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Bass pre-amp DI output
Bass pre-amp DI output
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Old 14th May 2013, 02:02 AM   #11
cyclecamper is offline cyclecamper  United States
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Join Date: May 2010
Location: Chicago IL, Long Beach CA, Vienna VA
Not just a transformer, but a DI transformer and a big coupling capacitor should be OK.

The DI transformer doesn't want to see any DC from a single-ended tube. The 1/2 Alembic is single-ended class A so it has DC that gets blocked by a cap in its output or the next stage input at the power amp. Single-ended output transformers are special things with air gaps and large cores to cope with the DC. So Jensen really wants you to provide a push-pull driver ideally, otherwise I guess you drive the transformer thru a large isolation capacitor so you don't lose bass. No need for another transformer for your normal high-impedance output. I would recommend a volume for each output, with a pull-switch range pad on the DI volume pot. A pull-switch on the main high impedance output to mute to a high-impedance dummy load might be convenient if you want to hear just the DI. The DI should have a ground lift, and if you plan to use it for monitors you want a DPDT polarity-reverse switch on the DI output too. Neutrik makes nice jacks that take an XLR, TRS balanced 1/4, or TS unbalanced 1/4, and the transformer will work balanced or unbalanced, so that's a nice jack to use and you can shove in just about anything. The load impedances should be much higher than the source impedances, so you shouldn't have trouble with the 2 volumes interacting as long as neither load is ridiculously capacitive or inductive, and they shouldn't ever be.

But get the opinion of a real EE on-list, not just me.
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Old 14th May 2013, 02:29 AM   #12
diybass is offline diybass  New Zealand
diyAudio Member
Join Date: Jun 2012
So the big coupling capacitor would connect to pin 1 of the 12ax7, then to the tranny, then other end of tranny primary to ground? What does the circuit for a volume control and pad look like for the DI out? Nice idea to have a phase reversal switch!

And what should I use for the dummy load for the high z output? Is a dummy load necessary for a preamp? I thought it was just for tube power amps?

What exact tranny anyway?

Sorry if some of my questions sound stupid, but I'm still fairly new to this. Having a heap of fun learning though!
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