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View Poll Results: a kit amplifier or home-brew amplifier
A kit amplifier 3 10.71%
Home-brew amplifier 18 64.29%
Both are equally good 6 21.43%
No opinion… 1 3.57%
Voters: 28. You may not vote on this poll

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Old 27th Dec 2011, 11:57 pm   #21
Don_Zalmrol
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Default Re: I want to build a stereo headset valve amplifier

@Rich: I'm also interested in that kit, thx for the link. Would it be a problem to connect my headphone (50 Ohms) to this kit as it states it accepts 300 Ohms headphones?

@Trevor would it be also possible to only make this setup for headsets?

PS: I've gotten a reply from Jukebox-Revival about the 6V6/12AX7 amplifier kit, it would cost € 250'
It has also the option to power a headset without a hassle

I'm starting to think to buy a kit (for the last time) and also start with gathering resources to start building the OTL headphone amp
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Old 28th Dec 2011, 12:19 am   #22
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Default Re: I want to build a stereo headset valve amplifier

Hi Don.
At first glance 250 euro is expensive but in the real world OK, the cost of transformers and the other parts it is actually OK.
I am not too sure about the 12AX7 in this circuit as you don't really need a lot of gain as the drive from the laptop will be quite high.
What you need is quality not quantity and that is what I was trying to say in post 20. Think about drivng your car at full speed all day, it will fail, but drive it moderately and it will run for many 100,000 KM. It is the same here, a throttled back amp using either an EF86 & EL84 or EF37A & 6V6 is the way to go.

I have included a circuit of the amp, the mixer wont be needed in this application and the EZ80 HT supply can be replaced by a silicon HT rectifier, also a headphone switching circuit is needed, PM me if you want the other circuits.
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Last edited by murphyv310; 28th Dec 2011 at 12:31 am.
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Old 28th Dec 2011, 9:24 am   #23
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Default Re: I want to build a stereo headset valve amplifier

Alright, thank you for the circuit!
It seems to be a mono amp, is this correct (from the view of chassis, right side input pin)?

The - of the silicon rectifier would be connected straight to the chassis of amp?

Can't I just use a direct headphones input?

Might be a silly question but wouldn't there be problem for ordering the components because some of the values aren't available anymore?

And how much would this design set me back?



About the 6V6/12AX7 amp kit, I can always change the 12AX7 to a 12AU7 for a smaller gain, the output of the amp 12Watts.
I would probably use a max of maybe 3Watts for my headphones? So 9Watts are wasted…

And what you say about running the car all day, this is what probably happened to my VT-96PP amplifier.

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Old 28th Dec 2011, 12:25 pm   #24
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Default Re: I want to build a stereo headset valve amplifier

I've just checked for the supplies online on Jukebox-Revival and the price is getting above €150' for just the EL84, EF86 and the main transformer… And unfortunately this amp is only mono.

So the kit is starting to look mighty attractive again and this kit includes some good quality components (transformer, volume resistor (alps),…). And I know that the kit is again overpowering for just powering headphones.

Piet from Jukebox-revival is looking for a headphone amplifier. And Trevor is going to supply me with another schematic.

I will then draw some more conclusions of what I will do.

Cheers,

Laurens
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Old 28th Dec 2011, 11:06 pm   #25
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Default Re: I want to build a stereo headset valve amplifier

Been googling a bit and found this rather nice headset amplifier project from HeadWize.
And this one is an improvement of the other one with a more stabilized power supply for the filaments.

Seems rather do-able for me not?

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Old 28th Dec 2011, 11:55 pm   #26
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Default Re: I want to build a stereo headset valve amplifier

I've looked for the parts online and the valves (and possible the main transformer) I can order trough JR (Jukebox-Revival) and the rest trough Elfa Distrilec from sweden, it's like Conrad but bigger…

The costs of the project would come on € 181,25' without the valves.
The valves (4) costs € 37,40' and there all from JJ and new.

So in total without transportation it would come on € 218,65'.

(Transport is roughly € 30' (€ 20' for Elfa and € 10' for JR)

To it would cost a little bit less than the 6V6/12AX7 kit from JR but it would be one heck of a headphones amplifier or not?
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Old 29th Dec 2011, 12:35 am   #27
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Default Re: I want to build a stereo headset valve amplifier

You could also build the Earmax reverse-engineered version by Morgan Jones, the White Cathode Follower, enhanced in this article: http://gilmore2.chem.northwestern.ed.../cmoy5_prj.htm
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Old 29th Dec 2011, 1:02 pm   #28
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Default Re: I want to build a stereo headset valve amplifier

That's true, but I'm starting to put my mind on the OTL Class A Valve amplifier because the schematics and pictures are quite clear on how to build and assemble it.

Just a quick question about the main transformer.
They used a 235V/75mA+2x3,15V/3A transformer from Transformator-Teknik a Swedish transformator company.

Can I use a Hammond 369HX?

Because the one from Teknik delivers 235V/75mA but the one from Hammond delivers 225V/150mA.

The filament supply is one 6,3V/6A instead of 2x3,15V/3A so the output is more with the one from Hammond.

So which one should I order? The price is almost the same (+- € 8' difference)
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Old 29th Dec 2011, 6:58 pm   #29
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Default Re: I want to build a stereo headset valve amplifier

Well the HT will be a bit low from the Hammond, but since the centre tap on the Teknik heater winding isn't used in the system you could use the Hammond OK. Since it has more power capability, it could be useful for something beefier later, if you decide to go that way. Also, since the DC heater supply looks a bit marginal in terms of input DC to the regulators, you may get a bit more voltage from the lightly loaded Hammond than the other one.

In fact after a bit of a think, I don't reckon that regulated DC 6V heater supply will work at all well, there just isn't enough voltage available. If you want DC heaters, I reckon use the Hammond and voltage double rectify the 6.3V output to get adequate input voltage to the regulators. The regulators will then need some reasonable heatsinks as they'll dissipate about 5W or so each.

Having looked at the spec for the Hammond, there's also a 5V 3A winding which might be more appropriate for the voltage doubling approach- the regulator dissipation will be significantly less.
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Old 29th Dec 2011, 7:09 pm   #30
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Default Re: I want to build a stereo headset valve amplifier

So if I use the Hammond, the amp will work. But with a marginal performance. What is the worst scenario if I leave it like this?

What do you mean with doubling the 6,3V or 5V.

Is it something like this? By tapping from another diode from the rectifier and feeding it directly into the regulators?

Sorry for all the questions, this is all new for me

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Old 30th Dec 2011, 12:59 am   #31
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Default Re: I want to build a stereo headset valve amplifier

Yes, indeed, something like that. For just a doubler, though, you only need two diodes.
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Old 30th Dec 2011, 11:03 am   #32
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Default Re: I want to build a stereo headset valve amplifier

So the circuit adjustment would look like this (please see the attachment)?
I've looked the Vin up for the 7806 and it needs a minimum voltage of 8Vin to work properly.

I've added 2 TO heatsinks in the basket of Elfa Distrelec that could disipate 27,3K/W of heat, I will also mount this to the chassis for more cooling.

Just a quick question, would this amplifier be better than the 6V6/12AX7 kit in terms of sound quality?
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Old 30th Dec 2011, 1:39 pm   #33
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Default Re: I want to build a stereo headset valve amplifier

What I meant was this:
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Old 30th Dec 2011, 2:43 pm   #34
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Default Re: I want to build a stereo headset valve amplifier

Too late to edit.....

You will probably need to use the 6.3V secondary, as even with doubling, the ripple voltage troughs could fall below the minimum recommended input to the 7806 regulators with only 5V AC input, and you'll need to increase the 2200uF capacitors to 3300uF or so.
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Old 30th Dec 2011, 3:08 pm   #35
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Default Re: I want to build a stereo headset valve amplifier

Thx, for your help!

Strange your schematic shows the 6,3V secondary in use.
The diodes D1 & D2 are 2x D1N5401 like in the previous schematic or not?
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Old 30th Dec 2011, 5:14 pm   #36
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Default Re: I want to build a stereo headset valve amplifier

Yes, 6.3V. I simulated the circuit with a 1.2A dc load (four 12AU7 heaters) and found the ripple voltage too high at the capacitors for 5V to work properly without ridiculously large capacitors.

The diodes are 1N5401 just for my simulation run (though that type would be fine). In reality they could be any 3Amp plus 100V plus silicon rectifier diodes or even a suitable bridge with both halves in parallel. (Feed top of AC secondary AC into both AC terminals linked together and treat the + terminal as cathode of "top" diode and - terminal as anode of "bottom" diode. The other AC connection from the transformer goes to the capacitor midpoint.
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Old 30th Dec 2011, 5:40 pm   #37
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Default Re: I want to build a stereo headset valve amplifier

I've put 2x fast recovery diodes in the Efas basket.

They can withstand 6A of current and a max voltage of 200V. (To be on the safe side, the transformer can put out a 6A current after all)

And also 2x 3300uF caps 16V.

Is there anything else peculiar or strange about the circuits?
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Old 30th Dec 2011, 5:58 pm   #38
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Default Re: I want to build a stereo headset valve amplifier

Those diodes should be fine.

The rest of the PSU looks OK.

So far as the amplifier is concerned, I really can't say, short of trying it. I have no way to simulate it. Hopefully it's better toleranced than the heater supply design was, though even that may have worked on a one off- it would depend on the tolerances of the transformer, diodes and regulators as to exactly how much ripple got through to the heaters, so the induced hum (not that much even if the heaters are run from raw AC) could still have seemed reasonable.
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Old 30th Dec 2011, 9:52 pm   #39
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Default Re: I want to build a stereo headset valve amplifier

If the hum would be excessive can't I minimize the ripple by adding a zenerdiode after the 2x 3300uF caps, and between the + line and GND line?

I think I have almost everything and will order the components after the weekend, after all it's the day before new-years eve and coinciding my birthday :p yay!
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Old 30th Dec 2011, 10:38 pm   #40
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Default Re: I want to build a stereo headset valve amplifier

Hi.
As an old timer with valves may I ask the silly question why we are feeding DC to heaters?
This is verging on audio foolery! When the valves we are discussing were designed the norm was to feed them with AC, an indirectly heated cathode valve (all of the types in question) is not susceptible to hum, any hum would be induced by poorly routed heater cabling or heater wiring that was separately run rather than twisted pairs.
I have been making valve amplifiers for 45 years and wouldn't contemplate a DC supply for the heaters.
In this application where the drive is high from a laptop, gain in the amp is low and hum in a balanced Push Pull drive is cancelled I see no reason at all for the extra components and complications.
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