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/attachment.php?s=&postid=673432&stamp=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
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/attachment.php?s=&postid=673432&stamp=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
musical fidelity pre-amps
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
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
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
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
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.
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.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
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
op amp replacement
one of the better solution,if your problem is the noise is the Linear op amp such LT1028/1128(single).In europe is hard to find but Distrelec have this one.
This increase dramatically the performance for all the parameter,but in this case you must verify whit a scope the stability margin and oscillation in power stage.
one of the better solution,if your problem is the noise is the Linear op amp such LT1028/1128(single).In europe is hard to find but Distrelec have this one.
This increase dramatically the performance for all the parameter,but in this case you must verify whit a scope the stability margin and oscillation in power stage.
Hi,
the line stages should be almost hiss free. Your hiss could indicate a component/s gone faulty.
Have you tried removing the socketed chips from their sockets and cleaning as best you can?
The designer has gone to a lot of expense to avoid electrolytics in the main board. But a few, very small electros, may usefully be swapped for polyester and better still polypropylene. It does depend on what the electros are doing.
224K is that 220nF+-10%?
105K is that 1uF+-10%?
the line stages should be almost hiss free. Your hiss could indicate a component/s gone faulty.
Have you tried removing the socketed chips from their sockets and cleaning as best you can?
The designer has gone to a lot of expense to avoid electrolytics in the main board. But a few, very small electros, may usefully be swapped for polyester and better still polypropylene. It does depend on what the electros are doing.
224K is that 220nF+-10%?
105K is that 1uF+-10%?
Hi,
You say the phono stage is of no interest, fine.
The output noise on the pre-amplifier is related to its gain. Gain
is needed for tape, radio etc but much less is needed for CD.
Tape etc is usually ~ 200mV maximum, whilst CD
is 2V maximum, a difference of x10 or 20dB.
I'm guessing the noise is related to CD use and due
to the topology not being optimised for CD inputs.
Could you sketch out the topology, i.e. the positions of
the gain stages, tape buffer, volume control relative to
the inputs and outputs etc and the R value of the
volume control would be useful.
Maybe all you need is another switch on the front, for CD.
/sreten.
You say the phono stage is of no interest, fine.
The output noise on the pre-amplifier is related to its gain. Gain
is needed for tape, radio etc but much less is needed for CD.
Tape etc is usually ~ 200mV maximum, whilst CD
is 2V maximum, a difference of x10 or 20dB.
I'm guessing the noise is related to CD use and due
to the topology not being optimised for CD inputs.
Could you sketch out the topology, i.e. the positions of
the gain stages, tape buffer, volume control relative to
the inputs and outputs etc and the R value of the
volume control would be useful.
Maybe all you need is another switch on the front, for CD.
/sreten.Thanks very much for your response. It's much appreciated.
The white noise is present regardless of what is connected to the amp.
I could try to sketch out the areas of the amp if you feel it's worth it. It will take me some time as I'm new to this.
edit: I should note that I get the same level of hiss regardless of volume control position.
The white noise is present regardless of what is connected to the amp.
I could try to sketch out the areas of the amp if you feel it's worth it. It will take me some time as I'm new to this.
edit: I should note that I get the same level of hiss regardless of volume control position.
Hi,
After some more testing I discovered that the hiss only affects the buffered outputs. The direct ones a dead silent. This is fantastic
The buffer is just two skanky old transistors, one for each channel I presume. Would this be something like an emitter follower? I'm going to have the board out of the chassis to investigate further.
I don't particular need to use the buffered outputs, so far the only difference I can tell is that they sound louder through the power amp. I've yet to test if the really sound different in tonal qualities and dynamics. Sitting hear now it seems they're a bit flatter in sound but that's perhaps my imagination.
Perhaps I could build a new buffer stage with a modern pair of opamps. I have a NE5532 lying around and a pair of LM394 amps. I removed them from the MC circuit as I'll never use it and I thought they may be handy.
Before I think too much about that though I'd like to know more about the buffering of outputs in general, for that matter, there's two sets of inputs on the power amp, one seems direct and the other goes through a set or resistors and caps that (when switched in via the external switch) seem to kill the bass and make the amp sound very thin. I use the direct ones.
Thanks again to everybody.
After some more testing I discovered that the hiss only affects the buffered outputs. The direct ones a dead silent. This is fantastic
The buffer is just two skanky old transistors, one for each channel I presume. Would this be something like an emitter follower? I'm going to have the board out of the chassis to investigate further.
I don't particular need to use the buffered outputs, so far the only difference I can tell is that they sound louder through the power amp. I've yet to test if the really sound different in tonal qualities and dynamics. Sitting hear now it seems they're a bit flatter in sound but that's perhaps my imagination.
Perhaps I could build a new buffer stage with a modern pair of opamps. I have a NE5532 lying around and a pair of LM394 amps. I removed them from the MC circuit as I'll never use it and I thought they may be handy.
Before I think too much about that though I'd like to know more about the buffering of outputs in general, for that matter, there's two sets of inputs on the power amp, one seems direct and the other goes through a set or resistors and caps that (when switched in via the external switch) seem to kill the bass and make the amp sound very thin. I use the direct ones.
Thanks again to everybody.
I still have an earlier model so please confirm if correct
Does the pre amp have two output options (active / passive)
By swapping between the two output type’s one could make a more informed decision as to the main culprit.
The input selector switch tends to be a source of bad and or intermitted connections
Does the pre amp have two output options (active / passive)
By swapping between the two output type’s one could make a more informed decision as to the main culprit.
The input selector switch tends to be a source of bad and or intermitted connections
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