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In Reply to: RE: Conrad Johnson Classic 2SE tube question posted by airtime on June 04, 2015 at 06:43:50
I don't think you quite understand what Kevin is saying.
He's pointing out that 27dB of gain is a ridiculous amount for a linestage, and that any hint of noise in such a circuit would be amplified to ridiculous proportions, especially since you will be attenuating your source signal way, way down before letting it into the active preamp circuitry.
This problem is not solved by you using a volume control before the preamp. The level control is the first part in the signal path of the preamp, so six of one, half dozen of the other.
He mentions another solution, which may only be available by phone, but I would suspect that in involves a pair of -20dB attenuators plugged into the input jacks of your power amplifier, which would go a long way to remedying these problems.
To put this into perspective, let's say you have a fairly sensitive amp, which will make 100W with 0.5V in, and you have a 2V source. With your preamp's 27 dB of gain, it could take that 2V source and put out 44V (it won't, there won't be enough available voltage). To get from 2V to 0.5V, you need -12dB of attenuation. To get 0.5V out of your preamp, you need -39dB of attenuation. If you do this, you are now left with 22mV of source signal entering the first stage of your preamp. This is a little bit more than a moving magnet cartridge!
The gnarly part about this is that I have assumed that you need all 100W that your speakers deliver, but if you have sensitive speakers, maybe you only need 5W for most listening situations, and then we are really down to delivering only a few mV of signal. In a phono preamp, this works out fine, as you take additional precautions to keep noise down, but we aren't exactly talking about a phono preamp here.
Follow Ups:
Are you saying to put an attenuator between the preamp and amp?
say for a 25db attenuation use a 85k resistor in line with a 10K to ground.?
charles
Yes, attenuation at the amplifier is a very good idea (or buying a preamp with a realistic design to begin with).
The amount of padding will depend a little bit on how sensitive the speakers are and how sensitive the amplifier is.
The 85K/10K combo will provide 20dB of padding. You will typically see morel like 8.5K/1K, which is more immune to the resistor installed across the RCA jack in the amplifier, and less likely to pick up noise when used.
So I'll try a 10k line 1K ground shunt coming out of the preamp to the amp input. I'm using the 10K because I have a 1/4 watt metal film of that value.
I'm at the plug and play part of my life. When I decided on this preamp I didn't sign on to having this kind of problem. I did do some research before choosing this preamp. And in all sincerity I had visions of rolling 6922 tubes dancing in my head. The one joy I have with tubes. Sonically I have no complaints. but truthfully if I was aware of this problem - eh - who knows?
Thanks
Charles
Be sure to use it right at the input of the amplifier, not the output of the preamp.
Harrison Labs makes a -12dB in-line attenuator that's easy to use. Sadly, it's not enough of a pad for you, but it's a good start.
Also, if you have level trim pots at the input of your amplifier, you can crank those way down to the same effect.
Is this basically the same if cranked up around a third or half way? This amp has a 100k variable input and 10K fixed switchable input.charles
Edits: 06/05/15
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