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valve preamp modify to make DI box?

M

Matt

Jan 1, 1970
0
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
 
K

Kevin Aylward

Jan 1, 1970
0
Matt said:
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.

Kevin Aylward
[email protected]
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.
 
T

Terry King

Jan 1, 1970
0
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??
 
M

Matt

Jan 1, 1970
0
Kevin Aylward said:
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
 
M

Matt

Jan 1, 1970
0
Terry King said:
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??

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
 
S

Spajky

Jan 1, 1970
0
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?

-- Regards, SPAJKY ®
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
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I

Ian Bell

Jan 1, 1970
0
Spajky said:
why not to use simple transistor emitter follower circuitry on the
input?

Becuase it would not have a high enough input impedance.

Ian
 
S

Spajky

Jan 1, 1970
0
Becuase it would not have a high enough input impedance.

it would; if not take FET or Mos-Fet one !

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

Rich Grise

Jan 1, 1970
0
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
 
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