300b lovers

In terms of load abuse, plugging planar headphones (20 ohm to 32 ohms) into the Raven preamp will definitely load down the preamp and is not recommended. No harm will result, but the preamp will slide over into Class AB operation and distortion will rise to higher levels. Headphones with impedances of 300 ohms or higher (most dynamics) are fine.

We may design a future project that can accommodate planar headphones, but these require a genuine 1 to 2 watt Class A power amplifier, which is outside the scope of a preamplifier intended to drive power amplifiers. In other words, a tiny but very high quality power amp with extremely quiet power supplies. Designing (good) amplifiers for planar headphones is not as simple as it appears at first glance.

Of course, it is tempting to design a special-purpose headphone amp. I own HifiMan HE1000 Stealth (V3) planar headphones myself, and I use a modest all-in-one DAC/headphone amp to power them.

But ... the headphone amp market is quite crowded, and is dominated by Chinese products selling from $200 to $1600. On the domestic front, Geshelli is cleaning up making AKM-based DACs and Sparkos discrete-opamp headphone amps at entry-level pricing. As far as I can tell, headphones, and headphone amps, is the most competitive sector of the audio market right now.

So even if we spent several months to a year on a special headphone amp, I’m not sure we’d sell many. Something to think about, though.

@audioquest4life 

I did not make it to the room you mentioned above.  I was quite busy running our room with Spatial Audio Lab and only made it to about half the rooms at the show.  The link you posted did not work.  Can you check and repost it?

The Raven is not very sensitive to loads. In practice, power amps range from 10K (typical solid-state) to as high as 470K for a handful of vintage tube amps. Most modern tube amps are 100K. Plus whatever cable capacitance is there, along with the Miller capacitance of the input section of the power amp. So 100 to 400 pF is typical. The range of loads is predictable and well known.

What dominates the transformer performance is the source impedance, not the load. The source impedance from the preamp tube is much lower than the load, so it heavily dominates the transformer performance. Transformers don’t much care where the low impedance is, primary or secondary, so long as it is there.

Transformers are aptly named as they transform impedance. They do not isolate impedance. If the source impedance is low that will change the correct loading on the output (as opposed to a high source impedance), which will be found to be exactly one value. Above that value the transformer will ring; below that value it will roll off highs.

This phenomena is well known and is why Jensen Transformers specifies the loading to be used with their SUTs depending on what cartridge is used with them.

If the load is too high impedance, the inter-winding capacitance will come into play as well, causing the FR to no longer be flat, and in extreme cases the transformer may fail to express its turns ratio. The proper termination will yield the widest bandwidth out of the transformer.

This why in the old days when balanced lines were used, the termination standard was always 600 Ohms so the designer would have a pretty good idea of what to shoot for.

A rheostat, placed across the output (between pins 2 and 3) can be used to load the transformer for optimal operation. Or the transformer can be designed for 600 Ohms with a termination switch using a 600 Ohm resistor built into the equipment (this is how Ampex solved this issue on their 351 tape electronics). That way any higher impedance load such as 10K or 100K is negligible and would not affect the transformer performance. The latter approach is why we did with our P-2 balanced line preamp 30 years ago.

One of the nice things about working with Cinemag is their custom design service. This gets us away from the many limitations of standard off-the-shelf transformers from other vendors. We specify source impedance, the range of loads (including capacitance), DC current flow, and expected DC imbalance. They show us what their computer model tells them, we test the physical prototype, see how it works on the bench and in-circuit, and go back and forth a few times until everyone is happy. Off the shelf, it's take it or leave it.

This is what made both the Raven and Blackbird possible. I didn't think a transformer using the 6SN7 in balanced mode could be realized because of the quite high impedances involved. To my surprise, Cinemag showed me otherwise ... very clean square waves at 10 kHz with minimal overshoot. This is not an off-the-shelf part ... it's been designed for us.

When we say the Raven and Blackbird use custom parts we aren't joking. Without them, they aren't practical. We use standard tube types because we want our customers to enjoy them for a long time, and we expect the 6SN7, KT88/6550, and 300B to be around a long time. Under the chassis, though, we use custom parts for critical functions.