Instrument Preamplifier

[mattnj]

Disclaimer: I don't really know anything about circuit design. I can build from schematics, and design reasonable power supplies but this is my first stab at a preamplifier and is just an idea I had.

I did not come up with this off the top of my head (as some of you can probably tell). It is an Ampeg B25 input stage feeding a 5532 balanced output stage from page 742 of the Handbook for Sound Engineers.

My questions are:
First, will it work? and how well? Is it worth building?
Secondly, what should I change on it? The original B25 had the Ultra Low switch that I omitted, except left the cirucitry. Can I leave any of this out?

I have a power supply built for it and a cool lunchbox style case for it. I've also built the 12AV7 input stage and it sounds good, except it is unbalanced, and I want a "DI" style preamp.

Thanks in advance.

Here's the direct link if this image is too big.
http://i130.photobucket.com/albums/p267/treestheband/MattsDI.png

 

[PRR]

Your bunch of 6.8K resistors wants a LOW impedance drive. The 270K+ is not going to do the right thing.

I'm not familiar with that output stage. It seems OK if pin 3 is shorted, but not if pin 2 is shorted.

I dunno if you really want all that gain.

I'd be inclined to drop a 6AU6 and 10K:600 transformer in there, sorta a mini-Champ.

 

[mattnj]

Thanks PRR. I'll keep working on it. I had a SRPP output stage with an OEP transformer, but again maybe it was an impedence thing because I couldn't get it to work (and possibly hit the primary with 250 VDC), so I moved to the 5532 output stage. Any suggestions on an electronically balanced output stage with this input stage?

Also, yes PRR I forgot to put the second cross-coupled feedback path in. So it would fail...

 

[mattnj]

Generally, how much gain should an active "DI" box have?

 

[PRR]

> Generally, how much gain should an active "DI" box have?

"Enough"?

I dunno. Figure pickup level may be 20mV or 500mV. WHat are you going into? Mike input? Line input?

 

[Andy Peters]

[quote author="mattnj"]Generally, how much gain should an active "DI" box have?[/quote]

Active DIs often have a loss of 20 dB, to match you'd get when using a passive DI into a mic preamp.

-a

 

[mattnj]

I never knew that active DI's had a "loss". I thought that was part of the purpose of using an active DI. I knew that passive DI's had a small amount of loss, but is it really 20? (I'm going to consult the handbook in a few minutes.)

I understand that I can figure out the gain using an average input signal and saying I want +4 dbu out of it, but I was just curious as to the common practice of such things.

Maybe I will just make the output +4 dbu as in going to a line input. I will be using this onstage so I can't say whether it will be a mic input with a pad on a console, or a line input.

Thanks guys,

 

[mattnj]

[quote author="Andy Peters"][quote author="mattnj"]Generally, how much gain should an active "DI" box have?[/quote]

Active DIs often have a loss of 20 dB, to match you'd get when using a passive DI into a mic preamp.

-a[/quote]

Do you consider a Type 85 active? Because they have about 1db of gain.... Am I thinking of something different? Is the -20 db describing the output level of the box, or the ratio of input to output? I'm confused here.

Thinking as logically as possible wouldn't unity gain be ideal for a direct box? And thinking further of my design, I would like the level adjust pot on my box to go from Unity Gain to +4 dbu. Is this possible with my current input stage? I'm going to go learn a little more before posting again....

Thanks guys,

-Matt

 

[Andy Peters]

[quote author="mattnj"]I never knew that active DI's had a "loss". I thought that was part of the purpose of using an active DI. I knew that passive DI's had a small amount of loss, but is it really 20? (I'm going to consult the handbook in a few minutes.)[/quote]

The loss is a direct result of the transformer action that presents a high impedance to the source. So the attenuation is a function of the turns ratio.

I understand that I can figure out the gain using an average input signal and saying I want +4 dbu out of it, but I was just curious as to the common practice of such things.

Maybe I will just make the output +4 dbu as in going to a line input. I will be using this onstage so I can't say whether it will be a mic input with a pad on a console, or a line input.

There's the rub, then. If you want to plug into a line input (moderate input impedance, +4 dBu nominal level) or a mic input (lower impedance, lower input level).

-a

 

[Andy Peters]

[quote author="mattnj"][quote author="Andy Peters"][quote author="mattnj"]Generally, how much gain should an active "DI" box have?[/quote]

Active DIs often have a loss of 20 dB, to match you'd get when using a passive DI into a mic preamp.

-a[/quote]

Do you consider a Type 85 active? Because they have about 1db of gain.... Am I thinking of something different? Is the -20 db describing the output level of the box, or the ratio of input to output? I'm confused here.[/quote]

The Type 85 is absolutely an active box. And RTFDS; its gain in pickup mode is -10 dB (with the speaker pad on, it's -46 dB). (Other boxes have different gains.)

The -20 dB number I quoted was voltage gain, the ratio of output to input.

Thinking as logically as possible wouldn't unity gain be ideal for a direct box?

No; if you plan to connect to a mic input it might be desirable to have some loss.

-a

 

[mattnj]

Thanks,

I think I can go from here.


Any suggestions on the hi impedance balanced output stage?

 

[mattnj]

would the sowter 9041 transformer work in here for the output stage? it's a 15K:600 ohm 5:1.

Thanks,