Nice phono stage with subsonic filter?

[Richard]

I'd try and ascertain the cause before putting something like that active filter in the chain. Only when you know for sure what the problem is can you fix it in the least invasive way. A simple passive filter such as used in Phono3S or the caps I suggested is the traditional way of handling warp or rumble when the records are still capable of being used and the deck assembly is otherwise well matched.

You should not be getting that cone movement unless a very LF signal is recorded in the groove of those albums or they are warped. Warping will be cyclical and obvious if you watch part of the record pass the stylus while also watching the cones so you should be able to confirm or eliminate it easily.

It could be that the problem has been there all along and your previous triode amp didn't reach down far enough to move the cones at LF.

Kit6550 will have a lot more LF power. Have you tried any other power amps or an integrated amp in the system to be sure the new amp isn't unstable as Greg mentioned?

[Infinitely Baffled]

Originally Posted by Richard

I'd try and ascertain the cause before putting something like that active filter in the chain. Only when you know for sure what the problem is can you fix it in the least invasive way. A simple passive filter such as used in Phono3S or the caps I suggested is the traditional way of handling warp or rumble when the records are still capable of being used and the deck assembly is otherwise well matched.

It could be that the problem has been there all along and your previous triode amp didn't reach down far enough to move the cones at LF.

I take your point. Better to see if a minimum-intervention solution can be made to work. I have dug out some old unused orange-drop type capacitors left over from the build of the Kat 6550. They say "BC 220n K 400" on the side. Would they be the correct rating? If it does work, I assume that it would work as well if installed as the last component before the RCA output sockets of the pre-amp? (Lots more spare room to accommodate it in the Croft case).

You could well be right about the problem not being apparent with the SET amp; the Kat 6550 has a good deal more punch at the bottom end.

Thanks for the idea; I'll let you know how it works out regards. Gary.

[Richard]

Yes those caps would be fine. Ideally they'd be fitted inside Kit6550 but they should work where you suggest (well enough to try anyway) if your interconnects are fairly short and low CR.

See this pic borrowed from Tony on this site of his integrated version below. You'd remove the orange wires on the vinyl RCA inputs and reconnect with the caps.

If you have a meter do check that the inputs measure 100K, if not you need a different cap value or resistors changing.

[Infinitely Baffled]

Originally Posted by Richard

If you have a meter do check that the inputs measure 100K, if not you need a different cap value or resistors changing.

This is helpful. Thank you, Richard.
I have consulted the diagram in the build instructions for the amp, and I remember now that the signal input wire connects onto two resistors. One of them is rated at 3k3 and is soldered directly to pin no. 2 of the ECF80 valve base. The other is rated at 100k and that is connected (eventually, via a capacitor [rated 2k2uF] and another resistor [rated 10R]) to pin no. 7 of the same valve. Does that sound like the inputs are 100k as you advised they needed to be?
Regards
Gary.

[Richard]

Yes that sounds correct. I don't know which version you built, the single input power amp version is usually built with 100K and the volume control version used 1M. It's also important to check that the end you describe as connected to the 2200uF cap is also connected to signal ground. If you have a meter put a probe on the RCA centre pin and the other on the RCA outer shell and check for 100K.

[bikerhifinut]

Daft question here, Why not just make a switch box with a selection of capacitor values to insert in series with the power amp input/preamp output?
And a "straight through" setting. Granted there may be a possibility of deterioration of the sound, that would depend on how good your ears are and the quality of switch etc. But it could make a cheap experiment to find an optimum coupling cap value.
And the other advantage is that putting the "fiilter" between pre and power amp is that you can use less expensive lower voltage rated capacitors as there should only be the signal voltage applied.
Anyone see why that can't be done?
PS the old Leak "Point One" preamps used this subterfuge to filter out subsonic signals, the filter consisted of a 10nF capacitor on the output that was simply shorted out to take it out of circuit.

Andy.

[Greg.]

Well, personally I'd bite the bullet. Purchase the WD Full Pre/Phono kit, fit it into a combined enclosure if that is the desire and ditch the Croft. I have every expectation the outcome will be absolutely positive and a major sound upgrade. Of course, in my case using the earlier WAD2 units, the pre now fitted with a John Brosky Aikido circuit, happily they can all be used closely together taking up no more space than a CD player. YMMV.

[bob orbell]

A bit off topic, so if the moderators need to move the post please do so, Greg, I don't remember you commenting on your Akido after giving up with the CF, I was going to order the kit but decided to build a few alternatives and try, first off was an Akido, (you sent me the circuit) second was a White cathode follower and last and what I think is the best for me, a cathode follower with active load, it is still a bit like a rats nest and needs to be tided up, but it dose sound very good. BOB

[pre65]

Bob, cathode follower with active load ?

Did you follow a published circuit ?

[bob orbell]

Hi Philip, yes, from Morgan Jones book, it's very simple, simply a cathode biased cathode follower using ECC88 with another ECC88 as the load. Why do you ask?. BOB