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A balanced 12AX7 based buffer

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Les Cargill
Guest

Sat May 12, 2018 3:45 am   



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


Guest

Sat May 12, 2018 7:45 am   



You can use a lower voltage if that's the idea. in fact you'll probably have to if you want low enough value plate resistors to directly drive the mixer or whatever. Making balanced is no sweat, feed the grid of one side, bypass 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 impedance, so tell them the impedance.

The site charges nothing, installs nothing but a cookie, and the only password 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 electronics question there and if you wanted to display the picture somewhere for people here for example, you can just stick the URL in your post. That will reveal 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 it. They might recommend a 6SN7 or whatever. If you don't use an output transformer I think the tube will be feeding a lower impedance than it likes, but I think it will do it.

Les Cargill
Guest

Sat May 12, 2018 7:45 pm   



jurb6006_at_gmail.com wrote:
Quote:
You can use a lower voltage if that's the idea. in fact you'll
probably have to if you want low enough value plate resistors to
directly drive the mixer or whatever. Making balanced is no sweat,
feed the grid of one side, bypass 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 impedance, so tell them the impedance.


That's a good lead - thanks for that.

Quote:
The site charges nothing, installs nothing but a cookie, and the only
password 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 electronics question there and if you wanted to display the
picture somewhere for people here for example, you can just stick the
URL in your post. That will reveal 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 it. They might recommend a 6SN7 or whatever.


Interesting. I just figured those were easy to find.

Quote:
If you don't use an
output transformer I think the tube will be feeding a lower impedance
than it likes, but I think it will do it.


I appreciate it.

--
Les Cargill

Phil Allison
Guest

Sun May 13, 2018 8:45 am   



Les Cargill wrote:

Quote:

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.



** 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

Les Cargill
Guest

Sun May 13, 2018 4:45 pm   



Phil Allison wrote:
Quote:
Les Cargill wrote:


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.



** When you wake up from your dreaming,


Ah, good old phil...

<insert Little Jimmy Dickens song here>


Quote:
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/


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.

Quote:
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.


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

Quote:

..... Phil








--
Les Cargill


Guest

Sun May 13, 2018 5:45 pm   



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 nothing.

If you really want to emulate tube sound you might consider building a micropower 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 placed vents and a properly placed mike would be about as close as it gets. Set 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 have noticed.

Phil Allison
Guest

Mon May 14, 2018 4:45 am   



Les Cargill wrote:
Quote:

Phil Allison wrote:


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.



** When you wake up from your dreaming,

Ah, good old phil...

insert Little Jimmy Dickens song here


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/


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.


** The design was a published as a project in Electronics Australia in 1998
- it proved very popular. Easy to build, considerably lower cost than commercial alternatives plus better performance.



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


** Really ?

Your string of naive questions made me think you were dreaming.

I still do.



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


** FYI, this is the transformer I used:

http://www.altronics.com.au/p/m0705-600-ohm-10k-ohm-shielded-microphone-transformer/

..... Phil

George Herold
Guest

Mon May 14, 2018 3:45 pm   



On Sunday, May 13, 2018 at 11:39:47 AM UTC-4, Les Cargill wrote:
Quote:
Phil Allison wrote:
Les Cargill wrote:


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.



** When you wake up from your dreaming,

Ah, good old phil...

insert Little Jimmy Dickens song here

"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.

Quote:


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/


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.

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.


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


..... Phil








--
Les Cargill


Les Cargill
Guest

Tue May 15, 2018 11:45 pm   



Phil Allison wrote:
Quote:
Les Cargill wrote:

Phil Allison wrote:


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.



** When you wake up from your dreaming,

Ah, good old phil...

insert Little Jimmy Dickens song here


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/


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.


** The design was a published as a project in Electronics Australia in 1998
- it proved very popular. Easy to build, considerably lower cost than commercial alternatives plus better performance.



Yeah, that looks like a very easy build. I've bookmarked that. Thanks,
Phil.

GND2 is case ground, and isolated from signal ground?

Quote:

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


** Really ?

Your string of naive questions made me think you were dreaming.

I still do.



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


** FYI, this is the transformer I used:

http://www.altronics.com.au/p/m0705-600-ohm-10k-ohm-shielded-microphone-transformer/

.... Phil


--
Les Cargill

Les Cargill
Guest

Tue May 15, 2018 11:45 pm   



jurb6006_at_gmail.com wrote:
Quote:
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 nothing.


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.



Quote:
If you really want to emulate tube sound you might consider building
a micropower 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 placed vents and a properly placed mike would be about
as close as it gets. Set 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 have noticed.


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



--
Les Cargill

Phil Allison
Guest

Wed May 16, 2018 3:45 am   



Les Cargill wrote:
Quote:


Phil Allison wrote:



** The design was a published as a project in Electronics Australia
in 1998 - it proved very popular. Easy to build, considerably lower
cost than commercial alternatives plus better performance.



Yeah, that looks like a very easy build. I've bookmarked that. Thanks,
Phil.

GND2 is case ground, and isolated from signal ground?


** Correct.

This is needed for use with PA systems to eliminate hum loops, with sources that are mains earthed like keyboards and guitar amps.

The original EA article is here, starting on P52.

http://americanradiohistory.com/AUSTRALIA/Archive-Electronics-Australia/1998/EA-1998-02.pdf


..... Phil

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