A balanced 12AX7 based buffer

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I'd like to "design" and implement a simple 12AX7-based
buffer for musical instrument DI purposes. I'd like the
output to be balanced for ... no good reason at all
beyond having balanced inputs to play with.

My questions are:

- Do I really need 300V for this? What's the trade here?
- Is there a nice single-package, preferably
switchmode power supply that does 300V and 12.6V
heater, or am I in purely linear PS territory?
- Can I use both halves of the 12AX7 in triode to
achieve balanced output?
- Do I need an op amp for balancing, or can I do it
with just the 12AX7?
- How hard is it to add switches to make the circuit single-ended
and the output unbalanced?

The input will be unbalanced and potentially hi-Z. I can afford
to buy a nice tube DI but I haven't built anything in a long time.

Thank you for any responses that follow.

--  
Les Cargill

Re: A balanced 12AX7 based buffer
You can use a lower voltage if that's the idea. in fact you'll probably hav
e to if you want low enough value plate resistors to directly drive the mix
er or whatever. Making balanced is no sweat, feed the grid of one side, byp
ass the other and tie the cathodes together.  

I don't usually send people other places but Audio Asylum has a "tubes DIY"
 section for people who build their own amps and other tube equipment. Some
 of them guys are pretty sharp and can probably give you good answers. They
 are also familiar with balanced lines but it might not be the same impedan
ce, so tell them the impedance.  

The site charges nothing, installs nothing but a cookie, and the only passw
ord requirement is 6 characters. Quite easy to deal with. And it has image  
hosting for attachments to posts so if you don't, you can post an electroni
cs question there and if you wanted to display the picture somewhere for pe
ople here for example, you can just stick the URL in your post. That will r
eveal your nick there but so what, people can get your real email here.  

It does sound doable though, but the ECC83 might not be the best tube for i
t. They might recommend a 6SN7 or whatever. If you don't use an output tran
sformer I think the tube will be feeding a lower impedance than it likes, b
ut I think it will do it.

Re: A balanced 12AX7 based buffer
snipped-for-privacy@gmail.com wrote:
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That's a good lead - thanks for that.

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Interesting. I just figured those were easy to find.

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I appreciate it.

--  
Les Cargill

Re: A balanced 12AX7 based buffer
Les Cargill wrote:

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** When you wake up from your dreaming, consider building something practical and affordable like a JFET input DI - plus hum and noise free.

At the bottom of his page is the schem for one I designed:

https://www.talkbass.com/threads/have-an-h4n-recorder-do-i-need-a-di-box-to-connect-a-bass-di-schematic.1331245/

It use the "48V phantom" supply included in the vast majority of mixing desks, even budget ones.

The output transformer is a 10K to 600ohms type and not expensive.


.....    Phil





  



Re: A balanced 12AX7 based buffer
Phil Allison wrote:
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Ah, good old phil...

<insert Little Jimmy Dickens song here>


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I already have plenty of DI boxes like that. You're right; those are  
easier. But they're strongly on the "buy" side of the "build vs. buy"
curve.

What I want this for is to make it into a distortion generator to create  
test vectors for asymmetric waveforms. Gain curves, mainly.

Those will be then used to write a VST plugin. I am already using
a couple of guitar pedals for this now.

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I may look at transformers later.... not initially.

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--
Les Cargill


Re: A balanced 12AX7 based buffer
Actually Phil has a point. If you are after the characteristics of a triode
, a depletion mode JFET is practically identical except that it can conduct
 in reverse. If you don't intend to use a transformer that trait means noth
ing.  

If you really want to emulate tube sound you might consider building a micr
opower output with tubes and actually feed a speaker. You could even build  
a box for the speakers and mike, a properly placed baffle with properly pla
ced vents and a properly placed mike would be about as close as it gets. Se
t the B+ low enough so it actually clips the output stage and you should be
 good to go, and I don't see many of that type of devices out there, but I  
don't look either. I did work in the field for a time and I think I would h
ave noticed.

Re: A balanced 12AX7 based buffer
snipped-for-privacy@gmail.com wrote:
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Huh. That certainly does simplify things. I just don't want to make a
lot  of measurements that depend on understanding a transformer well,
and I was thinking I could vary the gain between the two halves of the  
preamp to explore asymmetry.



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Interesting thought, but this is less a working DI and more like a test  
bench.



--  
Les Cargill


Re: A balanced 12AX7 based buffer
On Sunday, May 13, 2018 at 11:39:47 AM UTC-4, Les Cargill wrote:
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"Out Behind the Barn"?  I went to the grand ole opry once.  
(in the new digs.) All I can remember of it was Little Jimmy  
Dickens.  

Sorry, I can't help much with tubes.  We do buy a linear 200V supply
~$50.  I sometimes will hack it for lower or higher voltage operation.
I know which R to tweak.  

George H.  

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