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Something New in the Works
https://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6330
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Author:  ILoveHiFi [ 27 May 2019, 16:51 ]
Post subject:  Re: Something New in the Works

with small gain

I'm guessing you've used the wrong output transformer and is not matching with the valves
otherwise could be connected the output stage wrong somehow

Author:  gofar99 [ 22 Jun 2019, 22:01 ]
Post subject:  Re: Something New in the Works

Hi Everyone, Well some things don't go as planned. Using the 6AS7 either as SE or SRPP without output transformers is not really suitable for a headphone amp. Nice sound in either case, but limited output because you can't get more current out than you put in. :( With as much as 80 ma per tube the most you can send to phones via a capacitve coupling is ...guess what, the max you put in. Thus the output is limited to the 30-75 milliwatt range. Plenty loud on 100db/mw phones, but hardly worth the trouble. 75 watts in...50mw out. Bad math. I will be trying a transformer coupled version next.

Good listening
Bruce

Author:  ILoveHiFi [ 23 Jun 2019, 20:03 ]
Post subject:  Re: Something New in the Works

30-75mW Thats not what I would expect

But if your really keen on SRPP OTL I would give the one another try running on lower voltages.
75/2/1.5 = 25W
25/80mA = 312.5V = +-150V
The operating voltage your currently running at by guessing, I saw some running on +-60V on google

The main prolbem with the OTL is that the output power is current limited, not voltage swing limited.

Author:  gofar99 [ 23 Jun 2019, 20:23 ]
Post subject:  Re: Something New in the Works

Hi, Not what I expected either. The program indicated it would deliver +/-16 volts into 35 ohms. Well, I should have thought it through better. If you put 32 volts into 35 ohms you need a current of about 900ma. With only 35ma passing through the tubes there is no way that can happen. So I am looking into the program to see if I can find out what it is doing incorrectly. Possibly an out of range error that it was unable to warn me of. With 35 ma available the best it can do is about 1 volt with a 35 ohm load and ......guess what that is where it measured (actually a little more). I can get more power, but only when it uses either a lot more current with is limited by the dissipation, or an output transformer. It did sound very nice and measured quite flat (less than +/- 0.5 db) from 20HZ to 20KHZ) and 30mw is way more than needed for the phones (sensitivity of 100db/mw at 35 ohms). Still it is not a really good project. I get more than that with the simple 6DJ8 headphone amp and it is a whole lot less complicated and much more efficient.

Good listening
Bruce

Author:  ILoveHiFi [ 24 Jun 2019, 04:06 ]
Post subject:  Re: Something New in the Works

Whenver I do design something I never even bother to waste a second on the simulator.
Highly unaccuarate and totally waste of life.

Great for learning basics to medium schematics including lm741 internal I'd say
But not something to design a amplifier with.

Author:  gofar99 [ 24 Jun 2019, 10:59 ]
Post subject:  Re: Something New in the Works

Hi, Since most of my designs are audio related I often use a pc to get things close to the desired result. Then everything is by ear and finally by testing. I found that for some reason the program I used on the 6AS7s does not compute correctly the values. All other tubes are fine. My guess is that it is such an unusual tube for current, gain, anode resistance and so on that it is out of range on the computations and gives what seems like answers...but really are not correct. Now on to the next design, what ever that may be.

Good listening
Bruce

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