I hope this is not off topic here -

I am designing a mic premamp/mixer for our band's practice sessions. We
have one main singer and at least one or two backup singers. I built a
simple circuit that has 4 balanced inputs mixing to one output with about 52
dB maximum gain per channel. Channel One has an effects loop. There is no
phantom power as we are using dynamic mics for now. While it worked OK in
practice, we had trouble matching levels to the rest of the system. I am
wondering what other features common preamp/mixers have and how to best
implement them - like perhaps clipping indicators, VU meters, an FX bus
(rather than just a simple loop on channel one only), built in compression,
eq, etc. Also, I want to operate as a +4dB device but Im not sure what
that means when you dont even know where "zero" is (without a meter) Is
this correct? I am assuming that +4dB means 4dB over .775 volts into a 600
ohm load, which would be about 1.228 volts. From what I have read, its good
to shoot for a 20dB headroom which I am guessing means being able to swing
over 12 volts into a 600 ohm load.
Also, is it more common to have the fader for a channel affect the send
level for the effects or would the fader come after the send and return?
If anyone knows any good sources of technical info on such matter, I would
appreciate hearing from you. Are there any good web sites that outline the
process of desiging custom live/pro audio equipment? Most of my previous
design experience is in the areas of home audio and guitar amplification, so
some aspects of this project I have not enountered before now.

thanks in advance,


David Forsyth

"David Forsyth" wrote in message
...
I hope this is not off topic here -

Not really but in future might I suggest breaking your message up into
shorter Paragraphs consisting of individual questions/comments so that your
message does not look so imposing and people will be more likely to read and
respond to it.

I am designing a mic premamp/mixer for our band's practice sessions. We
have one main singer and at least one or two backup singers. I built a
simple circuit that has 4 balanced inputs mixing to one output with about
52
dB maximum gain per channel.

Fine. Would that Circuit be an Op-amp ? you can usually get better. Lower
noise performance if you use a discrete Long tailed pair up front. Although
if you want to go a simpler low noise/high performance solution I have heard
good reports about the Analog devices SSM2019.

Channel One has an effects loop. There is no
phantom power as we are using dynamic mics for now. While it worked OK in
practice,

an effects loop ? do you mean like an insert which breaks the signal from
the pre-amp before it goes into the mixer ?

we had trouble matching levels to the rest of the system.

As in there was not enough gain? - you might have to elaborate on this

I am
wondering what other features common preamp/mixers have

have a look - try
http://www.behringer.com/02_products...x.cfm?lang=ENG in fact with the
pricing of these mixers are you sure you even want to bother making your own
? I know there is a certain satisfaction from making your own but this stuff
is reasonable quality at a good price - especially when you consider the
amount of effort and time needed to roll your own - In fact in Australia I
couldnt even buy all the parts required for these for less than twice the
price of one of these desks.

and how to best
implement them - like perhaps clipping indicators,

Vital

VU meters,

Vital - you need a PFL option so the VU meter can show the level on
indivdual channels as well as mix buses

an FX bus
(rather than just a simple loop on channel one only),

Good idea ! - usually termed to as a Post Fade Auxillary send.
also a Foldback send or two is a good idea (Pre fade)

built in compression,

Actually quite Fiddly to get Compression right - it is easier and cheaper to
buy an outboard Box for this but this means you would need an Insert type
Jack

eq, etc.

Of course Sweep EQ is the preffered option here with shelvable high and low

Also, I want to operate as a +4dB device but Im not sure what
that means when you dont even know where "zero" is (without a meter) Is
this correct?

0dBu= 0.775V or 1mW into a 600 ohm load

I am assuming that +4dB means 4dB over .775 volts into a 600

Yep

ohm load, which would be about 1.228 volts.

Yep

From what I have read, its good
to shoot for a 20dB headroom which I am guessing means being able to swing
over 12 volts into a 600 ohm load.

Just run the thing off a +/- 15V rail

Also, is it more common to have the fader for a channel affect the send
level for the effects

Yes the Auxillary send for effects is usually Post fade - that is the
control comes after the fader. This is because the effects return usually
gets mixed back into the main mix on a different channel

or would the fader come after the send and return?

Foldback/ Monitor sends are usually before the Fader so that variations in
the main mix do not affect what the Musician is hearing.

If anyone knows any good sources of technical info on such matter, I would
appreciate hearing from you.

Well beyond suggesting the Yamaha Sound reinforcement handbook and such like
similair tomes - which are more about running systems than building
equipment I cannot think of anything off hand - of course all the common
electronics etc used in SR gear is not usually in electronic circuit
notebooks etc....

Are there any good web sites that outline the
process of desiging custom live/pro audio equipment? Most of my previous
design experience is in the areas of home audio and guitar amplification,
so
some aspects of this project I have not enountered before now.

Probably one of the best ways is to get some SR gear and see what it does
then figure out how it does it - I have been working on a collection of
various circuits in common use in Pro-audio Gear and a few related kits
(including a Mic preamp + level meter etc) however I was not planning to
launch the website until August (if I get it all together in time ;-)

thanks in advance,

No sweat

David Forsyth

"David Forsyth" wrote in message
...
I hope this is not off topic here -

Not really but in future might I suggest breaking your message up into
shorter Paragraphs consisting of individual questions/comments so that your
message does not look so imposing and people will be more likely to read and
respond to it.

I am designing a mic premamp/mixer for our band's practice sessions. We
have one main singer and at least one or two backup singers. I built a
simple circuit that has 4 balanced inputs mixing to one output with about
52
dB maximum gain per channel.

Fine. Would that Circuit be an Op-amp ? you can usually get better. Lower
noise performance if you use a discrete Long tailed pair up front. Although
if you want to go a simpler low noise/high performance solution I have heard
good reports about the Analog devices SSM2019.

Channel One has an effects loop. There is no
phantom power as we are using dynamic mics for now. While it worked OK in
practice,

an effects loop ? do you mean like an insert which breaks the signal from
the pre-amp before it goes into the mixer ?

we had trouble matching levels to the rest of the system.

As in there was not enough gain? - you might have to elaborate on this

I am
wondering what other features common preamp/mixers have

have a look - try
http://www.behringer.com/02_products...x.cfm?lang=ENG in fact with the
pricing of these mixers are you sure you even want to bother making your own
? I know there is a certain satisfaction from making your own but this stuff
is reasonable quality at a good price - especially when you consider the
amount of effort and time needed to roll your own - In fact in Australia I
couldnt even buy all the parts required for these for less than twice the
price of one of these desks.

and how to best
implement them - like perhaps clipping indicators,

Vital

VU meters,

Vital - you need a PFL option so the VU meter can show the level on
indivdual channels as well as mix buses

an FX bus
(rather than just a simple loop on channel one only),

Good idea ! - usually termed to as a Post Fade Auxillary send.
also a Foldback send or two is a good idea (Pre fade)

built in compression,

Actually quite Fiddly to get Compression right - it is easier and cheaper to
buy an outboard Box for this but this means you would need an Insert type
Jack

eq, etc.

Of course Sweep EQ is the preffered option here with shelvable high and low

Also, I want to operate as a +4dB device but Im not sure what
that means when you dont even know where "zero" is (without a meter) Is
this correct?

0dBu= 0.775V or 1mW into a 600 ohm load

I am assuming that +4dB means 4dB over .775 volts into a 600

Yep

ohm load, which would be about 1.228 volts.

Yep

From what I have read, its good
to shoot for a 20dB headroom which I am guessing means being able to swing
over 12 volts into a 600 ohm load.

Just run the thing off a +/- 15V rail

Also, is it more common to have the fader for a channel affect the send
level for the effects

Yes the Auxillary send for effects is usually Post fade - that is the
control comes after the fader. This is because the effects return usually
gets mixed back into the main mix on a different channel

or would the fader come after the send and return?

Foldback/ Monitor sends are usually before the Fader so that variations in
the main mix do not affect what the Musician is hearing.

If anyone knows any good sources of technical info on such matter, I would
appreciate hearing from you.

Well beyond suggesting the Yamaha Sound reinforcement handbook and such like
similair tomes - which are more about running systems than building
equipment I cannot think of anything off hand - of course all the common
electronics etc used in SR gear is not usually in electronic circuit
notebooks etc....

Are there any good web sites that outline the
process of desiging custom live/pro audio equipment? Most of my previous
design experience is in the areas of home audio and guitar amplification,
so
some aspects of this project I have not enountered before now.

Probably one of the best ways is to get some SR gear and see what it does
then figure out how it does it - I have been working on a collection of
various circuits in common use in Pro-audio Gear and a few related kits
(including a Mic preamp + level meter etc) however I was not planning to
launch the website until August (if I get it all together in time ;-)

thanks in advance,

No sweat

David Forsyth

A little clarification on levels:

+4dBm is 4dB over 0.775V into 600 ohms. (The latter is 1mW.) But these days,
almost nobody drives 600 ohm loads; most of the time, you're driving a load
of at least 10k unless you own a Studer, which is 5k. So most of the time,
now, we use dBu instead of dBm. 0dBu, like 0dBm, is 0.775V, but it's into an
undefined impedance. Practically speaking, you're talking dBu most of the
time unless you have some very old gear (or are driving a phone line).

20dB headroom is usually the standard for pro gear. That means +24dBu if
you're truly working at +4dBu nominal. A single opamp run from +/-15V
supplies can't do that (although it can if you run from +/-21V supplies, but
only a few chips will take that supply voltage without blowing up, most
notably including the NE5534 and the OPA604/2604). But if you are working
with balanced signals, then each leg of a nominally-+4dBu line is actually
at -2dBu, so your maximum output would be +18dBu on each side, and a +/-15V
supply will do that fine.

All that said, if you're using this for rehearsal and nothing else, really
and truly, go buy a Behringer or Tapco or something like that. It's not
worth putting in the time to do the design work.

Oh, in answer to another of your questions...effects feeds can come before
or after the channel fader, and if you're lucky, they're switchable. For
something like reverb, you usually want the send after the fader, so the
reverb level tracks the fader level. But not always (special effects, etc.).

Peace,
Paul