Croft preamp with headphone amp

[Nagraboy]

Got a new Croft 25 preamp today and have been thinking of using the line output to connect my Graham Slee headphone amp. I’m sure this is possible but I’ve seen someone have their Croft pre modified with a tube buffer on the line output for attaching a headphone amp. Do I need to have this done, or is it unnecessary? What’s the reason to do it?

 

[Vinny]

I very, very seldom use phones here, but the line out works fine into a Myryad.

Beyond that, you will probably be looking at output and input impedances, which are generally a bit odd for Croft pre's and quite possibly varies depending on what the output from the Croft is.

Likely the whole question is a bit of a can of worms and would need someone like Jez to actually see what the circuit is.

 

[Nagraboy]

I very, very seldom use phones here, but the line out works fine into a Myryad.

Beyond that, you will probably be looking at output and input impedances, which are generally a bit odd for Croft pre's and quite possibly varies depending on what the output from the Croft is.

Likely the whole question is a bit of a can of worms and would need someone like Jez to actually see what the circuit is.

Yes, I suspect you’re right. I had my 25 made with the cathode follower mod, so it has slightly less than zero gain with an ECC82 cathode follower instead of the usual ECC83 line stage valve.

Best thing I can do, I suppose, is just plug it in and see what I think.

 

[Vinny]

Agreed - suck it and see. It can only sound OK, or not.

Gain isn't really much to do with any particular valve, but the circuit design, especially feedback. That apart, using an ECC83 cathode follower in a Croft pre' is a reasonably recent change - if you go back not too far the cathode follower was commonly an ECC82 or, occasionally, a 12BH7. As to why, is way beyond my pay-grade, although I do know that Glenn's valve choices are not generally ones that would be made by any designer of "conventional" amplifiers. But they do sound just fine.

 

[Nagraboy]

Agreed - suck it and see. It can only sound OK, or not.

Gain isn't really much to do with any particular valve, but the circuit design, especially feedback. That apart, using an ECC83 cathode follower in a Croft pre' is a reasonably recent change - if you go back not too far the cathode follower was commonly an ECC82 or, occasionally, a 12BH7. As to why, is way beyond my pay-grade, although I do know that Glenn's valve choices are not generally ones that would be made by any designer of "conventional" amplifiers. But they do sound just fine.

Sorry, I wasn’t clear - mine does use an ECC82 as cathode follower, whereas the usual Croft 25 these days uses an ECC83 for line duties. I saw no need for gain in the preamp as the Croft 7 power amp only needs 0.5V for full output, and the CD player and built-in phono stage outputs plenty for that.

 

[Vinny]

Sorry, I wasn’t clear - mine does use an ECC82 as cathode follower, whereas the usual Croft 25 these days uses an ECC83 for line duties.

I did appreciate that, but gain has nothing to do with what valve is acting as cathode follower, it is all to do with circuit design, especially feedback. Within reason, you can get whatever gain from whatever valve.

 

[Nagraboy]

I did appreciate that, but gain has nothing to do with what valve is acting as cathode follower, it is all to do with circuit design, especially feedback. Within reason, you can get whatever gain from whatever valve.

I know, I only mentioned it to explain that there were different internals to the usual 25 pre.