Valve Headphone Amp

[simon]

Ah right. I'd checked Farnell, CPC, Mouser and DigiKey and it didn't turn up anywhere so presumed it an ex.

Was I any closer with my other thoughts?

[ed]

Hi Simon

I’m a bit slow, I didn’t realise the questions might still be outstanding.
C2 and C3 are the blocking caps because it’s single ended and there is a voltage on the FET source.
R8 is the output load and in the spice diagram it represents the headphones.
As Nick has pointed out, it is a buffer, in fact it has a voltage gain of 97% with the 35R load shown. With a 300R load the spice sim shows a gain of 99%.

[simon]

Thanks ed, this all makes more sense now.

Just a buffer I think as it's a source follower.

I'd completely missed this! Don't know how, but it would have saved much unnecessary research last night lol.

[simon]

I think I might just breadboard ed's circuit - how well matched are ECX10N20s? As they're "audio" does that imply they are well matched maybe?

Whilst not exactly cheap they are a different league to valves so it wouldn't be the end of the world if I did have to buy more than 2. And only having 2 and letting the magic smoke out of one would be very frustrating...

[Nick]

I would not think matching would matter for that circuit.

[simon]

Cool, 2 and a bit of care it is

[simon]

For the +12V supply I could piggyback the +12V 0V -12V PS I use for the active crossovers. It's a LM317 job using one of Andrew's general 3 leg reg boards - should be okay?

The master volume control feeds the active crossovers - I reckon I should be able to also feed ed's buffer too. Any reason not too?

[simon]

How much heatsinking do you reckon the ECX and LM need ed? I think I might just draw up a pcb to save messing about and could use a single on board sink with the chips back to back, if that's enough sinking.

[Nick]

4r7 should give 265ma, So with 12v rail that is 3.2W to dissipate between the two devices.

Maybe something like

https://uk.rs-online.com/web/p/heatsinks/1898151

4 K/W would mean a 13K temp rise, so that should be manageable.

[simon]

Brill thanks.

Is that really 4 kelvin per watt? 12.8C isn't much to worry about.

[ed]

Hi Simon

If it's of any use, here is the original circuit in rough. I mention it because It shows the relay that allows the hpa to sit between the dac and the power amp. With the hpa switched off the signal is routed directly from the dac to the power amp. when the hpa is switched on the relay routes the signal to the hpa alone.

hpa se circuit.jpg (219.06 KiB) Viewed 3409 times

[Nick]

Brill thanks.

Is that really 4 kelvin per watt? 12.8C isn't much to worry about.

Yep, they are chunky heatsinks, I use them in a phono stage. 50mm tall

[ed]

for reference

https://www.audio-talk.co.uk/phpBB3/vie ... 6&start=28

[simon]

Thanks Ed, your original sketch was really helpful and made me think a little more laterally. I'd been musing on how to switch the power supply between active crossovers and hpa - but switching the signal is a much better solution in my case I think. I might have a play with Eagle tomorrow.

[simon]

I tried to squeeze two channels with onboard sinks on to a 100 x 80 pcb but Eagle's limitations were just too great - it would have been really squeezy, the caps would have been too close for my liking.

A mono channel was easy enough though.
.

Mono Onboard Sink.png (168.34 KiB) Viewed 3327 times

.
I wasn't completely happy with it though and moved on to a side sink like ed and I feel it's a nicer layout with both channels on one pcb, and a bit of distance between the sink and caps.
.

Stereo Side Sink.png (132.88 KiB) Viewed 3327 times

.
I've used 12V TQ2 relays, hopefully I've implemented correctly as I've not used a relay before.

And I've used a relay per channel so the output to the amps maintains separate grounds. I went round in circles about the merits of this but convinced myself in the end. I've probably just made life harder for myself though.

I have some 1000uF Panasonic FC caps so I'll use those for the big output coupling cap, and I'll go for Wima MKP10 1uF caps for input coupling cap and lytic bypass. I did find a couple of 470uF Blackgates that would do as well probably.

Not sure there's much else to say apart from will it work?! Only one way to find out. Any obvious goolies dropped before I upload the gerbers?