Hum when old BSR connected to new op-amp/speaker circuit

[indigo.girl]

This is becoming rather a long thread Edward but I can assure you that I'm learning so much in this discussion. The desired outcome for me is to salvage as much as possible from an old 1950's radiogram. The receiver is beyond repair for me at my current skill level so instead of discarding the lot I'm trying to utilise the deck in some way. I may put it back in the old cabinet and use the original speakers or I may fashion a smaller case for it with some different speakers. Either way I needed to learn how to match this old deck with a modern IC amp and stumble across all the problems with buzz, impedance matching, attenuation and small speaker size along the way. I feel I've got a much better understanding of the issues now and really appreciate everyones help and advice

[indigo.girl]

Originally Posted by Clydeuk

Impedance matching would probably get rid of the distortion. Also paralleling the 2 channels on the cartridge would improve the sound as at the moment you're only hearing one channel of the stereo pair, alternatively since your little amp is stereo you could remove your bridge on the input and wire in both channels.

Just to double check what you mean here Clyde - paralleling the 2 channels on the cart means I can play both channels through the same speaker?

[Herald1360]

Yes. You'll hear everything that's on the record instead of only part of it. It will be in mono of course, you can't have stereo without two separate signal paths and speakers.

If you put the deck back into the Sobell cabinet along with the Bardwell amplifier and some passable speakers, you'd have a record player that would blow the socks off the original Sobell setup. 10 watts per channel from a domestic player is serious noise

[indigo.girl]

Originally Posted by peter_sol

What is the volume like with the new cart and better speaker?
You need the speaker on its baffle or in its case to judge or mount the small speakers on a baffle you can't just can't leave them out in the open. For a test you could get a piece of cardboard an cut holes and mount them on that.

I'll have a go with the speaker mounted in the cabinet. I've ordered a 1M pot too so I can have a play with changing the resistance I add in series with the amp. I want to explore this attenution vs impedance matching concept in practice. I will also try Andy's idea of putting the 1M pot in place of the 10K pot in the PCB. If I understand it correctly having the pot in this position will still increase the total input impedance of the amp as required but most of the voltage will drop across the 1M pot (and hence the input of the amp) rather than it being lost across the big resistor if its simply positioned in series with the cart.

I also need to sort out the issue of combining the two mains supply power sources. I have one mains supply plug running straight to the motor at 240V and also a mains adaptor plug supplying 12V to the amplifier. I'm presuming there's just a simple transformer inside the casing of the adaptor plug that I can get out and connected it to the other mains supply one I only have one power lead trailing from the machine?

[peter_sol]

I would leave the 12v adaptor alone (it may be a transformer type or maybe a switch mode type) and just use a 2 way mains socket. using a 1M pot instead of the 10K one will still leave you with 100K input impedance but it is on the way, and yes the pot to vary the resistance and see what happens is a good idea.

[indigo.girl]

Originally Posted by Herald1360

Yes. You'll hear everything that's on the record instead of only part of it. It will be in mono of course, you can't have stereo without two separate signal paths and speakers.

have joined the two channels and works great

[indigo.girl]

Originally Posted by peter_sol

I would leave the 12v adaptor alone (it may be a transformer type or maybe a switch mode type) and just use a 2 way mains socket. using a 1M pot instead of the 10K one will still leave you with 100K input impedance but it is on the way, and yes the pot to vary the resistance and see what happens is a good idea.

The mains adapter I'm using has a 12V / 2Amp output. I'm presuming that 2A is not necessary for this application. Whats the lowest Amps I could get away with ? 500mA?

[Herald1360]

That mini amp you have is capable of about 1 wpc so wet finger in the air, say 40% efficient that's 5 watts from the psu. So yes, 500mA should be OK at 12V.

My take to tidy it up would be to build or obtain a small open chassis type psu and mount that under the deck somewhere so you only need one mains lead from the plinth to the wall. If there's room the psu could be a wallwart in a mains socket.

[Edward Huggins]

Ah, you just said "speakers", whereas earlier you said 9" speaker. Are there 2 speakers in the old Sobell radiogram cabinet then? Be aware 9" is an odd size. They are either 8" or 10" and will be 3 ohms impedence. I am getting more confused and this thread continues......Edward

[GeoffK]

Philips 654A #90 (353A chassis) is that also a scrap radiogram?

[indigo.girl]

My plans is to now put the deck back into the original sobell radiogram cabinet and use the exiisting speaker thats inside. (I'm effectively just swapping the valve amplifier and receiver for my own solid state amplifer that I will impedance match to the cart). If the speaker inside is only a 3ohm can I boost this up to the necessary 6-8ohms but putting a resistor in line with it?
Nicola

[ms660]

If you connect a resistor in series with a loudspeaker the power to the loudspeaker will be reduced.

Lawrence.

[peter_sol]

the 3 Ohm one should do fine.

[Clydeuk]

Personally I think the amplifier you're using is a bit feeble for the job you want it to do. It will work but I don't think you'll get much volume at a decent quality. Whether what you get is sufficient only you will be able to determine by trying it.

[indigo.girl]

Originally Posted by peter_sol

the 3 Ohm one should do fine.

Thanks Peter - so I can't damage the amp by the low impedance speaker drawing too much power from it?

[ms660]

Originally Posted by pwdrive

If you connect a resistor in series with a loudspeaker the power to the loudspeaker will be reduced.

Just to add to the above for the OP, there's quite a lot to learn with all this electronics stuff and understandably it can sometimes be hard for beginners, one of the first ports of call is the Ohms Law triangle and the Power triangle, Googling will reveal all, the Ohms Law triangle is as good for impedance as it is for resistance, those formula's are the first ones most folks learn and memorize, well worth doing if you haven't already done so, they can answer lot's of questions.

Lawrence.

[Herald1360]

In principle you could, but most modern ic amps should be idiot proof to a first approximation. Just don't run it flat out.

[peter_sol]

The IC is OK for 4 ohms and 3 ohms is the old 4 ohms
A speakers impedance varies across the audio frequency band so its quoted impedance is nominal.
Should not do damage but if it does it does
Nobody can be sure
I should just try it and see how it goes.

[Edward Huggins]

In my post #109 I asked about the Sobell's speaker or speakers. In your post #111, you said "speaker". Do you mean there is only one speaker in the old Sobell cabinet? Edward

[indigo.girl]

Yes. Just one speaker