Help with Gain on old Foreplay II

[Rollie]

I
In 2001 I built a Foreplay 2, I upgraded with the C4s. I have been happily enjoying it attached to my Carver SS TFM-15CB power amp, running through a pair of old Klipsch Heresy ones.

 My nephew recently got me on old Dynaco ST 70 amp. It is the original kit one, totally stock from the day it was completed  We played it at his house using the Dynaco preamp he has. It sounded darn good. I took it home and hooked it up to my foreplay and aside from some hum issues it still sounds good. (cleared up most of the hum by putting on the old tubes I got with the foreplay kit). I will work on cleaning up any remaining hum.

 My only problem is that the volume controls are unbelievably sensitive. I have them set almost to nothing, If I move them a 1/16 of an inch the volume goes up a great deal. I called and ordered the sweetest whispers hoping this would help. My question is will this reduce the gain issue, or will I need to do something else?  On the Carver it had its own volume controls so I guess I just kept them very low and didn't notice the gain issue.

Thanks for any help. 

[Grainger49]

Rollie,

To eliminate most of the hum short the 10 ohm resistors that float the input grounds.  That is, take the input shield, outer conductor directly to ground.  In the stock amp it goes to ground through two 10 ohm resistors on the circuit board.

A search of the old forum will yield a hundred or so posts on the FP 2 hair trigger.  The answer is generally to pad the input of your amp.  A voltage divider of 2:1 will be a big help.  The ST-70 has an input resistor of 470k ohms.  Buy two more and put them in series with the input resistor.  Take the input to the tube from the junction between the two resistors.  If that isn't clear I can make it clear in another post.

This is the old forum:   http://www.audioasylum.com/forums/bottlehead/bbs.html

[Rollie]

Thanks.

Will the sweetest whispers upgrade help the gain problem, or will I need to go ahead with putting the 470 ohm resistors in the dynaco 70? Thanks for the link to the old forum. Using the phrase Hair Trigger brought a lot up.

[Paul Joppa]

The Sweetest Whispers  is normally installed as a shunt-mode attenuator, which provides at least 10dB extra attenuation. The kit comes with several resistors, so you can select whether you want 10dB, 16dB, 22dB, or 28dB extra attenuation.

You may find that the ST-70 is still so sensitive that the Foreplay's hum and noise is audible, in which case an input attenuator at the power amp (and less attenuation at the preamp) is an effective solution. The "search" function on the old site can lead you to several posts over the years by ST-70 owners who have had these problems. Once this problem is solved, most of them have been very happy with the combination - they do seem to go well together.

[Rollie]

Thanks for the help.

[slomatt]

Rollie,

It sounds like you and I are working on the same issue. I also have a Foreplay II and would like to lower the gain on my amplifier to have more usable volume knob travel and to reduce the level of hum. There is some discussion related to the gain at the end of this thread.

http://www.bottlehead.com/smf/index.php/topic,327.0.html

One option is to pad down the input to the foreplay by adjusting the voltage divider implemented by the volume control. The downside to this is that while it will reduce the signal output level it does not reduce the level of hum output by the Foreplay. The better solution seems to be to either reduce the gain of your amplifier or to attenuate the input signal to the amplifier as mentioned above, this reduces the signal level and also the level of hum.

- Matt

[Grainger49]

Rollie,

Poster Paully had a ST-70 that hummed with his system but not with mine.  After a couple of years I said, "I'm going to jump out those 10 ohm resistors," and the ST-70 fell silent.  If you have a pair of alligator jumpers locate the resistors on the circuit board and jump them.  I bet your hum drops drastically.