Reducing input sensitivity of the Linex FM microtransmitter - UK Vintage Radio Repair and Restoration Discussion Forum

Nickthedentist · 29th Jan 2008, 11:27 pm

Hope this is in the right section...

These excellent and very cheap FM microtransmitters are spoiled by the fact that their input seems overly sensitive for direct connection to the line output (e.g. tape sockets) of a typical hifi system.

After a bit of reverse engineering, I found that the audio input from each channel goes via a 10uF cap to a 6K8/47K potential divider, then via another 10uF cap to the IC.

Increasing the value of the two 6K8 resistors (R7 and R17) to about 100K makes things much better.

Access is easy - 2 screws to release the cover then 2 more to remove the PCB.

A variable pot would be a better solution, but fitting one internally is impossible. It's a shame they don't include one. An alternative, of course, is to drive the unit from a headphone socket and use the amp's volume control to adjust the levels.

Cheers,
Nick.

Sean Williams · 30th Jan 2008, 12:02 am

Or find an old set of PC headphones with an inline volume control - just chop the lead off and use that with suitable plugs/sockets...

paulsherwin · 30th Jan 2008, 1:09 am

Like most cheap microtransmitters, these are designed to be driven by the headphone output of an MP3 player. Some of these don't produce a lot of output even with the volume turned up all the way, hence the high sensitivity.

I found by trial and error that a standard line signal needed to be attenuated by about 6dB to drive a Linex properly. I can't remember what resistors I used to achieve this now, but they are inside a Radio Shack switch box that sends one of the amp tape outputs to either the Linex, a Wild Planet Radio DJ AM micro TX or a second tape deck.

With this attenuation my Linex works quite well with all the sources I've tried - CD, FM tuner, Freeview box etc. A CD with lots of compression recorded at close to 0dB pushes it a bit and it would probably sound less than perfect on hifi equipment but it's OK for radios, ghettoblasters etc. A Freeview signal with quite low levels (BBC N24 sound say) sounds quiet on a radio but is OK if you turn up the volume a bit.

Paul

igranic · 30th Jan 2008, 1:41 am

Based on my experience driving the Linex from my Tesco Technca MP3 player, or pocket CD player, I tend to agree with Paul regarding sensitivity. With normalised MP3 files, setting the player's volume to 7/10 seems to produce the optimum sound quality from the Hacker Sovereign with minimum sibilance and no attenuation necessary.

Driving it from the Freeview STB is a different matter, requiring the volume control set near to minimum thereby introducing noise vs. signal problems, so an attenuator is definitely needed here.

29th Jan 2008, 11:27 pm	#1
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,845	Reducing input sensitivity of the Linex FM microtransmitter Hope this is in the right section... These excellent and very cheap FM microtransmitters are spoiled by the fact that their input seems overly sensitive for direct connection to the line output (e.g. tape sockets) of a typical hifi system. After a bit of reverse engineering, I found that the audio input from each channel goes via a 10uF cap to a 6K8/47K potential divider, then via another 10uF cap to the IC. Increasing the value of the two 6K8 resistors (R7 and R17) to about 100K makes things much better. Access is easy - 2 screws to release the cover then 2 more to remove the PCB. A variable pot would be a better solution, but fitting one internally is impossible. It's a shame they don't include one. An alternative, of course, is to drive the unit from a headphone socket and use the amp's volume control to adjust the levels. Cheers, Nick.

30th Jan 2008, 12:02 am	#2
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Re: Reducing input sensitivity of the Linex FM microtransmitter Or find an old set of PC headphones with an inline volume control - just chop the lead off and use that with suitable plugs/sockets... __________________ Engineers make things work and have spare bits when finished

30th Jan 2008, 1:09 am	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,943	Re: Reducing input sensitivity of the Linex FM microtransmitter Like most cheap microtransmitters, these are designed to be driven by the headphone output of an MP3 player. Some of these don't produce a lot of output even with the volume turned up all the way, hence the high sensitivity. I found by trial and error that a standard line signal needed to be attenuated by about 6dB to drive a Linex properly. I can't remember what resistors I used to achieve this now, but they are inside a Radio Shack switch box that sends one of the amp tape outputs to either the Linex, a Wild Planet Radio DJ AM micro TX or a second tape deck. With this attenuation my Linex works quite well with all the sources I've tried - CD, FM tuner, Freeview box etc. A CD with lots of compression recorded at close to 0dB pushes it a bit and it would probably sound less than perfect on hifi equipment but it's OK for radios, ghettoblasters etc. A Freeview signal with quite low levels (BBC N24 sound say) sounds quiet on a radio but is OK if you turn up the volume a bit. Paul

30th Jan 2008, 1:41 am	#4
igranic Rest in Peace Join Date: Jan 2005 Location: North Bedfordshire, UK. Posts: 395	Re: Reducing input sensitivity of the Linex FM microtransmitter Based on my experience driving the Linex from my Tesco Technca MP3 player, or pocket CD player, I tend to agree with Paul regarding sensitivity. With normalised MP3 files, setting the player's volume to 7/10 seems to produce the optimum sound quality from the Hacker Sovereign with minimum sibilance and no attenuation necessary. Driving it from the Freeview STB is a different matter, requiring the volume control set near to minimum thereby introducing noise vs. signal problems, so an attenuator is definitely needed here.