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In Reply to: RE: First stage phono preamp transformer output coupled posted by KevinC on March 06, 2017 at 16:54:32
Hi Kevin,
in my design 7788 is triode strapped, bias is around Va=140V, Ip=30mA, with Rp=10k to get max transconductance (50mA/V) that gives a total gain around 40 (32dB).
LCR RIAA network 600ohm is the load on secondary trasformer. With 4:1 transformer it is reflect to primary around 10k that match 7788 Rp needed.
My reasoning is that correct?
LL1692A/30mA version is in stock or is a special order version ?
What do you think about LL1660/30mA version 4.5:1 connection (100H) in my SE ?
Thanks.
Best regards.
Follow Ups:
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
I can not fully understand your explanations, so in order to not make you lose too much time chatting, I sent you an email with the circuit design (ltspice).
In this way you can well understand what I'm trying to build...
Thanks.
Carlo Alberto.
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