Bijou All Tube Futterman Headphone Amplifier - Page 6

I suspect once the group buy finishes and people complete their amps, there'll be plenty more people to share impressions and "flak"

The interest is pretty overwhelming for an amp that really has just left the prototype stage.

Kudos DBel84 and Mazuki! That helps those of us who are waiting in the wings and anticipating the golden flutter of notes entering our earholes. ;-)

I'll easily take myself out of the experienced builders class, but the cavalliaudio site shows a pretty clear diagram of connecting the 270, including the center tap.

Ah, but I see the error of my ways. Would there be more than one CT wire on the 370? And would the bias tap wire just get lopped off and not used? (See...here's that "someday, somebody" thing you were talking about...)

I think if the 370 is a more robust design, I'd sure like to just build the Bijou one time and then enjoy it from then on out!

That's the CT of the high-voltage section, which the 370DAX also has. What Marshall was referring to was the fact that the filament (6.3V) portion of the 370 also has a CT, which you wouldn't want to use (as far as I can see, taking a quick look at it). But I'd want to look more into it before giving actual advice on how to wire it up for this amp. One thing is for sure, you are correct that you'd just ignore the bias tap all together.

If it were me I'd use the 370 and just make darn sure to ask enough questions and do enough planning to be able to use it. It's certainly not that difficult if I can figure it out and not zap myself or the amp. Proceed with caution is the name of the game.

Just buy the 370DAX and when it comes time to wire it up, post some pictures and someone can help you out. It should be fairly straight forward.

Its possible to use the 6H30 for the input which has half the gain. You could probably do away with the NFB, I suspect this would have very good sonics. T

he problem is the 6H30 requires more heater current and I am not sure how to deliver this (no info on the Bijou website.)


I am waiting on more detail on this and also the best price/benefit output caps. Someone used 330uF Auricaps which would be great for high Z phones but they are $180. I can't find any thing resembling a boutique 250V 470uF electrolytic.

Yes, but the resistor values for the front end will need to be adjusted to account for the differing characteristics of the tubes. A straight drop-in of the 6H30 would place it at a fairly non-linear operating point. A more robust transformer will also be needed as the 6H30 uses a lot more heater and plate current than 6922 when operating in its linear range.

No, you can't, unless you only use 600 ohm headphones such as the AKG K240M. NFB is required to reduce the output impedance so that it can drive medium to low impedance headphones.

All you have to do is change the cathode resistor to 5.1k.

Are you sure about the NFB? I think it won't be needed with 6H30, its more linear than the 6922, the Zo only climbs up to 32 ohms with no NFB. I don't see any problems with Senns with this output impedance. I think the NFB is mainly to reduce gain unless you are trying to listen to Grado's or Denon's with a tube amp! Low Z phones are meant for Solid State IMO.

Wrong. Absolutely wrong. Running a 6H30 at ~1mA which is what your proposed change will do is beyond stupid. Have you even looked at the curves & specs of a 6H30? The tube is practically in cut-off at your proposed operating point, there will be loads of distortion and the tube may even shut off entirely at higher volumes. 6H30s need at least 10mA going through them, preferably around 20-30mA depending on the plate voltage.

A 6922 operating into a 100k plate load is going to be quite linear, and the same is true of the phase-splitter portion. It is far more linear than the 6H30 in the same circuit, let alone your proposed 6H30 circuit. The 6H30 is more linear than the 6922 only if the circuit is optimized for it, which is not a trivial task given the voltage constraints and the need to keep current draws and so forth in a manageable range without blowing the budget.

With regards to NFB and output impedance, 32 Ohms is too high unless you're using Sennheisers or the 600 Ohm AKGs. It's unacceptable with Grados, K701s, Audio-Technicas, Denons, and Ultrasones among others.

It's there to adjust the Zo so that a sufficient damping factor is provided for the headphones.

Also, driving low Z headphones with a tube amp is a trivial task, it's called using an output transformer and/or using the right tube. With a transformer I can drop the Zo into the low single digits with ease, an OTL with the right tubes can easily have a Zo in the single digits, good enough to drive Grados.

The designer of the Bijou recommends the 5.1k cathode resistor its under tweaks here:

The Bijou Stereo Headphone Amplifier

I don't think his designs are stupid, in fact I greatly appreciate that he shares his designs,

As far as low Z phones with a transformer output, the tube is doing very little right I mean how much gain do you need with 30 ohm headphones?

And he'd be wrong. It appears he's looking at the voltages of the circuit without regard to the operating point of the tubes. The 6H30 will work, but it'll be far from optimal and a complete waste of the tube's capabilities. The 6H30 is a lot closer to an ECC99 than a 6922, it's a driver tube which is meant to be run with a fair bit of current, it's not happy being run at a couple mA, it wants a couple dozen mA for best results.

Current. Not much voltage gain is needed, but a fair amount of current may be required.

Which is the realm of solid state, just my personal preference to keep low Z phones away from tubes. I just don't see the point.

Thanks for the info about the 6H30, I think I will nix that idea.

The transformer I used has 6A of filament power. It also has the 260-0-260 tap.

It's also cheaper than the 370DAX.

Roam, you need to cool it. Whether you are right or wrong, the DIY forum only works due to collegiality. Whether you agree with his design philosophy or not, the designer of that amp is a talented and respected designer. If you think he has made a mistake, please post it, but do so in a respectful manner. There isn't a lot of moderation here, but you will find yourself on a lot of ignore lists pretty quickly.