Help a total beginner with sweet whispers anatomy

[Bribase]

Hey guys

I built an extended foreplay and it has been providing me with ethereal sound for a few months now. The stupid thing is I have almost no experience in how it actually works, I'm a living testament to how easy to put together these kits are! But I've got to admit I have a very silly question for you.

I'm changing my sweet whispers for some Noble 50k precision potentiometers and I'm very confused as to what is input, Output and ground.

Please forgive my ignorance

B

[Grainger49]

There are three connections to a pot or attenuator.  There is a Top, Bottom and Wiper.

To identify these turn the device all the way CCW.  Measure resistance between the three connections on each channel.  Two lugs will be shorted together (wiper and bottom), the other lug (top) will show the resistance of the attenuator.  You have identified the top.  Then move the attenuator to about midpoint and the one with the least resistance to the top is the wiper. 

In a FP III the wiper is fed from the selector switch, the grid is fed from the top and the bottom is grounded.

Does that help?

[Bribase]

Whoa Grainger! Way too complicated for me!

I was looking for answer like, looking at the sweetest whispers unit

what is terminal 11, 1 and the center pin A?

11 must be to ground right?

But I'm not sure about the STP from the center of the selector switch leading to terminal B and 1 on the sweetest whispers. Stupid of me not to know

Thanks for your help

B

[Grainger49]

Ok, I'm looking at the manual.  Yes, on The SW terminal 1 is the ground (bottom), terminal 11 (top) goes to the tube through the 220 ohm resistor and terminal A (wiper) connects to the selector switch via the STP.  The other wire in the STP is grounded. 

You will still need to know the top and bottom of the new attenuator otherwise the volume control might work backward.  That is, you turn it CCW and it gets louder.

[Bribase]

Thanks for bearing with me Grainger! I'll fire up my multimeter and check. Last question, how do you think the nobles will compare to the sweetest whispers?

I really appreciate your help!

B

[Grainger49]

I haven't heard the Nobels.  I have heard that they are good and smooth.

[Paul Joppa]

The Foreplay III Sweetest Whispers connection is unusual, possibly even unique in the market place. That may be a source of confusion, especially if you have tried to read up on volume control connections!

This arrangement causes a 15K resistance seen by the tube's grid, fairly independent of the level control setting. That's because the Whispers itself is equivalent to a 15K potentiometer; the additional 33K series resistor (which makes it a shunt-mode control) brings it up to 33K-48K depending on the setting. Substituting a 50K pot will increase the resistance seen by the grid. I believe this will be OK; the Miller capacitance should lead to about -1dB at 40kHz, or -3dB at 80kHz. But I have not tried it, measured it, or listened to it this way.

For this reason, I have in the past recommended a 10K to 20K pot if the original connections are to be retained. Just want you to be aware that you will be exploring untested ground, however slightly. Please report your findings and add to the experience base for everyone! Don't worry, in the (very unlikely) event that you get some dulling of the sound, the Forum will help make a recovery.

[Bribase]

Paul, you're only instilling fear in me!

Could you make sure that I've identified the top by following grainger's instructions?

The nobles' instructions identify as ground, input and output resistances read:

gr.   Out.   In.
0.     50.    50

and turned half way

17.   50.    31.

So it should be:

bottom. Wipe. Top.

Tell me I should leave well enough alone! There are some nice pots in it for you if you do!

B

[Grainger49]

Paul, you're only instilling fear in me!

Could you make sure that I've identified the top by following Grainger's instructions?

The nobles' instructions identify as ground, input and output resistances read:

gr.   Out.   In.
0.     50.    50

and turned half way

17.   50.    31.

So it should be:

bottom. Wipe. Top.

Tell me I should leave well enough alone! There are some nice pots in it for you if you do!

B

You will need to verify what direction the Noble pots are turned in your first example. Do your Noble pots have 3 or 4 terminals on them?

Three terminal pots almost always have the wiper, the one that moves, in the center.  Reread my post #2.  Look at the shaft of the pot and turn it all the way counter clockwise.  Find the two that are zero ohms, shorted together.  The other one is the top of the pot and it is connected to either Terminal 17 or Terminal 37.  

Turn it 1/2 way between the stops and measure between each of the two that were shorted together to the third.  The one that shows 50k ohms is the one to be connected to ground (either Terminal 16 or Terminal 40).  The other one is the wiper and it connects to the selector switch.

[Bribase]

Got it Grainger

turned half way, the center is the wiper (about 30k). There is a fourth terminal marked as not connected. Thanks for the clarification!

Wish me luck!

B

[Grainger49]

Good luck!

[Bribase]

This is proving a little tougher than I thought! The damn locking nut for the whispers is too big for the nobles! And the little notch that stops the pot from turning is in the wrong place! I'm off to find the right nut and seeing if I can use milliput to build a new notch.

On the plus side of my foreplay laying exposed and helpless on my work desk is that I'm listening through my old preamp (linn pretek) and boy can I tell the difference! I shall persevere!

B

[Grainger49]

The notch can be anywhere as long as you have the space for the pot lugs.  Just set the line on the knob at 7:00.

The Noble pots should have had two nuts.  So you shouldn't need one from the SW.  The worse that would happen is that the shaft stick up too far. 

[Bribase]

If only it could be that easy, The nobles arrived nutless! I'll find my some tomorrow.

B

[Bribase]

Wow! That was one hell of a trial! I walked across london looking for the right nuts. Finally found some at a guitar repair shop. I decided that instead of making a new notch for alignment I stopped the pots twisting with double sided automotive tape.

It's all wired up and playing great but I'm not sure what to think. I'll let you know once I get a proper listen.

B