Foreplay III New Build + adjusting listening levels + ST-70 troubleshooting

[Paul Birkeland]

How many clicks on the volume controls before you have too much output?

[syncro]

That depends on who you ask, but since I get to answer and not my family, I'll say I've had it up to 3 with the CD (loudest input).  It is really rocking the house.

[Paul Joppa]

At first look, you are not that bad off with the stock build. Your speakers are pretty inefficient, so you will probably use all 35 watts on musical peaks. You should be able to get that with a volume setting four clicks below maximum, using the stock 33K resistors.

The other sources have their own level controls, and you can set them to match the CD player.

If you find this is too loud, you can change the resistor on the CD player inputs. The supplied resistors come in 6dB increments, and the Whispers are 3dB per step except for the lowest few, so it should be easy to decide which resistor to use. I base my levels on statistical averages for musical genre, personal preference, room acoustics, etc. so it's not unusual for your situation to differ by as much as 6dB either way.

[syncro]

Thank you for the recommendation based on my input data.  Seems like I have a lot more gain than the numbers may be indicating.  I'd like to have the CD be able to play with a full volume control range, from  low-level, background-to-family-dinner music to I'm-in-the-kitchen-and-still-want-to-hear loud.  I cannot approach four-below-max, but rather two above silent.  It is just too loud.  One above mute is louder than I often need, and two above is driving out the family.  So, I'm inclined to attenuate the ST-70 with the series resistor technique suggested above by Granger49 who said:
"....Then the input is taken from the "junction" between these two resistors; the solder joint you just made.  The input wires should reach since this point is closer to the circuit board.  The value of the series resistor will be determined later."

Can someone suggest a resistor value, type and/or part no?  I have a few floating around (like from the FP III kit I have pairs of 80.6 K ohm, 182 K ohm and 360K ohm.)  Are these the correct type to try?  Are the attenuation levels from the FP3 instructions for these input resistors going to hold if they are installed in series in the ST-70 inputs?  I need to open up the ST-70 to replace some tube sockets anyway.  Another option would be to rewire the ST-70 with one of the upgrade kits to operate at a lower wattage.  Suggestions are welcome, but this will not be an option for me immediately.

[Paul Joppa]

OK, now we have some data to work with! Two above silent is -27dB, so a 27dB reduction will give you a full range. A good split would be -20dB in the ST-70 and -6dB in the Foreplay. So replace the 33K in the Foreplay with 80.6K; that will reduce the Foreplay sensitivity by 6dB and keep its signal levels well above the noise floor.

Then reduce the ST-70 by 20dB, which is a 10:1 resistance ratio. Grainger is probably better suited to describe this since he has an ST-70. I'd suggest you keep the stock 470K resistor  at the center of the RCA, and from the other end (the junction) use a 56K or something near that value to ground, i.e. the outer connector of the RCA jack. Then take signal from the junction to the circuit board input. The outer connector of the RCA jack is connected to ground, I gather this is through the 10 ohm resistor that Grainger says you may want to replace with a piece of wire, but this is the part of the circuit I'm having a hard time visualizing.

I like quality metal film such as the Vishay/Dale RN series resistors, but in this application with no DC current carbon film can be an excellent performer as well. Most listeners seem to think it is a bit warmer sounding.

[Grainger49]

David,

Radio Shack, now just called "The Shack," sells (I almost said "stocks" but that is up to the local store) a metal film resistor assortment.  It lists the contents on the back of the blister pack.  At least they used to list them.  You will likely find what you need in this assortment.

Paul has described the installation very well.  If you need more I might be able to cobble a picture into a post.  (I have previously had problems with pictures.  The new board hosts them so I'm doing much better.) 

I might have a couple of 56k ohm resistors, or something near the value I can mail you.

[syncro]

OK, now we have some data .... So replace the 33K in the Foreplay with 80.6K....

Thank you both, again.   I like the notion of splitting the attenuation (and the assurance of keeping off the "noise floor" of the FP.)   Regarding the recommendation quoted, above:  Would that be the 33K ohm resistor pair on the input I am using for the CD?  (Input #1 looks the easiest terminals to access to make this change.)

Or is the intent to attenuate all the FP inputs by 6dB?  I know I have volume control on my phono preamp and my digital music player, but I'd rather have them wide open for ease of control and best sound.  Another recent thread was discussing attenuating all the FP inputs with a resistor between each Sweetest Whispers and the selector switches.  This seems a simple mod, but

Even if so, I think I should try the series resistors in the ST-70 first and listen again for a while. 

Granger,
Thank you for the offer of resistors.  I have some leftover from the HagermanRipper build, so I'll check around for what I have and get back to you via the forum's p-message feature, if that's best. 

[Paul Joppa]

I was suggesting you change the CD input only, as in the manual.

Once that is done, you can use the level controls on the other sources to match all levels. It may take several albums to find the best-matching compromise, since album signal levels are not exactly constant (!)

Once THAT is done, you can (if you like) maximize one of them, and use the Foreplay controls to bring it back in line. Since the Foreplay controls are 3dB per step (except for the first 2 or 3) you can then estimate how much attenuation you want on that source, and implement it in the Foreplay. Repeat as necessary. Yes, it;'s a lot of work, but you only have to do it once.  :^)

[Grainger49]

David,

The volume controls on your phono stage and digital music player are the equivalent of padding each input on the FP III between the input and the selector switch.  But using the volume controls would be much faster.

I'll assume that dropping the level of all inputs has already been explained since you mention that in your second paragraph.

Just send me a PM if you need the resistors and I will look to see if I have some.

[syncro]

I'm attaching two diagrams of the suggested modification to attenuate my ST-70 amplifier (one channel only is diagrammed.)   I have the blank diagrams I can post if someone wants to start from scratch.  If I'm close, please let me know, or if I got something wrong.(http://)

[syncro]

Schematic version... and here I have circled the 10 ohm resistor spoken of earlier in the thread as something to possibly eliminate if a hum develops.

[Grainger49]

David,

In both diagrams you need to swap the positions of the two resistors.  Edit: I looked at it backward.  Both sketches are right. That is, the input feeds one end of the 470k ohm resistor, the other end is soldered to one end of the 47.5k ohm resistor and the other end of the 47.5k ohm resistor goes to ground.  The input wire that feeds the circuit board at eyelet 17 is attached to that eyelet and to the solder joint that connects the two resistors.

Yes, that is the 10 ohm resistor that you can jumper out and see if it eliminates noise or causes it.  It seems to be a crap shoot as to whether it will help in an ungrounded house.  I bet it has a lot to do with whether you use a grounded power cord too.

[Paul Joppa]

The sketches look correct to me, though a little hard to unscramble mentally. Grainger's description is of course correct, it just looks to me like you have in fact sketched what he wanted.

[Grainger49]

Paul, I was looking back at this post, maybe I should take another look at the two sketches:

OK, now we have some data to work with! Two above silent is -27dB, so a 27dB reduction will give you a full range. A good split would be -20dB in the ST-70 and -6dB in the Foreplay. So replace the 33K in the Foreplay with 80.6K; that will reduce the Foreplay sensitivity by 6dB and keep its signal levels well above the noise floor.

Then reduce the ST-70 by 20dB, which is a 10:1 resistance ratio. Grainger is probably better suited to describe this since he has an ST-70. I'd suggest you keep the stock 470K resistor at the center of the RCA, and from the other end (the junction) use a 56K or something near that value to ground, i.e. the outer connector of the RCA jack. Then take signal from the junction to the circuit board input. The outer connector of the RCA jack is connected to ground, I gather this is through the 10 ohm resistor that Grainger says you may want to replace with a piece of wire, but this is the part of the circuit I'm having a hard time visualizing.

I like quality metal film such as the Vishay/Dale RN series resistors, but in this application with no DC current carbon film can be an excellent performer as well. Most listeners seem to think it is a bit warmer sounding.

Yup, both sketches are right.  My brain was in reverse.

[syncro]

Thanks again, and again!  I'm following this, scratching my head, and seeing what we all see, now.  I'll let you know once I've completed the addition.  New RCA's should arrive tomorrow.