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 Post subject: C3m preamp
PostPosted: 27 Nov 2012, 02:13 
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If you wish to follow how this thread was started you may want to go here first: http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4151

I have ordered sockets and tubes (4 of) for the preamp. I'll re-post Matt's schematic here for those not wanting to wade through the previous thread.

Matt thanks once more for the effort you took in designing a preamp around this (yet another) mysterious looking tube. For me it's another standard PS (I now build them with my eyes closed) based around an external 24VAC 1A wall wart. I should have no problems producing the 20V for the filament at 125mA. When the tubes and sockets arrive I'll progress with the build.

In the meantime I will knock-up the PS and develop the 20VDC for the filament.
Attachment:
C3M schematic.jpg


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 Post subject: Re: C3m preamp
PostPosted: 27 Nov 2012, 04:37 
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Nice and simple :)

But why the volume control on the output side? :confused:

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 Post subject: Re: C3m preamp
PostPosted: 27 Nov 2012, 05:07 
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This comes from Pass (I think - memory is drifting a little). The signal you receive from your CD player or phono preamp is all you have to work with. Why reduce it then amplify it/ Why not amplifier the the whole signal, intact and then reduce it down to the levels you want when it is bigger and able to drive the next stage.

Your preamp has to able to handle the full input signal, unattenuated, generally around 2 to 3V.

I might be wrong about Pass.

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 Post subject: Re: C3m preamp
PostPosted: 27 Nov 2012, 05:22 
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It was Pass :)

But he uses 5K pots and not 250K ones.

Consider cable capacitance and the low pass filter effect may be seriously rolling off the highs.

Cheers!

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 Post subject: Re: C3m preamp
PostPosted: 27 Nov 2012, 05:48 
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Matt is probably better to answer this. But it is to do with the drive capabilities of the single stage amp. I use to build and are yet to compete one last, two stage 12AX7 preamps. The last stage was a cathode follower. Great drive capabilities into low impedance loads.The single stage tube preamp does not have a high drive capability.

Pass probably uses 5K pots for SS preamps. Tubes have a higher output impedance and will not drive low impedance loads well.

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 Post subject: Re: C3m preamp
PostPosted: 27 Nov 2012, 16:40 
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This has to be a very special pre-amplifier with a unique sound! :D

Please let us know your listening impressions as soon it sings to you...


Have fun! :)

Miguel


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 Post subject: Re: C3m preamp
PostPosted: 27 Nov 2012, 16:57 
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It would want to be exceptionally good to beat the 6N7 - "Heavy Metal" preamp.

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 Post subject: Re: C3m preamp
PostPosted: 27 Nov 2012, 23:29 
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Geek wrote:
But why the volume control on the output side?
This is simple. Most potentiometers are carbon devices and this means that they are the most noisy elements in the circuit. The circuit is really two stages, an amplifier and a lossy control. The total noise figure of the circuit is dominated by the first element in the chain. By placing the noisy potentiometer after the high gain amplifier, the potentiometer's contribution to the overall noise figure is divided by the gain of the first stage. In effect one gets about a 20dB noise figure improvement from putting the control on the output rather than the input.

For more information on noise figure, look here (http://en.wikipedia.org/wiki/Noise_figure). At the bottom of the description is Friis' formula for cascaded noise figure (http://en.wikipedia.org/wiki/Friis_formulas_for_noise).

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 Post subject: Re: C3m preamp
PostPosted: 28 Nov 2012, 00:40 
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Thanks Matt.

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 Post subject: Re: C3m preamp
PostPosted: 28 Nov 2012, 02:20 
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Suncalc wrote:
Geek wrote:
But why the volume control on the output side?
This is simple. Most potentiometers are carbon devices and this means that they are the most noisy elements in the circuit.


Incorrect... the tube is.

Use conductive plastic for real low noise :)


Quote:
The circuit is really two stages, an amplifier and a lossy control. The total noise figure of the circuit is dominated by the first element in the chain. By placing the noisy potentiometer after the high gain amplifier, the potentiometer's contribution to the overall noise figure is divided by the gain of the first stage. In effect one gets about a 20dB noise figure improvement from putting the control on the output rather than the input.


High gain, yes. But a line stage?

Cheers!

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