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Old 13th July 2005, 12:58 AM   #1
Tweeky is offline Tweeky  United Kingdom
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Default preamp/power amp matching

Help, I have a Ming-Da preamp with a stated output of 39V, an output impedance of 50k, and an input? sensitivity of 200mV. It is connected to a Gamma Space Reference 300B power amp of unknown specification. I know that each signal input feeds directly into a 6SN7GTB valve which has a 100k resistor connected from the signal input to one of the other pins on the same valve. I read that there is supposed to be a 200X factor but do not understand this. Can someone please explain what this means and how to optimise the matching of these two components in terms that a relative beginner can understand?

Thanks

Steve
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Old 13th July 2005, 09:56 AM   #2
EC8010 is offline EC8010  United Kingdom
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Welcome to the forum. Now, to your question. It'll work. There, that's simple.

More detail? Well, the 39V is probably the maximum output that your pre-amplifier can produce without distortion. Obviously, the voltage it actually produces is set by how enthusiastic the music is and how far you advance the volume control. The 100k resistor you found is probably the grid-leak resistor of the power amplifier and this is effectively its input resistance. Almost any valve (and any semiconductor) stage should be able to drive this without problems. I haven't a clue what the 200x factor might be. You have nothing to worry about.
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Old 13th July 2005, 10:01 AM   #3
Giaime is offline Giaime  Italy
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Hi, I think he means the plate resistor of the input tube to be 100k, and no, that doesn't provide a gain of 200. It is probably around 30/40 to fully drive a 300B.
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Old 13th July 2005, 10:27 PM   #4
Tweeky is offline Tweeky  United Kingdom
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Thanks for your help guys. The combination goes loud at quite small movements of the volume control so I wanted to be sure that things were ok.

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Steve
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Old 13th July 2005, 10:57 PM   #5
mcs is offline mcs  Denmark
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Quote:
Originally posted by Tweeky
Thanks for your help guys. The combination goes loud at quite small movements of the volume control so I wanted to be sure that things were ok.
The problem is that you have a preamp with gain, and you don't actually need any gain. It won't cause any problems apart from only being able to use a small bit of the volume control range

Are you sure about the 50k output impedance? It sounds very high.

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Mikkel C. Simonsen
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Old 14th July 2005, 08:09 PM   #6
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Hi,
I am working on my own pre-amp design. I am surprised to hear 39V. I have come to understand that all you need to drive most amps is around 2V p-p.
Given this, is would not surprise me that it is very loud. Either the preamp gain is way to high (my design will be around A=20) or the attenuation of the volume control is not all that well suited for the application
I am hoping that some one can back me up on this.
Marc
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Old 14th July 2005, 08:49 PM   #7
mcs is offline mcs  Denmark
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The mentioned 39V is probably the maximum output.

For a normal linestage a gain of 0-5 is enough. But many linestages have gains in the 10-20 range which causes the problem.

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Mikkel C. Simonsen
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