Adding Splitter resistors to a speaker level output - UK Vintage Radio Repair and Restoration Discussion Forum

'LIVEWIRE?' · 6th May 2019, 9:49 pm

Are these necessary when connecting the speaker level output of a mono car radio to the (speaker level) inputs of a stereo amplifier. If so, what value & wattage of resistor should be used? The radio in question is a 1971 AM/FM model which has a maximum output of ca, 6 watts into 4 ohms, and my assumption is that the output from the radio should be split between the two channels of the added amplifier in a similar way to that which would be done at line level. A fellow forum member in Canada has asked me about this, and, despite my having spent over 23 years servicing & repairing car radios & stereos, I'm not certain how to advise him. He has tried simply connecting the two inputs in parallel to the radio's output with no problems, but for long-term use, I'm not certain that is a good idea. The radio in question has a typical DC coupled transistor amplifier with a complementary symmetrical output stage built around AD156/157 germanium transitors with the usual 470uf or thereabouts coupling cap. from the junction of the AD156/157 emitters to the speaker(s).

G8HQP Dave · 6th May 2019, 10:21 pm

Paralleling inputs should create no problem. It is paralleling outputs which requires some ballast resistors, to ease the load when one output tries to drive the other one.

paulsherwin · 6th May 2019, 10:27 pm

Agreed, no resistors needed.

Refugee · 7th May 2019, 12:12 am

It is worth loading the radio with something like 47 ohms in series with 10 ohms to put a little bit of load on the amplifiers. Take the input to the big amplifier from the 10 ohm resistor the low end this being ground.
This potential divider will improve the range of the volume control on the radio and damp any noise that might be generated by the radio that was originally meant to drive speakers.

'LIVEWIRE?' · 7th May 2019, 9:22 am

Thanks, all, for your replies. As the owner is a forum member in Vancouver, Canada, there's no way I'm going to see the radio or amplifier in question, unless I get him to e-mail pictures to me. I'm familiar with the radio, a 1971 Blaupunkt Mannheim, but have no idea what amplifier is involved, though I've PM'd him to confirm that it has dedicated speaker-level inputs. If so, these should incorporate 'dummy load' resistors, plus the necessary attenuating circuits, otherwise I'll advise him to add them.

6th May 2019, 9:49 pm	#1
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Adding Splitter resistors to a speaker level output Are these necessary when connecting the speaker level output of a mono car radio to the (speaker level) inputs of a stereo amplifier. If so, what value & wattage of resistor should be used? The radio in question is a 1971 AM/FM model which has a maximum output of ca, 6 watts into 4 ohms, and my assumption is that the output from the radio should be split between the two channels of the added amplifier in a similar way to that which would be done at line level. A fellow forum member in Canada has asked me about this, and, despite my having spent over 23 years servicing & repairing car radios & stereos, I'm not certain how to advise him. He has tried simply connecting the two inputs in parallel to the radio's output with no problems, but for long-term use, I'm not certain that is a good idea. The radio in question has a typical DC coupled transistor amplifier with a complementary symmetrical output stage built around AD156/157 germanium transitors with the usual 470uf or thereabouts coupling cap. from the junction of the AD156/157 emitters to the speaker(s).

6th May 2019, 10:21 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Adding Splitter resistors to a speaker level output Paralleling inputs should create no problem. It is paralleling outputs which requires some ballast resistors, to ease the load when one output tries to drive the other one.

6th May 2019, 10:27 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: Adding Splitter resistors to a speaker level output Agreed, no resistors needed.

7th May 2019, 12:12 am	#4
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Adding Splitter resistors to a speaker level output It is worth loading the radio with something like 47 ohms in series with 10 ohms to put a little bit of load on the amplifiers. Take the input to the big amplifier from the 10 ohm resistor the low end this being ground. This potential divider will improve the range of the volume control on the radio and damp any noise that might be generated by the radio that was originally meant to drive speakers.

7th May 2019, 9:22 am	#5
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Adding Splitter resistors to a speaker level output Thanks, all, for your replies. As the owner is a forum member in Vancouver, Canada, there's no way I'm going to see the radio or amplifier in question, unless I get him to e-mail pictures to me. I'm familiar with the radio, a 1971 Blaupunkt Mannheim, but have no idea what amplifier is involved, though I've PM'd him to confirm that it has dedicated speaker-level inputs. If so, these should incorporate 'dummy load' resistors, plus the necessary attenuating circuits, otherwise I'll advise him to add them.