Hum when old BSR connected to new op-amp/speaker circuit - Page 7 - UK Vintage Radio Repair and Restoration Discussion Forum

PsychMan · 6th Jun 2016, 12:14 pm

Indigo girl, nice to see you're trying to make use of the old kit and learning along the way. I think its important we help people learn their way with audio like this.

I've attached a simple circuit, which may or may not be of use to you. It will enable you to send the ceramic cartridge output into a modern magnetic phono preamp with good results. So if you wanted an easier path to using the deck on its own with modern equipment, this might work well for you while you learn how to repair the rest of it

Obviously you will need 2 of these networks for stereo, very simple with 1 capacitor and 2 resistors per channel. I used this myself a while back as I wanted to run an auto changer through my hifi rig. I understand such networks used to be in amplifiers that featured a "ceramic / magnetic" switch on the phono input.

(excuse the poor diagram, should do the job though!)

Edward Huggins · 6th Jun 2016, 12:30 pm

If you only have one speaker in your Sobell cabinet, how are you going to connect up your new IC Stereo amplifer? Are you going to connect a second speaker somewhere else, say, in a seperate cabinet? Edward

indigo.girl · 6th Jun 2016, 12:34 pm

Quote:

Originally Posted by PsychMan

I've attached a simple circuit, which may or may not be of use to you. It will enable you to send the ceramic cartridge output into a modern magnetic phono preamp with good results. So if you wanted an easier path to using the deck on its own with modern equipment, this might work well for you while you learn how to repair the rest of it

Thanks PsychMan, this is fantastic. It looks very simple to construct and a very easy way to get an old autochanger deck playing through the phono input on a hifi. In laymans terms, what is this capacitor/resistor combination doing to the signal? I know the output from a ceramic will be very high compared to a magnetic cart so is it reducing that down a bit or is it something to do with equalisation?

I'm not sure what I'll end up making out of this project but I feel like I've learnt so much and have so many options to try out and compare. I'm an ex-scientist so love all this kind of stuff!
Nicola

PsychMan · 6th Jun 2016, 12:48 pm

Its both attenuation and equalisation. The resistors form a voltage divider to attenuate the signal down to the level of a magnetic cartridge (or thereabouts), and the capacitor provides a rising high frequency response so that when the RIAA curve is applied you get the results you'd expect. You're essentially making the signal similar enough to that of a magnetic cartridge

It worked well for me, but then I started using a Garrard SL65b with magnetic cartridge so it became redundant. You can experiment with different resistor values if that combo doesn't work. Ceramics can vary in output, from what I recall the circuit posted worked well with a BSR SC5H high output cartridge.

Clydeuk · 6th Jun 2016, 9:19 pm

Can this amplifier be modified so the channels are bridged, as this should give more output (mono) to a single speaker?

I think it was suggested above but in light of the OP using the original speaker it might be worth revisiting.

Ignore this comment if it's too complicated.

Edward Huggins · 6th Jun 2016, 10:47 pm

Ohh - not a good idea! Edward

indigo.girl · 6th Jun 2016, 11:41 pm

With only one speaker in the cabinet I only need mono from by stereo cart so I've bridged the cart outputs so there's just one ground and one signal wire coming from the pick up arm. This gets amplified through one channel of the amp and then into one speaker.

Clydeuk · 6th Jun 2016, 11:47 pm

By bridging, I didn't mean joining the outputs together. I was thinking along the lines of a car amplifier, where it can be configured in bridged mode to power a subwoofer, or used as 2 channels for normal stereo. In bridged mode generally the output power in increased.

Actually doing it would involve careful study of the i.c datasheet to see if it's possible. I certainly wouldn't suggest parallelling the outputs as this would very likely destroy the i.c.

peter_sol · 7th Jun 2016, 6:48 am

It can be bridged as per the data but lets see how the single channel goes first.

Edward Huggins · 7th Jun 2016, 7:29 am

What about the loading on the other IC channel where no speaker is planned to be connected? Edward

julie_m · 7th Jun 2016, 7:36 am

IC amplifiers are as near as possible ideal voltage sources, and don't mind running an into open circuit.

indigo.girl · 7th Jun 2016, 4:46 pm

Bit confused here then - so what is the best way to connect a stereo cart to a stereo amp with only one speaker? I get good audio when just one channel from the stereo cart goes through the amp with only one speaker connected. I can't really notice the difference by ear when both cart channels are joined so that a mixture of the two channels gets played through one speaker. I'd prefer to have both channels from the cart playing through the speaker ideally. Is this a problem? Alternatively, why is it not a good idea to have both channels from the amp playing through one speaker?

indigo.girl · 7th Jun 2016, 5:03 pm

Quote:

Originally Posted by bikerhifinut

Its difficult to say.
For sure matching the impedance is a good thing.
I have a feeling that your little amp and speakers just ain't that good and you might not be getting the best out of things.
You could try a pair of headphones and that would indicate if the amp itself is ok and its the speakers that are the issue. They'll sound pretty tinny anyway whatever you feed into them.
The main thing about impedance matching so to speak, is that you don't lose the low frequency parts of a signal. This is very simplified. You also lose a bit of the signal but I doubt its that much in this instance.

A.

I've just done a test comparing impedance matching vs signal attenuation by putting a 1M pot in series with the cart output (in place of the 430K resistor Peter suggested to me). With the pot set to zero ohms the audio is much louder but uncomfortably tinny and an emphasis on cymbals. With the pot set to 1M ohms the audio is quieter but much richer and fuller. This has really nicely demonstrated to me the trade off between impedance matching and the attenuation of the output. Using the pot I can set it to optimal which to my ear is at about 100-200 ohms. With just a small increase in impedance the balance of the frequencies is much improved but I still retain good volume. So actually its not crucial to perfectly match the impedance, but increasing the amps impedence by just a factor of ten is enough to make a big difference. For my purposes I think this is enough

Thanks Andy for your idea about the pot.
Nicola

Clydeuk · 7th Jun 2016, 5:05 pm

You shouldn't join the outputs together as this will probably damage the i.c

The suggestion that seems the best is to connect the outputs of the cartridge together in parallel (+ to + and - to -) then feed this into one channel of the amplifier.

Perhaps the reason you've not been able to notice any difference when connected this way is that you are maybe playing a mono record? A mono record will provide the same information to both channels of a stereo cartridge.

Clydeuk · 7th Jun 2016, 5:07 pm

I wouldn't worry too much about the impedence matching, it's not going to do any harm to either the cartridge or the amp, and the best suggestion with what you've done is to leave it they way it sounds best to you.

indigo.girl · 7th Jun 2016, 6:06 pm

Thanks Clyde

peter_sol · 7th Jun 2016, 6:11 pm

You say 100 to 200 ohms Did you mean K ohms?
I would parallel the cart for single amp use.

indigo.girl · 7th Jun 2016, 6:57 pm

Yes, sorry, I meant to say 100 K ohms. I've paralleled the cart and compared it to just a single channel (but this time using a stereo record - thanks Clyde). Much better with both channels mixed

indigo.girl · 7th Jun 2016, 7:01 pm

Quote:

Originally Posted by Nickthedentist

I have an unused Bardwell module that was kindly given to me by a forum member, but which I'm unlikely to ever use. You can have it for the cost of the postage. PM me if you want.

Just wired up your Bardwell amplifer Nick - wow what an improvement! The 5W output per channel is powerful. I can get away with turning the 1M ohm pot up all the way because it doesn't matter that the input signal drops - I still get more than enough volume

peter_sol · 7th Jun 2016, 7:29 pm

You have still learnt a great deal first hand and hopefully although frustrating at times, I hope,had fun as well.
Those carts are best, if possible, to play into between 500 K and 1 meg.
as you have found out.

6th Jun 2016, 12:14 pm	#121
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: Hum when old BSR connected to new op-amp/speaker circuit Indigo girl, nice to see you're trying to make use of the old kit and learning along the way. I think its important we help people learn their way with audio like this. I've attached a simple circuit, which may or may not be of use to you. It will enable you to send the ceramic cartridge output into a modern magnetic phono preamp with good results. So if you wanted an easier path to using the deck on its own with modern equipment, this might work well for you while you learn how to repair the rest of it Obviously you will need 2 of these networks for stereo, very simple with 1 capacitor and 2 resistors per channel. I used this myself a while back as I wanted to run an auto changer through my hifi rig. I understand such networks used to be in amplifiers that featured a "ceramic / magnetic" switch on the phono input. (excuse the poor diagram, should do the job though!) Attached Thumbnails

6th Jun 2016, 12:30 pm	#122
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 If you only have one speaker in your Sobell cabinet, how are you going to connect up your new IC Stereo amplifer? Are you going to connect a second speaker somewhere else, say, in a seperate cabinet? Edward

6th Jun 2016, 12:48 pm	#124
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: Hum when old BSR connected to new op-amp/speaker circuit Its both attenuation and equalisation. The resistors form a voltage divider to attenuate the signal down to the level of a magnetic cartridge (or thereabouts), and the capacitor provides a rising high frequency response so that when the RIAA curve is applied you get the results you'd expect. You're essentially making the signal similar enough to that of a magnetic cartridge It worked well for me, but then I started using a Garrard SL65b with magnetic cartridge so it became redundant. You can experiment with different resistor values if that combo doesn't work. Ceramics can vary in output, from what I recall the circuit posted worked well with a BSR SC5H high output cartridge.

6th Jun 2016, 9:19 pm	#125
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: Hum when old BSR connected to new op-amp/speaker circuit Can this amplifier be modified so the channels are bridged, as this should give more output (mono) to a single speaker? I think it was suggested above but in light of the OP using the original speaker it might be worth revisiting. Ignore this comment if it's too complicated.

6th Jun 2016, 10:47 pm	#126
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 Ohh - not a good idea! Edward

6th Jun 2016, 11:41 pm	#127
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 With only one speaker in the cabinet I only need mono from by stereo cart so I've bridged the cart outputs so there's just one ground and one signal wire coming from the pick up arm. This gets amplified through one channel of the amp and then into one speaker.

6th Jun 2016, 11:47 pm	#128
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: Hum when old BSR connected to new op-amp/speaker circuit By bridging, I didn't mean joining the outputs together. I was thinking along the lines of a car amplifier, where it can be configured in bridged mode to power a subwoofer, or used as 2 channels for normal stereo. In bridged mode generally the output power in increased. Actually doing it would involve careful study of the i.c datasheet to see if it's possible. I certainly wouldn't suggest parallelling the outputs as this would very likely destroy the i.c.

7th Jun 2016, 6:48 am	#129
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 It can be bridged as per the data but lets see how the single channel goes first.

7th Jun 2016, 7:29 am	#130
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 What about the loading on the other IC channel where no speaker is planned to be connected? Edward

7th Jun 2016, 7:36 am	#131
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Hum when old BSR connected to new op-amp/speaker circuit IC amplifiers are as near as possible ideal voltage sources, and don't mind running an into open circuit. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

7th Jun 2016, 4:46 pm	#132
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 Bit confused here then - so what is the best way to connect a stereo cart to a stereo amp with only one speaker? I get good audio when just one channel from the stereo cart goes through the amp with only one speaker connected. I can't really notice the difference by ear when both cart channels are joined so that a mixture of the two channels gets played through one speaker. I'd prefer to have both channels from the cart playing through the speaker ideally. Is this a problem? Alternatively, why is it not a good idea to have both channels from the amp playing through one speaker?

7th Jun 2016, 5:05 pm	#134
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: Hum when old BSR connected to new op-amp/speaker circuit You shouldn't join the outputs together as this will probably damage the i.c The suggestion that seems the best is to connect the outputs of the cartridge together in parallel (+ to + and - to -) then feed this into one channel of the amplifier. Perhaps the reason you've not been able to notice any difference when connected this way is that you are maybe playing a mono record? A mono record will provide the same information to both channels of a stereo cartridge.

7th Jun 2016, 5:07 pm	#135
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: Hum when old BSR connected to new op-amp/speaker circuit I wouldn't worry too much about the impedence matching, it's not going to do any harm to either the cartridge or the amp, and the best suggestion with what you've done is to leave it they way it sounds best to you.

7th Jun 2016, 6:06 pm	#136
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 Thanks Clyde

7th Jun 2016, 6:11 pm	#137
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 You say 100 to 200 ohms Did you mean K ohms? I would parallel the cart for single amp use.

7th Jun 2016, 6:57 pm	#138
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 Yes, sorry, I meant to say 100 K ohms. I've paralleled the cart and compared it to just a single channel (but this time using a stereo record - thanks Clyde). Much better with both channels mixed

7th Jun 2016, 7:29 pm	#140
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 You have still learnt a great deal first hand and hopefully although frustrating at times, I hope,had fun as well. Those carts are best, if possible, to play into between 500 K and 1 meg. as you have found out.