Volume Pot Advice Please

[yungin's-dad]

Hey all,

I recently received a Bottlehead Foreplay bastardized between an FP II and III, so I guess I'll call it a Foreplay 2.5. My question is: What would y'all suggest as good replacement of the so called Sweet Whispers stepped attenuator? As of now it has a (what I believe to be), home made 12 step attenuator. The tech that built it did a great job, but the 12 steps is not quite what I'm looking for. I would like a little more adjust-ability. Right now it seems to be either too low in volume or too loud. I love the sound of the pre, but wish I could have a little more control over the volume level, but I would like to keep the sound quality where it is or even improve upon it. I am thinking of replacing it with an Alps, or what ever. I will attempt to this myself, I am pretty decent with a soldering iron, but not so good at reading a schematic. You guys think this is something a beginner like myself can accomplish?

Please give me your thoughts, suggestions, and opinions.

Thanks,

Ron

 

[yungin's-dad]

Here's the best photo I have of right now.

 

[Grainger49]

This is what was known as the FP Hair Trigger Problem. The hair trigger can be tamed by the insertion of a resistor in each channel. If you have different volumes from phono and CD then you can tweak each input separately.

For an overall volume reduction place a resistor of the same value as the attenuator (measure from end to end to get the value) between the selector switch and the attenuator. This will cut the volume in half. The lower steps will be much smaller, you will use more steps to get to a volume that usable. Doubling or tripling the value might be necessary.

The output of the selector switch is the two lugs nearest the center of the SS. There is Teflon wrapped wires coming from the SS to the attenuator.

Looking at the attenuator it is a simple ladder type so the series resistor will work fine.

This will give you the opportunity to find the right value with a cheap assortment of 1/8W resistors from RS. Then, since it is in the signal path buy some $4 resistors.

If you are interested in tweaking each input put the wires at the selector switch input lugs. Wrap one leg of the resistor onto the lug and attach the red wire to the other end of the resistor.

I have padded my CD input and my phono input is running straight into my FP 2. My amps have less gain than yours does so I need all the gain for my phono stage.

 

[gadget73]

Would there be any reason to not replace the stepper with a conventional audio taper pot of good quality though?

 

[sebrof]

Check out the New stepped attenuator=happiness thread.

I think I can post a link: http://www.audiokarma.org/forums/showthread.php?t=315472

Yes, you can replace the pot yourself, pretty easy to do.

 

[shelly_d]

....

For an overall volume reduction place a resistor of the same value as the attenuator (measure from end to end to get the value) between the selector switch and the attenuator. This will cut the volume in half. The lower steps will be much smaller, you will use more steps to get to a volume that usable. Doubling or tripling the value might be necessary.
...

Not in half. Volume in this case is reduced by 3dB (1/2 the power into the input stage of the preamp.) This is only a small but noticeable reduction in volume. Over 100 years ago Alexander Graham Bell (for whom we have named the Bell unit of sound pressure and use 1/10 of that - the deciBell) discovered that to half or double the perceived volume of a sound the sound pressure must raise or lower 10 dB.

Shelly_D

 

[shelly_d]

Yungin's-dad:

Here is how to wire in a new volume control.

You should find 2 sets of wires coming off the existing volume control. Each has 3 wires. One wire goes to the input jack (probably), one to ground and one to the input of the first tube. With a replacement pot, you will find either 3 or 4 terminals. If it has 4, one of them will be grouped apart from the other 3 and is intended to connect to a loudness contour circuit. If your preamp does not have such a circuit, just leave it un-connected. There will then be 3 terminals grouped together. The center terminal is the wiper and that is the one that goes to the first tube. On the other two, one will go to ground and the other to the source. If you get them backward you will have a volume control that works but turns backward from every other volume control in the free world so here is what you do. Rotate the control full right (max volume). Measure the resistance between the center terminal and both outside terminals. The outer terminal that measures very low is the one to wire to the input. The one that measures very high is the one you wire to ground.

Repeat this for the other channel as well.

Hope this helps.

Shelly_D

 

[yungin's-dad]

Thanks for all the help!! It's going to be a couple of weeks before I can get to it, but I really appreciate the input.

Grainger and Shelly, you two always help me out! Thanks for the help with this and my amps!!

Ron

 

[Grainger49]

Would there be any reason to not replace the stepper with a conventional audio taper pot of good quality though?

Yes, the FP 2 has a large amount of gain. Regardless of the resistance of the volume device, from say 10k to 500k, the device only "picks" a percentage of the input voltage and sends that to the grid of the VA stage. So unless the stepped attenuator is assembled wrong the key here is percentage. The current doesn't matter other than not loading down a source too much.

 

[Grainger49]

Yungin's-dad:

Here is how to wire in a new volume control.

You should find 2 sets of wires coming off the existing volume control. Each has 3 wires. One wire goes to the input jack (probably), one to ground and one to the input of the first tube.

With a replacement pot, you will find either 3 or 4 terminals. If it has 4, one of them will be grouped apart from the other 3 and is intended to connect to a loudness contour circuit. If your preamp does not have such a circuit, just leave it unconnected. There will then be 3 terminals grouped together. The center terminal is the wiper and that is the one that goes to the first tube. On the other two, one will go to ground and the other to the source. If you get them backward you will have a volume control that works but turns backward from every other volume control in the free world so here is what you do. Rotate the control full right (max volume). Measure the resistance between the center terminal and both outside terminals. The outer terminal that measures very low is the one to wire to the input. The one that measures very high is the one you wire to ground.

Repeat this for the other channel as well.

Hope this helps.

Shelly_D

Shelly,

Hi, I'm an Electrical Engineer. Doubling the resistance will cut the input voltage in half.

You will see in my post that I own a FP 2. And that the signal comes from the inputs to the selector switch to the volume device to the first tube stage.

There are no loudness nor tone controls on the FP.

 

[shelly_d]

Shelly,

Hi, I'm an Electrical Engineer. Doubling the resistance will cut the input voltage in half.

Grainger49

That is absolutely true. I am not arguing that point with you. However when you cut the voltage in half you reduce the signal by 6 dB. [ 20 * log (v1/v2) ] It takes a 10 dB change to double or half the volume. Volume is based on perception not power output. Double the power to a set of speakers and the volume increases by 3 dB not double. Double the voltage and the power goes up by the square of the change - 4 times. That's a 6 dB change.

It's a non linear scale that people not in the business of using dB on a regular basis sometimes miss. I dealt with these puppies at one phone company or another for 26 years.

Shelly_D

 

[gadget73]

Yes, the FP 2 has a large amount of gain. Regardless of the resistance of the volume device, from say 10k to 500k, the device only "picks" a percentage of the input voltage and sends that to the grid of the VA stage. So unless the stepped attenuator is assembled wrong the key here is percentage. The current doesn't matter other than not loading down a source too much.

not quite sure I follow here. Does a standard potentiometer not "pick" a percentage of the input voltage provided its not set all the way up? If the attenuator thing is set up to always have a minimum resistance, you could just add that fixed resistor in series with the pot. I've never used an FP, but in general I favor fully variable controls over stepped controls. If it takes a little re-curving of the pot via some fixed resistors to get desired behavior, thats easy enough.

 

[Grainger49]

not quite sure I follow here. Does a standard potentiometer not "pick" a percentage of the input voltage provided its not set all the way up? If the attenuator thing is set up to always have a minimum resistance, you could just add that fixed resistor in series with the pot. I've never used an FP, but in general I favor fully variable controls over stepped controls. If it takes a little re-curving of the pot via some fixed resistors to get desired behavior, that's easy enough.

Yes, that is right but changing the pot/attenuator by itself, as you suggest in post #4 above, doesn't change the range of the volume pot/attenuator. As you say you need a series resistor.

If the current pot/attenuator is 100k ohms then with the series resistor needed the input impedance will probably get high enough that it might cause noise. This is the reason I didn't suggest putting a Meg ohm or even a half Meg ohm resistor in series in post #3 above. A lower resistance pot/attenuator, say a 20k ohm might be a better substitute. There are a number of attenuators available out there.

 

[gadget73]

I was under the impression that it was either a too loud or too soft complaint, which would tell me the "steps" in the attenuator are too course. I didn't really see where the issue was the total range, but the granularity of the control. My thoughts were to simply determine the total end to end resistance of the attenuator array and replace it with a pot of same value, which shouldn't mess with the impedance but give fully variable control. You'd still have the same total gain obviously, but unless its too loud at say one or two clicks, then it shouldn't be a big problem.

 

[Grainger49]

Gadget,

There are 3 volume/attenuator threads I am participating in right now on 2 boards. So I might be a bit confused.

Yes, a standard pot will have greater resolution. It will also only have a limited range without a series resistor. But as I said above, this is the classic hair trigger problem which is best solved with the addition of a series resistor. I'm a frequent poster on the BHBB, makers of the preamp kit. Stepped attenuators are a nice thing to have but need to be tuned to the device. The series resistor is cheaper and easier to install. If he uses 4 steps now the right resistor will give him 8 or 10 steps for the same volume range. That will give higher resolution on the low end and only get loud near the end of the existing 12 step range.

Being able to tweak each input is one of the best features here. On my Foreplay 2 I don't need to shed any gain for Phono in my system since I changed driver tubes in my power amps (lower gain, I need all the Foreplay Gain now). But to get the CD to match I have a series resistor on both of those inputs. Nothing in my tuner or SACD inputs yet.