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In Reply to: RE: A question on driving input transformers posted by FlyCast on April 01, 2016 at 07:02:43
a 1:2 step up will quarter the input impedance of your amp from the DAC's perspective. Most SETs will have an input impedance in the 50K Ohm to 100K Ohm range, so you'll convert it to 12K5 to 25K. That should be plenty easy for the DAC. Even the worst case scenario of a 10K impedance converter to 2500 should not be a problem.
Follow Ups:
Thank you, as a follow up
1) discussion about input transformers being difficult to drive requiring low source impedance, does this mostly apply to RC coupled tube output preamps?
2) input transformers being bandwidth limited, is this mostly talking about older transformers, ie should not apply to a high quality transformers like LL1676 (I would like the option to wire 1:4 as well)
Input transformers are not in and of themselves difficult to drive, rather as was previously described, it's the amplifier input resistance reflected back to the driving circuit that makes it a tougher load to drive. However, if you connect an input transformer to you SE amp, you can raise the value of the grid leak resistor substantially, because the secondary of the transformer is now providing a very low DC resistance path to ground for the input tube bias current. This will reduce the loading, if it needs to be reduced. However, I am with the previous responder in believing that this won't be an issue.
All circuits (and transformers) are band limited. A good transformer of the type you need should be capable of at least four decades or, most likely more, of bandwidth. With a reasonable source resistance (like 600ohms) the LL1676 is capable of 10Hz to 40KHz at +/- 0.5dB using it 1:4. Bandwidth at the high frequency end will likely be somewhat greater at 1:2.
Kevin Carter
K&K Audio
www.kandkaudio.com
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