A blast from the past (well, hopefully not!)

[Aussiebottlehead]

Hi folks, my first posting for many years! I have been happily listening to my Foreplay pre-amp as part of my home-made system until a couple of years ago. I then made the decision to retire my CD player and use a PC with JRiver Media Centre, which allows me to take advantage of some parametric EQ, high-res downloads and other 21st century innovations. As a consequence I bought a Wadia DAC/pre-amp and have been using that for a couple of years. What I now want to do is connect a DAC to my PC and start to use my Foreplay again. I bought a Topping D10 DAC and as a result of a long story I ended up with a balanced output unit rather than the unbalanced D10s unit. I think I have a handle on making a suitable cable to convert the TRS output on the DAC to an RCA input for my Foreplay, but I suspect that I should also attenuate the signal, as it will be 4V and therefore too high for my Foreplay. I searched the Forum and found an old thread ("Add XLR Input to BeePre2") which indicated that I should install a resistor on the Foreplay input. Is that all I have to do and if so, what value resistor should I use?
PK

 

[Paul Joppa]

Foreplay has had several different input controls over the years.What version do you have? Maybe a photo of the inside would help.

 

[Paul Birkeland]

I searched the Forum and found an old thread ("Add XLR Input to BeePre2") which indicated that I should install a resistor on the Foreplay input.

No, that thread says you should add two input transformers and two resistors...

 

[Doc B]

Without knowing the source's balanced output circuit the transformer method is the best way to go from balanced to single ended. There can be simpler ways to do it with certain output circuits in the source, though those methods can cut the source level in half.

 

[Aussiebottlehead]

Hi folks, thanks for the replies! Paul B you are quite right, I misinterpreted the thread, it does say transformers, and BEYOND THAT, resistors. The DAC I wish to use has a TRS output, so what I'm thinking now is to just use a cable with TS plug at one end and an RCA plug at the other end for my Foreplay. But that will still leave me with a 4V source, which will be too high for the Foreplay wouldn't it? Paul J, I can't get at my Foreplay to photo it, as I'm repainting my listening room ATM and everything is stacked in one corner. My Foreplay instruction book is dated 1998 and I have installed the Anticipation upgrade. I also installed an Alps 438t 50kAX2 controller - is that the input controller information that you wanted?

 

[Paul Birkeland]

But that will still leave me with a 4V source, which will be too high for the Foreplay wouldn't it?

Do you have the Sweetest Whispers attenuator or the two mono carbon potentiometers? 

The Foreplay can be made to accept any source voltage by adding a resistor either between the selector switch and volume controls to reduce the sensitivity of the Foreplay as a whole, or between the RCA jacks for one input and the selector switch to just pad one input.

 

[Aussiebottlehead]

Thanks for that PB! With the constraints around where the ALPS volume control is placed in my amp, I think I'll pad the input. I also made a newby mistake by saying that I'd have 4 V input. If I use just one of the active wires from the balanced output, it should be half that, So there shouldn't be too much padding needed.
Once again, thanks for all the advice folks!
Regards
PK

 

[Aussiebottlehead]

Hi folks,
After a bit of tinkering I find that my DAC puts out 2.09 V at the end of my home made XLR/RCA cable. This was measured using a 40 Hz 0dB signal from my laptop going to the DAC (i.e., no attenuation at the PC using a test CD signal). Ideally, to what level should I reduce the sensitivity of my Foreplay (as per PBs suggestion of reducing overall sensitivity by putting a resistor between the source plug and the volume control)? What I was hoping to do was to measure the voltage at B2 at with the volume control at max using the same 40 Hz test signal and swap out the padding resistor until I get an optimal V reading at B2, but I don't know what an optimal voltage would be - should I aim for 1 V?
I may also be wildly naive in assuming that I should be measuring a voltage at B2 in order to test the overall sensitivity, it wouldn't be the first time that I underestimated what is required!
Any advice will be gratefully received!

 

[Paul Birkeland]

I have no idea how much padding would be optimal for your system.  This will depend on the sensitivity of your speakers, the gain of your power amp, the output voltage of your source, and what sounds loud to you with your system in your room.  You would have to know all of this information in order to set the padding resistors optimally on the first go, otherwise you'll want to experiment to see what works best for you.

 

[Paul Joppa]

It seems to me that you should be able to adjust the gain in the PC software to find the desired attenuation. It may even be satisfactory to just do that instead of modifying the Foreplay.

The needed attenuation may be quite high, over 20dB - Foreplay has a LOT of gain! As I recall, the Sweetest Whispers was designed to provide a good bit of extra attenuation, which your Alps potentiometer does not provide.

You could also buy inline attenuators from places like Rothwell or Harrison Labs, which would eliminate the need to modify the preamp. I normally recommend the Rothwell because they present a higher load impedance to the source, but a DAC would likely be happy with the lower impedance of the Harrison (and many others).

 

[Paul Birkeland]

It seems to me that you should be able to adjust the gain in the PC software to find the desired attenuation.

This is a great idea! 

 

[Aussiebottlehead]

Hi folks,
thanks for the replies!  My question was a little poorly-expressed, what I was wondering is whether there is an input voltage that is too high for a Foreplay in regards to safety/longevity/noise. I previously used my Foreplay with a CD player, which I suspect put out about 2 V (didn't test that before I sold it though). It worked and sounded fine and the Foreplay gave no problems, so I also suspect that the Topping DAC would also give no problems with its 2 V output. The only downside was that I didn't get a full range of use out of my Foreplay volume control (Alps) as it was just too loud at the top end.
Now, the PC attenuation is an interesting consideration I hadn't considered! I'm currently using JRiver Media Centre to implement parametric filters to control for my listening room characteristics and a slight mismatch between the sensitivity of tweeters and mids. I use a Wadia DAC/preamp with my kit amp and speakers and the parametric filters are all cuts, no boosts. As a start I'll work with those filters when I slip in the Foreplay and Topping DAC in place of the Wadia. So, due to the cuts from the parametric filters the Foreplay should receive a signal from the DAC attenuated by quite a few dB. Hopefully that could be enough, but I could also put in an additional broadband attenuation to take the output back further. Great!

 

[Paul Joppa]

Hi folks,
thanks for the replies!  My question was a little poorly-expressed, what I was wondering is whether there is an input voltage that is too high for a Foreplay in regards to safety/longevity/noise.

...

The answer to that question is "no, there is no such problem". That's because the level control is at the input.

[Off Topic: It is a potential problem for our current preamps (Moreplay and BeePre 2.0). Inputs greater than about 7vrms for Moreplay, or 10vrms for BeePre 2.0 have the potential to overdrive the tube and introduce increased distortion.]

 

[Aussiebottlehead]

Great, thanks Paul!