Hum when old BSR connected to new op-amp/speaker circuit - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

peter_sol · 1st Jun 2016, 10:38 am

No that cable supplied is unscreened. Use the old cable from the gram for now, but you still have to connect the inner to the outer of both the input terminals of the amp and the earthed side to the centre of the 3 input terminals on the PCB.

Lucien Nunes · 1st Jun 2016, 10:40 am

The output of the iPod dock would be low impedance and will work with an unscreened cable without picking up hum. The cartridge is very high impedance and the input will be much more prone to hum pickup unless the screened cable is used, as it was originally.

When choosing an amplifier to work with a particular source, one needs to consider not only the signal level but also the source and load impedances. These days, most electronic sources are relatively low impedance and rather uncritical of connected loads. But vintage stuff, and low level devices such as mics and pickups, need the correct impedance for proper operation.

Regarding earthing, I would earth the deck, and the signal ground of the amplifier input if the mains adaptor is floating (i.e. the DC -ve is not connected to mains earth). Only earth it in one place to avoid loops and instability.

I echo the above thoughts, what is wrong with the original amp that makes it necessary to use this alternative?

bikerhifinut · 1st Jun 2016, 2:21 pm

Quote:

Originally Posted by Edward Huggins

There has been some suggestion of using the original speaker in the Sobell Radiogram. Although we don't have the model number, it is likely to be 3 ohms and not suitable to be run off that Chip. Edward

I'd wager it would drive the speaker ok, most modern chip amps aren't that fussy. The amp in question would be a waste as its a stereo job anyway and probably 100mW or so I bet.
In any case my idea wasn't to use that particular amplifier but to get a mono one, of say 3watts or so. Most of the kits/modules I have seen will drive any speaker from 3 ohms upwards without any drama.
What I would recommend though is making a high impedance buffer for the cartridge load. Easy enough to adapt from one of the available opamp kits from maplins etc etc. Just tweak the input and feedback resistors.

Like Lucien though, I am wondering what was so wrong with the original amp and radio?

A.

Edward Huggins · 1st Jun 2016, 2:34 pm

Thanks bikerhifinut for the heads-up on the speaker matching. Yes, I might have been a bit over-cautious. Yes, what is so wrong with that Sobell - not that we know as yet which model it is. Edward

indigo.girl · 1st Jun 2016, 2:43 pm

Quote:

Originally Posted by peter_sol

No that cable supplied is unscreened. Use the old cable from the gram for now, but you still have to connect the inner to the outer of both the input terminals of the amp and the earthed side to the centre of the 3 input terminals on the PCB.

I've added the resistor on the audio channel (I made up 430K from what I had to hand) and changed over to the original screened wire and its made a world of difference. What does the screening of the wire do? How does it work?

I still have a slight buzz so will try a capacitor across the DC adapter input next.

indigo.girl · 1st Jun 2016, 2:53 pm

When I got the Sobell there was no audio from the radio and lots of buzzing so I decided to extract the record deck and try building my own small amp/speaker unit to house it. I'm not experienced enough yet tackle all the valves in this one. I'm going to use the sobell 1950's cabinet to convert into a cocktail cabinet.

bikerhifinut · 1st Jun 2016, 2:54 pm

Quote:

Originally Posted by Edward Huggins

Thanks bikerhifinut for the heads-up on the speaker matching. Yes, I might have been a bit over-cautious. Yes, what is so wrong with that Sobell - not that we know as yet which model it is. Edward

No worries Edward, My failing is that I tend to forget that earlier semiconductor amplifiers were quite sensitive to load impedance to the point that they could be damaged.

A.

peter_sol · 1st Jun 2016, 2:55 pm

430K is fine.
Basically the screening "screens" or shields the inner sensitive live wire so that hum and other interference is not picked up.
The hum comes from the mains with wires under the floor and around the room.
Interference from power supplies, Radio transmitters ETC. You name it.

bikerhifinut · 1st Jun 2016, 2:55 pm

Quote:

Originally Posted by indigo.girl

When I got the Sobell there was no audio from the radio and lots of buzzing so I decided to extract the record deck and try building my own small amp/speaker unit to house it. I'm not experienced enough yet tackle all the valves in this one. I'm going to use the sobell 1950's cabinet to convert into a cocktail cabinet.

NOOOOOOOOOOO!!!!

If it can be saved please save it. Any one of us here will gladly help you and guide you through the tricky bits.

A.

bikerhifinut · 1st Jun 2016, 3:01 pm

Sorry I get a bit emotional about saving these little bits of history.

cheerfulcharlie · 1st Jun 2016, 4:04 pm

Does the hum (or noise) change when you touch the deck? lifting the arm for example.

indigo.girl · 1st Jun 2016, 5:12 pm

hmmm, damn things playing up now. I have reduced the buzz alot now using the screened cable, earthing the casing of the pot and putting a 4700uF/35V capacitor across the power source*. However, although the buzz is much quieter its still over-powering the audio as this is very quiet and barely amplified at all. So rather than reducing the buzz I now need to focus on increasing the gain of the amplifier (I thought the audio was louder earlier today but perhaps that was just because the hum was reduced and it 'appeared' loud rather than it actually being louder.... either way its too low to listen to. What have I done wrong?? I've connected the inputs of the two channels in the PCB together now so I can get the same signal to both speakers. Have I done this wrong somehow?

*note: for ease I have connected the smoothing capacitor into the holes in the circuit board made for the +ve and -ve for the battery supply as these both connect directly to the +ve and -ve of the power adaptor (as seen on the circuit diagram). Am I right with positioning here? I'm not sure since there seems to be two +ve terminals coming out the back of power adaptor input in addition to the -ve so it was not clear where to connect the capacitor...

indigo.girl · 1st Jun 2016, 5:13 pm

Quote:

Originally Posted by cheerfulcharlie

Does the hum (or noise) change when you touch the deck? lifting the arm for example.

Yes, the hum does change if I touch the metal on the deck and if I touch the outside of the metal screened cable...

indigo.girl · 1st Jun 2016, 5:21 pm

Quote:

Originally Posted by bikerhifinut

Sorry I get a bit emotional about saving these little bits of history.

Sorry bikerhifinut, I've already used some bits and pieces out of the old amplifier so I've passed the point of no return. Its a tricky one to judge isn't - how to restore things. Where I can I try to keep things as original as possible and would only take the deck out if there's no way for me to mend whatevers wrong. I'm not ready yet to tackle the big complicated valve amps and receivers so to me this was a non starter. But rather than discard the lot or leave it to rot in a loft somewhere I decided to salvage the deck and get that playing independently outside the radiogram. That way at least some of the vintage stuff can be used once more. Also, my house is pretty full already and so I need to down size some of my projects!

ms660 · 1st Jun 2016, 5:22 pm

Looking at the last lot of pictures reminds me of a 43:1 attenuator.

Lawrence.

indigo.girl · 1st Jun 2016, 5:24 pm

Quote:

Originally Posted by Lucien Nunes

The output of the iPod dock would be low impedance and will work with an unscreened cable without picking up hum. The cartridge is very high impedance and the input will be much more prone to hum pickup unless the screened cable is used, as it was originally.

When choosing an amplifier to work with a particular source, one needs to consider not only the signal level but also the source and load impedances. These days, most electronic sources are relatively low impedance and rather uncritical of connected loads. But vintage stuff, and low level devices such as mics and pickups, need the correct impedance for proper operation.

Thanks Lucien for explaining the high impedance issue for vintage stuff - this is really useful to know.

Lucien Nunes · 1st Jun 2016, 5:40 pm

Quote:

I'm not ready yet to tackle the big complicated valve amps and receivers so to me this was a non starter.

Don't give up on stuff just because you don't think you can fix it straight off. I have many exciting things that I certainly don't have the technology or knowhow to restore yet, it might take the help of 20 different engineering specialists and 20 years to get everything workable in my collection working, but it's all possible and I'll learn from them as we go along. This forum is the perfect resource to help you in the same direction...

indigo.girl · 1st Jun 2016, 5:52 pm

I think I've found the problem - the cart has now broken hence no audio. I have just exchanged the cart for a TC8H that I know works in another machine and I get a good volume from my little IC amp and speaker set up

I would still love feedback on whether my capacitor is wired in correctly just to be sure...

Also, this IC amp/speaker kit doesn't have a tone control. How complicated would it be for me to wire in another pot so I can adjust tone?

Clydeuk · 1st Jun 2016, 5:58 pm

Have to say I too feel it's a shame for the original radiogram. Obviously it's not extremely valuable but since it uses the UA8 deck that dates it to the late 1950's, so it's over 60 years' old. Much better than the later transistor stereograms of the late 60's and early 70's. Obviously you've made your decision on that one, but most of us here find pleasure in saving these vintage items, after all there probably won't be many of them left now as most will have gone to landfill in the 70's and 80's.

Clydeuk · 1st Jun 2016, 6:00 pm

Just a thought on your capacitor connected to the "battery" terminals. If this is designed so that a battery could be connected while still allowing operation from the adaptor, the power socket may be switched to disconnect the battery when the adaptor plug is inserted. So your capacitor in place of the battery may be disconnected. A meter check would tell you if this is the case.

1st Jun 2016, 10:38 am	#21
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Hum when old BSR connected to new op-amp/speaker circuit No that cable supplied is unscreened. Use the old cable from the gram for now, but you still have to connect the inner to the outer of both the input terminals of the amp and the earthed side to the centre of the 3 input terminals on the PCB.

1st Jun 2016, 10:40 am	#22
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: Hum when old BSR connected to new op-amp/speaker circuit The output of the iPod dock would be low impedance and will work with an unscreened cable without picking up hum. The cartridge is very high impedance and the input will be much more prone to hum pickup unless the screened cable is used, as it was originally. When choosing an amplifier to work with a particular source, one needs to consider not only the signal level but also the source and load impedances. These days, most electronic sources are relatively low impedance and rather uncritical of connected loads. But vintage stuff, and low level devices such as mics and pickups, need the correct impedance for proper operation. Regarding earthing, I would earth the deck, and the signal ground of the amplifier input if the mains adaptor is floating (i.e. the DC -ve is not connected to mains earth). Only earth it in one place to avoid loops and instability. I echo the above thoughts, what is wrong with the original amp that makes it necessary to use this alternative?

1st Jun 2016, 2:34 pm	#24
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,327	Re: Hum when old BSR connected to new op-amp/speaker circuit Thanks bikerhifinut for the heads-up on the speaker matching. Yes, I might have been a bit over-cautious. Yes, what is so wrong with that Sobell - not that we know as yet which model it is. Edward

1st Jun 2016, 2:53 pm	#26
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: Hum when old BSR connected to new op-amp/speaker circuit When I got the Sobell there was no audio from the radio and lots of buzzing so I decided to extract the record deck and try building my own small amp/speaker unit to house it. I'm not experienced enough yet tackle all the valves in this one. I'm going to use the sobell 1950's cabinet to convert into a cocktail cabinet.

1st Jun 2016, 2:55 pm	#28
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Hum when old BSR connected to new op-amp/speaker circuit 430K is fine. Basically the screening "screens" or shields the inner sensitive live wire so that hum and other interference is not picked up. The hum comes from the mains with wires under the floor and around the room. Interference from power supplies, Radio transmitters ETC. You name it.

1st Jun 2016, 3:01 pm	#30
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: Hum when old BSR connected to new op-amp/speaker circuit Sorry I get a bit emotional about saving these little bits of history.

1st Jun 2016, 4:04 pm	#31
cheerfulcharlie Heptode Join Date: May 2009 Location: Birmingham, West Midlands, UK. Posts: 708	Re: Hum when old BSR connected to new op-amp/speaker circuit Does the hum (or noise) change when you touch the deck? lifting the arm for example.

1st Jun 2016, 5:12 pm	#32
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: Hum when old BSR connected to new op-amp/speaker circuit hmmm, damn things playing up now. I have reduced the buzz alot now using the screened cable, earthing the casing of the pot and putting a 4700uF/35V capacitor across the power source. However, although the buzz is much quieter its still over-powering the audio as this is very quiet and barely amplified at all. So rather than reducing the buzz I now need to focus on increasing the gain of the amplifier (I thought the audio was louder earlier today but perhaps that was just because the hum was reduced and it 'appeared' loud rather than it actually being louder.... either way its too low to listen to. What have I done wrong?? I've connected the inputs of the two channels in the PCB together now so I can get the same signal to both speakers. Have I done this wrong somehow? note: for ease I have connected the smoothing capacitor into the holes in the circuit board made for the +ve and -ve for the battery supply as these both connect directly to the +ve and -ve of the power adaptor (as seen on the circuit diagram). Am I right with positioning here? I'm not sure since there seems to be two +ve terminals coming out the back of power adaptor input in addition to the -ve so it was not clear where to connect the capacitor... Attached Thumbnails

1st Jun 2016, 5:22 pm	#35
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Hum when old BSR connected to new op-amp/speaker circuit Looking at the last lot of pictures reminds me of a 43:1 attenuator. Lawrence.

1st Jun 2016, 5:52 pm	#38
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: Hum when old BSR connected to new op-amp/speaker circuit I think I've found the problem - the cart has now broken hence no audio. I have just exchanged the cart for a TC8H that I know works in another machine and I get a good volume from my little IC amp and speaker set up I would still love feedback on whether my capacitor is wired in correctly just to be sure... Also, this IC amp/speaker kit doesn't have a tone control. How complicated would it be for me to wire in another pot so I can adjust tone?

1st Jun 2016, 5:58 pm	#39
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: Hum when old BSR connected to new op-amp/speaker circuit Have to say I too feel it's a shame for the original radiogram. Obviously it's not extremely valuable but since it uses the UA8 deck that dates it to the late 1950's, so it's over 60 years' old. Much better than the later transistor stereograms of the late 60's and early 70's. Obviously you've made your decision on that one, but most of us here find pleasure in saving these vintage items, after all there probably won't be many of them left now as most will have gone to landfill in the 70's and 80's.

1st Jun 2016, 6:00 pm	#40
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: Hum when old BSR connected to new op-amp/speaker circuit Just a thought on your capacitor connected to the "battery" terminals. If this is designed so that a battery could be connected while still allowing operation from the adaptor, the power socket may be switched to disconnect the battery when the adaptor plug is inserted. So your capacitor in place of the battery may be disconnected. A meter check would tell you if this is the case.