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Modify preamp to headphone amp

If you mean line level out, as would normally go to a tape (in the past), that is the output required for a headphone amp, no mod's required. You'd need a headphone amp' though, there's no way around that.

If I understand your question correctly.

I asked a poorly worded question along similar lines a couple of days back and it was misinterpreted, but got the right answer indirectly.
 
thanks very much. I mean to use a line level out directly to the headphones not to the headphones amp. Some people say there is an impedance mismatch but it will work
 
Headphone impedance varies vastly from a few tens of ohms to k-ohms, depending on make/model.
With no headphone amp', you'll have no volume control though, unless you fit one between amp' and 'phones.

You'd need to know the output impedance of the line output, which may be tricky to find, but you'd need to stick to the rule of thumb - 10x the input impedance for the 'phones as the output impedance of the line output.
 
It will work after a fashion but be really poor.... in fact, being a valve stage, it would be unusabley crap for this and as Vinny says there would be no volume control.
 
Whether it will work or not depends very much on the exact nature of your valve linestage design, so what valve pre-amp are you looking to use?

Valve headphone amplifiers generally use either small power valves, to drive direct; or transformer -coupling to make it work at all.

edit - crosspost with Arkless -far more succinct!
 
i built a kondo m7 clone which i like the sound very much. when you say no volume control, i presume the existing volume doesn't work then because the output level is fixed ?
 
Tape out is just a direct connection to whatever source is selected. No circuitry. IE select CD and the "tape out" connects to the "CD in" sockets!

Your only other output is the main outputs to the power amp. These of course are volume controlled but at far too high an impedance and too low a current capability to drive headphones without gross distortion.

A suitable headphone amplifier could be built in to it if there is enough space etc.
 
martin clark said:
Just looked up the shematic - an M7 hasn’t a hope of driving headphones directly. Build an add-on amplifier for that task
Click to expand...

Yep! I have the original schematics from pre internet days... at least the nice high value of the paralleled cathode resistors mean it should work pretty good at its intended job.
 
philiphifi said:
thanks for your input. What would be needed to turn one output to drive a headphone? Easiest option please
Click to expand...

I'm not sure what you mean by "turn one output to drive a headphone".... A headphone output can be added and would obviously need a headphone amplifier to be built into the M7, sufficient space and provision of suitable PSU voltages etc allowing. You cant "turn" another output into a headphone output...

Easiest option would be to use one of the many headphone amplifier IC's which are available. Depending on how you decide you want to do it you will have to decide on what types of 'phones you want it to drive as they vary from as low as 8 Ohms up to as much as 600 Ohms for hi fi quality models that I'm aware of. To drive all types is certainly possible... but not for all technologies used to do it.
 
To add to that, there are many many IC's available for driving headphones these days. Do not be put off by them being intended for laptops, headphone output of TV set etc as many have superb specs. Your biggest problem is likely to be their incredible miniaturisation which makes many virtually unusable outside a mega factory with precision pick and place robots etc! They could be 2mm X 2mm and have 20 pins!

An op amp with a class A output stage added would be how I would do it...

Valves may be possible, again mainly depending on space and how much more power you can take from the M7's mains transformer.... BUT knowing what impedance 'phones you are going to drive becomes really crucial when valves are involved.... unless it is to become a beast of an external headphone amp with power valves etc!
 
Or just try a Bottlehead Crack kit perhaps...?
While I know nothing about valve headphone amplifers, it's a simple build with a beefy cathode-follower output which has remained popular for a decade or so, now updated, and that suggests it's agreeable to many /gets much acclaim in some places. Up to you to read up and see if such / the headphones people use with it appeal; but if you like simple valve circuits, and obviously like valve diy - could be somewhere to start looking.

(NB I've not heard one - and I'd like to. I've built dozens of headphone amps of various homebrew solid-state varieties though over last 25yrs... )
 
The Bottlehead certainly gets lots of votes online, both sides of the Atlantic. Ready-built ones come up in the UK for around £375 every once in a while.

A full spec' kit from the US would come out around £330, plus shipping, plus duty, so £420+ total.
 
ah... thanks for clarifying. From what i read on the internet, i was mistaken in thinking that what is needed is a O/T of some sort. I think if i have to buy a headphone amp then i'll get one from ebay. It just seems a waste not able to leverage the existing M7 clone by adding some passive components to match impedance etc
 
The basic problem is, the output 'drive' available from the M7 is derisory; it is direct, from the high-impedance output port (the anode, c.>8Kohm effective) of a teeny-tiny signal triode: via an utterly-feeble value coupling-cap. It just does not, cannot, does not posess the spare AC current to drive even 300ohm headphones - you'd get a bass roll-off below c 5Khz (so fizz and noise only) and the current draw would utterly collapse the preamp's performance: the M7 is a preamp only suitable for loads above c 100,000ohms ( and even that has caveats...)

Sorry!
 
martin clark said:
The basic problem is, the output 'drive' available from the M7 is derisory; it is direct, from the high-impedance output port (the anode, c.>8Kohm effective) of a teeny-tiny signal triode: via an utterly-feeble value coupling-cap. It just does not, cannot, does not posess the spare AC current to drive even 300ohm headphones - you'd get a bass roll-off below c 5Khz (so fizz and noise only) and the current draw would utterly collapse the preamp's performance: the M7 is a preamp only suitable for loads above c 100,000ohms ( and even that has caveats...)

Sorry!
Click to expand...

The schematic I have shows a cathode follower at the output...
 
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