Loud popping noise with Sweeter whispers

[Rollie]

Hello all,

I hadn't done anything with my foreplay 2 that i built in 2001, I used it stock with the c4s upgrade. I just recently got a Dynaco 70 amp and the gain on the Foreplay was too great. I just installed the Sweetest Whispers. Now my problem is that one channel has a LOUD pop/scratchy sound between clicks. I first installed the shunt attenuator pad 33 ohm for 10db and it was still to loud. I then went to the other extreme putting the 360 ohm 28db pad. It was after I made the change that the LOUD snap/ crack started. I went back removed the resistor and tried replacing it with the a 180ohm still a LOUD pop/crack between clicks. If I press the Knob down hard the click is not as loud and it isn't always there. I went through all the joints reheated then, took out the pad resistor reinstalled the 360 ohm and retried, still the LOUD noise between clicks. 

While I was assembling the attenuators I would ohm out each step to make sure I did not make a mistake, each one matched what it should be and the total resistance was 15kilo ohms. It still shows 15kohms in the foreplay.

I'm wondering if when I re soldered the 360 ohm resistor I didn't fry something in the attenuator, or fill it up with flux. I checked all my ground points they appear okay. I again reheated all the joints including the ones on the attenuator. I loosened the nut holding the attenuator and now it sounds worse.

I took the whole bad attenuator out cleaned it out using rubbing alcohol and a can of computer compressed air let it dry, I checked the attenuator while it was off and the ohms matched what they should be, i then re soldered everything and it is still making the Loud pop. Also sometimes while I change the level it will go real loud and I have to tap it before it goes to the proper level.

I reviewed postings on the old board , checked the grounds. I'm still at a loss. I would appreciate any guidance on how to troubleshoot this.

P.S. The other channel sounds great!

Thanks for any help

 

[Paul Joppa]

Here's one thing that may help - connect the Sweetest Whispers as they are connected in the Foreplay III. That just means to connect the tube grid (A2 or B2) to switch terminal 11 instead of the "A" tab in the center of the switch. (Leave the wire from the selector switch to the "A" tab unchanged.)

When the switch makes a momentary bad connection, in normal shunt mode the full input signal is connected to the tube through the 360K resistor, which provides very little attenuation - that's why the "pop" is so loud. With this change, a momentary bad connection disconnects the input from the tube completely, resulting in a much quieter "pop" - often inaudible.

Of course this does not address the momentary bad connection problem. No switch is perfect, and unfortunately inexpensive switches are quite imperfect. We think this switch, which is more enclosed than the one used in the old Sweet Whispers, is at least better protected from dust and likely to last longer, but sadly even quite expensive switches have these glitches occasionally.

Incidentally, I would recommend getting some spray contact cleaner and avoiding the use of rubbing alcohol, which usually has a bit of glycerin dissolved in it and can leave a residue. It won't solve mechanical problems, but it will prolong the time before the contacts get gummed up.

One final point, the Stereo 70 is quite sensitive, and would benefit from an input attenuator so the preamp can operate at a higher signal level. There are a few posts on this forum about it, and on the Bottlehead Community page you can read my white paper on signals and noise if you are interested.

 

[Rollie]

Thanks Paul.

I did use contact cleaner in addition to the isoproyl alcohol, After using it I blew it out with canned air compressed computer cleaner. I am aware of using the input attenuator. You helped me with this previously. (thanks) I was going to do that later after the sweetest whispers. The good sweetest whispers switch is dead quiet and works well. I am tempted to just get another set of switches and hope that I get/make a clean switch to replace the popping one. Is there anything in the wiring other than the switch that could cause my problem? It seems pretty straight forward there are only five or six joints and I have re soldered and rechecked them a number of times.

 

[Grainger49]

I think I remember this problem cropped up quite a while back.  The culprit is the switch.  Didn't someone suggest a resistor in parallel with the whole thing, high value, that took over when the switch opened?  That is, the problem is the switch doesn't make before break.  So when it opens, like a break before make switch, the high value resistor keeps the volume from going wide open.  And maybe the high value resistor is between the input and the wiper.

I don't remember clearly, I've slept since then.

Somebody help us out here!

 

[Rollie]

THanks all,

I did as Paul recommended. I removed the wire from the A2 to the A on the attenuator and attached it to terminal 11 on the attenuator and it got rid of the loud pop scratch etc.

Having done this I have a few of questions:

1) Did I do something wrong to cause this problem? If so what.

2) Is this a permanent solution? Should I replace the attenuator?

3) Is there any advantage to having the Wire from the A2/B2 on the tube to the A spot on the attenuator?

Thanks I'm just trying to learn and understand.

 

[Grainger49]

1) No, the problem was reported on the old board.  Possibly it was limited to early SW, how long have you had them?

2) Yes, this should work for a long time.

3) Your current configuration presents a changing impedance, resistance, to the source as volume is changed.  The series resistor is the minimum impedance.  That should be no problem to your sources.

 

[Rollie]

Thanks Grainger49,

The sweetest whispers were just purchased. I still don't know what I did wrong,but never mind, it sounds good. My next project will be to put additional resistance in the input on the Dynaco 70 as Paul advised.  I will put that off for a couple of weeks.

 

[Grainger49]

Rollie,

You did nothing wrong.  The SW  switches are good but inexpensive.  Hey, price some Goldpoints!  Only a few of the switches have this problem.  After assembly no one wants to assemble a new one.  That is a real PITA and the solution is easy.

Yes, the attenuators are a good idea for the ST-70.  

One more question, on your Dynaco ST-70 have you tried shorting out the 10 ohm resistors that "float" the input RCA ground from the circuit common?  I have found that it injects hum into the system if it isn't shorted.  Try jumpers first.  It works on most systems.

 

[Paul Joppa]

The fix is permanent. This arrangement is standard on the current Foreplay III. The only reason the Sweetest Whispers instructions do not mention it is that it's a change from the older Foreplay I and II, and we feared anything but an exact replacement would confuse more customers than it would help. (Incidentally, this arrangement is NOT suitable for a stand-alone passive preamp, only for something with a direct connection to an adjacent tube grid.)

For initial problems like this, I know of three possible causes:

1) it's a crappy switch, or

2) some solder flux got into the switching mechanism during soldering and is interfering with the contacts, or

3) some mechanical pulling or twisting on the solder lugs resulted in bending of the internal parts, just enough to make the contact unreliable.

While the latter two are potentially avoidable if you are careful enough, I would not say any of these causes is your fault. If you want to replace the switch, just contact Eileen. It's the same for things like the power switch (easily damaged by excess heat during soldering) or bad tubes (a few show up now and then, and we don't test every one).

 

[Rollie]

Thanks again Paul and Grainger49

If the patch/fix on the foreplay cause no problems I consider it a good thing and will take no further action.

Grainger49  I did try putting a jumper across the 10 ohm resistors, there were two options the 10 ohms on the top of the board on the Dynaco and the two 10 ohms underneath attached to points 8 and 10 on the Dynaco board, I tried both options with no apparent change. The hum is really minimal.

Paul You advised that I put an additional 470 ohm resistor in series with the existing 470ohm attached to the inputs on the Dynaco. My understanding is that I then will basically split the difference by putting the wire to the tube centered between the two 470 ohm resistors. I assume after I pad the Dynaco input I may have to go back and change out the 360 ohm resistor on the attenuators. I will cross that bridge when I get there. Right now I am again listening to my system and enjoying it.