upgrading this 12B4A headphone amp

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colin.hepburn
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upgrading this 12B4A headphone amp

Post by colin.hepburn » Thu Dec 18, 2008 12:24 pm

Hi All
Well revisiting this amp again this upgrade will have a bigger PSU and anode choke on V1 plus anode choke or C4S Kit on V2 and ASC Oil caps on the output there are no Elect Caps in this amp anywhere not even in the PSU and it sounds very good driving 300 ohm sennhiser HD650s so why change it I here you say well the amp is also a learning curve for me as well as experimental given that you can clearly hear the sound of different components it’s the ideal platform to work with here is the Sch as last upgraded
The HD200 Sch is the intended upgrade however I'm not sure which way to go on V2 the 12B4a i.e. anode choke or a C4S if choke what spec would best suited to the 12ba4 HT @300volts
All choke from Hammond on Phil's bluebell site
all other ides you may have welcome pleas post them
Cheers
Colin
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Post by colin.hepburn » Thu Dec 18, 2008 9:27 pm

Hi All
Having a play with Duncan PSU II software for the Psu idea as I have a second choke I was thinking of adding but is smaller than the 30H@565ohems I already have in its a 3H @100 ohm I presume that the 30H choke is first followed by the 3H choke or the other way round can anyone advice me on that
Will require bigger TX but I think Mike H can supply that in the VT426
So we have some thing like this
TX 250volts CL CL C CT RC CT
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Post by Andrew » Thu Dec 18, 2008 10:40 pm

Why not try a regulated supply?

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Post by colin.hepburn » Thu Dec 18, 2008 10:58 pm

Andrew wrote:Why not try a regulated supply?

-- Andrew
Hi Andrew
|Yes I was thinking along those line first a C4S on the anode of the 12B4a as for the psu I haven’t seen a regulator Schematic that i may be able to copy i require some help in understanding this type of circuit can anyone point me to one that will integrate in too this PSU thats simply and easy to build
:)
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Post by colin.hepburn » Sun Dec 21, 2008 4:00 pm

[

Hi All
Well it seems most of you are busy at the mo with your projects reading your posts there’s a lot of decision about the LW DHT Monkey amps and thinks and as im about to do a rebuild on my 12B4a Headphone amp so got me thinking can it work on this circuit so had a play and came up with this one
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I am very aware it may not be correct but I had a go again it’s the math that gets me every time if it won’t work a small explanation as to why and if it will work ok pleas post corrections to circuit for best results
Thank you
Colin

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Post by Andrew » Sun Dec 21, 2008 4:14 pm

colin.hepburn wrote: Hi Andrew
I haven’t seen a regulator Schematic that i may be able to copy.
:)
Check out the recent phono thread, that points to the regs circuits, layouts and schem, and offers some tips on what bits get hot.

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Post by Nick » Sun Dec 21, 2008 4:49 pm

The big problem is that unlike the other amps, you want the volatge on the cathode of the output valve to vary, so you are starting from a moving voltage point.

The other problem is you have 131v on both sides of a 100k resistor which implies there is no current flowing through the first valve.
Resistance isn't futile it's V / I.

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Post by colin.hepburn » Sun Dec 21, 2008 5:36 pm

Nick wrote:The big problem is that unlike the other amps, you want the volatge on the cathode of the output valve to vary, so you are starting from a moving voltage point.


The other problem is you have 131v on both sides of a 100k resistor which implies there is no current flowing through the first valve.
OK not sure by what is meant by
output valve to vary, so you are starting from a moving voltage point.


Yep got a bit lost with the 131 volts at the anode of V1 lost it some whare there with adding in the choke I think it should be about 126volts at that point
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Post by Nick » Sun Dec 21, 2008 8:29 pm

OK not sure by what is meant by
output valve to vary, so you are starting from a moving voltage point.
For a normal common cathode amplifier, the goal of the bipassed cathode resistor is to develop a constant DC voltage to bias the grid. The bipass cap prevents the voltage varying as the signal on th egrid is amplifield and taken from the anode. So the DC voltage on the cathode will remain constant so can be used to feed the previous stage.

In your case, you have a cathode follower, so the voltage developed across the cathode resistor will not remain constant and will vary by almost the same amount as the signal. In fact thats the point the signal is taken to the output from.
Resistance isn't futile it's V / I.

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Post by colin.hepburn » Sun Dec 21, 2008 10:42 pm

Nick wrote:
In your case, you have a cathode follower, so the voltage developed across the cathode resistor will not remain constant and will vary by almost the same amount as the signal. In fact thats the point the signal is taken to the output from.
thanks nick
for the expansion :) it was just an inexperienced Sunday after noon play about with ohms law OK so its back to Sussing out a simple but good Regulated PSU :?
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