Pass F5 Help

[MWalt]

I bought a Pass F5 clone second hand and have a couple questions. Although it sounds great, the toroidal transformer buzzes. It isn't audible through the speakers, but it does drive me nuts. I sent pics to Ak'er Marantzfan and he pointed out a lack of a ground and advised me to check DC offset. The numbers were:

Left channel: .119v
Right channel: .063v

Hopefully, I did that check correctly. Can anyone tell me how to correct the problems and add a ground. Here are pics under the hood:

 

[mr_RBG]

Normal offset is =< 50mV. First you should remove all these huge wire loops. You have lot of space, so do some wire work. AC -- twisted DC straight. Make a ground star on the PSU cap board. AC input wires twist and put some where far from "sound" part, especially input. You can post photos so we can help further.

 

[westend]

The offset and bias are set using the variable resistors, the process is documented here: http://www.firstwatt.com/pdf/prod_f5_man.pdf Look under "Initial Adjustment".

Sometimes, just shifting the toroid and retightening will be all that is necessary. If you can get something under the toroid, that might help, as well.

I would suggest that you install a bolt through either the back of the chassis or through the bottom and use a star-ground topology.

 

[MWalt]

Normal offset is =< 50mV. First you should remove all these huge wire loops. You have lot of space, so do some wire work. AC -- twisted DC straight. Make a ground star on the PSU cap board. AC input wires twist and put some where far from "sound" part, especially input. You can post photos so we can help further.

I am an extreme rookie here. So, I need to be spoon fed this stuff :yes:
Are the AC wires I need to twist, the ones from the transformer? Also, how do you make a ground star? I also worry about stored current even after I shut off the amp and disconnect the PC. Does it just require a waiting period after the amp is shut down to drain the PSU caps?

The offset and bias are set using the variable resistors, the process is documented here: http://www.firstwatt.com/pdf/prod_f5_man.pdf Look under "Initial Adjustment".

Sometimes, just shifting the toroid and retightening will be all that is necessary. If you can get something under the toroid, that might help, as well.

I would suggest that you install a bolt through either the back of the chassis or through the bottom and use a star-ground topology.

What should I put under the torroid?

I appreciate the help guys!

 

[mr_RBG]

Here how I would do it. Sorry did not have time for a good drawing. Dark dirty spot(no eraser near) is explained on the right. Look into F5 article above: connect middle pin from AC inlet -- earth to chassis and use CL-60 thermistor to connect it to your ground star.

 

[Russellc]

For some really clear pics, also try DIYaudio.com and click on the Pass forum. Then search in the search box for member 6L6. Click on any of his posts. In his signature, click on hi F-5 project. Scroll down. He has VERY clear pics of how things should be wired.


here you go:http://www.diyaudio.com/forums/pass-labs/188691-illustrated-guide-building-f5.html

Russellc

 

[marantzfan]

What should I put under the torroid?

I appreciate the help guys!

Hey there.

Usually when you buy a transformer it comes with rubber disc(s) that you can use to mount the transformer so that it isn't metal to metal on the chassis. If you have one laying around, I would think you could use an old TT platter mat and trim it to fit. Not the prettiest solution but it should work.


As for the star ground, all it need be is a hole drilled and tapped in the bottom of the chassis with a screw and lock washer. I actually used one of the feet of my F5 so that I wouldn't have to drill and tap anymore holes.

 

[marantzfan]

BTW, what's up with the elctrical tape next to the power switch? Is that an original "Rawson Mod"?

 

[MWalt]

Thanks for all the help and fast response, guys. I'll try to get this figured out.
I am assuming I have no need to worry about any life threatening voltage once I disconnect the powwer and let it sit for 30 minutes, or is there something I have to do to drain the power caps?

 

[westend]

Thanks for all the help and fast response, guys. I'll try to get this figured out.
I am assuming I have no need to worry about any life threatening voltage once I disconnect the powwer and let it sit for 30 minutes, or is there something I have to do to drain the power caps?

Nah, don't worry about the caps, there should be a resistor on the PS board that dissipates the current from the filter caps. That is the original Pass F5 design and is on all of the licensed PSU boards, AFAIK. I wasn't going to say anything about the wiring but...whoever wired it up didn't really care about detail. There looks to be nothing fundamentally wrong (the loops are livable) but a straightening out of a few wires and some connections would be a better deal. One thing that you should have (don't know if it's there) is a chassis ground to the AC power inlet. That is a primary safety issue. As far as "star ground topology", do a search. Many have written and describe what it is better than I can.

The Westend Labs way to star ground is to drill a 9/32" hole in the bottom panel, close to the power inlet. Insert a 1 1/4" 1/4-20 bolt from the bottom. Onto the bolt is installed a serrated edge lock washer and a nut. WL then runs all of the grounds, starting with the power inlet ground, to this bolt. Wires are terminated with the appropriate crimped ring connectors. The terminated wires are held fast onto the bolt with a flat washer, a lock washer, a flat washer, and a nut. I usually don't mess around with caps or thermistors on ground leads because I haven't found the need but that method can also work. Good luck on fixing up the F5!

 

[marantzfan]

Nah, don't worry about the caps, there should be a resistor on the PS board that dissipates the current from the filter caps. That is the original Pass F5 design and is on all of the licensed PSU boards, AFAIK. I wasn't going to say anything about the wiring but...whoever wired it up didn't really care about detail. There looks to be nothing fundamentally wrong (the loops are livable) but a straightening out of a few wires and some connections would be a better deal. One thing that you should have (don't know if it's there) is a chassis ground to the AC power inlet. That is a primary safety issue. As far as "star ground topology", do a search. Many have written and describe what it is better than I can.

The Westend Labs way to star ground is to drill a 9/32" hole in the bottom panel, close to the power inlet. Insert a 1 1/4" 1/4-20 bolt from the bottom. Onto the bolt is installed a serrated edge lock washer and a nut. WL then runs all of the grounds, starting with the power inlet ground, to this bolt. Wires are terminated with the appropriate crimped ring connectors. The terminated wires are held fast onto the bolt with a flat washer, a lock washer, a flat washer, and a nut. I usually don't mess around with caps or thermistors on ground leads because I haven't found the need but that method can also work. Good luck on fixing up the F5!

Hey Lance, this is a Rawson build. So far I'm not impressed with his workmanship.

 

[westend]

Hey Lance, this is a Rawson build. So far I'm not impressed with his workmanship.

Had to look up Rawson. I guess the guy turns out a few amps. The OP's F5 is just as ugly as one of mine, I guess, lol.

Besides the excess wire length, the location of the outputs puzzled me, why not in the middle of the sink? I saw another picture of the sinks in another of his builds and it looks like they are predrilled. He probably has to adhere to those hole spacings. One more thing that bothers me (and then the OP can return to enjoying his amp,lol) is the big holes in the bottom plate. Westend Labs would have drilled more and smaller if our highly accredited engineering staff thought that the additional airflow was necesary. Actually, I may give that a try if I build anymore with the smaller HeatsinkUSA sinks. Using the M&M heatsinks, the interior temps are so close to ambient that holes in the bottom would only be a roach door.

 

[MWalt]

My amp is UGLY! The good news is that I bought the amp used for less than it would cost to build a new one. But it does sound very nice and for that reason it is worth keeping and possibly improving. Now if one of you experts would allow me to send you this amp, you could do your magic to it and I would be glad to send you some coin for the effort. I really don't have the electrical knowledge to screw around with this. Hell, I had to google "CL-60 thermistor" to figure out what that was.

 

[westend]

Yeah, the F5 is a great amp. If you're hesitant about messing with it, don't. To fix the main problem, the transformer buzz, just loosen the nut on the top and rotate the tranny just a little and retighten. That might be all that is necessary. If it still buzzes, it may be necessary to remove the bolt and add a rubber pad as Anthony suggested. If you can't find any rubber (how could you not find rubber in Ohio? ), I'll send you a piece of EPDM rubber with a hole in it. PM me.

Maybe someone close to you could pay a visit and adjust the bias and offset or you could look at one of the F5 build threads and see what needs to be done. Specialidiot's build thread has some pictures, IIRC. He even invented the Fantabulous Ergonomic Resistor Deployment (FERD). That is: bending one leg of the resistors up so that meter leads are easily attached. Before FERD, we were all at a disadvantage. :thmbsp:

 

[TLCW]

The explanation and cue may be obtained from here:
http://www.ehow.com/how_7819729_eliminate-buzzing-electrical-transformers.html
http://forum.allaboutcircuits.com/showthread.php?t=36298

 

[chicks]

Assume you mean a mechanical buzzing from the transformer? Make sure the rubber washers are in place on top and bottom, and that the wingnut is snug.

 

[ashok]

Good advice in the posts above. The toroid will have some kind of rubber sheath underneath it to prevent magnetic shorting with the metal chassis. Repositioning and tightening the transformer, or perhaps inserting a slightly thicker rubber sheet might be all that is needed.

The other issue that I have read about is the presence of distortions or even dc offsets in the 120V ac input to the toroid. Electronic ballasts, hair dryers and switching supplies can inject asymmetry into the ac input voltage, and it is no longer symmetrical about the horizontal axis. This asymmetry and hash can cause the toroid to buzz.

Rod Elliott has a nice article on this subject: Transformers and dc, and you can find several other discussions on Audiocircle and Diyaudio.

The usual solution (also described in Rod Elliott's article) is to add back-to-back electrolytic capacitors in the ac input in an effort to block the dc offset.

Does this hum/buzz get worse or improve depending on time of day? That could help confirm that it is indeed variations in the ac input that is causing the problem.

 

[MWalt]

Does this hum/buzz get worse or improve depending on time of day? That could help confirm that it is indeed variations in the ac input that is causing the problem.

The buzzing gets worse as it warms up. It doesn't transmit through the speakers, but it irritates me. I appreciate the help and will try take that torroid apart and try and make an adjustment on it.

 

[marantzfan]

You probably won't need to disconnect any of the leads coming from the transformer to do this as it looks like there is enough slack to unbolt it and tip it on its side to install the rubber sheeting/washer and then set it back down.

As for adjusting the DC offset and bias, all you will need is a couple of meteres (3 would be better) with aligator clamps and a small flat head screwdriver to adjust the trimpots. Just make sure that you turn them VERY slowly. Also, it'll take a little back and forth with both trimpots on each board to get it zeroed in.

Once you get those two tasks completed we can walk you through adding a ground. :thmbsp:

 

[MWalt]

I can assume that the trim pots are the small blue boxes on the circuit boards. Am I correct?

At first, I felt that the whole thing was daunting, but the more advice I receive and the more I read, the more condidence I am getting that I can probably do this myself. You guys are a wealth of knowledge and I truly appreciate the assistance.