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phrarod 10th July 2010 02:04 PM

Proper load resistor for I/V secondary?
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I'm using a pair of Jensen 11p1 1:1 transformers as interstage treansformers between my dac and tube output. I'm no transformer expert but I believe Audio Note who does a similar arrangement has I/Vs with a 600 Ohm secondary. This one has a 2.5K

Right now I have a grid to ground on the secondary of 10K as spec'd by Jensen for this transformer. However, a couple friends felt that was too high and I'm over dampening the transformer plus in its intended application it would be driven harder. The suggestions ranged from dropping the grid to ground 1K at a time to putting in a 2.5K resistor matching the ouput resistance of the Jensen.

I need to lower gain before it hits the grid as it has too much at this point and is amplifying some hum. As well as I want the best performance.

Since I'd have to order the resistors, break them in for 100 hours, etc I was wondering if someone with experience in these areas could guide me to the proper resistance. The tube its going to is like a 6922 tube.

Ciu 13th July 2010 10:46 AM


Have a look :

Seems very close to your project
And whole subject : SYclotron Audio The Heretical Preamp


analog_sa 13th July 2010 12:23 PM


Originally Posted by phrarod (
I'm no transformer expert

Maybe i suffer reduced reading comprehension today but the above is the only part of your post i understand.

Although the title mentions I/V it seems no I/V is required at all. Is this actually a current output dac?

phrarod 13th July 2010 03:41 PM

Yes, the audio note board using a 1865 dac chip is current out until you use an I/V resistor to create voltage out. Which is what everyone who uses the 1865 dac chip (loads of posts about this chip) does. I have been able to find another thread that recommends a 200R or less I/V resistor and have spoken to Jensen so I think I'm pretty clear on the path I should take unless you have something new to add to it. Jensen redirected me to the primary as mentioned above because they stated the 10K is the proper secondary loading. I may drop down to 100R off the dac chip to lower the gain there.

Thanks CIU for the info.

analog_sa 13th July 2010 05:41 PM

Well, this is essential information. The best thing you can do in order to lower the output voltage is to reduce the I/V resistor. As a side effect the DAC will also work better. If Jensen recommend 10k secondary load this probably minimises ringing in the secondary. Not having experience with this particular unit i cannot comment but generally interstage transfomers sound better when lightly loaded. I would definitely try a load of 100k as well.

analog_sa 13th July 2010 08:30 PM

Btw, why is a transformer required at all?

phrarod 13th July 2010 09:07 PM

Better isolation. Filtering the nasty digital bits before it hits the grid of the tube.

analog_sa 13th July 2010 09:21 PM

Fair enough. Just curious if you have tried listening while bypassing it.

phrarod 13th July 2010 09:24 PM

Yes I have. I've had many configurations and this is the best sounding.

Ciu 15th July 2010 01:03 PM

I had a "deeper" look to the AD1865 datasheet : this dac has a particularity, and having both types of output, but not symetrical (voltage or current) you can choose which you want , so using a transformer would not be my priority (but in current mode, a resistor with a tube amp stage...)


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