• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Any comment on Vacuum State DPA300B?

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#61
It’s the fully differential topology that distinguishes it from the conventional push-pull amplifiers, which do not have differential push-pull output stage, i.e., there are some differences in the operation of the output stages.
 
#62
grannilsson said:
I'm not the right guy to say what the"Differential" part of the amplifier has advantages over a more traditional connection of the output stage, but it would have been fun if comparisons had been made between these, but with blind tests of course, otherwise you do not know.
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The amplifier accepts a differential input, and thus doesn't require a phase-splitter. This can only be a good thing. Allen contended that the output stage being tied down with a CCS meant that it essentially functioned as two single-ended amplifiers joined together at the output transformer primary. I think this is at least nine parts marketing to one part engineering, and runs counter to his own preference for output stages to be push-pull rather than single-ended, which in his opinion are too syrupy. So why he would boast that his DPA-300B was a single-ended amp twice over is lost on me. Allen also said that the CCS at the output was something of an engineering breakthrough, but it has definitely been done before.

After posting this comment I am going to try to (finally) post some pics of the DPA-300Bs I built more than 10 years ago from a kit Allen supplied.
 
#65
As it happens, I've built both the earlier EL34 design (without CCS in the output stage) and the newer one with the CCS. I thought the CCS was much cleaner sounding but not in the way SS can sound etched. I did try this with 300b's but always had a bit of a filiment hum problem (no, didn't try DC, just AC) so I eventually went back to EL34's (in triode). Really nice amps. In the schematic on Allen's site it just shows the early version without CCS, he had sent me the schematic for the newer version privately (not sure where it is now). Also, he told me that he used his Super Reg for regulating the first stage, which also isn't in the schematic.
 
#71
planet10 said:
For a suboptimal design it is damn good… i didn’t’t get a chance to hear it, but my partner Chris thinks it the best tube amp he had heard.

dave
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Comments from people are very favourable about this design, maybe I am missing something.

blek stena said:
DPA uses combination of CSS and fixed bias to get the symmetry right.
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Then, in the picture, Allen was drawing his powerstage after all.

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Fixed bias combined with CCS.
 

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#75
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This is the CCS circuit that I implemented in my PP amp which provides 120mA for two EL34 tubes. The IRFP140 can stand very high voltage and current. I had it mounted to the chassis as heat sink. The PP amp was with Williamson's design. I had the common cathode resistor replaced with this CCS circuit. The sonic improvement is amazing and I like it so much.
 
#76
kmtang said:
View attachment 1106426

This is the CCS circuit that I implemented in my PP amp which provides 120mA for two EL34 tubes. The IRFP140 can stand very high voltage and current. I had it mounted to the chassis as heat sink. The PP amp was with Williamson's design. I had the common cathode resistor replaced with this CCS circuit. The sonic improvement is amazing and I like it so much.
Click to expand...
Thanks for that. The only issue I can see is that a pair of 300Bs would likely draw more than 120mA (my 300B push-pull amp has 80mA per output valve). What changes would you make to increase the current capacity?

Did you have to add an extra negative supply when you added this?

Alex
 
#77
A push pull output stage with a constant current sink (CCS) that is connected to parallel cathodes . . .
Generally has Soft Clipping.

Could Soft Clipping be one of the characteristics that many prefer the sound of,
Versus the sound of Hard Clipping?

Just my question of the day.
 
#79
kmtang said:
View attachment 1106426

This is the CCS circuit that I implemented in my PP amp which provides 120mA for two EL34 tubes. The IRFP140 can stand very high voltage and current. I had it mounted to the chassis as heat sink. The PP amp was with Williamson's design. I had the common cathode resistor replaced with this CCS circuit. The sonic improvement is amazing and I like it so much.
Click to expand...
Kmtang, that looks like a good CCS circuit, but it will only work in Class A mode. If your PP amp is class AB, as are most, the 120mA limit will prevent the active tube from drawing any more current when the other tube reaches cut-off.

A big cap from the cathodes to ground (across this CCS) will allow the “on” tube to draw more than 120mA, allowing full power beyond the small class A region.

See my posts #16 and #19 in this thread.
 
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