P3A-MORE UPGRADES

[sakis]

Sorry this one will be kinda long ....

Most of you know that i work with the P3A more than 5 years now i end up changing minor things that had quiet serious effects on it .Meaning of the post is that i am looking for more suggestions for the best but without changing the original concept of the P3A .

Dear Friend Wahab suggested to remove the bootstrap and make it a ccs but this will alter the character of the P3A and on the other side i like the bootstrap so this type of changes is not my go .


bellow i will summarize and comment the changes ive made in categories and wait for your comments and suggestions .


LTP
for the LTP all my PCB feature thermal junction that is better for stability and offset,then always semis are chosen for high hfe over 400 and closely matched improved and made the bass more tight and generally improved the all amplifier by far . also tried to choose semis from a trusted manufacturer and also use semis that have lower voltage than BC 546 to force them work on the edge . Also played with LTP current +- a few ma with good results .

FILTER-MILLER CAPS
for filter and miller caps i try to use best of quality styroflex for the input and silvermica 500V 2% for VAS and driver depending on the choice of VAS semi i have lowered the values low as 68 pf with no obvious stability issues but with some sonic difference for the best. install a DC input and skip input cap when its possible to do .

VBE multiplier
It seems that after i common heatsink both of the drivers and in the middle attached the VBE transistor i ve noticed that the amp become much more stable and sound was far better and in a better "order " i have added 0.1mfd as a bias stabilizer ....

VAS /DRIVERS
been trying many excluded from the process MJE series since sounded worst eventhough pretty rugged device, and realized that this process is seriously related to the choice of outputs ....For the record the best combination ever was the BD829-830 paired with 2SA1302-2SC3281...BD 139-140 with 2SA1943-2SC5200... obviously different combinations will produce different sonic signature.

These are in a nut shell the small changes i ve made and most of my P3A run at no more than 40+40 volts and with 60.000 mfd for 2ch amplifier as about the bias always depending on the choice of semis an optimum 70-120ma is the best while tests ive made to 250ma didn't improve much and actually i think made the amp worst .

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As many know i am in the repair business so many of my costumers often take a P3A amplifier as a replacement amplifier from my shop while their amplifier is under repair ...At 99% of the cases the costumer will call the office in order to know more about the amp since he is thrilled with sound and even express the intention to buy one .

For the record i will state the type/brand of amplifiers that the P3A kicked *** off at least according to the costumer
---Plenty of Musical Fidelity
---Plenty of AUDIOLAB
---Few Goldmund
---Plenty Of QUAD
---One ADCOM
---2 NVA amplifiers
---Few Rotels
---Few Hardman Cardon



Changes that produced bad results

---High bias 250ma i think made the amp stiff and slow
---High rail voltage 52+52 volts produced more power and heat, some stability issues But no sonic improvement ...

---been thinking to make the all thing run from a regulated power supply but i am not sure if this will change anything for the best

---Been thinking to separate output from drive circuit in order to supply input circuits from other regulated voltage probably higher but never done that yet

---Been trying very fast drivers with relatively slow outputs with bad results

---Been trying the opposite like normal drivers but with very fast outputs

now suggestions please ppl !!!

kind regards
sakis

(Thanks for your time )

[sakis]

There is also something else to notice especially in the part that costumers take a P3A as a replacement amplifier ....

The amplifier and PCB i make are well made with order and proper wiring and cable rooting and with good and rugged material so obviously this will make any amplifier better than a commercial amplifier difference is that often commercial amplifier have a quite pricy tag on them .....

[boyet0173]

Hi Sakis, could you attach the modified schema P3A? Thanks in advance.

Regards,
Boyet

[sakis]

(????) there is no modified schematic

[jlester87]

Hi,

Can you share your power supply? What rectifier are you using?

Thanks

[sakis]

2 caps 15.000 mfd + standard rectifier 25A 250V +1.0 mfd 400V bypass

[p robertson]

Hi sakis i allways find your posts interesting. I have built 3 p3a amps the current one is using a jlh regulated supply(see ETI international 1983) of +-35v. i was wondering since you like to modify have you tried different method of compensation,i.e cap from vas collector to ltp base? thanks and good luck

[sakis]

thanks for the input ...i may as well check this also out
Do you have anything to add regarding the regulated PSU ?
have you made before and after tests ? is there anything better with regulated PSU ?

Kind regards
Sakis

[wahab]

If nothing is modded in the amp topology itself the best it can gain from
would be to add filtering to the front end since this amp has not
basicaly good supply noises rejection.

100R + 470uF in each side would already improve supply rejection
quite efficently considering the simplicity but it wont be as good , by far ,
as adding also cap multipliers consisting of BD139/140 with a 4.7K resistor
and a 100uF cap.

[Jay]

Originally Posted by sakis

Dear Friend Wahab suggested to remove the bootstrap and make it a ccs but this will alter the character of the P3A and on the other side i like the bootstrap so this type of changes is not my go .

The question is: have you tried it? The reason for the question is:

1) The CFP has "good" character, and is difficult to design. But Rod has made a very good work here. What I mean is, it should be a good design whether using bootstrap or not.

2) The P3A has been "dominated" by odd order distortion anyway, may be the usual perception about a bootstrap (e.g. its effect on vocal) not so dominant here?

EDIT: what i mean with "dominated by odd order is..."...