Foreplay II buzz-need help with snubbers

[manfred99]

I searched other areas for this topic and couldn't find anything, if I am posting in the wrong area, please point me in the right direction.

I recently purchased a very clean, well built example of a Foreplay II preamp with C4S upgrade, works very well, better sonics than my ASL preamp. But it does have a slight upper frequency buzz in both channels that comes through the speakers. I have viewed many posts in the archived Bottlehead forum about this problem and need some help in installing the snubbers which may(or may not)help alleviate the problem. I have changed tubes with several different sets, turned off all electrical devices on the circuit, ran a ground between the Foreplay and the amps(Dodd Audio mono blocks), removed the cable TV line. The hum remains. The hum does go away when preamp is turned off. The hum was not present with the ASL preamp.

Can anyone explain where the HV snubbers go in the preamp, and what parts do I need to at least try this. I do have soldering skills, though I would be better off looking at pics of the installed snubbers rather than try and decipher schematics.

An e-mail with pics would be of great help.
Tom
Portland, Oregon

 

[Grainger49]

Before diving into this, are you certain that the problem arises in the preamp?  If it is a power supply induced noise you can turn the preamp off and before the capacitors discharge the buzz will stop.  That is, as soon as the switch is off the buzz will be gone.  Maybe it will be more apparent if you play some music and hear the buzz go away before the music fades out.

VoltSecond has a site with a lot of information on Legacy products.  This page has the HV Snubber on it, but the page also says filament snubber:

http://www.siteswithstyle.com/VoltSecond/foreplay_filament_snubber/Foreplay_filament_snubber.html

There is not a picture of the snubber on that page.  IIRC, it takes 2 - 5 terminal, terminal strips to construct the circuit.  These are still sold at RS.

 

[manfred99]

When I turn the preamp off, the buzz immediately stops. So where do I find "RS".

Tom
Portland, Oregon

 

[JC]

I expect he is referring to "Radio Shack".  If you look at the underside of your FP Chassis plate, you will see that the parts are connected to 5 position terminal strips.  They have these very same strips at Radio Shack, or something remarkably like them, that they sell in a package of 3 or 4, IIRC.  In the '274-xxxx' series of parts, I believe.  Useful for mounting snubber parts.

If the sound you are hearing is a "buzzy" sound with a fundamental of 120 Hertz, the snubber will put a big dent in it.  If it is more like a hum at 60 Hertz, that will require something else.

Sorry, I don't have a picture for you, but surely someone else does; there had to have been quite a large number of snubbers installed in Foreplay 2's back in the day.  If all else fails, ping forum member Wardsweb; he takes excellent pictures of his work, and I would have to think he made and FP 2 with snubber at some point!

 

[Paul Joppa]

You might consider just replacing the UF4007 rectifier diodes with a pair of Cree Schottky diodes; the 600v ones should be sufficient. Schottkys don't have a reverse recovery spike, not even an ultra-fast one...

The virtue of this is that there is no modification of the circuit or layout to make.

 

[manfred99]

Thanks very much, I'll see if I can find where these diodes are and take it from there.

Tom
Portland, Or.

 

[manfred99]

Anybody have a source in the USA for  UF4007 Schottky 600v diodes?

Thanks,Tom

 

[Len]

Not sure about the US, though I think I saw them at Mouser.

In Canada, partsconnexion has them in different flavors.

http://www.partsconnexion.com/rectifier_diode_cree.html

 

[manfred99]

Thanks so much, I hate to be a pest but what amperage should I use, or does it matter?

Tom

 

[JC]

The UF4007 is a 1 Amp diode, with a reverse Voltage of 1000 Vdc.  As Paul noted, 600 Vdc should be adequate.  Truth be told, you could probably get by with less current, too.

I'm pretty sure the Schottkys will have a different designation.

 

[Len]

Since JC says it replaces a 1 amp diode, the cheaper 1 amp Cree should be fine.

Check with others on this forum whether you need a heatsink. I always heatsink mine, so I never checked the ratings for when a sink is required.

 

[manfred99]

Thanks very much for the help, hopefully this will get rid of the buzz.

Tom
Portland, Or.

 

[Paul Joppa]

Cree CSD01060 is what we use in the Paramount and Eros. 600v, 1A, no heatsink. Less than $1 at DigiKey among others.

 

[manfred99]

The 4 diodes in my Foreplay II have 3 legs, all the Cree diodes I see at the 3 online sources I've found have 2 legs, including the CSD 01060. I may learn to live with this buzz, I may be in over my head on this one.

Tom
Portland, Or.

 

[Len]

I don't have a Foreplay, but by definition a diode should only have an input and an output.

A transistor would have 3 legs. Are you perhaps confusing them?

 

[Paul Joppa]

Sounds like you got a kit preamp without the manual. Never a good idea. Ping Eileen next week when they are back from CES and see about getting a manual so you know the terminal designations and have some pictures of the parts - it will help a lot. Make sure you ask for the C4S upgrade manual as well.

The 3-legged TO-126 black flat-pack transistors are on the four C4S boards. Somewhere further back there is a power supply, with some large round electrolytic capacitors. Scrunched down among the terminal strips are two diodes - black cylinders about 1/2 inch long and 3/32 in diameter, with a wire out each end. Those are the rectifier diodes; they have a white band around one end to indicate the cathode. The Cree diodes are TO-220 flat packs, quite a bit larger; and one of the leads has a wide chunk near the plastic body, indicating the cathode end.

Do be aware that buzz can have many sources, so nothing is a guarantee that it will kill the buzz. Just thought I ought to say that. Very many hum and buzz problems have been siolved on this forum over the years, sometimes on the first try and sometimes only after many experiments and exchanges. It can be frustrating while it happens, but it's a real thrill when you succeed!

 

[manfred99]

The only components that look remotely like the Cree diodes are on these 4 small boards, pic enclosed. I also have enclosed another pic of 4 small components that are connected to a cap. While I was in the preamp, I checked for any cold solder joints, redid any questionable  joints and it still has the buzz.

Where in the preamp are these 4 diodes located?

Thanks, Tom
Portland, Or.

 

[Grainger49]

Tom,

The four diodes are the black cylindrical black components with silver bands around them in the first picture.  (red wires coming in and blue wires going out)  The ones you think are Cree diodes are on the constant current source (C4S) circuit boards.  Those are transistors.  The C4S boards have two transistors, one with black plastic on the outside one with a metal can for the outside.  Both can be seen in your second picture.

 

[manfred99]

Got it. When I put in the new diodes, do they need to go in a specific orientation, i.e.( +)(-), and is the silver band on the current diodes positive or negative?

Thanks for all this help, I do have a lot to learn. Next week I'm replacing the driver boards in my Dynaco Mark III's.

Tom
Portland, Or.

 

[JC]

The silver band on the UF4007 indicates that that end is the Cathode of the diode.  The un-marked end is the Anode.

I'm not sure how this is designated on the Cree package, but you do indeed need to pay careful attention to which lead goes where.