Musical Fidelity preamp p3a

[hoshy]

Hi,

I'm a first poster here so, hello to everyone.

I have the above pre amp and it's a little hissy. I thought about sending it back to Musical Fidelity but I understand it's like that by its nature?

I've been recommended to upgrade caps and op-amps.

I'm looking for some help with this little project. any or or all of the following.

what op-amps / caps to use
which items to replace
schematics

I'm confident about my my ability to carry out the work but I don't know enough electronics to be sure of which components are really worth replacing. Obviously I can just drop an upgraded op-amp in with high spec and see what happens but that's a bit hit and miss.

I looked at this thread: http://www.diyaudio.com/forums/attac...amp=1120201026

which has a schematic for the p170 - is this likely to be close enough? The bottom right mentions a 70watt version which I believe the p140 is (2x70 or 140 bridged).

Thanks in advance.

Ashley

[hoshy]

I have a feeling I may have posted in the wrong section. Would Solid State be more appropriate?

[pinkmouse]

Yes.

Moved.

[AMV8]

Hi
I own a preamp3 and a preamp and p172.

I have changed the caps and the chips in each.

Changing the capacitors in the pre3 made the sound clearer. My preamp 3 - which is fully class a - is all transistor so no chip to change. However changing the chips in the preamp extended the treble and made the mid range and treble clearer.

I hope this helps. If not I am just down the road in Walton on Thames and you can phone me on 07766 160733.

Don

[hoshy]

Thanks for moving the thread

Don, I'd be very interested in hearing more about what you did.

Yesterday I replaced the two LM318 devices in the power amp with a dual NE5532 - This has helped the amp no end. I just wanted to clean up the excessive hiss on the amp but the improvement goes far beyond that. Detail and clarity has been dramtically improved.

i'd like to attack the preamp next as I can now hear the little bit of hiss from the preamp itself, before is was masked by terrible hiss on the p140.

I didn't quite follow your mpost. do you have the preamp 3a? In mine there are two LM348 op amps. and a host of poly caps. Am I right in thinking that poly caps don't deteriorate or is it worth replacing them also ?

I have some op amps on order - NE5534, OPA134 and NJM2114. As for capacitors I'm not too sure where to start.

Cheers,
Ashley

[hoshy]

Excuse me, it's not LM348 devices that's in the preamp - I'm going to check now what they actually are.

[hoshy]

LM394

[sreten]

Hi,

As I remember it the 3a did not contain op-amps, but it did contain DIL transistors.

That is a set of 100 transistors wired in 2 sets of 50 parallel to
form a pair of low noise transistors that can be used in MC inputs.

Depending on the MC circuit the LM394, which is the above device,
could be replaced by lower noise transistors, or possibly lower
noise DIL equivalents to the LM394, don't know what they are.

/sreten.

[AndrewT]

Hi,
which input is troubled with hiss? or is it all of them?

[hoshy]

Hi all,

sorry for the slow reply. the chips in the amp are 2xLM394 and 4xVQ1000J

It looks like the LM394 ops are used in the phono stage and that's not important to me. the VQ100J devices are quad n-type mos transistors. I stand fully corrected

Still, I'd like to know which parts are ripe for the upgrades. On the capacitors side of things theres a mix of ploy and electrolytics. I know the electros degrade with time but I'm guessing they polys are in the signal path so the electros aren't much of an issue?

Would it be worthwhile finding modern replacements to the VQ1000J devices?

There's a high res pic of the internal here: http://hoshy.co.uk/~hoshy/amp/p3a.jpg

It is all inputs that are the issue - the hiss is not related to the volume setting either.

Thanks to everybody who's helped so far,
Ashley