Active DI, Ground Lift, and Me

[slor]

I've been searching the posts for info on these topics, and found bits and pieces here and there, but I'm still lacking the whole picture. Here's my situation:

I'm building an active DI based on the Altec 1566A. I'm not including the Lo-Z input as I don't believe I'll need this feature (plus the chassis I have is limited in size). Here's a schematic:



My questions are:

-Because there's an output transformer (an Altec 15095), do I need to provide a ground lift? And if I do, is it as simple as breaking the ground connection or shunting it (as Jensen suggests <a href="http://www.jensentransformers.com/as/as066.pdf">here</a>) through a small-value resistor and capacitor?

Or, as one poster suggests elsewhere, should I just make a telescoping ground by terminating the signal cable's shield before it hits the grid of the first preamp tube?

-Speaking of preamp tubes, the Altec uses a pair of 12AX7s, with the capability to bypass the first stage via a "hi/lo" switch. What are the ramifications of substituting a completely different tube for the first gain stage (I'm thinking perhaps a 5879) and dispensing with the switch? Obviously they're very different tubes, but as long as the plate resistances and B+ supply were dealt with....

Many thanks,
Seth

[Nigel Goodwin]

You don't need a ground lift as long as you're using the low impedance output, as there's no ground connection to disconnect.

If the alternative valve is a pin for pin replacement, then just try dropping it in and see how it works - most of the double-triodes are 'reasonably' interchangeable.

What are you intending using it for?.

[slor]

Well, actually the 5879 is a pentode, so the circuit would definitely change...I'm just wondering if everything "downstream" of the first preamp stage could remain more or less the same....

I'm thinking this box will primarily be used to track bass, as I dislike the sound of most passive DIs I've heard....

[Nigel Goodwin]

Originally posted by slor
Well, actually the 5879 is a pentode, so the circuit would definitely change...I'm just wondering if everything "downstream" of the first preamp stage could remain more or less the same....

You could always simply strap the pentode as a triode, and try the existing values - it's a simple enough circuit, you could probably throw it together in under an hour?.

I'm thinking this box will primarily be used to track bass, as I dislike the sound of most passive DIs I've heard....

Well a passive DI is just a transformer, and may well not handle bass too well? - an obvious snag here is using a transformer?.

Personally I've no interest in valve circuit for their supposed 'sound', but I come from the valve days - for lead guitar, particularly heavy metal, fair enough, massively overdriving the entire amp (preamp, power amp, and PSU) gives that unique sound that some people love. But for bass?, I can't see it.

Have you tried normal active DI's?.