[sdiy] Preamp question

metasonix at earthlink.net metasonix at earthlink.net
Sat Aug 3 05:57:17 CEST 2002


At 01:01 PM 8/2/02 -0700, you wrote:
>I was perusing the latest TapeOp magazine, and came across a schematic for
a DIY Hamptone tube preamp.
>(http://www.hamptone.com/mic-pre_kit.htm)

This is a good kit--the price is very reasonable, and if
built properly, it will perform as well as most of the
high-priced tube mic pres on the market.

>The input stage has two, er, areas: one for a balanced (?) mic (which goes
across some optional pad and phantom power circuity and a 1:10 transformer),
and one for an instrument DI (which cuts out the mic input, no extra
circuitry).  The article states that a large part of the preamp's "sound"
comes from this input transformer.
>The jist of my question is... can I modify the circuit so that the
instrument DI *also* goes through the transformer?

Yes, but you will have to drive it with a 600 ohm or lower source
to match the transformer input. It might load down a guitar pickup
or some high-impedance microphones.

>Since the DI input is NOT balanced (hot and ground only), would the
transformer work with one end tied to ground and the other to hot?  For the
mic input, one end is hot, and the other is cold.  (Is that the right
terminology?)

Yes, you would just put a 1/4" jack where the XLR is. One input wire on
the transformer goes to tip, the other to ground/sleeve. Doesn't really
matter which, unless you are a perfectionist and want to preserve phase.

>As an aside, the article refers to the "totem-pole" architecture, but I
can't tell what that means by looking at the schematic.  Any clues?

The output stage is a totem pole--a class B low impedance output buffer.

>Thanks for any advice!  (As you can tell, I'm not at all familiar with
transformers...)

Transformers are very useful when dealing with tubes. But you can't 
skimp on them and expect great sound. Use good quality transformers.
Dunno about the ones Hamptone is selling--they aren't shielded--not good. 
You can't go wrong with Jensens or Lundahls.




E. Barbour
metasonix




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