preamp/power amp matching

Tweeky · 12th July 2005, 11:58 PM

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

EC8010 · 13th July 2005, 08:56 AM

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.

Giaime · 13th July 2005, 09:01 AM

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.

Tweeky · 13th July 2005, 09:27 PM

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.

Regards

Steve

mcs · 13th July 2005, 09:57 PM

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.

Best regards,

Mikkel C. Simonsen

photo_marc · 14th July 2005, 07:09 PM

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

mcs · 14th July 2005, 07:49 PM

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.

Best regards,

Mikkel C. Simonsen

12th July 2005, 11:58 PM	#1
Tweeky diyAudio Member Join Date: Jul 2005 Location: East Sussex	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

13th July 2005, 08:56 AM	#2
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	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. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

14th July 2005, 07:49 PM	#7
mcs diyAudio Member Join Date: Aug 2003 Location: Denmark	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. Best regards, Mikkel C. Simonsen __________________ My personal electronics website Remote control kits website

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13th July 2005, 09:01 AM	#3
Giaime diyAudio Member Join Date: Oct 2004	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.

13th July 2005, 09:27 PM	#4
Tweeky diyAudio Member Join Date: Jul 2005 Location: East Sussex	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. Regards Steve

14th July 2005, 07:09 PM	#6
photo_marc diyAudio Member Join Date: Jun 2005 Location: boston	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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