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Apt Holman preamp : hearing an FM station on phono input
Apt Holman preamp : hearing an FM station on phono input
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Old 22nd May 2020, 03:45 PM   #1
georgesgiralt is offline georgesgiralt  France
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Default Apt Holman preamp : hearing an FM station on phono input

Hello,
I use this preamplifier since a little more than a year. It was in the estate of my deceased elder brother.
It had suffered a flaw in the phono preamp, with one channel a lot lower than the other and you helped me solve it. (it was a dead electrolytic capacitor).
I was advised to change all the electrolytic caps in the device but did not proceed because I am afraid to break something.
Anyway, i live near a lot of FM broadcast stations (I'm on the only hill above the city... ) and I've noticed that one station is audible when I listen to a vinyl. If I'm not mistaken, it was not the case before, but I wont bet my life on this...
As the cart has been changed ( an old Grado with a Black2 diamond has given room for an AT VM95E cart), I thought I could play with the input impedance setting and the capacitance adapter switch but to no avail.
So I'm seeking advice here.
Many thanks in advance for your help.
Have a nice day and stay safe !
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Old 22nd May 2020, 04:07 PM   #2
MarcelvdG is offline MarcelvdG  Netherlands
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Clipping a split-core ferrite on the cable from the turntable to the amplifier, as close to the amplifier as possible, might help. Moving the cable around a bit might also help.
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Old 22nd May 2020, 10:54 PM   #3
duncan2 is online now duncan2  United Kingdom
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Location: Scotland.
Personally I would not use a ferrite core in a quality amplifier especially near the input .
The second choice is a better trial ,does moving the preamp 90 degrees make any difference ?


If not then it could be a case of using higher quality co-ax or a very small value cog ceramic connected at the input to earth.
Its initially the connecting cable acting as an aerial that amplifies the RF signal .


This is a well known fault in audio equipment of those living near transmitters and there are many fixes but not all are conducive to keeping the distortion down but your music signal must have the RF signal piggy-backing it so that's not too good.
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Old 22nd May 2020, 10:59 PM   #4
rayma is offline rayma  United States
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First try placing the turntable right next to the preamp, and bundling up the audio cable as small as possible.
If this does not help, decoupling each of the preamp's RCA input socket common to the chassis with a small
ceramic capacitor may work.
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Old 23rd May 2020, 06:17 AM   #5
MarcelvdG is offline MarcelvdG  Netherlands
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The ferrite core works as a common-mode choke. I'd expect it to have less impact on the sound than any other filter would.
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Old 23rd May 2020, 11:44 AM   #6
Mark Tillotson is offline Mark Tillotson
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Location: Cambridge UK
Quote:
Originally Posted by duncan2 View Post
Personally I would not use a ferrite core in a quality amplifier especially near the input .
That makes no sense at all, this is one of the primary ways to defend against RFI ingress for sensitive audio equipment and is often used successfully where close to a radio transmitter. High levels of RF floating in on the outside of the cable are definitely something you don't want entering a quality amp. RF field strengths can be surprisingly high if some part of the system just happens to be resonant and acting as a tuned antenna.


Any cable entering the amp box is a potential route for RF energy, some experimentation might be involved - get only the spilt-core
style ferrites as the one-piece can't be added to existing cables, only useful when building the cable from scratch..

Last edited by Mark Tillotson; 23rd May 2020 at 11:48 AM.
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Old 23rd May 2020, 11:51 AM   #7
ET160 is offline ET160
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Have you tried different interconnects?
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Old 23rd May 2020, 01:48 PM   #8
georgesgiralt is offline georgesgiralt  France
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Join Date: Jul 2008
Hello,
Thank you all for your answers.
The turntable is an old Technics fitted with an SME 3009 Series II non improved arm. The arm is of 1970 vintage . As such, it uses a fixed cable to link to the preamp. (it has a proprietary plug at one end of the cable and two nice silver coated RCA plugs at the other end and need a dismantling of the turntable to have access to the plug inside). So this is the last i'll try to change ;-)
As the Apt Holman input has a switch with various capacitive load to adjust the input to the best recommended value, i tried this. IMHO it is no different than putting another capacitor at the RCA plugs.
You are right that the cable is the antenna. If I remove the cable, the RF signal is so feeble it is not bothering even at full volume.
For now, my goal is to find a split ferrite on one of my cable in the house and try it. Could take a while given the mess I have in the house. (do you have a lead for a model and a shop where I can find some ? )
So I will reply hopefully during the week end or next week ;-)
Again thank you all !
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Old 23rd May 2020, 02:38 PM   #9
georgesgiralt is offline georgesgiralt  France
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Join Date: Jul 2008
Found it !
And it did the trick ! At full volume, I just hear a high pitched hiss which is absent on the Phono 2 input (unplugged). But the level is low so won't be noticeable at normal listening level. It took a whooping 30 sec to fit the ferrite.. So a very difficult repair ;-)
Thank you !
See the picture of the ferrite I used. The LR6 shows the size and I do not know the origin not brand.
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File Type: jpg IMG_20200523_162839.jpg (978.1 KB, 40 views)
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