Problems with FPIII (Humming and gain)

[Dwight]

Hello 
Finally I finished my Foreplay III Kit some weeks ago. Everything worked fine, thanks to the very good documentation. I only have a problem with to much gain and humming. Due to the first tests I connected the FP III between one of my DACs (I think it was the CEC DA1N) and my amp (TA3020 based). First thing I noticed was a very loud humming. Then I switched the volume to the first step of the sweetest whispers attenuators and the music was immediately very loud. The sound was very nice but a lot to loud for the first step of the sweetest whispers volume control. Normally I would expect this level of volume at one or two o'clock of sweetest whispers attenuators. What resistor value should I use to solve the volume problem, at the moment the standard 33k is installed?

To solve the humming problem, I tried to isolate the FPIII from the protective ground from the wall outlet. Sometimes this helps to avoid humming, but this did not help. Any ideas where the humming could come from and what to do agianst it?
I also connected the FPIII between my DAC and my solid state preamp, in this configuration the humming was much lower.

Here are some pictures from my FPIII:

http://www.dropbox.com/gallery/20770445/1/bottlehead?h=d118a5

Best Regards

Philipp

 

[Paul Joppa]

I speculate that the root problem is that your amp has too much gain for the system. That would cause both excess noise (hum and tube rush) and too much loudness. Unfortunately this situation is quite common - a search will turn up many frustrated posts of mine on the subject! We can confirm this guess if you can identify the amplifier's gain and power, and your speakers' sensitivity.

If I am correct, then we should solve the hum and noise problem first, since solving it may also solve the gain problem. The gain reduction resistors will not solve the hum and noise problem.

If the amplifier and speaker sensitivities are high, the best solution is to reduce the amplifier's gain. Is this a DIY amp? (You said it was "TA3020 based".) If so, you can probably make the necessary adjustment easily. If you do not want to modify the amplifier, then we can probably attenuate the Foreplay output since you appear to have a large amount of excess gain in the system.

I have written a short technical article on the subject, which is on the "Community" page - check it out if you are interested in more details.

 

[Dwight]

Hello Paul

Thank you very much for your fast reply. Sorry for my late answer, I had a lot to do the last days.

Is it possible that a broken tube was responsible for the humming?

I was going to do some tests with the FPIII in my main system with active speaker as I can change the input sensitivity of them.
After I connected everything, the humming was directly noticeable and also a new phenomenon: A very big volume difference between left and right.
After some measurements I was sure that one of the ECC82 must be broken. A few minutes ago I changed the tube and connected the FPIII to my
test system (with the TA3020) again. I was very surprised, there was no more humming and of course the volume difference between left and right was also gone.

Later I will test this in my main system; I hope there will be also no more humming.

The standard input sensitivity value from my active speakers (Manger MSMc1) is +6dBu (1,54V), I can set it to a higher sensitivity of 0dBu (0,775V) and additionally
I can trim the input sensitivity between -2,5dB to 2,5db. As I'm not an expert with dBu and dB, can you give an advice which setting I should best choose for the FPIII?
I really hope that I can solve the gain problem without modifying the hardware.

Best Regards

Philipp

 

[Paul Joppa]

Sorry I missed this; I seem to have missed anything posted on Feb. 13.

I would start with the lowest sensitivity on the active speakers. That is the highest input, in other words +6dB. You do not say how much loudness that produces, so I can't be more specific yet.

If that is not the right volume range, then identify which position of the Foreplay level controls gives you the loudness you want, and which position you would like to produce that volume, and we'll calculate the appropriate adjustments. I'll post the method in case someone finds this thread in a search at some future date.

Is the test system louder than the main system, at the same level control setting? If so, we can address that situation as well - but let's get the main system optimized first!