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Re: Selecting Output Transformers for WOT Line Preamp

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In Reply to: Selecting Output Transformers for WOT Line Preamp posted by Snoopyma on June 27, 2005 at 11:35:37:

Any of the choices you name have enough primary inductance to make a reasonable low frequency limit and the 437A has low enough Rp to also provide a reasonable high frequency limit, so the real quesiton is how much gain do you want. The 437A has a pretty high mu, so unless you really need the gain, the LL1689 at 18:2 is probably the best choice. You will get low output impedance and good frequency response performance.
As far as the gap to specify, of course it depends on the operating point and maximum output level you choose. Many transformer users are very careful to get a transformer gapped for exactly the current that they plan to use, however, that is only necessary if you are planning to use the full output level swing. For example, if you chose the LL1660/18mA in connection alternative Q and operated your tube at 18mA of bias, you would have a gain stage capable of putting out 57V RMS at 30Hz. I have never needed that output level from a line stage, but maybe someone would. So if I operated the tube at 25mA with this transformer I would reduce my maximum output to about 30-35V RMS at 30Hz. Put that way, it doesn't seem to be such a big deal! With your tube (low Rp) it probably makes almost no real difference whether you choose 25 or 30mA appropriate gaps, unless you want really high output levels.

Kevin Carter
K&K Audio
www.kandkaudio.com

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Re: Selecting Output Transformers for WOT Line Preamp - Snoopyma 23:34:00 06/29/05 (2)
Thank you for your reply and suggestion.
I am using a 97db horn speaker system, so inadequate gain is not a problem. On the contrary, as I am using a transformer as the volume control of my preamp. too much gain is a problem particularly for some CDs with higher output.
As the LL1689 does not have the frequency specification, can you give me some hints on the predicted freq. response in the case of WE437a running at 22ma using LL1689/18ma 18:2 connection that you recommend?
To me, LL1689 has the advantage of having 18:4, 18:2 and 18:1 connection alternatives so that I can play around to choose the most suitable one. Is there a difference in terms of freq. response for the 3 connection options?
For specifying the gap, I have the false common belief that running the tube at a current higher than the gapped one will saturate the core and deteriorate the performance at all levels, won't it?
What is the loss of gapping the cores at higher current assuming the same core and coil in terms of technical performance and sound quality? Could you elaborate.
Thanks in advance.
- Re: Selecting Output Transformers for WOT Line Preamp - KevinC 12:53:46 07/01/05 (1)
  "As the LL1689 does not have the frequency specification, can you give me some hints on the predicted freq. response in the case of WE437a running at 22ma using LL1689/18ma 18:2 connection that you recommend? "
  Not having tried your tube, but having used this transformer, I would expect response close to 1KHz levels at 20Hz and 20KHz, but I wouldn't be able to predict -3dB points/
  "To me, LL1689 has the advantage of having 18:4, 18:2 and 18:1 connection alternatives so that I can play around to choose the most suitable one. Is there a difference in terms of freq. response for the 3 connection options?"
  I expect that in the audio band there will be little difference, but if you are a bandwidth oriented designer there will be diffferences. Again, I couldn't presume to predict them with your circuit.
  "For specifying the gap, I have the false common belief that running the tube at a current higher than the gapped one will saturate the core and deteriorate the performance at all levels, won't it?"
  The Lundahl current spec is the current at which the transformer core is about 55% saturated. To reach saturation you will have to have current flowing at a proportionally higher level. It is easy enough to calculate what that would need to be for a given transformer and tube combination. From that you can calculate a maximum signal level and see if you are going to approach this under normal operating conditions. For line level signals of a few volts RMS, there really isn't a big need to worry if you are within 25% of the Lundahl current spec. In point of fact, until you get so high on the magnetic curve that you reach the saturation knee, you are in a region where the iron is more "linear" than it is at low current levels, due to the dominating presence of the air gap. Some believe that this is one reason why SE stages sound better than PP stages where current is flowing through the primary of the output transformer. With PP stages, the opposing currents cancel and there is little magnetic flux present under no signal conditions, whereas SE stages are biased in the region of 35-65% of saturation.
  "What is the loss of gapping the cores at higher current assuming the same core and coil in terms of technical performance and sound quality? Could you elaborate."
  More gap means less primary inductance and, therefore, for a given set of operating conditions, higher low frequency cutoff.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  - Re: Selecting Output Transformers for WOT Line Preamp - Snoopyma 07:56:46 07/05/05 (0)
    THANK YOU FOR YOUR INFORMATION!

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