Low voltage triode headphone amp

[robjkmannering]

I wish to make a small headphone amp for use as late night entertainment as I like seeing the glow as I fall asleep.
The problem is, having a toddler in the house, I do not wish to have anything with high HT living in easy reach!
The valves will be sheilded with a metal grid so that they can not be touched.
I have read that you can run triodes on very low HT and wondered if an ECC83 for example, could be used as the output power would only be milliwatts.

I would be gratefull for any advice.

Rob

[G8HQP Dave]

ECC83 is not too happy at low voltages. Try ECC82. As a general rule, for low voltage operation you want a low mu valve.

[robjkmannering]

Thanks for that, what would be best for output transformers or can I simply use an anode load resistor?

Rob

[Radio Wrangler]

Most headphones are lower impedance than the optimum anode load you'd be needing.

The anode voltage is still going to be significant and you're going to stick those phones on your head (A favourite movie quote: 'My brain? That's my second-favourite organ' ) so transformers are good for safety isolation and they tend to fail to a DC path to ground.

Capacitor isolation is a lot less reliable. These pages are laden with terrible gloom and despondency over the short life and great smoke content of capacitors... more than all other components put together.

Cheers
David

[robjkmannering]

Thanks, a rummage through the spares have revealed a small(ish) output transformer so it will have to be a trial unit in mono untill I find a matching pair.
Looking on other sites, the other option appears to be to buffer the output with mosfets or even op-amps. If I try this, I could use one half of the triode to make a Baxandall type tone control.

This does not have to be Hi-Fi, I do not have golden ears, but it must be at least pleasant to listen to.

Rob

[turretslug]

It's not so many moons ago that RS (and probably other catalogue suppliers) did small multi-ratio (primary and secondary) output transformers for valve circuits that would give you plenty of scope for "suck it and see" experimentation and late production ones would use excellent modern enamelling- I, too, am wary of sticking anything over my head that is microns of insulation away from severe unpleasantness. Don't be put off by dogmatic "6QX6 needs 8.3k load" type stuff- what they mean is "a study of possible load-lines shows optimal compromise between output power, efficiency, frequency response and distortion is likely to be at around 8k loading but a far spread around this point won't be disastrous".

Unfortunately, these components are in keen demand but you can occasionally find "more than one" for not too bad a price,

Colin.

[G8HQP Dave]

If you are running at low currents and voltages then an old transistor output transformer might do. These turn up on ebay from time to time, or may still appear in some catalogues. You would need to check whether they can cope with DC (as many will be for P-P), or you can use a capacitor to feed the transformer.

[broadgage]

How do you propose to obtain the relatively low HT voltage that is needed ?
If it is derived even indirectly from the mains supply, then I would agree that an isolating output transformer is advisable for something that is applied to the head.

If however the equipment is to be battery powered, and at a low HT voltage of perhaps 30 or 40 volts, then an isolating transformer is not needed for safety since the supply is "earth free", no shock to earth can be received.

Depending on the valve chosen, something like a 6 volt sealed lead acid battery for the LT and a few 9 volt transistor batteries for the HT might serve.

A transformer might still be needed for impedance matching, unless you can obtain relatively high impedance headphones.

[KeithsTV]

RS still sell a multi ratio transformer.

http://uk.rs-online.com/web/p/audio-...PSF_430714|cav

Keith

[paulsherwin]

I do think you will have your work cut out producing an adequate headphone output with very low HT. The output required for hifi listening is very significantly more than that for, say, SW DXing.

Unless you have a very specific reason for doing this job with valves, it would make a lot more sense to build a headphone amp using a simple dual op-amp, like this one. This will be totally safe, much cheaper to build, and can be run from batteries or any spare wall wart PSU lying around. It will probably sound better too. For visuals you could just light a candle

[turretslug]

Thanks for the link, Keith. I take it that secondary impedance of "3-15kohm" is a typo!

[robjkmannering]

Thanks for all the information and taking the trouble to reply.
The HT envisaged would be no more than about 30 volts and would be derived from a transformer as would the LT.

I have now found several designs published that all overcome the various problems by using a triode as a voltage amp followed by an op-amp, mosfet or even bi-polar transistor stage feeding the headphones. This obviously does not require an output transformer and would keep the output volts away from my ears!

Thanks again for all your help.

Rob

[richrussell]

If you're using transistors or op-amps to buffer the output to the headphones, then they might as well do the amplification too. OK, so they don't glow, but if they're already there, it kind of makes sense to use them.

[G8HQP Dave]

You could always use the valve for purely cosmetic purposes, like some Chinese audio products. Waste of a good valve, though, so make sure you use a cheap common one. At 30V HT, most valves will simply add some distortion if they are actually in the circuit.

[kalee20]

For cosmetic reasons, just energise the heater. You could have a couple of B9A valveholders and with pins 4, 5 connected to a 6.3V supply, plug in any E-series valves (except ECC81, 2, 3 which need pins 9 and pins 4+5 together).

It's a waste of valves though - to me, if it's there, it's got to be doing something. But it might satisfy your own requirement.

I would be surprised if you couldn't use an ECC82 with HT of around 50V to drive headphones via a transformer, at decent volume. But it would need some development.

[turretslug]

I wonder if triode-connected EF80 would work? A few of these used in the odd headphone amp is unlikely to generate KT66-style stampede/scarcity/price escalation!

[Alan Stepney]

EF91's work quite well at low voltages.

OK, I know they arent triodes, but they do work.

[GrimJosef]

There was a series of valves designed specifically for operation from 12V car batteries. The 12K5 http://www.mif.pg.gda.pl/homepages/f...137/1/12K5.pdf looks like it might meet your needs although you will still need an output transformer. The step down ratio will be small though. The valve needs to see something like 800 ohms so if your headphones are 32 ohms that's a factor of 25:1 in impedance or 5:1 in turns ratio. And at these voltages it could safely be an autotransformer.

Cheers,

GJ