DIY Musical Fidelity P170

[MassiAudio]

Hi, I'm Massi from Italy, sorry for my bad english.

I'd like to buid a power amplifier like Musical Fidelity P170.
I searched a lot in WEB, but I found only a schematic:
http://www.diyaudio.com/forums/attac...amp=1120201026
Also I found some photos of P170 that have some modifications, like two transistor for each power supply to go to 12volt zener.

Do you know if there is a sure schematic diagram for this amplifier?

Or someone could help me to understand why these modifications?
I searched and read many 3ads, but I am not able to understand everything.

Thank you very much.

Bye bye
Massi

[MassiAudio]

Update :

I dismount my P-170 ...

This is the schematic (I hope it's right):



This is the "other" schematic:



And this is the schematic of A-370 (class A):



Now, I have 3 problems:

1) In my P-170 the 22pF capacitor between pins 6 and 8 of LM318N there is not, there is the place on PCB, but no capacitor:



Is the capacitor important, or not?

2) Always in my P-170, instead of 1.5k and 1.8k resistors to 12 volt zener, there are are two little PCBs with TIP31C (TIP32C) and BC414C (BC416C): are they a current limiters?



These PCBs there aren't in A-370 too.

Are they very important and need to be there?

3) There are two power transformers of 42+42 VAC, one for each channel:
how many watts or VA could are they?

I hope you can help me .

Best Regards
Massi

[AMV8]

hi Massiaudio

I currently own a few of these amps namely 3 Dr Thomas Mk11's and an A370.

I believe the circuit was designed by Michael Vaughan Thomas. I have always liked the sound.

The company that made them for Musical Fidelity is nearby where I live. They tell me that the difference between the A370 and the Dr TMk11, P170 and P270 is that the A370 has an extra stage added before the output transistors. It is shown in your drawings.

All of my amplifiers have the capacitor next to the op amp. I understand that the circuit is not especially stable and that probably helps stability.

The extra pcb you mentioned was added to later versions since all the earlier amps had a problem with the resistors in the power supply lines overheating. That is the 3w resistors between the output and input stages on the power supply lines.

In my amps I replaced the these 3w resistors with 20w resistors - and they still get very hot. I think I will add your circuit as it looks a good modification. My amplifiers have a 1k5 3w ( now 20w ) resistor in the positive power supply to the front end and a 1k5 3w resistor paralled with a 1k8 resistor both 3w ( now 20w in mine ) in the negative power supply to the front end. 9 I really should replace these with regulated supplies.)

The early versions of the A370, I am told by the makers, ran at almost completely pure class A. Not surprisingly few survived and they tell me that later models ran a 25% class A.

I think it is a good sounding circuit. I find the A370 circuit with the added stage slightly better than the version without. I also think that the versions with more output transitors seem to produce more and deeper base - but I may be kidding myself.

The only change I have made to my amps is to change out all the capacitors for current 105 degree capacitors and I have added much more capacitance to the power supply; about four times more than in the originals.

The transformers in my amps are 625va with 40-0-40 secondaries. Weight 5.7 kilo each.

I hope that helps.

Don

[AndrewT]

Hi,
is the 10k at the input a bootstrap to raise the input impedance?

Can the opamp be changed to some higher performance version to yield any improvements?

[MassiAudio]

Originally posted by AndrewT
[...]
Can the opamp be changed to some higher performance version to yield any improvements?

Perhaps the answer is in this 3ad:
Mf A 370

Regards
Massi