Hello!
I've been thinking about building a headphone amp with the following constraining designs:
1) OTL
2) Capable of driving my GRADO's impedance (32 ohms)
So given the above, looked at a simple white cathode stage using the lovely 6e5p in triode mode and using SY's great servo idea used in his Heretical preamp (The Heretical Preamp, p5) to remove the output cap.
To do so, we need a symmetrical HT supply. I'm sure this can be improved further, but a quick simulation shows that driving the valves at 45mA (a bit too hot perhaps) can deliver a low output impedance to around 16-20 ohms and distortion is an acceptable 0.25% at 90mVrms. I remember measuring my headphones and finding that 100mVrms was loud enough... (if I'm not mistaken).
Yes, 6e5p are running very hot and a lot of power is dissipated in their anodes, but is worth trying this in a breadboard to listen to?
6e5p is a great sounding valve and with no output cap, this seems to be a promising solution.
How could I improve this circuit?
Cheers,
Ale
I've been thinking about building a headphone amp with the following constraining designs:
1) OTL
2) Capable of driving my GRADO's impedance (32 ohms)
So given the above, looked at a simple white cathode stage using the lovely 6e5p in triode mode and using SY's great servo idea used in his Heretical preamp (The Heretical Preamp, p5) to remove the output cap.
To do so, we need a symmetrical HT supply. I'm sure this can be improved further, but a quick simulation shows that driving the valves at 45mA (a bit too hot perhaps) can deliver a low output impedance to around 16-20 ohms and distortion is an acceptable 0.25% at 90mVrms. I remember measuring my headphones and finding that 100mVrms was loud enough... (if I'm not mistaken).
Yes, 6e5p are running very hot and a lot of power is dissipated in their anodes, but is worth trying this in a breadboard to listen to?
6e5p is a great sounding valve and with no output cap, this seems to be a promising solution.
How could I improve this circuit?
Cheers,
Ale
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Yes I do see it. But having HT2 with a value of -150 is the same as inverting HT2 (i.e. negative side to cathode) with a value of +150....
thanks
Ale
Add an output cap? For reasons of safety for you and your headphones. A cap ought to be more trustworthy than a servo, if only because it is simpler.mogliaa said:
If you don't like the 'sound' of caps then you may have to invent a cap-less servo too.
R6 at 10M may be high enough to allow for thermal runaway in the upper valve. What does the datasheet specifiy as maximum grid resistor for this valve?
What about adding protection diodes at the output to protect the headphone from an unlikely failure of the servo circuit? I'd be able to avoid an unwanted electrolytic there.
Circuit still have an input cap and the cap on the servo. The input cap can be a good quality one as its value is 100-220nF. Need to tweak value depending on the servo resistor which as suggested with 100K then 220nF is ok.
Not sure what would be the sound impact of the servo op amp feedback cap?
@Lars: Yes going for an OT version could be probably the best option. Just was toying with the idea of an alternative OTL without output cap.
What if I parallel more valves to increase effective gm and reduce HT voltage? HT supply would be cheaper (and smaller that way).
Otherwise I may go for a simple spud using a good quality OT...any recommendations?
thanks
Ale
Circuit still have an input cap and the cap on the servo. The input cap can be a good quality one as its value is 100-220nF. Need to tweak value depending on the servo resistor which as suggested with 100K then 220nF is ok.
Not sure what would be the sound impact of the servo op amp feedback cap?
@Lars: Yes going for an OT version could be probably the best option. Just was toying with the idea of an alternative OTL without output cap.
What if I parallel more valves to increase effective gm and reduce HT voltage? HT supply would be cheaper (and smaller that way).
Otherwise I may go for a simple spud using a good quality OT...any recommendations?
thanks
Ale
What if you combined the diodes with a pair of 62mA pico fuses in series with the supplies. You might need to drop your bias current slightly to be sure you don't blow the fuse during normal operation.
Increase R2 to 35R and R4 to 60R and your bias current drops to 52mA, Zo is around 17.6R, and THD is 0.12% at .109Vrms out. Anode plus screen dissipation drops to 7.8W
Increase R2 to 35R and R4 to 60R and your bias current drops to 52mA, Zo is around 17.6R, and THD is 0.12% at .109Vrms out. Anode plus screen dissipation drops to 7.8W
Hi Bigun,
On the last edition of Morgan Jones' book he found that 6BX7 shows a big variance in distortion so these need to be manually chosen for audio use...
Cheers,
Ale
Hi Steven,
Many thanks. Looks like a sensible approach! Will need to breadboard this to find out how it sounds then....
BTW never used pico fuses, are they easy to buy?
Cheers,
Ale
Hi Lars,
I'm not familiarised with the circlotron, it's in my To Do list! Any recommended reading like "circlotron for dummies"
Cheers,
Ale
Hey,
Did another testsim. By parallelling two 6E5P and using them in SY´s Heretical topology(sand CCS) you get Zout below 20ohm and about the same distortionfigures as the White.
But note that no input cap is needed if an input transformer is used.
What 6E5P modelo do you use? These are the models discussed at Slagles forum: :: View topic - 6E5P triode-strap. I suggest using Dmitrynizh´s.
Did another testsim. By parallelling two 6E5P and using them in SY´s Heretical topology(sand CCS) you get Zout below 20ohm and about the same distortionfigures as the White.
But note that no input cap is needed if an input transformer is used.
What 6E5P modelo do you use? These are the models discussed at Slagles forum: :: View topic - 6E5P triode-strap. I suggest using Dmitrynizh´s.
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I just assumed the stage should be designed to take what I have seen in some specs being max level. Just did some Googling and found these high levels are hazardous to your hearing. So I back off: Why not design for 98dB=180mV?
You listen at a low level as 100mV corresponds to something like 92dB. Good enough, you will keep your hearing intact.
You listen at a low level as 100mV corresponds to something like 92dB. Good enough, you will keep your hearing intact
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