Microphone Preamp design - battery powered

[DanSteely]

Hi all,

 

I’m looking around for a circuit diagram for a microphone preamp.

There are lots of designs around but I have some specific requirements:

 

Balanced input

Balanced output (if possible)

Variable gain adjustment (+- 40-60dB)

Pad switch (if possible)

Battery powered (single 9V or 2x 9V)

Primarily for use in live environments so studio type noise levels are not a must have.

Simple design that can be made on strip board.

(Possibility a small batch of PCB’s could be made overseas. I’d therefore need a layout in a compatible format)

Nice to have: A battery level LED: Bi-colour that indicates good/low.

Small overall footprint - similar to a large smart phone with a height of around 25mm/1"

Nice to have: low voltage phantom power

 

 

Searching around I’m a bit snow blinded by the designs but none seem to fit my spec.

 

Any pointers or thoughts would be welcome.

 

Thanks

[sunray]

I have (very buried under loads of unused kit) a 'Drake' card frame with a number of cards containing one pre-amp each running on +/- 15v with a fairly good input transformer. They are obviuosly larger than your suggested size and will take a while to dig out. But they may make a starting point. It's been a long time since I've seen the unit so I'll predict there are about 6 cards.

 

 

Hi all,

 

I'm looking around for a circuit diagram for a microphone preamp.

There are lots of designs around but I have some specific requirements:

 

Balanced input

Balanced output (if possible)

Variable gain adjustment (+- 40-60dB)

Pad switch (if possible)

Battery powered (single 9V or 2x 9V)

Primarily for use in live environments so studio type noise levels are not a must have.

Simple design that can be made on strip board.

(Possibility a small batch of PCB's could be made overseas. I'd therefore need a layout in a compatible format)

Nice to have: A battery level LED: Bi-colour that indicates good/low.

Small overall footprint - similar to a large smart phone with a height of around 25mm/1"

Nice to have: low voltage phantom power

 

 

Searching around I'm a bit snow blinded by the designs but none seem to fit my spec.

 

Any pointers or thoughts would be welcome.

 

Thanks

[DanSteely]

I remember Drake kit from broadcast studios, pretty high quality I'd assume. The only downside is the split rail which would need 4x 9V batteries.

 

I'd be interested to look at the PCB and research the circuit diagram. Not sure if Drake are still going.

 

 

I have a Canford Audio mic preamp which has the footprint of a house brick, but not as tall. My ideal would be less than half that size.

[skinda0]

Have you seen the Elliott Sound Proudcts Website? He has a few different designs on there for mic preamps:

https://sound-au.com/project66.htm

This one if for the design of a mixer input but may be of use for a few ideas:

https://sound-au.com/project30a.htm

[Shez]

You could look at using something like an SSM2019. That takes care of almost all of your functionality in one IC. Just need a gain pot and a few sundry components for your other functionality. It's about as simple as it get if you're building on stripboard.

[DanSteely]

Many thanks all,

 

The ESP stuff has always looked good. Has anyone imported their stuff?

 

The SSM2019 looks very interesting, I need to do some more research.

[Jivemaster]

Ferret through https://www.profusionplc.com

 

The dimensions of the case will be limited by the connectors in use, and your ability to punch suitable holes.

[sandall]

I have a Canford Audio mic preamp which has the footprint of a house brick, but not as tall. My ideal would be less than half that size.

Likewise - makes an excellent stage-weight or doorstop :) The Behringer PS400 fits you size criteria, but has no gain adjustment, & needs 12V, which isn't ideal for battery-powering. I bought a Studiospares Red-something at a trade show some years back, which did have a gain control. Unfortunately this cut from zero to about 20dB gain as you moved the pot from near zero . Used once, causing a highly embarrassing moment, & promptly returned for refund. This was the last time I trusted any of their specs.

Edited October 26, 2019 by sandall

[sunray]

I have a Canford Audio mic preamp which has the footprint of a house brick, but not as tall. My ideal would be less than half that size.

Likewise - makes an excellent stage-weight or doorstop :) The Behringer PS400 fits you size criteria, but has no gain adjustment, & needs 12V, which isn't ideal for battery-powering. I bought a Studiospares Red-something at a trade show some years back, which did have a gain control. Unfortunately this cut from zero to about 20dB gain as you moved the pot from near zero . Used once, causing a highly embarrassing moment, & promptly returned for refund. This was the last time I trusted any of their specs.

At school (1970-72) we had a microphone mixer - metal box 2x2x6", 5 sockets, 4 rotary pots and a PP3 but no power switch so it was always flat when we tried to use it. The first 1/4 of movement did nothing for the channel concerned but it did affect the other 3 channels, then to full volume in about 30^o and the remainder of the travel did nothing.

 

[sunray]

I managed to get to some buried kit and pics here:

 

 

https://photos.app.g...ZHWHyJAEVDEjHk7

Show a rack shelf with 8 mic pre-amps and PSU.Cards are 100x110x23mm and contain I/p and o/p transformers, inbetween them are +/- 12V regulators and caps.

the rack shelf has 8 XLR's

and EDAC multiway which seems to have 8 pairs for line level audio and 8 control lines (+ 3 or 4 more),

 

since in my posession this has been used in a school as a temporary arrangement to get mic lines from a stage to desk via cat 5 infrastructure, this was not set up by me so I have no idea if there were any complications.

There is also a second rack shelf available, containing 2 more pre-amps, XLR's and EDAC which was powered from this rack shelf.

Also pictured is 1 of 3 amplified loudspeakers (including original 7 pin XLR from rip out) which was also powered from the main shelf.

Please ask for any details, I'll answer what I can,

 

Ignore the Sony video switcher in pics.(unless anyone has a use for it, there is also 2 halves of the multicore cable that went from switcher to CRT gun projector)

Edited November 3, 2019 by sunray

[paulears]

Cpc link

 

Quite cheap 12v, save a lot of work, led clip and even phantom.

[sunray]

Cpc link

 

Quite cheap 12v, save a lot of work, led clip and even phantom.

 

And noisy

[paulears]

Want me to send you one? There's one on the shelf?

[sunray]

Want me to send you one? There's one on the shelf?

I had a couple and found them very useful as long as the noise wasn't an issue.

 

I used them for a school play for impromptu show relay feeds into their speakeramps and sold them on when they wanted to keep them.

 

EDIT: I'm not slagging these, they are useful devices especially as they have balanced and unbalanced I/p and o/p's but, apart from price, they do not compare well with a Canford or Alice pre-amp.

 

Edited November 3, 2019 by sunray