what Headphone amp do you have

For anything about cans, and the amplifiers that drive them
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colin.hepburn
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Post by colin.hepburn » Tue Jul 17, 2007 1:23 am

hi Simon
One day I'll get around to a nice wooden case to mount on the mdf and some nice shiny knobs - then everyone who visits will think I have a really expensive amp from a shop

Nice to see you are near completion of your head phone amp yep I'm now some twenty months on and still not got the Case work dun on my 12B4A H/Amp :oops: its still looking the same as the pic in the post at top of page perhaps this is the way a valve amp should look 8)
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Post by simon » Tue Jul 17, 2007 8:55 am

Hi Colin,

Well, when I said it was pretty much finished, I was obviously kidding myself :D . I'm already thinking about a slightly different PSU, and possibly replacing some of the resistors in the Aikido with booteek ones. These things are never finished really, are they :wink:

And now I can break the other one up for bits. Already got an idea for the mains tx...

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Post by simon » Wed Aug 01, 2007 10:20 pm

I've had a play with the PSU tonight. Before I had:

GZ30 -> 10H 82DCR -> 40uF Sprague -> 390R -> 94uF Cerafine

And I've just tried:

GZ30 -> 14H 429DCR -> 47uF Cerafine -> 14H 429DCR -> 47uF Cerafine

They model pretty similarly in PSUD but the 2 14H chokes give a smaller ripple and approx 10V less HT.

The hum maybe a smidge lower but I'm not sure I can hear any other difference between the two. The two chokes may have faster deeper bass, but I couldn't be certain. Question is, do I have cloth ears (probably) or are the two PSUs too similar to hear any real difference anyway? Any thoughts?

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Nick
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Post by Nick » Wed Aug 01, 2007 10:24 pm

Or is the circuit generally immmune to power supply changes?

I would guess that the current demands of a phone amp would not be that high, so altering the source impedance may not make that big a difference.

But it is just a guess.
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Post by Darren » Thu Aug 02, 2007 12:48 am

My experience would tend to agree with Nick.

When I last re-built my phono stage I tried some very different PS configurations starting with many smoothing stages and slowly removed various sections.

I couldn't tell any change, so left them out and stuck with a simple set-up.

I too put this down to a low ma draw. But on the other hand I also found the same with a 120ma 6B4G amp and ended up with two small chokes and a single 10uf cap.

But then again their ain't much in my 833 supplier either, so what do I know.

Got no hum though....that's rare for me with DHT's, but nice all the same.

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Post by simon » Thu Aug 02, 2007 8:55 am

Ah, okay. The amp's drawing about 40mA so that would make a lot of sense.

To keep the hum in perspective it's very small. With my old 414SLs it wasn't noticeable, it's only the 650s which are obviously a fair bit more sensitive where I picked some up. I was interested as much in whether this was HT ripple or something being picked up off the AC heaters perhaps. With two chokes it's barely audible with no music playing. Compared to some of the hum monsters I've built I'm quite impressed :D . Considering what's in the amp (a mains tx, 2 chokes, a Cerafine, 4 valves, 2 big Solens and 2 Russian PIOs) I reckon it's pretty good. I'll stick it in the rack and forget about it for a while now I think.

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Post by simon » Thu Aug 02, 2007 9:14 am

Funny, the more I think about PSUs, the more complicated they become. Sometimes simple is better, sometimes not.

The phono I'm working on has a little hum. That could be the AC heaters, or HT ripple, or both. I have 3 chokes in the PSU at the moment - perhaps I should try just one and see what happens to the hum and the sound. Maybe it's time to have a serious think about regulated supplies? I've been thinking about a Steve Bench "experimenters" type supply with variable HT. I know Nick likes regulated supplies so it should be worth trying one.

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colin.hepburn
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Post by colin.hepburn » Tue Aug 14, 2007 8:34 pm

Hi All
OK I'm thinking of increasing the current on my 12B4a Headphone amp as it could do with a little more output in to my 300ohm HD650s the amp is drawing 30mA total
As my math’s is crap can I change the cathode resistor R4 to say 7k giving the 12B4A about 20mA? Giving a total of 42 mA for the amp mains TX 230 volts @50 mA
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Post by Nick » Tue Aug 14, 2007 11:47 pm

In theory yes, the valve will try and keep its cathode close to the voltage on the grid (unless the catkode resistor is so small, it can't pass wnough current), so altering the value will as you say alter the current.

BUT, it won't alter the outout volumn, it could make it worst, as the valve might run out of headroom earlier. The cathode follower will have a gain of somwhere aroung 0.9, and altering the current won;t change that, as the cathode will follow the signal on the grid (hence the name),

What is job is, is to change the higt impedance signal on the grid, into a low impedance one that can produce current to drive the headphones.

It might be better to look the at the previous stage to see if there was any extra gain to be had there.
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Post by colin.hepburn » Wed Aug 15, 2007 3:15 am

Hi Nick
Well it’s not Volume that is required as such but a bit more body and depth the drive stage is a 5751/12ax7 with a af 70/100 running @ 1mA] 12B4A is running @ 15mA plate current for 12B4A is 34mA max
I was thinking of a lower value for the cathode resistor 7k giving about 20mA on the 12b4a giving a total 42 mAs both channels mains TX 230 @50mA
Headphones impedance 300ohms
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Post by Nick » Wed Aug 15, 2007 7:49 am

Well it’s not Volume that is required as such but a bit more body and depth
Ok, well, its worth trying then.
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colin.hepburn
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Post by colin.hepburn » Wed Aug 15, 2007 12:09 pm

Thanks nick
i will give it a try and see if that improves things a bit
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Post by Nick » Wed Aug 15, 2007 12:48 pm

Given the max diss of the valve of 5.5w, and the 98 volts across it anything under 50ma should be ok in theory, so 2k2 and above I guess.
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Post by pre65 » Fri Aug 24, 2007 10:12 pm

Hi Colin-i have used the 832A tube in my PP amp and i am very pleased with the results.
It would seem quite an unusual valve to use,you don't see many others on the net !

Philip

[quote="colin.hepburn"]Hi All
I have been thinking of building an other headphone amp with the 6as7 valves for some time now and came across this one the head hunter see also the Martian A headphone amp using a rare 832a double Tetrode tube just a couple of ides if your looking for some thing a bit different :idea: 8)
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G-Popz THE easy listening connoisseur. (Philip)

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colin.hepburn
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Post by colin.hepburn » Sat Aug 25, 2007 6:13 pm

Hi Philip
thanks for your reply on the 832A tube as you say a bit unusual looking but thats the attraction a good talking point although they do seem difficult to get hold of as for building the Headphone Amp well maybe its on my to-do list but i have too much projects on and not enough beer vouchers at the Mo
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