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In Reply to: Re: Choke loaded triode output impedance posted by Tre' on April 7, 2007 at 14:55:51:
At high freq. the choke is a very high impedance and with a 100Hy choke the low freq. impedance is fairly high as well. The output impedance of the stage is (or very close to) the Rp of the tube.
Follow Ups:
I was looking at a choke loaded 1st tube for a phono amp and was wondering if I could get the accuracy for the RIAA calcs. A D3a at its normal operating point has an RP of 1.9K , a 100H 50ma choke would be OK as a load. So the output impedance would be ? Is 1.9K good enough to work out the rest?
Between 100hz and say 10K i think it is fair to say that the impedance of the choke will be large enough that it can be ignored and the 1K9 value can be used as the starting point for the filter. above and below those frequencies you get into a gray area where things may get a bit interesting. Worst case, the 1900R value you are basing your filter on will go down. This means you need to understand what will happen at say 20hz and 20K IF your source Z suddenly drops below 1K9.depending on your filter type, you might be able to use the inductors behavior to help tune the response of your riaa filter. It is also important to understand that the 1K9 you are using for your source impedance from the D3a is also a very soft number.
all in one RIAA filter what will happen when the impedance drops below the 1K9 below say 100hz? Will the bass go or be boosted.
Hi!At the point where the impedance of your chokes starts to have an effect on the output impedance, your driving stage will start to have a roll off.
Typically one would select the input impedance of the RIAA network to be about 10 times the output impedance of the driving stage. A drop of the output impedance would theoretically boost a bit, but much less than the rolloff which it would have itself.
With a rp of 1.9k it should not be difficult to find a choke which gives a very low roll off frequency.
Best regards
The choke that I have has -1db @ 12hZ, -3DB @ 3hZ and -.1 @ 128Hz at an Rp of 1K9.Is this good enough?
Hi!Would be good enough for me!
oops... forgot to close the bold tags on the earlier post...your bet bet is to simply sim it and see for yourself how the ideal should work. I have been dealing with LR and LCR networks lately so i'm not sure what will happen with your particular network.
IMO that's a problem. A plate choke load is a reactive part. The RIAA filter has more reactive parts. Too complex for me to work out. But don't let that stop you. You could always build it, listen to it, tweak it and listen again.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I thought as much. You do not see chokes in phono stages.I suppose it would be suck it and see approch. Thanks for your help.
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