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View Full Discussion Has anyone looked into the actual engineering design of this amplifier? I found a schematic for it here: http://bilder.hifi-forum.de/max/415727/bravo-v2-schematic_691080.png
It shows some interesting things, not all of which look particularly 'good.' I don't know if that schematic is accurate, or is representative of the units that are actually shipping.
1) The input potentiometer (volume control) is 100k ohms. That is a common value for vacuum tubes. However, R4 and R5 (grid leak resistors) are only 24k ohms. That means the input impedance is 100k||24k = 19k ohms. That's only a little higher than you'd see from a typical solid state headphone amp. Maybe this is not a problem in this instance. BUT...
2) The input capacitors, C1 and C2 are 1uF (one micro-Farad). That creates an RC high pass filter (HPF), with bass roll-off that is -3dB at 8.4Hz. That means this little amp can only be truly flat (-0.1dB) in the bass at about 80Hz, and probably -1.5dB at 20Hz. That is kind of high, and would probably be audible used with cans that go really low. This thing won't make really deep bass, but this is probably not a problem. People who buy this are probably not looking for that, but rather are looking for that 'warm tube sound.'
3) The LM317 (IC1 and IC2) current sinks in the sources of the output MOSFETs IRF510 (Q1 and Q2) are set to 167mA (0.167A). Just FYI.
4) The plates of the 12AU7 (triode tubes V1A and V1B) are running with very low voltage, only 16VDC. If you look at the characteristics for type 12AU7, you can see that it must be running with practically no grid bias. That would mean the control grid must be drawing at least a little current. When the tube is drawing grid current, its input impedance goes low. The grid's input impedance is in parallel with the grid leak resistance (defined by the volume control with R4 and R5 in parallel), which is already pretty low. Audio sources with low output impedance and the ability to sink some current into that load will be necessary for good sound.
At any rate, I can see that this design would be very, very sensitive to tube characteristics that the tube itself wasn't designed for. Tubes in general are designed to be used with plate voltages of 100VDC and higher (up to 250VDC is normal for the 12AU7). Characteristics at plate voltage of 16VDC are likely to vary a great deal between different 12AU7s. Tube rolling indeed!
This looks like a fun little "tube-ifier", but it doesn't look like a truly high fidelity headphone amplifier. I might pick one up just to play around with, maybe swap out a few parts, play with some minor modifications. Looks like it could be fun. I think I'd change that blue LED for an amber one. I'd want warm colors for a hot little 'tube amp.'
Anyone else with experience with tube amp designs care to take a look at that schematic and comment on it?
So I bought one. It's as expected. Not exactly hi-fi.
Compared to my Objective 2 amp, this Bravo V2 sounds a lot more aggressive in the upper mids. Low impedance (50 ohm) headphones playing straight from the PC line out sound less bright. But this amp is kind of cute.
This amp does sound better driving high impedance headphones, but still very colored sounding. Its bright and aggressive character remains, just less apparent than when driving low impedance cans.
Incidentally, the output MOSFETs are IRF630, marked as such on the PCB. The schematic I linked to says IRF510. I'm not sure how much difference that would make.
Oh well. Maybe I shouldn't have bothered. I might try some mods to see if I can lower the gain and/or get it to sound more mushy and tube-y. That would be nice for mellowing out the hashy highs from MP3 streams. I can't think of any other use for this thing...
Um... Right!! What you wrote sounds very good, but you might as well have said, "may I mumble dogfish to the banana patch?" Definitely way over my head :)
BTW: quote is from Steve Martin