valve preamp modify to make DI box?

M

Matt

Guest
Can anyone advise me on following?

I have a simple valve preamp (based on a 12AX7) which I want to modify so
that it works similar to a DI box. What I want the device to do is provide
a
high impedence ( > 1 Meg) input for a device such as a guitar pickup, and
convert the signal to a lower impedence suitable to feed to a microphone
input or line input of a mixer. Prefereably the output would be balanced
but its not a requirement.

I understand that the output from the 12AX7 might need a transformer to
convert it to a lower impedence. I have a quality MC step-up transformer
(Dynavector DV-6A). Could this be used in reverse if the signal was
suitable attenuated first?

Can anyone advise me or point to schematics on the web?

Matt
 
Matt wrote:
Can anyone advise me on following?

I have a simple valve preamp (based on a 12AX7) which I want to
modify so that it works similar to a DI box. What I want the device
to do is provide a
high impedence ( > 1 Meg) input for a device such as a guitar pickup,
and convert the signal to a lower impedence suitable to feed to a
microphone input or line input of a mixer. Prefereably the output
would be balanced but its not a requirement.

I understand that the output from the 12AX7 might need a transformer
to convert it to a lower impedence. I have a quality MC step-up
transformer (Dynavector DV-6A). Could this be used in reverse if the
signal was suitable attenuated first?
Click to expand...
May be, but I wouldn't bother. Its best to avoid transformers.

How is the pre-amp configured. You could use one half, as the gain
stage, then use the other half as a cathode follower to get the
impedance low.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"quotes with no meaning, are meaningless" - Kevin Aylward.
 
Matt wrote:
Can anyone advise me on following?
I have a simple valve preamp (based on a 12AX7) which I want to
modify so that it works similar to a DI box.
Click to expand...
A typical DI (Direct Input AKA Direct Injection) box takes an unbalanced
High-impedance input at 100mV or more from a musical instrument pickup
(typically with a 1/4" 'phone' plug) and matches a standard professional
microphone input (with an XLR plug) which is a balanced output at
microphone level (say 1 to 2 Mv). Most DI boxes are passive and do use a
transformer for isolation and good common-mode balance.

Just why do you want to use the 12AX7, and what 'special' features are
you looking for??

--
Regards, Terry King ...In The Woods In Vermont
terry@fredking.us
The one who Dies With The Most Parts LOSES!! What do you need?
 
"Kevin Aylward" <kevindotaylwardEXTRACT@anasoft.co.uk> wrote in message
news:s1Zac.1114$Ny4.188@newsfep3-gui.server.ntli.net...
Matt wrote:
Can anyone advise me on following?

I have a simple valve preamp (based on a 12AX7) which I want to
modify so that it works similar to a DI box. What I want the device
to do is provide a
high impedence ( > 1 Meg) input for a device such as a guitar pickup,
and convert the signal to a lower impedence suitable to feed to a
microphone input or line input of a mixer. Prefereably the output
would be balanced but its not a requirement.

I understand that the output from the 12AX7 might need a transformer
to convert it to a lower impedence. I have a quality MC step-up
transformer (Dynavector DV-6A). Could this be used in reverse if the
signal was suitable attenuated first?

May be, but I wouldn't bother. Its best to avoid transformers.

How is the pre-amp configured. You could use one half, as the gain
stage, then use the other half as a cathode follower to get the
impedance low.
Click to expand...
The amp configuration is shown in this schematic
http://www.musicyo.com/planet/graphics/tube10scmtc.gif. It's actually a
preamp plus power amp. Since my knowledge of circuit theory is fairly
limited it's probably best for me to show you the schematic rather than me
attempt to explain it.

Thanks for your input.

Matt
 
"Terry King" <terry@fredking.us> wrote in message
news:MPG.1ad64625dbb78327989869@news.newsguy.com...
Matt wrote:
Can anyone advise me on following?
I have a simple valve preamp (based on a 12AX7) which I want to
modify so that it works similar to a DI box.

A typical DI (Direct Input AKA Direct Injection) box takes an unbalanced
High-impedance input at 100mV or more from a musical instrument pickup
(typically with a 1/4" 'phone' plug) and matches a standard professional
microphone input (with an XLR plug) which is a balanced output at
microphone level (say 1 to 2 Mv). Most DI boxes are passive and do use a
transformer for isolation and good common-mode balance.

Just why do you want to use the 12AX7, and what 'special' features are
you looking for??
Click to expand...
The features I want are high input impedance and the use of equipment I
already own if possible. The main purpose is to feed an electric guitar
signal directly into a mixing desk without significant loading of the guitar
pickups.

I specified the 12AX7 preamp because a) the impedence is high enough that I
don't lose significant high-frequencies from the single-coil guitar pickups
I use, and b) I have a spare 12AX7-based amp available to modify, and c)
I've read of high-end DI boxes that use a valve input such as a 12AX7, and
use a transformer output stage.

I've looked at buying an active DI boxes but the ones suited to my budget
don't have a high enough input impedence for my guitar pickups. The passive
DI boxes that I've seen don't have a high enough input impedance either.

Matt
 
On Fri, 2 Apr 2004 13:25:03 +1000, "Matt" <not@this.address.com>
wrote:

The features I want are high input impedance and the use of equipment I
already own if possible. The main purpose is to feed an electric guitar
signal directly into a mixing desk without significant loading of the guitar
pickups.
Click to expand...
why not to use simple transistor emitter follower circuitry on the
input?

-- Regards, SPAJKY ÂŽ
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
E-mail AntiSpam: remove ##
 
Spajky wrote:

On Fri, 2 Apr 2004 13:25:03 +1000, "Matt" <not@this.address.com
wrote:

The features I want are high input impedance and the use of equipment I
already own if possible. The main purpose is to feed an electric guitar
signal directly into a mixing desk without significant loading of the
guitar pickups.

why not to use simple transistor emitter follower circuitry on the
input?
Click to expand...
Becuase it would not have a high enough input impedance.

Ian
 
On Fri, 02 Apr 2004 20:02:24 +0100, Ian Bell <itb@yahoo.com> wrote:

why not to use simple transistor emitter follower circuitry on the
input?

Becuase it would not have a high enough input impedance.
Click to expand...
it would; if not take FET or Mos-Fet one !

-- Regards, SPAJKY ÂŽ
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
E-mail AntiSpam: remove ##
 
Spajky wrote:

On Fri, 02 Apr 2004 20:02:24 +0100, Ian Bell <itb@yahoo.com> wrote:

why not to use simple transistor emitter follower circuitry on the
input?

Becuase it would not have a high enough input impedance.


it would;
Click to expand...
Simple contradiction is no argument.

if not take FET or Mos-Fet one !
Click to expand...
Precisely i.e don't use an emitter follower, use a source follower.

Ian
 
A schematic! Awesome! ;-}

Replace R3, the 3.3K resistor, with a 1K pot to ground, with _no_
cap across it, in series with (a 2.2K resistor in parallel with C1).

i.e.:

12AX7-3 (cathode)
|
+----+----+
| |
\ |
2K2 / = 20 uF
\ |
| |
+----+----+
|
/ 20 uF
1K \<-----||----- output
/ +
|
Gnd

That's your low-impedance emitter-follower output.
If the volume gets too high too soon on the knob, use
a smaller pot and larger fixed R. The change in feedback
will probably change the overall response of the rest
of the amp, but I'm ass-u-me-ing, from your description,
that you're willing to live with that.

Good Luck!
Rich


"Matt" <not@this.address.com> wrote in message
news:406cd87d$0$25655$afc38c87@news.optusnet.com.au...
"Kevin Aylward" <kevindotaylwardEXTRACT@anasoft.co.uk> wrote in message
news:s1Zac.1114$Ny4.188@newsfep3-gui.server.ntli.net...
Matt wrote:
Can anyone advise me on following?

I have a simple valve preamp (based on a 12AX7) which I want to
modify so that it works similar to a DI box. What I want the device
to do is provide a
high impedence ( > 1 Meg) input for a device such as a guitar pickup,
and convert the signal to a lower impedence suitable to feed to a
microphone input or line input of a mixer. Prefereably the output
would be balanced but its not a requirement.

I understand that the output from the 12AX7 might need a transformer
to convert it to a lower impedence. I have a quality MC step-up
transformer (Dynavector DV-6A). Could this be used in reverse if the
signal was suitable attenuated first?

May be, but I wouldn't bother. Its best to avoid transformers.

How is the pre-amp configured. You could use one half, as the gain
stage, then use the other half as a cathode follower to get the
impedance low.

The amp configuration is shown in this schematic
http://www.musicyo.com/planet/graphics/tube10scmtc.gif. It's actually a
preamp plus power amp. Since my knowledge of circuit theory is fairly
limited it's probably best for me to show you the schematic rather than me
attempt to explain it.

Thanks for your input.

Matt
Click to expand...
 
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