called with 100pf on the filter, the sound is excellent, but there is a little bit of bass, I'll test it. In general, the sound of the amplifier is very good, I like listening.
 
#982
get ready this one is going to be long .....

Its a15 year birthday since i start dealing with P3A ...Through such a long period of time i have been dealing with many changes and tried almost everything around this amp .
Some of the things i ve done you already read in this thread ....

The biggest struggle was the PCB and sorry folks but the pcb's you post is not my style.... These PCB 's might be operative and might produce sound but there is nothing innovative in them ...

A proper pcb should have
--Starting from left to right power supply caps, bypass and bleed , output area , small things ...
--drivers and vbe on common heat sink
--LTP thermally coupled
--current source as close as possible to the LTP
--Narrow ground spread over the PCB
--clean and/or star ground
--Keep dirty things away like zobel or coil if you use one
--proper/practical size to fit in any enclosure of the market or even surplus
--designed in a way that will operate with available heat sinks of the market or surplus
--make everlasting job
--Make sure thermals are fine at all operation conditions

So trust me on this IT CANNOT BE DONE
It can be only done if you locate the filter capacitors horizontally and this is something I DONT LIKE !!!!

Then again i designed one PCB that can do that IE keep everything vertical and have a heat sink on a top BUT
A) that was a pain in the a** to assembly and tune
B) heat that concentrates under the heat sink + Ambient+ operating conditions will produce variations in bias which is something that i dont like .

So i had to cut a deal with the devil .....Produced another PCB your style (sad to say) and added to it a few goodies ....
( i was about to replace my home amplifier that was made as an integrated amplifier P37+p3A with a new preamplifier yet again P37 but PLUS ECHO phono stage + dual mono amplifier )

So my latest project is like that .....

--2 torodials 250VA 32+32AC ( sound king surplus )
--4*15.000uf /63/105 nipponfarrad 2* per ch
--3.3uf +220nf +100nf bypass ( times 4 )
--18K 4W bleed resistors
--1 uf /400v rail to rail capacitor Perreaux style
-- 4mm supply leads to board +- twisted excluding ground from the twist Yamaha style
--220uf/63V +1uf 63 v decoupling on board

------------------------------------------------------------
Input capacitor ...
been playing with any available film capacitor available in the market to notice differences between them . So this time i added 47uf bipolar of good quality+ 1uf bypass +100nf bypass Sansui style
feedback bypass capacitor 100nf
since it looked nice i also bypassed bootstrap capacitor
bias stabilizer of 100nf

-------------------------------------------------------
vbe scheme

Now its clearly important that you run your output transistors ( in our case the drivers ) at the same temp .... ( dont know how many of you understand that ...if not you have to stop looking P3A and continue to think EFP )
So previously i had a small piece of heatsink 2-2.5 mm thick to locate my drivers on with the VBE multiplier
I noticed though that for this application a thinner fin of 0.8mm works better since the point there is not to cool the drivers but to equalize the temp between them and that makes the BD139 VBE multiplier to sense better and keep extremely stable bias at most operating conditions + ambient ...
-----------------------------------------------------------------------------------------------

Drivers now are A1837 C4793 70 mhz combo and pistons remain 2SA1302 and 2SC3281 since i have so many of them original that i can make a treasure bath in them ...

My load at home is easy 8 ohms typically and i have seen no signs of stress at the machine even if rails are a bit on the risky side ....85 W that i actually dont need is produced while higher rails will probably increased a bit the current on the LTP and the VAS stages

sound is mind blowing ..... separations are far better with a dual mono and the few add ons continue to surprise me .... week from now might post more detailed differences between what i had before and and the latest version ....

Happy birthday
Kindest regards

Sakis
 
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#989
Yes, please support Rod Elliot if at all possible. He makes an enormous amount of great information and projects available for us DIY ers on his site.....all for no charge......and the P3A is his intellectual property. With the current pandemic delivery may be difficult, but drop him a line and ask if he can help.
 
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#993
Setting/measuring bias

Am a beginner so question might sound stupid.
Is this the correct way to measure DC bias using a multimeter as shown in the attached image? Or do I need to connect the -ve probe to ground?


Regards,
Kay.
 

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#996
BD137/138 as driver for +/- 35V?

I accidentally found some nice pairs of PHILIPS BD 137 / 138 available on hand. It could be nice if they can be used in P3A (even with a lower voltage version since the transformer is not yet bought).

My understanding is that -- the driver see only half of the power supply (i.e.35V) while the max Vce in spec is 60V. With 35% derating on Vce, it's still save to use BD 137/8 as drivers for +/-35 volts.

Am I right?
If not, what the max rail voltage should be used then?

Thanks in advance.
 
#997
No. You probably imagine that the drivers and power transistors voltages slide from either rail to half the output terminal potential, like the audio signal. That's a typical misunderstanding that comes from poor texts and teaching, such as you find on the 'net. Actually the 4 power transistors are exposed to almost the full +rail to -rail potential difference at all times and they effectively "slide" up and down on the P3A's 70V rail differential.

You really need to have power transistors that withstand the full rail-rail potential voltage swing plus a safety margin which means using BD139/140 transistors, as specified. Otherwise, you would need to reduce the supply voltage to +/- 30VDC to have a safety margin and that also means a significant power reduction. Your choice though, since you could probably use Chinese copies of 2SD669A/ 2SB649A which are quite cheap and likely better performing, providing you can check for oscillation, since these have more gain but at least similar capacitance.
 
#998
Reducing in power output is fine to me - since it's for near fields listening.

I've seen many design using BD 139/140 as VASand drivers on +/- 35V. That's margin of safety is only 12.5% Using the same number of safety margin, the BD 137/8 could be safely used at 52.5 volts (full swing). I'll go for +/- 25V max then.

Thank you Ian for your clarification. I do appreciate for your response.
 
#1,000
Thank you Sakis.

You once mentioned a model of Hegel integrated amp earlier. Based on their website, Hegel's internal circuit seem to be much more complicate than P3A. Sadly, I never have chance to hear it myself.

Any chance to have further A/B (P3A vs Hegel) audition tests esp. at low level listening?

PS: Just notice that this post is 1000th. !
 
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