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Old 18th January 2003, 07:18 PM   #1
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Default Low power tube amp for headphones

I currently have a portable headphone amp with the OPA2227P chip and a two-9V battery power supply. It would work great for travelling... but I am a home listener more than anything. So I want something better, but not too expensive as I am limited on funds. The tube amp below seems as though it could be quite affordable, and I have always liked tubes, and have been wanting to build a tube amp for a while now. Some of you may remember the idea to make an HF-87... it just got too expensive, unfortunately. I could have bought a rebuilt one in perfect chape for the same price. This amp will power Sennheiser HD580s in case it is necessary

I have some questions, about this amp, not a lot on how to build- I can get that part, but about safety. I am not one to enjoy strapping something to my head that could have more than 9V running through it- for my safety and that of the headphones. Are OTL headphone amps safe? I really don't wanna blow my $200 cans. Can this amp damage anything- the turning on and off, etc. Will it ever harm my phones?

The only real question I have about the amp diagram is the heater supply. It says use a shielded cable- can anyone give me an example of that type this is and what I should use? And also on the heater supply, am I reading this right when I am thinking the power is attached to pin 4 on both 6CG7's and again power attached to pins 4 and 5 on the 12AU7's and then pin 5 on the 6CG7's and pin 9 on the 12AU7's is going to ground? Or is that supposed to read that power goes to all pins (4,5,9) and then from 9 out to ground?

Also, just thought of a question... the headphone output jack/RCA input jacks have a ground. Do I connect this ground to the ground of the amp? Won't that give me some voltage across my ground that could damage my cans?

Thanks
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Old 18th January 2003, 07:27 PM   #2
dhaen is offline dhaen  Europe
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Please post a diagram or link.
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Old 18th January 2003, 07:41 PM   #3
halojoy is offline halojoy  Sweden
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Default Yes, please do!

Quote:
Originally posted by dhaen
Please post a diagram or link.
It is always recommended with some diagram.
It is almost impossible to desrcibe a circuit, just with words.

Makes you get many more answers and better discussion,
if you give as many and easily understood facts, as possible.
This is something many more should think about.

/halo - and all wants this forum to work as good as it can
for all to benefit
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Old 18th January 2003, 07:52 PM   #4
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As I hit the submit button, I hat this odd feeling I was forgetting something... but it just never dawned on me.

Anyway, here it is:

Click the image to open in full size.

and the power supply:

Click the image to open in full size.

That should help considerably.
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Old 18th January 2003, 08:19 PM   #5
dhaen is offline dhaen  Europe
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trespasser_guy,

As long as it constructed properly, there should be no safety issue with this OTL design, for you, or for your cans.

IMO the screening of the heater wiring is unnecessary, providing you keep it away from signal carrying wires and components.

I don't understand your earth/ground question.

Cheers,


Halojoy,

Sorry. But don't you think you were stating the obvious?
He had not ommited the diagram on purpose. It was obviously a mistake.
BTW Ranking stopped some time ago.
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Old 18th January 2003, 10:26 PM   #6
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Default author, author!

Hi trespasser_guy,

Mind sharing where you lifted the schematic from? TIA.

Hi John, Came across your site for the first time. Nice!

thanks
Craig Ryder
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Old 18th January 2003, 10:54 PM   #7
dhaen is offline dhaen  Europe
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Default Some more thoughts

trespasser_guy,

Looking again at the heater circuit, I would predict less hum (and noise) if you just used AC through twisted wires "daisy chained" from the heater winding on the transformer. If there is no centre tap, you can use 2 resistors to ground.
Indirectly heated valves were designed with AC in mind. IMO there is no reason for DC except in mic and phono inputs.
There'd be a bit less heat too.

Cheers,


Craig,

Thanks, but it's got a long way to go.
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Old 19th January 2003, 12:06 AM   #8
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Default HEADPHONE AMP.

Hi,

With a little bit of imagination this circuit could be turned into a grunty little linestage as well.

In fact,the term OTL is a tadd misleading,it's nothing more than an asymmetrical SRPP.

I would also add a mute switch at the output and adorn it with a decent PSU.

Cheers,
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Old 19th January 2003, 12:51 AM   #9
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Thanks so far.

Ryder, I found the schematic on HeadWize- the first one under the tubes section by Kurt Strain. Here

So, the power supply could use work? What should be done? Change the heater supply to AC? I am guessing that would be done by removing the diode bridge, but what else would be done? And I am still uncertain how to wire the heaters... For the 6CG7's it shows pin 4 on top and 5 on the bottom next to the ground. Does this mean that pin 4 gets the power and then pin 5 is just grounded? Then the same with the 12AU7's, except pins 4+5 are powered and 9 is grounded?

Also, when I run the ground from my input/output jacks, if they connect to the amp ground, won't they have some voltage running across them?

fdegrove, this mute switch I am guessing is a simple switch like a DPDT that is just an on off at the output section, correct? Where in the diagram is it located exactly?
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Old 19th January 2003, 12:59 AM   #10
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Default MUTE.

Hi,

Quote:
fdegrove, this mute switch I am guessing is a simple switch like a DPDT that is just an on off at the output section, correct? Where in the diagram is it located exactly?
Very well, it is to be positioned behind to output cap.

Re:heaters...I need to check pinouts for the tubes.

Just a guess, if pins 4+5 are connected on the the 12AU7 you run them at 6.3 V with twice the current.

I'll check and get back on this.

The SRPP circuits are rather finicky about the PSU,go either regulated or brute force.

Cheers,
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