Thanks for sharing!
How do you like UTС A-23?
Do you connect it regular way or as autotransformer for your headphones?
I have a couple of these beauties too but I haven't tried them for such a smart purpose yet.
Hi, this was very useful but I'm still - as a learner of electronics - wondering, how the hell are OTL-s function in the sense of voltage amplification - or are they rather current amplifiers ?
Normally, the rule of thumb would say to me: more output power against lower impedance, applicable for silicone amps and also pretty much everything in room amplifier league too.
Now this is completely the opposite in this OTL design, I get only low/moderate power with low impedance load and high power with high impedance load.
The same applies to Tim Mellow's OTL amp, 25W 8R or 50W 16R, lol. Weirdo animals.
What's the trick here ? What happens against even more impedance ? (Any natural current-limiting protection by design maybe, or the amp will simply fail at some point ?).
The trick is tubes are current limited and voltage happy. You can push lots of power at a few mA if you use 100's of volts 🙂
So the trick is if you only have 20mA of current, and the load is 32R, Ohm's law says you get 12.8mW of power... If the load is 470R but you still have 20mA, you now get 188mW.
So the trick is if you only have 20mA of current, and the load is 32R, Ohm's law says you get 12.8mW of power... If the load is 470R but you still have 20mA, you now get 188mW.
Doesn't source impedance vs. load impedance enter into it?
A 6AS7G Cathode Follower (CF) will have a high Zout of about 140 ohms. If you connect a 32 ohm headphone to that, it's going to load down severely (it won't be able to drive current into that low impedance load and will clip prematurely).
Aren't all tube OTL amp output stages based on Cathode Follower or White Cathode Follower (or sometimes SRPP)?
A 6AS7G Cathode Follower (CF) will have a high Zout of about 140 ohms. If you connect a 32 ohm headphone to that, it's going to load down severely (it won't be able to drive current into that low impedance load and will clip prematurely).
Aren't all tube OTL amp output stages based on Cathode Follower or White Cathode Follower (or sometimes SRPP)?
If Ri=280 ohm and mu=2 of each half of the tube we would get Zout=35ohm with the halves in parallell.
I'm using a WCF for headphones now though... A 6N6P will put out 180mW into any load from 32R to 470R in that config.
I'm using a WCF for headphones now though... A 6N6P will put out 180mW into any load from 32R to 470R in that config.
For a cathode follower, Zout is approximately 1/gm.
A 6AS7G has gm of about 7mA/V, which is similar to the gm of a 6N6P triode.
1/gm = 1/0.007A = 142.857 ohms.
parallel the two triodes and you get half that, or Zout = approx 71 ohms.
I'm not sure how mu enters into that.
Of course a WCF is different (and better for use as a headphone amp output stage).
A 6AS7G has gm of about 7mA/V, which is similar to the gm of a 6N6P triode.
1/gm = 1/0.007A = 142.857 ohms.
parallel the two triodes and you get half that, or Zout = approx 71 ohms.
I'm not sure how mu enters into that.
Of course a WCF is different (and better for use as a headphone amp output stage).
Julius Futterman would be very surprised to hear that.
Anyway if you want voltage gain, put a small tube in front, don't ask the big tubes to do it.
???
Isn't this essentially an active loaded CF output?
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Schematic of hot chassis device removed.
Isn't this essentially an active loaded CF output?
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Schematic of hot chassis device removed.
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The lower 12B4's is driven and not cathode-loaded. And as Lampie is hinting, not the full Futterman but an early stage.
Post #169 is a hot-chassis design, and really shouldn't be posted here, for safety reasons.
All good fortune,
Chris
All good fortune,
Chris
Oh, you're right!
Sorry, I posted it as an example of the Futterman topology, not as an example of something to actually build.
Should I report it to the mods and get it removed?
Sorry, I posted it as an example of the Futterman topology, not as an example of something to actually build.
Should I report it to the mods and get it removed?
Certainly not my call of course, but maybe? safe enough in context of further comments? Your call. I'd guess that a lot of this rule is intended to not mislead unwary or folk not completely knowledgeable about modern safety standards. Call me a Safety Sally, I can live with it.
All good fortune,
Chris
All good fortune,
Chris
I don't think that the Futterman schematic should be removed. Yes the rule here is no hot chassis designs, but no one was building it.
I did build one of those as an experimental thing around the time it was published
in Audio Magazine on a piece of 1/8" extruded Aluminum ( Aluminium??).
And I'm still here 65 yrs later, what a coincidence!
It 'sounded OK' running a 16R speaker. But as usual I soon went on to other things.
The cct should be left here in DIY, it is a great example of Futterman thinking outside the box.
And can easily be made safe with an isolation transformer. I may have done that, the lab I worked in had them.
People here on DIY need to see a lot more than DHT amps from 1930 & all the warts that come with them. 😀
in Audio Magazine on a piece of 1/8" extruded Aluminum ( Aluminium??).
And I'm still here 65 yrs later, what a coincidence!
It 'sounded OK' running a 16R speaker. But as usual I soon went on to other things.
The cct should be left here in DIY, it is a great example of Futterman thinking outside the box.
And can easily be made safe with an isolation transformer. I may have done that, the lab I worked in had them.
People here on DIY need to see a lot more than DHT amps from 1930 & all the warts that come with them. 😀
Good evening again ! Coming back to your design again and again 😎
Swinging 42mA -> how would you spec the power filtering choke for that ? (Choke of LC filtering).
Do I need 300mA (for one side of stereo) or can it be lower, at around 100mA ? Given the fact I'm using 250 Ohm Beyer DT 770 Pro-s now but might also try 600 Ohm headphones too and still being able to get a quite healthy-strong power out of it ?
- I studied this article about PSU filtering.. -> I think with a double LC stage like shown on the very bottom I'll go into "deep dark silence" regaring PSU noise
- This is the whole list of Hammond offerings just as a starting baseline but there are also toroidal chokes with air gap, seemingly quite okay..
I'd use a cascaded LC stage with 100uF for each choke to suppress PSU noise effectively.
Trying to stay with resistance as low as possible for not losing that much B+ voltage (or I reduce the 620R/1W resistor at the beginning) ..
If I can oversize this filtering, why don't do that ? -> e.g. 4H - 225 mA - 65R model(s), same weight, what do you think ?
I'd have them custom-made here locally in toroidal form.
I never use a filter choke, the circuit takes roughly 50mA for the stereo pair. There was a miscalculation however, it will swing about 20mA.
A choke needs a certain baseline current to become "critical", oversizing in this case wound'nt work as well.
Triad C-7X would be fine. Hammond 150M. Hammond 193B if you want a fancy one.
Personally I run it from 12VDC using a DC-DC boost converter.
A choke needs a certain baseline current to become "critical", oversizing in this case wound'nt work as well.
Triad C-7X would be fine. Hammond 150M. Hammond 193B if you want a fancy one.
Personally I run it from 12VDC using a DC-DC boost converter.