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Old 25th May 2007, 10:44 PM   #1
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Default what causes noise in a SS preamp?

Hi all,

I have an old electrocompaniet SS preamp - its one of the early lot that were made in Scotland and its labeled "the electro".


On tuner input its all but silent. On MM input its a bit noisy when turned up, and on MC input its very noisy when turned up. In other words I suppose the s/n is a bit low - the more amplification it provides, the noisier it gets. Other than that the SQ is OK for what it is, ie I think the RIAA is OK.

The noise is a kind of "rushing" white noise type sound.

I'm pretty unfamiliar with SS stuff, so would anyone have any ideas as to what might be causing this? Would anyone have a schematic?

I also have a DC offset on one channel to the order of a couple of volts!

any help greatly appreciated.... (I'll try and get some pics tomorrow)


Fran
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Old 26th May 2007, 05:37 AM   #2
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Default Noise in SS Preamp

Fran,

Noise is an unavoidable consequence of the power amplificaion process. The higher the gain of an amplifier, the more noticable noise becomes. This is in part due to the intrinsic noise of the source resistance and the amplifier itself. In fact, a simple resistor has a thermal noise component that is a function of its resistance and its temperature.

The gain of the tuner input stage is probably no more than ~20 dB, while the unequalized gain for the MM cartridge is ~50 dB, and the gain for a MC cartridge is ~70 dB. In other words, the higher the gain, the more the noise of the input stage and source resistance is amplified. this explains why the tuner input was quieter than the MM or MC inputs.

The value of the source resistance is also important, and an amplifier will produce excessive noise if too high a source resistance is employed. For example, a MM cartridge has an impedance of ~47K, while a MM cartridge typically has a resistance of less than 100 ohms. If the MC input is left open or driven with a high impedance source, then the amplifier will sound noisy.

The rushing sound you heard is primarily thermal noise from the input stage and source resistance. The phono equalization generates a decreasing gain at higher frequencies. Otherwise the noise you would hear would be spectrally flat and would sound like a hiss.

the DC offset indicates that something is wrong with the preamp. That is not normal.

Hope this helps.
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JCM
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Old 26th May 2007, 09:13 AM   #3
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Thanks for that info.....


So what are the typical causes of DC offset in a SS pre like this? Where would I go to look first? (I know its not the source or the amp following it). I haven't got the thing right in front of me now, but would it be the transistors? Are old transistors available still?


Fran
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Old 26th May 2007, 10:04 PM   #4
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Maybe it was a stupid question, but I'd really appreciate any help as to where I should look first when sorting out this DC offet issue? Anyone any suggestions?


Fran
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Old 27th May 2007, 01:19 AM   #5
IanAS is offline IanAS  England
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Did you have a record deck, ie, cartridge plugged in? If not that'll be noisy. Try connecting one and see if the noise goes.

DC offset could be a pot needing adjusting or a faulty component. Just have to measure back to a point where there is no offset and see if you can identlfy where it starts.

You'd probably need the schematic.
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Old 27th May 2007, 10:33 AM   #6
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Many thanks,


I'll measure back as far as I can and see where it comes in. I'd say the chances of a schematic are small - I can't even find a pic of this thing on the web!

Fran


EDIT: yep, I did have the cart connected up..
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