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Inductor coupled Preamp

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With this tube you need a 2:1 or 4:1 transformer. Very unlikely these transformers will be suitable. From what i recall Audio Note do use extreme impedance ratios in some of their kis for reasons best known to themselves. Keep in mind it is not just a question of extra gain. The tube has to be linear at much larger voltage amplitudes...
 
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I wrote an article for Positive Feedback Online about 14 years ago about my first 26 transformer coupled design, since then there have been 3 more generations. The design is somewhat popular in Asia, and has been substantially revised twice..

The original is here: http://www.positive-feedback.com/Issue16/kennedy.htm
It's pretty long in the tooth and I would not recommend anyone build this version today.

Currently using the EML 20A and 20AMs in a new design.
 
Hey All,

I thought you might like to see the circuit. The output impedance is 9.5k. The amp uses half a 6N6P as a CCS and the other as a voltage amplifier. Audionote specs an output impedance of less than ten ohms for the PRE10. The Telefunken OPT's I have are meant for EL95 so 10k which made me think they might work. Question is where to attach them? It seems standard SE would have the two triodes coupled by the primary of the OPT. But I don't know what would happen to the CCS? Plus that would put DC through the primary which I'm not sure is a good idea? I would rather leave the circuit as is if possible and connect the primary of the transformer plate of the voltage amplifier where the output cap is. But if I do that where do I ground the OPT primary?
And how low of an output impedance is too low?
 

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Tony, as laid out in post #11 it would be closer to the Audionote PRE10. I've redrawn the schematic using and EDCOR matching transformmer. Either a 10k/600 or a 10k/150 2.5 watt. Any suggestions?
 

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I noticed on the Audio Note schematic they use a 600 ohm load resistor across the RCA output. Is that necessary or a listen to and see kinda thing?
An amplifier represents a very light load for a pre-amp with an OT. This can lead to resonance at high frequencies and the best way to tame this is to increase the overall load. This can be achieved with either a resistor or a zobel, but the zobel needs more skill and better test equipment to get right.
My small experience is a PP 5687 preamp with OT and I found in initial test that a load as low as 1K across the secondary took away a hard edge to the sound. I will attempt to replace this with a suitable zobel when I get the preamp up and running again. With the step down I am using this still represents a very easy load for the over half a watt of power output from the preamp.

I suspect, but cannot confirm, that the choice of 600R as standard for professional equipment is in order to remove the chance of resonance's between a wide range of different equipment.

Shoog
 

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> An interesting read

Yes; but I had to tease Google to let me see it, and it is a later-day explanation which misses the truths.

And reference impedance was 500 Ohms until 1938 (VU meter).

The trouble comes with very long lines, miles, true. But not at "one wavelength" of audio on any practical wire. And what we want to approximately "match" is the line characteristic impedance. What you see looking into an infinite length of line, L and C and R(series) and R(shunt). This reduces total loss. A "600 ohm" line of enough length to matter is a VERY expensive proposition. The wires have to be spaced feet apart. I just bought a house built in 1948 where the studs are telephone-pole cross-arms 9 feet long, and that would have been local service. Since before that house was built the standard line is twisted-pair, generally in cable, and shows more like 100 Ohm characteristic impedance. Line amps met the line near this impedance, not 600r.

600r *was* the standard inside studios. For the main part, this matched the specs of the VU Meter, which was sold for use with 600r nominal impedance. If amplification is too expensive to support <60 Ohm outputs and >10K inputs, a uniform 600r in and out is a fine way to cross-patch various gear and drive "short" lines (up to a mile between studios, 5 miles back from the ball-park announcer). FWIW, NBC stuck with 150 Ohm interfacing, transformering at VU meters.

600 Ohms is useful with transformers. Treble resonance becomes troublesome for wide-band audio transformers over a few K Ohms loading. Of course a little top-end rise may suit some put-together hi-fi systems or rooms; or not. Mostly designing for 600r loads in a one-room audio system is extravagance which might best be put elsewhere. However sometimes it is all about the extravagance.

> necessary or a listen to and see

Yeah, do the 12 cent experiment.

It depends very much on the transformer. Jensen or Sowter could wind you one that was was flat to 50KHz with or without loading. Edcor is fine stuff but not so sophisticated. Easy to try.
 

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600r *was* the standard inside studios. For the main part, this matched the specs of the VU Meter, which was sold for use with 600r nominal impedance.
600 Ohms is useful with transformers. Treble resonance becomes troublesome for wide-band audio transformers over a few K Ohms loading. Of course a little top-end rise may suit some put-together hi-fi systems or rooms; or not. Mostly designing for 600r loads in a one-room audio system is extravagance which might best be put elsewhere. However sometimes it is all about the extravagance.

I think it's for old-school 600R headphones.
 
There is an undoubted fact that an underloaded transformer is more prone to ringing. Think of the ultimate example of an unloaded transformer which can create voltage spikes in the thousands of volts.
A 600R loaded transformer will have relatively flat frequency response across the whole audio range - regardless of quality.

Shoog
 
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