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K&K Audio / Lundahl Transformers Welcome! Need support, you got it. Or share youe ideas and experiences. |
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In Reply to: RE: First stage phono preamp transformer output coupled posted by d888sp4@gmail.com on March 7, 2017 at 00:27:30:
If you use a plate resistor of 10K, the plate impedance is about the inverse of 1/10K+1/4K or 2.86K. Formally speaking the plate resistor and the tube are in parallel, so the parallel combination of their impedances are the gain stage impedance. If you use a transformer to load the 7788, then the stage impedance is that of the 7788 transformed by the turns ratio of the transformer squared. So if you assume the 7788 has a plate impedance of 4K and you use an LL1692A/30mA with a turns ratio of 3.5:1, then the output impedance of you stage is 326 ohms. The use of a 600ohm load resistor is unnecessary with this set-up, but we should try to get closer to 600ohms with the 7788 and the transformer. Using the LL1660/30mA at 4.5:2 gives about 790ohms, which is closer. To get closer yet will require adjusting the operating conditions of the 7788.
If I haven't lost you in all of this, you can see the process. If you are lost, well let's chat more.
Kevin Carter
K&K Audio
www.kandkaudio.com
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Follow Ups
- RE: First stage phono preamp transformer output coupled - KevinC 17:34:19 03/08/17 (1)
- RE: First stage phono preamp transformer output coupled - d888sp4@gmail.com 23:57:32 03/08/17 (0)
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