my tube headphone amp-> need feedback
Ok, I am not really that good with tubes... I am out of school on summer break, and need a project. Summer assignments can wait for a nice audio gadget. :) I was thinking about doing a solid-state class A amp, but I just have to keep myself focused... I have always wanted tubes, so I must do that. I have lots of time now, so it should be do-able.
I have drawn up the attached schem for an OTL tube amp... I neevr find a design to my liking, so I draw up my own. Approximate parts values:
C2- 470uF, 25V
C3- 1000uF, 25V
C4- 470uF, 400V
P1- 100k audio/log
R1- 1M, 1/2W
R2- 1k, 1/4W
R3- 15k, 1W
R4- 1k, 1W
R5- 100, 1/2W
R6- 4.7k, 1W
R7- 470, 1W
R8- 47k, 1/2W
R9- 22k, 1/2W
What do you think? I would prefer an octal-base tube to a 9-pin one, if you think I should change drivers, etc. Will it efficiently drive Sennheiser HD580's? They are 300-ohm cans. Thanks for any help you can provide...
Get rid of C1. Why do you need R1 and R2, get rid of them and adjust P1 for desired impedance, although I think just the pot would do fine. Also, I think you should get rid of R5. I am also guessing that the second half of the 6SL7 is choke loaded? R9 not necessary, you headphones provide low impedance as it is. That's all I could find as I went along, please correct me if I am wrong on something.
Agreed, you don't need C1. A 6SN7 probably doesn't need R2, the grid-stopper. Technically, R1 isn't needed because its provided by the wiper of the volume control to ground, but wiper contacts can be intermittent, causing noise, so I normally leave it in. You definitely need R9, otherwise you will blow up your headphones the first time you plug them in as C4 and C5 charge through the coils of your headphones.
300 Ohms is a low impedance load for a valve, and although you are right to use a cathode follower, it needs to pass substantial current. Staying with Octal, you could use half of a 6BL7 or even 6080 for the cathode follower. They should be biased to pass 20mA, or more. Headphones don't need many volts to drive them, so V2 isn't really needed, and V3 could be directly driven from the anode of V1. Driving V2 directly from the anode of V1 would not have worked because V2's grid would have had half the HT on it, so it's cathode resistor would have needed to be very large to achieve the correct current, and there would have been hardly any volts left for the valve to operate. V1 has rather a small value of anode load, which will increase its distortion - you might want to consider increasing this value. Don't be disheartened by these, we all have to make our mistakes...
Way more gain than you'll ever need. Parallel both halves of the 6SN7 as a cathode follower and enjoy.
And yes, loose all the resistors and the cap in the front end. The volume pot's total value sets the load impedance (I'd use 500k), and acts as the grid resistor. Functions very well that way.
(I've built this exact circuit... feel free to search on the 10,000 posts regarding it - "complete 6SN7 preamp", etc)
If you rethink it for headphone use you best lose some gain and let some more current flow through the output.
You could easily use half a 6SN7 for the voltage amplification and a paralelled one CF for the output.
If it were for my own use I'd think along the lines of the dissimilar triodes where you can have the entire headphone amp from a single tube and it could easily double as a CDP preamp.
6EW7 would do nicely...
Amongst a whole raft of others if you don't stick to octals...;)
Octals look cooler. hehe
(though they are actually QUITE warm...)
Anyway, if another that is non-octal would sound better, then by all means... so, it is possible to have just ONE tube amplify the whole thing? That would really cut down on parts cost... I am intrigued. Can anyone elaborate more? Thanks :)
You could use half of a 6SN7 driving half of a 6BX7/6BL7 as a cathode follower (or 6SL7 first stage for more gain). The other halves, of course are for the other channel.
Or use dissimilar dual triodes, one per channel. 6EM7/6EA7 or 6GL7 for high gain, 6DN7 if you don't need so much. The output half is a 7-10W triode for plenty of drive.
All are octal. 6EW7, 6FD7, 6DE7, 6DR7 are some 9 pin possibilities.
I apologize for now sounding noob-ish... but I must ask in order to learn...
What exactly is a dissimilar triode? I understand the triode part, and how to use a triode (dual), but not the dissimilar part. What makes it different?
I have heard of 6EM7, but not any of the others. 7-10W? :eek: For headphones? :dead: How? Out of one triode? Golly...
How high a gain are we talking about with a 6EM7, etc.? And what about the 6DN7? My solid-state headphone amp has a gain of 10. I notice the amplification factor of a 6SL7 is 70- is this meaning gain=70? That is insanely large...
Another question I just thought of- people say the Sennheiser HD580's and 600's, along with some Grado headphones are 'dynamic.' What exactly is meant by this? There are some amp circuits that are also labeled 'dynamic' and are meant for these types of cans. What distinguishes this, and would a headphone amp like this be 'dynamic?'
Thanks all :)
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