Preamp mods

Most electronics these days use electrolytic capacitors (caps) to couple audio signals to different parts of the circuit. This is a poor use of these devices since they were designed more for power supply filtering (below 1 KHz). But they are cheap and compact, which is why they are used.

The first job is to add 0.01 uF “Film” type caps (the value is not critical, anything near that is fine) across all of the electrolytics that are in the audio section. Any type of film; mylar, polypropylene, polystyrene, etc will do. If power supply caps are not bypassed, you should add either film or ceramic caps there too. At the lower left hand of the picture below, I added a ceramic cap to a power supply electrolytic.

 

 

 

 

 

 

 

 

 

 

 

Here is a close-up view. This shows the leads of the film caps are cut and bent over the solder pad and leads of the electrolytic caps. I like to add solder to the lead of the film cap and the hold the film cap in place while placing the solder iron (with some solder on it) on the joint until it all flows together. The solder should support the weight of the film cap with no problem. If you can wiggle the film cap and break the connection, try again.

 

 

 

 

 

 

 

 

 

 

 

The VP29 requires a 220 pF capacitor across the input to keep a flat, balanced response using normal inexpensive cables. The picture below shows ceramic surface mount cap across each input. The ground connector relies on making good connection through the case and a standoff to the circuit board. It wasn’t working real well, so I added a wire directly between the connector and board…. No noise at all now.

 

 

Some of you may wonder why I add a film cap across an electrolytic, rather than change the electrolytic out. First, I’m cheap. Second, it really isn’t necessary. Bypassing works because the loss of electrolytic capacitors is significantly higher at high frequencies than film caps. The film cap ends up “shorting out” the electrolytic in this range, so the electrolytic contributes very little to the sound. This next picture shows regular leaded ceramic caps instead of the surface mount version. Either way, it works the same.

 

 

 

 

 

 

 

 

 

 

 

 

Parts List:                                                                           Quantity:

0.01 uF Film type capacitor, 15 Volt or more                      6 (7 if you add one to C2, top pic)

220 pF ceramic capacitor, 15 Volt or more                         2

and a couple of inches of any kind of hook-up wire.

A cheap place to buy is Jameco.com; here are their part numbers:

920464, .01uF 100V PLYE capacitor; $0.15 each

15413, 220 pF 50V ceramic capacitor; (10 minimum) $0.09 each

You can’t go wrong with a modified VP29. To get anything better, you would have to spend a few hundred or more. I use this with a tube amp and ribbon speakers, and I’m pretty happy with it!

 


Comments

Preamp mods — 11 Comments

  1. I have one of these also. Does yours have a 4560 OpAmp? I see it is missing C14, C15. Your C7, C9 is different than mine. I plan to replace the audio path caps with better, once I find them, and also change R4, R5 to 62k for my Shure cartridges. These are a great deal for the money.

  2. Jeff,
    Yes, it is a 4560. I did notice that there was a C16 in the schematic, but no C14, C15. They must have decided to delete this at some point…. Do you know what they were used for? I am not too surprised that C7 and C9 may look different during different production runs. There are so many different kinds of film caps and packages, it likely depends on what they can get a good deal on for each production run.
    I have heard that the M97 is a little shy in the highs, but since many modern speakers seem to favor the highs, that may be a good way to get a more full balanced sound. My M92 with 104E stylus seems to have pretty well balanced and clean highs now that it has had plenty of break-in time. And with solid bass and a smooth natural midrange, Shure is making a believer out of me! But it is fun and educational to experiment.

  3. Does it matter what kind of film capacitors are used? I have a good source for box type polyester caps. Will those do the job?

    • It does not really matter much what type of film or type of package. I have used the box type with good results. Even old stock surplus film caps are usually good, unlike some other types. Enjoy!

  4. What an awesome mod you came up with. I have a few questions concerning the caps. Does the voltage matter? Do I have to unsolder the legs of the electrolytics and fit the caps and the electrolytics through the board? Maybe you can come up with a shopping list for me? Also, I’m not sure if I understand the 220 pF connection on the back of the board. Any clarification for me would be more appreciated. Thanks!

    • Thank you Aaron,

      The voltage just needs to be higher than the power supply voltage; 15 Volts or more. Don’t unsolder any of the existing caps, just lay the leads of the film caps next to the leads of the electrolytics and solder. The solder will support the film cap. If you wiggle the film cap and it breaks off at the solder joint, it was not a good solder joint.

      Soon, I will add a picture showing leaded 220 pF caps at the input, which should clarify things, and a parts list.

  5. Kent,

    Although I built my first amp and tuner back in 1962 at age 16, I had not tried anything similar since until this mod. I wanted the Rolls VP29 to do better than an ART DJ Pre ii I had (it had a rather harsh attitude) if for no other reason than it was built a few miles away in Murray, UT. So I splurged a whopping $2.00 at my local electronic parts place and did your mod. It sounds excellent. Thank you so much for sharing this modification.

    Richard Gress
    Salt Lake City, UT

  6. Very nice write up. Thank you so much for taking the time to share your knowledge.

    I ordered my parts from Jameco today. I am looking forward to this project and will post my results when finished.

    • I completed these Mods to my rolls vp29, lastnight, with parts ordered from jameco. The .01uf film capacitors look exactly like the ones used in your write up. However, the 220pf capacitors I recieved were light green discs. I used them across the inputs and completed the rest of the mods, including the ground wire. When the moment of truth arrived, I put needle to vinyl on a few familiar albums. I noticed high frequency voice melodies sounded distorted (I am not an audiophile, please excuse my layman’s terms). After a few albums of listening and deciding that the rest of the audio range had noticeably improved, I concluded that I could not live with the high frequency vocal distortion. I decided to remove the input capacitors and that fixed the distortion. Now the vocals are warm and almost enveloping. Bass is improved in my opinion, sounding tighter and more immediate (drums that used to sound like a “thump” to me, now sound like a more crisp and natural “bump”.

      I am very happy with the sound now and would recommend this mod to anyone with a soldering iron and a Rolls VP29.

      I would appreciate any comments on the 220pf input capacitors. My turntable is a Pioneer PL-12D and my Cartridge is an AT95E mounted on an Ortophon Headshell with 2.0 grams of vertical tracking force.

      Thanks again for sharing your knowledge and help.

      • It is hard to say what the issue was, it could be that it was too much capacitance on the cartridge and cable combination, or the 220 caps themselves. Quality can vary on ceramic caps, and they can have a piezo-electric or microphonic effect. You did a good job on finding and correcting the problem. Other readers may want to keep your example in mind. I would prefer to use film caps there rather than ceramic, but they are hard to find in such small values. Of course, if the highs sound good without them, go with it! Many thanks for your comments!

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