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May 24, 2019, 12:19:50 pm

Author Topic: Problem Seduction pre-amp_radio station in background  (Read 1849 times)

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Offline choff

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Problem Seduction pre-amp_radio station in background
« on: January 29, 2018, 02:43:06 pm »
Hello,

I recently moved my listening room to another location in my house.  Unfortunately, I now have a local radio station playing (fairly loudly) in the background during playback of records with my Seduction amp run thru a Foreplay to a Dynaco 70.  I have no idea what to do to correct this issue?  Tried searching forum but couldn't find any relevant topics. Would really appreciate any feedback or suggestions. Thank you.

Carl

Offline Paul Birkeland

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Re: Problem Seduction pre-amp_radio station in background
« Reply #1 on: January 29, 2018, 03:31:03 pm »
If you unplug the cables on the input side of your Seduction, does the noise go away?
Paul "PB" Birkeland

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Offline choff

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Re: Problem Seduction pre-amp_radio station in background
« Reply #2 on: January 30, 2018, 03:07:34 am »
No, the radio station noise remains.  It only goes away when I remove the output cables going to the Foreplay.

Offline Paul Birkeland

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Re: Problem Seduction pre-amp_radio station in background
« Reply #3 on: January 30, 2018, 03:59:57 am »
Can you try temporarily connecting the chassis plates of the Seduction and Foreplay together with a piece of wire. 

It might also help to see the build underneath the Seduction, but if you're getting the radio station in both channels, it's doubtful that there's something unusual in the build. 

What may end up being helpful is adding 100-330pF of capacitance across each 47K resistor across the input jacks, or pursue additional shielding of the base (you are using the tube shields I presume).
Paul "PB" Birkeland

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Offline choff

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Re: Problem Seduction pre-amp_radio station in background
« Reply #4 on: January 30, 2018, 04:43:14 am »
I am indeed getting the radio signal in both channels.  I am using the tube shields. This Seduction amp has worked flawlessly for me since I built it about 8 years ago.  How would I connect the chassis with a wire?  How would I shield the base?  Can you explain a little further how I would perform this task.....100-330pF of capacitance across each 47K resistor across the input jacks.   I am pretty good at following instructions but don't know or understand all the inner workings of these amps.  Thanks for your feedback and assistance!!

Offline Paul Birkeland

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Re: Problem Seduction pre-amp_radio station in background
« Reply #5 on: January 30, 2018, 05:57:06 am »
To do the wire experiment, use a piece of wire and touch it on the ground post of the Seduction and the chassis of the Foreplay.

The Seduction was originally designed so that a Hammond metal box could be mounted over the circuit (you have to file out parts of the box where the wires come and go).  So few folks needed this that it barely came up, but it's an option.

On each input RCA jack, there's a 47K resistor from the center post to the ground buss.  To add the capacitors, solder one end of each cap to the end of the 47K resistor going into the center post of each input RCA jack, then solder the other end to the ground plane (or the other end of each 47K resistor that connects to the ground plane).
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline choff

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Re: Problem Seduction pre-amp_radio station in background
« Reply #6 on: January 30, 2018, 07:49:16 am »
Thanks for the reply.  I tried the wire and it reduced the amount of the sound, but did not eliminate it.  Probably cut it in half to 3/4.  It seemed to work best if i layed the bare wire across the face of the chassis rather than just touching at a point.

Any further thoughts on how to proceed?  Is there anything that might be causing this as far as the house wiring, or is that not associated with this type of problem.  Thanks again....much appreciated.

Offline Doc B.

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Re: Problem Seduction pre-amp_radio station in background
« Reply #7 on: January 30, 2018, 08:04:10 am »
A box covering the underchassis circuitry is the solution we proposed for this problem originally (17 years ago), so it probably makes sense to try that next. IIRC a Hammond 1590J was what we suggested, but I don't recall if that accommodates the C4S board or just fits the basic circuit.
Dan "Doc B." Schmalle
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Offline fullheadofnothing

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Re: Problem Seduction pre-amp_radio station in background
« Reply #8 on: January 30, 2018, 08:27:14 am »
Also at some point the screw holes for the Hammond box were removed from the chassis since no one ever seemed to actually need them. A picture would help here.

Is there any space in the setup to try different positioning/orientations...?
Joshua Harris

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Offline choff

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Re: Problem Seduction pre-amp_radio station in background
« Reply #9 on: January 30, 2018, 08:49:10 am »
Here are a few pics

Offline Paul Birkeland

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Re: Problem Seduction pre-amp_radio station in background
« Reply #10 on: January 30, 2018, 08:53:41 am »
It looks like you have the holes for the Hammond box (though perhaps with some added screws?).

The ground connection that you added should have been in the preamp, but IIRC the original Foreplay preamps didn't have signal ground tied to chassis.  I believe the recommendation was to jumper terminal 13 to terminal 14 to make this connection (which is now standard on all of our preamps).

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline choff

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Re: Problem Seduction pre-amp_radio station in background
« Reply #11 on: January 30, 2018, 08:55:55 am »
Hope these pics help clarify things.  Unfortunately, I am pretty locked into the layout of the system without much flexabilty to make changes, unless you see something I can easily tweak to improve the situation.  I do have CS4 in the Seduction. 

I will also note that the new location is a room in an addition on top of my house.  It is lightweight framing with alluminum cladding on the exterior, whereas the original room was lower and masonry exterior wall construction.  I imagine this higher more transparent location is allowing the radio signal to get to the amp more easily?

Offline Doc B.

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Re: Problem Seduction pre-amp_radio station in background
« Reply #12 on: January 30, 2018, 09:15:28 am »
Can we see a pic of the underside of the Seduction?
Dan "Doc B." Schmalle
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Offline choff

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Re: Problem Seduction pre-amp_radio station in background
« Reply #13 on: January 30, 2018, 09:21:52 am »
For some reason it won't let me upload the image? I'll keep trying, but it looks like I already have the Hammond. 

Offline choff

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Re: Problem Seduction pre-amp_radio station in background
« Reply #14 on: January 30, 2018, 09:35:37 am »
fingers crossed this finally works