saving the planet

[val33vo]

Hello fellow enthusiasts, I am building an ipod docking station using a homebrew push pull valve amp, no probs there as ive built quite a few over the years, however being a tight fisted git I dont want to use a mains transformer if I can help it , apart from the cost they generate magnetic fields and are bulky, Ive also aquired a job lot of PCL86,s for £2 quid each (much cheaper than the ECL86 ) my problem is with the heater supply I am toying with 2 ideas firstly wire all 4 heaters in series 4 x 13 = 52v and then drop the other 200v using a 60watt mains bulb or use a 5UF non polarised capacitor ( reactance = 666 ohm at 50 hz), I like the eco friendly aproach of the cap but am a bit scared in case it fails short cct

your comments appreciated

[ppppenguin]

I hope you are using a transformer for the HT. Otherwise you are connecting your ipod directly to one side of the mains. There is an answer to this, it's called audio isolating transformers but I really wouldn't recommend a live chassis approach unless you are really confident about keeping humans and mains separated. Not just you but visiting kids and dogs too.

As for the heater supply, what's wrong with an ordinary 12V or 15V transformer? Even a toroid with nice low field is pretty cheap. Otherwise a surplus PC power supply will give 12V which might be tweakable to 13V if necessary.

[val33vo]

I am aware of the dangers of AC/DC equipment, no uninsulated part of the amp will be touchable and the i/p will be via capacitors

[Studio263]

This form of heater supply would still be dangerous even if you had a transformer for the HT. The heaters may short to the cathodes (far from unknown!) and as they will be near chassis potential this would result in the audio connections (etc) becoming live.

Obviously anything involving an I-pod is never going to be hi-fi but all the same you should aim for decent performance. A nice way to power the heaters is to make a 300mA constant current DC source for each one (the LM317 IC is ideal for this, you will even end up with a decent heatsink like a proper amplifier has), that will give you the lowest noise and best valve life. A 15V 2A transformer will power this lot nicely.

There is no practical way to avoid bulk in an amplifier like this, the two output transformers will be quite big for a start if you want to use the full output capability of the valves (a mighty 6W if commercial designs are anything to go by).

[ppppenguin]

Originally Posted by val33vo

I am aware of the dangers of AC/DC equipment, no uninsulated part of the amp will be touchable and the i/p will be via capacitors

If the caps are big enough to give adequate bass response they may well be big enough to pass a dangerous amount of mains. Even if they are low value then they must be class Y/Y2.

I assume you are relying on the output transformers to isolate the speakers from mains.

If you want low bulk then what's wrong with transistors.

[val33vo]

yes I am using o/p trannies ( well actually normal power types again to save money ) ran a sig gen on them and they seem flat way above 10khz

Whats wrong with transistors ? quite simply they sound different to valves ( valve sound ) due to im told the way a valve amp handles peaks of signal, ie it causes a different type of distortion from the severe clipping produced by transistor ones

[Tim]

How about a transformer on the input? It would only need to be small and MP3 players are used to driving fairly low impedance loads(hearphones). Microphone transformers used to be available.You might need a handful of passives for equalization, but it would certainly be safer.

[Studio263]

Originally Posted by val33vo

Whats wrong with transistors ? quite simply they sound different to valves ( valve sound ) due to im told the way a valve amp handles peaks of signal, ie it causes a different type of distortion from the severe clipping produced by transistor ones

I allways thought this was a load of old guff, the "valve sound" is really the effect of a high-impedence device driving a low-impedence load through a (wretched) transformer. You could do this with transistors if you really wanted to. To make a valve amplifier sound even half decent is pretty tricky (and mains transformers for output matching is not the way to go, the cores are constructed differently as the design goals and not the same), just putting a valve in does not automatically make for a decent result.

Transistor amplifiers do not clip if they are not overdriven, this old excuse comes from the guitar field and is not really relevant to quality audio. A well designed transistor amplifier (and I hold up the excellent B&O Beomaster 4400 of 1978 as my absolute favourite) is subjectively completely neutral due to the care taken in it's design at every stage. I've not heard a valve amplifier that can match it.

[Tractorfan]

What's wrong with a mains transformer? You could always screen it with mu-metal to keep the "nasty" waves in check. There's a huge one in your local substation anyway!
Cheers de Pete.

[Nickthedentist]

Anyone remember Adi Brooke's thread on building a guitar amp from a live-chassis record player amp, from a few years ago?

Maybe one of the mods could resurrect it to save us all from doing everything all over again?

Nick.

[evingar]

I don't think I would personally contemplate a non isolated PSU design, be it using droppers or caps. The heat from the droppers would be bad and there are safety issues with capacitive isolation as pointed out by Jeffrey. The safety issues with un isolated supplies are also just too great IMHO.

As has been pointed out, a traditional output will require a bulky transformer in any case, so it's never going to be "slim line".

A possible way forward is by using a high frequency inverter for the HT, then powering the whole thing from (say) an isolated +12V DC supply. The heater supply could then be derived easily and you would have no isolation issues - Maybe it could be done with a transform less output stage ? Potentially the whole thing could then be quite small, the only transformer being a small high frequency pot core type for the HT.

Naturally, the switching part would need careful screening. and it would of course require silion in the PSU

[paulsherwin]

If you cut corners to this extent you won't get 'valve sound', you'll get 'lousy sound' and you may well kill yourself while listening to it. There are good technical reasons why audiophile valve amps cost as much as they do, it isn't all snake oil and snobbery.

You will get an incomparably better sound if you get a good quality 1980s hifi transistor amp from your local freecycle list: http://www.freecycle.org

Paul

[TrevorM6TSE]

Do you know it's funny that the very thing you are doing is something I've just done (literally). I decided to build a simple valve amp for my ipod that I could use in my conservatory. I tried not to cut corners too much, but it didn't cost an arm and a leg either:

2x ECL82 - £9 each from a member here
kit of components (caps, sockets, diodes et al) £12 from Bardwells in Sheffield
Box and sockets etc - £20 from Maplins
Old valve based extension speaker from a junk shop gave me the mains tranny and one OP tranny - £4
Old valve radio beyond repair gave me the other OP tranny - £2 car boot.
Ipod docking base - £10 Argos.

The result.. A REALLY nice sounding valve amp for not much at all. Yes I know it's only 2 WPC but hey, it's a small room!

But then the funny thing is, just exactly like Paul Sherwin said my neighbour kindly gave my a lovely old Leak 2000 Tuner Amp in a sorry state. I've restored it (Cleaned the case, replaced dead bulbs, cleaned ALL the many switches and controls) and tried that in the same setup and it sounds every bit as good, in fact it sounds better!

So it probably is true that after all that valve hype that I got myself into, a free Leak 2000 outsmarts it.

I did toy (very briefly) with direct mains feed for the amp but all the Health and Safety came into my mind and I decided on a transformer.

[TrevorM6TSE]

Hi,

Just to throw this into the mix, have a look at this thread: https://www.vintage-radio.net/forum/...ad.php?t=28227

I'm sure Stewart wouldn't mind be linking to it.. He's offering the bones of a rather nice simple amplifier that is comlete bar the valves.

This would save quite a bit of money and save the planet by recycling too!

Id'd have it myself if I was nearer to Surrey!