Different Switch Throw Number?

[Quiet_Storm]

It seems like my two volume knobs have a different throw number. The left channel volume has 10 positions while the right channel has 11 positions. Any idea why the two switches are different?

Overall I'm very pleased with my first Bottlehead experience. Everything else seems to be working fine. I even got the Foreplay III plus upgrade to work in a single build. Well my heater voltage bias was a little off the first time due to a bad solder joint on a resistor, but that was easily corrected.

[porcupunctis]

Are these Sweetest Whispers?  There is a little metal piece that works as a stop-lock.  I'm sure that someone will have a better term for it.  You can set it to prevent the knob from turning past a certain point.  Yours may be set one off from full turn.  It is just a little metal tab that pokes down into one of several holes.  Make sure that it is in the most clockwise position.

[Laudanum]

Are these Sweetest Whispers?  There is a little metal piece that works as a stop-lock.  I'm sure that someone will have a better term for it.  You can set it to prevent the knob from turning past a certain point.  Yours may be set one off from full turn.  It is just a little metal tab that pokes down into one of several holes.  Make sure that it is in the most clockwise position.

I thought the stops were internal.  My selector doesnt have any as discussed in another thread.  Doc mentioned that they were internal.  I assume the same would apply to the attenuators.  But you're saying they are external?   I'll have to take a look but am I reading you right or do you have to open the switch?  ... Sorry, I Dont mean to hijack from the OP but this should help provide some info for his situation.

[porcupunctis]

The ones I got fairly recently with my FPIII have the lock-tab right under the nut and lockwasher.  It looks just like another washer but it has a small tab that bends down at a right angle into one of several slots.  Depending on the slot you put it in, it will stop the rotation accordingly.

I only know this because during assembly, it slipped out and I must have replaced it in the wrong hole.  My right channel would barely rotate more than about halfway around.  I thought the problem was internal and took the thing apart and almost didn't get it put back together at all.

Since then, I've replaced them with some goldpoints. 

[Laudanum]

Sounds familiar.  I will have a look.  Maybe my selector works the same way and is missing the tabbed ring.  Hmmmmm.  Thanks.

[Grainger49]

If I am reading Randall's post right the tab can move when assembling.  That might be the problem, might not.

[Quiet_Storm]

I vaguely remember one of them slipping out when I was installing them. I'll check it out and report back. Thanks everyone for the responses.

[Quiet_Storm]

The "lock tabs" were definitely the issue. Thanks everyone. I can't go past a few "clicks" with my 97 db speakers in a tiny apartment, but I'm a bit of a perfectionist.

[Grainger49]

Check page 24 of your manual for the cure to the "hair trigger" volume control.  Most preamps, the Foreplays included, have much more gain than needed so Bottlehead supplies you with the parts and guidance to cure this. 

Once you replace the 33k resistor with an appropriate one you will get a finer control of the volume all the way up to the maximum you want to listen to.

[Quiet_Storm]

Thanks Grainger. I'm trying to show mercy on my neighbors.

For my turntable, I like it at about 2 clicks (-27 db on the sweetest whispers) with the stock 33k resistors. I figure changing to 182k resistors (-12 db according to the manual) will put me just about in the middle of the volume control (-15 db).

For my CD player, 1 click (-39 db on the sweetest whispers) is almost too loud. So I think I need to drop it about -24 db to be in the same range as the turntable. Does this logic sound right? Is there a formula for calculating the padding resistors for a given change in dbs? If I use the 360k resistors (-18 db), it'll certainly work, but it won't be as close in volume to the turntable.

[Quiet_Storm]

Alright I fit a log curve to the values:

33k, 0 db
80.6k, -6 db
182k, -12 db
360k, -18 db

This gave me the equation:

db = -7.489ln(R) + 26.529

I plan on using this to size my resistors. So for my turntable, I want -15 db. The equation gives me a value of 256k. For my CD player, I want -27 db, and I get a value of 1.27M.

If this sounds like a good approach, I'll order some 1/8 watt metal film resistors near these values.

[Paul Joppa]

Cool! Very close, too. The actual formula is

dB = 8.69 ln (15K/(R+15K)) + 10

dB = 20 log (15K/(R+15K)) + 10

dBs are tenths of a Bel, which represents a factor of 10 in power - so 10 log (power ratio), using a base-10 logarithm. But power is proportional to voltage squared, so 20 log (voltage ratio). The stock setup is a 15K attenuator with 33K in series, which gives -10dB - so we add the 10dB back to reference the stock setup.

[Quiet_Storm]

Oh very cool. Excel got pretty close. Based on availability, I think I'll go with 249k and 1M resistors. I'm assuming Vishay 1% 1/8-watt metal oxide resistors will do?

And thanks again PJ! This should be my last tweak until I get upgraditis.

[Paul Joppa]

With that much additional attenuation, you may have more hum and/or noise than necessary, and might want to put some attenuation at the input of the power amp. See my white paper on signals and noise on the Community page. I apologize in advance for the complexity of the subject!