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Old 24th October 2005, 09:46 PM   #1
pjpoes is offline pjpoes  United States
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Default Modifying a Musical Fidelity X-Pre

I have this old Musical Fidelity X-pre thats been sitting around for ages. It was my second or third preamp I ever owned, I bought it back when they first came out. The tubes went in it, and I had already upgraded to a new amplifier, so rather than replacing the tubes, I put it aside, and havent touched it. Recently I took it apart and decided to change things a bit, and fix it up. I just ordered a set of Phillips NOS 6922 tubes for it.

However, I figured, while its opened, why not tweak it a little. I emailed the company about a schematic and parts list, but have not recieved a response yet. Would taking a multimeter and testing the capacitance be a bad way to decide on values for the capacitors? I am thinking so because of variance, and the possibility that they have lost it because of age. Although, they are filmy types, so maybe not. What I believe to be the power supply section has 4-1000uf@35v electrolytics from from Jamicon. There are also two at 470uf in the same area, and following the traces, they appear to be just bypassing the bigger ones, I think, though they are not much smaller. there area also two 100uf@16v ones, which I think are going either right before or right after the two regulators. There appear to be two Triacs, 5 diodes, and 5 resistors, one of those diodes and resistors I think is for an LED. Would the Panasonic FC caps be a good inexpensive upgrade for the power supply caps. Should I bypass them with some low value Black Gates, or leave it as is. I was thinking of using Black Gates where the two 100uf@16v ones sit, as those are probably cheap. The regulator is marked with a strange M, and then says 6d2, then on a lower line says AN7815. The other says 673, then AN7915T. Are those very good regulators, are there any recomended upgrades over them.

The resistors are Blue, with the color stripes, unsure of the brand or values yet, I will note those later. Not sure if I want to go through the effort of upgrading those yet. The Film caps are the rolled and coated kind, and they have a big L with a circle around it. Unsure of that brand. Would Wima be a good choice as a replacement for these, should I consider something better yet, a mylar film, or some type of axial leaded one, which may be too difficult to install here. I intend to put it all back into the tight fitting tube it came from.

Finaly, the volume control is very small, about half an inch on every dimension, accept for the control shaft. Its green with just some numbers on it, no brand that I can find. I am assuming most of these parts are common to England, as I believe thats where the Caps come from, I am not familiar with the brand here. There are also two small polystyrene caps at each tube socket, are those worth upgrading, I thought I heard once that was a pretty good cap.

One last thing I can't identify for sure, and would like to know if anyone knows if there is an upgraded version, or if its fine as is, or needs to be left as is. Its labeled IC1, and has an M with a circle, I think thats Motorola. Then MC33079P and on the next line, KCR9645.

Any help on this little venture would be greatly appreciated, along with some advice on prioritising my upgrades. Thanks again.
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Old 25th October 2005, 08:39 PM   #2
kepa1 is offline kepa1  France
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Hi,

There's quite a bit of info on the net about modifying MF X ponents already, especially for the XCAN, so you should find answers to your questions.

Anyway, I can tell you that I have been upgrading my MF XPRE for some time now and that it plays really better than before, as one would expect it!

1. The first thing I did was building a clone of the MF XPSU, meaning connect a 12VAC toroid transformer instead of the cheap wallplug. That didn't make a lot of improvement.

2. After that, it was time to upgrade all major caps inside the pre. And it sounded way better, the sound was more open with better details. For safety I used the same capacitance but chose caps with higher voltage. I used BC 037 series polarised caps for the PSU (Panasonics shouls be a better match), MKC for the 2,2F non polarised caps and Wima MKP for the 0,22F (the MKP4 fits perfectly for these instead of other series).

As space is tight inside the thingy and you need to respect the PCB architecture, you should always consider the size of the components you chose...

3. Lately, I've decided to change the rectifiers as well. This time the XPRE is transformed from what it used to sound like. I cannot compare with any other gear (but you'll obiouvsly be able to if you go on with that) but it quite impresses me now; one can hear micro details not heard before and the image has also been improved. I follow the advice of Anthony and went for BYW98-200 fast rectifiers bypassed with 103 ceramic caps (all but the 1n4007 LED rectifier which I let as is). Be sure to change the rectifier near the power connector (it must be D5 or 6) and to respect polarity. Changing rectifiers really made sense for me because the wallplug was gone long ago!

Also, changing rectifers is not as easy as the rest, especially if you work with ones with big "legs"; if so you need to carefully enlarge the PCB with a proper tool and fit the rectifiers on the back of the PCB (it won't fit on the top).

4. ( To come) - As I only use one source I'll connect the RCA socket directly to the volume control and thus avoid the cheap selector and everything connected to it! So far i didn't change the tubes which are 6922 on mine (issue 2), but they are ECC88 on other series.

Sad thing is that today I heard an horrible sound coming out the pre, it must be a bad solering point.

To finish with this post, I warmly encourage you not to let yours collect dust because you can get a very decent sound out of it just by upgrading the (cheap and ugly) components that it is made off.

Oh and here is a photo of operations during step 2 - The red Wima caps were changed to MKP4 with smaller size but same features.


Alain


Click the image to open in full size.
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Old 25th October 2005, 10:37 PM   #3
pjpoes is offline pjpoes  United States
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Your information is most helpful. you have gotten me the component values I need for replacing the components I didnt know values of, namely the film caps. I appreciate that.

Do you have any reason not to go wit a TO-220 packaged diode, that was what I intended to use, I think they will fit no problem. I have some I can measure with. I intend to change the small electrolytics (100uf) with Black Gate NK types, and the Medium sized (470uf) with Elna's, and probably the largest with the panasonic FC. This will keep the cost down while using differnt caps for different area's, and getting some of the benefits of the higher end ones.

Any chance you know the value of the small ceramic caps that surround the quad opamp. Which by the way, the IC I couldn't figure out is an opamp, I finalyl figured that much out. I am also curious to know how you figured out the values, do you have a schematic, or did you simply measure them and get the closest value you could. The reason I ask is I also have an X-phono that I want to update. I have debated upgrading it or replacing it with a kit, but have never been able to decide. Thanks in advance.
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Old 26th October 2005, 08:20 AM   #4
kepa1 is offline kepa1  France
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Glad you own an XLP too, I'll have to get one for me too! If you ever make yourself a XPSU clone, you'll be able to power them both with one twin secondaries transformer, nice!

Your choice for new caps is sure a good one.

In fact, all values shoul be written on the components, including the film caps, but it is coded.
224 = 220 000 pF = 0,22F and then 225, or L225J = 2,2F, now you can deduce the values of all caps on the PCB!

For the TO-220 package, you'll just have to try them, if they fit they should work ok - remember that the XPRE is fed with 12VAC, so 100V rectifiers should do. Beware of the space between rectifier holes in the PCB, it must be like 10mm.

I could not find any info on replacing the IC, which is not an opamp, is it?

To complete the modifications, you could also change the resistor that are on the signal path (just follow the copper traces on the back of the PCB).

I wish you a lot of fun with this project, and not too many problems!
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Old 26th October 2005, 08:54 AM   #5
Luke is offline Luke  New Zealand
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Ive got one in storage and think its quite ordinary. Does anyone have a schematic for this unit? I think it uses voltage doublers to get workable volts for the tubes. Remember it takes 12V AC in. I would expect a proper power supply would sort it out.
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Old 26th October 2005, 11:11 AM   #6
pjpoes is offline pjpoes  United States
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I put the part number into Google for the IC and what I got back was a quad opamp. It seemed to be a decent one from what I could gather, but none the less, a quad opamp. If its not, then I dont know what it is I found.

I will agree that this device does not use a very advanced cuircit or even a great design compared with what I have seen before, but it, I believe, is a Single Ended Class A preamp. I believe it uses an opamp for the first stage of amplification and the tubes for like a buffer afterwords. This was based one something I was told once from someone looking at it. I dont have a schematic and am yet to find one.

Now for this coding business, I appreciate the help on that, I wondered if that was so, but wasn't sure. Does that mean the ceramic disc capacitors are labeled 10 are 10pf, and the ones labeled 104z are ?, and the one labeled 221 is 221pf. The one labeled 220j, the polystryrene type, are those 220,000pf?, doesnt that seem high for a polystyrene.

Oh I almost forgot, I have an actual X-PSU. Currently the X-psu and the X-lp are my phonostage. I bought the whole set as a demo set when I got it. I retired the preamp basicly because it didn't have enough inputs for me at the time, and seemed cheaply made. I came across a deal on a Perreaux SM6C, and I have been using that since. Though I also now have a Bosoz preamp almost finished, though it has no volume control at the moment, just two input controls, as I am still waiting on the 23 position switch to make the volume control from.
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Old 26th October 2005, 11:39 AM   #7
kepa1 is offline kepa1  France
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Getting the schematics would really help to identify the weaknesses of the circuit, that's for sure. I do have the schematics of the X-10D which is an input buffer and must use the same PSU (?), I'll have to post it later.

For the capacitors code (correct me If I'm wrong) the first two digits give the reference number and the third one give you the number of 0 to add.
For instance, 224 gives you 220 000pF, that is 0,22F. But I'm not sure how to read 10, is it 10pf or 1pf?
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Old 27th October 2005, 12:49 AM   #8
pjpoes is offline pjpoes  United States
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I agree, and I will work harder at it, but at the moment, I dont know how to get the schematics. I asked Musical Fidelity for them, and never recieved a response.

I haven't listened to this preamp for some time, but what I recall is it had a warmth and sweetness to it that was addicting. I felt that it had too many deficiencies to use it over the other preamp I use now. However, I also remember feeling a loss when I went to it.

You know what, I should open up my Perreaux preamp and see what its insides look like. I believe I looked at it once before, and it had some pretty good parts, what we would probably qualify as the best of the lower priced stuff. I think it had Elna caps, Wima caps, toroidal transformer, no opamps, etc. I dont recall seeing any super high end caps, and the resistors looked like fairly normal ones, it didnt have halco's, Vishays, or Caddock's as I recall. The Volume control is an electronically controlled discrete latter type, I think, and never worked very well. Oh and it uses relays for switching, I wonder if upgrading those would improve sound.
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Old 27th October 2005, 01:06 AM   #9
Luke is offline Luke  New Zealand
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Perreaux has allways had exceptional build quality and used quality parts.
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Old 30th October 2005, 03:09 PM   #10
kepa1 is offline kepa1  France
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...Unlike Musical Fidelity who apparently drop cheap Jamicon caps and 1n4007 rectifiers in all their products!

By the way here are the schematics of the XCAN and X10D. I've been told that the XPRE has the PSU of the XCAN and the "signal" section of the X10D...

Pjpoes, have you gone any further with this proect?
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File Type: gif mf_x10d schematics.gif (34.6 KB, 1733 views)
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