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Old 23rd February 2014, 10:04 AM
Richard Richard is offline
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Join Date: Dec 2005
Location: Notts
Posts: 5,204
Default Re: Phono 3 with 5751 valves

Originally Posted by bevensee View Post
Well, I didn't exactly make any calculation because I was a bit unsure of how to calculate the load. I therefore looked at the original diagram, and 0.022 is what you get in the middle position where 0.01 and 0.012 are added. Since the middle position works for me in the sense that it seems to reduce the rumble of these problematic records without really reducing useful bass range. Besides, this would be a easy and reversible way for me to go as the caps I would need are already there. Doing this will allow me to simply take the switch out, i.e. only change one thing at a time. Later on, I can try with other caps to see if it will make a difference to me.

So I suppose the mthodeology is something like "a pragmatic apporach to what's on the shelf..."

Well, as you say, there's a parallel load of 316K +39K +reactance of 8.2nF (970K at 20Hz). For 20Hz that brings the 1M down to 570K. The coupling cap which gives -3dB 20Hz with 570K load is 0.014uF so we'd use preferred value 0.015uF. That may be the wrong way to look at it which is why I wondered if you had a method

In the original notes 0.015uF was used but it said that was for 10Hz.

In the Phono3S 0.015uF is used as the lowest cap value to which the others are added so must be the 40Hz setting.

Hence my wondering if the later values were chosen on test.

Anyway, I built Phono3S and only ever used it on the lowest switched frequency (whatever that was!) as the other 2 seemed obviously light compared with CD. When a friend wanted Phono3S, and I was not using the MC input, I sold it to him and built another for MM using the 3S pcb in a regular unswitched case which is the circuit at the start of this thread.

Your pragmatic approach seems the best if you've already found a setting you like but it's interesting to try and understand these things isn't it! It was getting late last night but I'll dig out that article and email it in a minute, cheers, Rich
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