Upgrading LM318 to OPA627 in Musical Fidelity P170 may have problem. Below is an extract of a thread on a DYI forum regarding swapping LM318 for OPA627 – there are a few issues with dropping them straight in


“They’re LM318’s – it was suggested some improvement might be had by swapping
them for 627’s, which I have coming with the caps, as I’m sure they will get
used elsewhere if not on the P170… I think I’ve been infected with DIY..

I’ll have a go at creating a full schematic of the amp, and I’ll upload it
here and send it to Mr Hennesy, maybe he’ll archive it on his site…

Maybe on the weekend…

Michael

A 8
Hennesy should have the schematics, I think I sent it to him some time ago.
Regarding swapping the 318, I seem to remeber that there are no dc ofset
adjustments only a freq-compensatio pins/curcuits/components that you need
to disconnect. Other then that it should work as is. The 637 will probably
make the amp go into oscillation.

maf_au
Ok, I have the schematics, thank you Mark and A 8 🙂

Here is what I see on pins 1 and 5 on the 318:

Pin1 via 3K3 and 2n2 to Pin5

So should I just hook them out and swap in the 627?

Regarding replacing the caps, especially the 22,000uf jobs on the PS:

How should I (safely) discharge them before working on them?

Is running the amp disconnected from the power for a while enough to
discharge them, or should I ground the + terminal as well? Not wanting to
get fried this week, and aware of the potential for stored current in these
puppies…

At this stage, I’m going to replace caps, repair board and re-assemble. If
everything behaves at that point, I’ll swap the opamps.

Michael

amplifierguru
Firstly, if you have any power resistors around say 5W’ers then work out
from the supply voltage stored on the caps whether they would be in their
ratings if placed across one C to ground i.e. P=VxV/R. Then just hold it on
-the the other(s).

The RC on pins 1,5 sound like comp for the 318. I haven’t used one for
decades. Can you post the amp schema..

Cheers,
Greg

maf_au
quote:
Originally posted by amplifierguru
Firstly, if you have any power resistors around say 5W’ers then work out
from the supply voltage stored on the caps whether they would be in their
ratings if placed across one C to ground i.e. P=VxV/R. Then just hold it on
-the the other(s).

The RC on pins 1,5 sound like comp for the 318. I haven’t used one for
decades. Can you post the amp schema..

Cheers,
Greg

Thanks for the reply!

Ok, bear with me, I’m new to this. I’ve got the bit about grounding the cap
to earth via a power resistor, but calculating the load:

P=VxV/R

Am I on the right track here:

I’m thinking that V= the tranny output volts, R= the power resistor value
ohms, so assuming I had a 5k/5w resistor, and the tranny was putting out
63v:

P=63 x 63/5000 = 0.79 (watts?)

So, my 5watt/5k resistor would be fine, and turning the eq around, I’d need
a 793.8ohm/5w resistor or better?

I’ve asked A 8 if it’s ok to post the schematic…

Thanks,

Michael

A 8
They are for comp so yes desolder them, there should also be a 5p Mica
between output and pin 8, disconnect pin 8 and do the swap. You may want to
keep it but connect the other end to the inverted opamp input. Cant remember
what I did.

I think I also added filmcaps to the opamps powersupply.

A 8
I just sent you a private message.

BTW my unit had the K1058/J162 mosfets in them and it worked great with the
627, your unit seems to have a lot of components replaced which
theoretically can change how it behaves with new opamps. Be watchful of dc
level on the output of the amp and current drain when you first power it up
after placing the 627 in there.

amplifierguru
Yes maf_au you’re on track. generally when I design an amplifier I include
drain down resistors for the power supply. They only need bleed 5 mA or so
.. so that by the time the lid is unscrewed they’re as good as discharged.

A8 – you designed this unit? So the pins 1,5 and 8 can be relieved of
components to implant a OPA627?

Sorry I don’t know the design so without a schema.. I’m working blind.

Cheers,
greg

maf_au
Ok, here it is.

Not sure what the limits are here, hopefully this is small enough…

Michael

maf_au
quote:
Originally posted by amplifierguru
Yes maf_au you’re on track. generally when I design an amplifier I include
drain down resistors for the power supply. They only need bleed 5 mA or so
.. so that by the time the lid is unscrewed they’re as good as discharged.

A8 – you designed this unit? So the pins 1,5 and 8 can be relieved of
components to implant a OPA627?

Sorry I don’t know the design so without a schema.. I’m working blind.

Cheers,
greg

Thanks.

Looks like the replacement caps aren’t going to make it here today, I might
spend some time on the weekend identifying and recording the differences
between my amp and the schema, just in case there is something that
shouldn’t be…

Michael

amplifierguru
You’re doing well. If you can permanently fit 22k 1W resistors across all
your PS C’s that would ensure some safety. They won’t degrade performance.

The components on pins 1,5,8 can safely be removed if you want to use a
OPA627. The 3K3 at the input filter may be picking up excess hum and noise,
depending on the layout. Replacing with 1K or a parallel equiv might be
worthwhile.

I wouldn’t use an OPA637 in this cct.”