I just wanted to thank Monte P. McGuire, Scott Dorsey, Steve O'Neill
and all the others who chimed in with input on my mic pre project.

It is based on a Burr Brown INA217 and an OPA604 for a DC control
servo. I used 4 layer boards on the preamp boards (internal ground
plane) and 2 layer for the cap boards and power supply. The cap boards
have ground plane on the bottom layer. I used relays to switch the
gain resistors. It uses an external transformer.

The only signal capacitors are the large Polypropylenes for phantom
power blocking.

The relays were Monte's idea, and I am sure glad I went ahead and
tried it.

The pre sounds great (to my ears) and is quieter than my Summit
MPC100A by 7db at low gain to 2db at 60db gain.

Here are some links to pictures of the preamp. I sure would be stoked
if you guys would check them out!

http://www.mixposse.com/attach/Front_small.jpg

http://www.mixposse.com/attach/Inside_small.jpg

http://www.mixposse.com/attach/PS_small.jpg

Thanks again guys! It wouldn't have been anywhere near as successful
witout all the input I got on RAP!

Roger Foote

Roger,
That is forgen outstanding work....beautiful.

peter

Here are some links to pictures of the preamp. I sure would be stoked
if you guys would check them out!

http://www.mixposse.com/attach/Front_small.jpg

http://www.mixposse.com/attach/Inside_small.jpg

http://www.mixposse.com/attach/PS_small.jpg

Thanks again guys! It wouldn't have been anywhere near as successful
witout all the input I got on RAP!

Roger Foote

The pix are real pretty, nice neat work. How much? When do you start shipping?


R. Foote wrote:
I just wanted to thank Monte P. McGuire, Scott Dorsey, Steve O'Neill
and all the others who chimed in with input on my mic pre project.

It is based on a Burr Brown INA217 and an OPA604 for a DC control
servo. I used 4 layer boards on the preamp boards (internal ground
plane) and 2 layer for the cap boards and power supply. The cap boards
have ground plane on the bottom layer. I used relays to switch the
gain resistors. It uses an external transformer.

The only signal capacitors are the large Polypropylenes for phantom
power blocking.

The relays were Monte's idea, and I am sure glad I went ahead and
tried it.

The pre sounds great (to my ears) and is quieter than my Summit
MPC100A by 7db at low gain to 2db at 60db gain.

Here are some links to pictures of the preamp. I sure would be stoked
if you guys would check them out!

http://www.mixposse.com/attach/Front_small.jpg

http://www.mixposse.com/attach/Inside_small.jpg

http://www.mixposse.com/attach/PS_small.jpg

Thanks again guys! It wouldn't have been anywhere near as successful
witout all the input I got on RAP!

Roger Foote

Looks like a very nice unit....

You might want to consider changing your grounding, a 6 inch loop antenna
inside the case may cause rf problems... Use the case near each of the
input connectors as your common ground point, preferably with the two
connectors mounted next to each other...

Rgds:
Eric


"R. Foote" wrote in message
om...
I just wanted to thank Monte P. McGuire, Scott Dorsey, Steve O'Neill
and all the others who chimed in with input on my mic pre project.

It is based on a Burr Brown INA217 and an OPA604 for a DC control
servo. I used 4 layer boards on the preamp boards (internal ground
plane) and 2 layer for the cap boards and power supply. The cap boards
have ground plane on the bottom layer. I used relays to switch the
gain resistors. It uses an external transformer.

The only signal capacitors are the large Polypropylenes for phantom
power blocking.

The relays were Monte's idea, and I am sure glad I went ahead and
tried it.

The pre sounds great (to my ears) and is quieter than my Summit
MPC100A by 7db at low gain to 2db at 60db gain.

Here are some links to pictures of the preamp. I sure would be stoked
if you guys would check them out!

http://www.mixposse.com/attach/Front_small.jpg

http://www.mixposse.com/attach/Inside_small.jpg

http://www.mixposse.com/attach/PS_small.jpg

Thanks again guys! It wouldn't have been anywhere near as successful
witout all the input I got on RAP!

Roger Foote

"Eric K. Weber" wrote in message ...
Looks like a very nice unit....

You might want to consider changing your grounding, a 6 inch loop antenna
inside the case may cause rf problems... Use the case near each of the
input connectors as your common ground point, preferably with the two
connectors mounted next to each other...

Rgds:
Eric

Eric, Yeah, I tried that too but ended up with more noise. I used #12
wire for the pin #1 gnds all the other gnds are shorter and in a star
configuration. The unit is quieter than my Summit pre and everything
else I was able to compare it with.

roger

S O'Neill wrote in message ...
The pix are real pretty, nice neat work. How much? When do you start shipping?


There is one more version in the works... I need to get the labor down
from where it is to make them attractive (in price)

The new version will have the gain resistors directly behind a pc
mounted rotary switch, eliminating the reed relays. I will be putting
the polypro's on the samp pcb as the pre also. Initially, I thought I
would split up the caps from the pre to save money on the 4 layer
board, but it added labor, so...

The new version will have 12 stops instead of the current 9 on the
gain switch.

In article ,
R. Foote wrote:
I just wanted to thank Monte P. McGuire, Scott Dorsey, Steve O'Neill
and all the others who chimed in with input on my mic pre project.

You're quite welcome!

Here are some links to pictures of the preamp. I sure would be stoked
if you guys would check them out!

http://www.mixposse.com/attach/Front_small.jpg

http://www.mixposse.com/attach/Inside_small.jpg

http://www.mixposse.com/attach/PS_small.jpg

Wow - nice work!! You should be very proud!! With construction as
solid as this, you should be able to use this preamp for quite a long
time...


Have fun with it...

Monte McGuire

"Eric K. Weber" wrote in message ...
Looks like a very nice unit....

You might want to consider changing your grounding, a 6 inch loop antenna
inside the case may cause rf problems... Use the case near each of the
input connectors as your common ground point, preferably with the two
connectors mounted next to each other...

Rgds:
Eric

Just so there is no confusion, there is _no_ loop antenna inside this
case. I spent a lot of time planning out the star ground on this pre.
Every sub-section has it's own return to ground and none are looped in
any way. The only thing you _could_ say is that the ground point is
further away from the XLR than my previous design.

By each sub-section, I mean phantom power de-coupling, relay power,
preamp ground, XLR input, and each output jack have a "spoke" to the
central ground "hub"

The dual pre is 2 of my singles in a single case with exactly the same
grounding, except that it is further from the XLR.

Picture of the single preamp:

http://www.mixposse.com/attach/217_Inside_01.jpg

Roger Foote

Greetings:

I was just trying to point out a possible source of problems if the preamp
is used in an area with strong RF fields. The resistance to ground is not
the issue... with 100 foot of antenna (microphone cable).... and 6 inches
or so of it inside the case, the issue is the wavelength of the piece inside
the case. The non RF bypassed power supply wires entering the case could
also be a source of trouble.

If your venues are not near high power transmitter it may not be an issue.

And again a very nice layout and construction job.....

Rgds:
Eric



Just so there is no confusion, there is _no_ loop antenna inside this
case. I spent a lot of time planning out the star ground on this pre.
Every sub-section has it's own return to ground and none are looped in
any way. The only thing you _could_ say is that the ground point is
further away from the XLR than my previous design.

By each sub-section, I mean phantom power de-coupling, relay power,
preamp ground, XLR input, and each output jack have a "spoke" to the
central ground "hub"

The dual pre is 2 of my singles in a single case with exactly the same
grounding, except that it is further from the XLR.

Picture of the single preamp:

http://www.mixposse.com/attach/217_Inside_01.jpg

Roger Foote

"Eric K. Weber" wrote in message ...
Greetings:

I was just trying to point out a possible source of problems if the preamp
is used in an area with strong RF fields.

Thanks, I just wanted you to know that none of the grounds are daisy
chained...Rf is something we don't have here in our area... We are 70
miles from any radio gear except the scarce municipalities and vehicle
radios.

The resistance to ground is not
the issue... with 100 foot of antenna (microphone cable).... and 6 inches
or so of it inside the case, the issue is the wavelength of the piece inside the case.

So, if I understand, having the pin1 gnds that much closer would make
that much difference? Like I have them in the single pre? I may only
make single pres in half rack cases from now on so I can have symmetry
in the grounding, since this is something I struggled over with the
dual unit.

The non RF bypassed power supply wires entering the case could
also be a source of trouble.

The dual pre has AC entering the case as will the single unit by weeks
end. How would I rf bypass the transformer wires entering the case? Or
would this only be an issue with remote dc supplies? (since the
internal dc supply has .1uf bypasses on each rail after the filter
cap) Any suggestions?


If your venues are not near high power transmitter it may not be an issue.

And again a very nice layout and construction job.....

Rgds:
Eric

Thank you!

Roger

For the AC power part of the issue have a look at.....
http://www.corcom.com/catalog/filters/default.htm

Their products are available from all the standard parts houses.... and they
are approved by the regulatory bodies. You certainly could make your own
but if it could cause a fire it's best to buy it.

As for the Microphone input issue..... my solution on a Millennia Media HV-3
was to connect the pin 1 ground to a lug on one of the XLR mounting bolts
with a very short piece of wire..... then run shielded cable from the XLR
to the PCB with the shield connected at the XLR pin 1 end and floating on
the PCB end. You should be able to try this with minimal change to
your existing unit. The shields on the input will not be part of your star
grounding scheme but I believe you will not find any negative effects.

If you are in a very strong RF environment it would also be good to put a
couple of ferrite beads or RF chokes in series with the signal leads at the
XLR.... then add RF bypass capacitors on the input of the PCB.

The HV-3 had the pin one ground at the PCB about 4 inches inside the
case.... I had trouble with it before modification in a venue with bad light
dimmer noise. I have previously had problems with an Ampex mixer picking
up an FM broadcast station about a mile from the transmitter prior to it
being modified in a similar fashion.

Regards:
Eric


"R. Foote" wrote in message
om...
"Eric K. Weber" wrote in message
...
Greetings:

I was just trying to point out a possible source of problems if the
preamp
is used in an area with strong RF fields.

Thanks, I just wanted you to know that none of the grounds are daisy
chained...Rf is something we don't have here in our area... We are 70
miles from any radio gear except the scarce municipalities and vehicle
radios.

The resistance to ground is not
the issue... with 100 foot of antenna (microphone cable).... and 6
inches
or so of it inside the case, the issue is the wavelength of the piece
inside the case.

So, if I understand, having the pin1 gnds that much closer would make
that much difference? Like I have them in the single pre? I may only
make single pres in half rack cases from now on so I can have symmetry
in the grounding, since this is something I struggled over with the
dual unit.

The non RF bypassed power supply wires entering the case could
also be a source of trouble.

The dual pre has AC entering the case as will the single unit by weeks
end. How would I rf bypass the transformer wires entering the case? Or
would this only be an issue with remote dc supplies? (since the
internal dc supply has .1uf bypasses on each rail after the filter
cap) Any suggestions?


If your venues are not near high power transmitter it may not be an
issue.

And again a very nice layout and construction job.....

Rgds:
Eric

Thank you!

Roger

"Eric K. Weber" wrote in message ...
For the AC power part of the issue have a look at.....
http://www.corcom.com/catalog/filters/default.htm

Aren't these for 120vac service? The preamp has an external
transformer with a secondary voltage of 18-20 vac. Would a filter be a
good thing at these voltages?

As for the Microphone input issue..... my solution on a Millennia Media HV-3
was to connect the pin 1 ground to a lug on one of the XLR mounting bolts
with a very short piece of wire..... then run shielded cable from the XLR
to the PCB with the shield connected at the XLR pin 1 end and floating on
the PCB end. You should be able to try this with minimal change to
your existing unit. The shields on the input will not be part of your star
grounding scheme but I believe you will not find any negative effects.

I see what you mean. I guess I was hesitant to think about changing
the grounding, but that sounds like a very logical solution. Consider
it done!

If you are in a very strong RF environment it would also be good to put a
couple of ferrite beads or RF chokes in series with the signal leads at the
XLR.... then add RF bypass capacitors on the input of the PCB.

I have ferrite beads on the inputs and outputs of the preamp.

http://www.mixposse.com/attach/CB01.jpg
http://www.mixposse.com/attach/MP01.jpg

What value of cap would you use for rf bypass at the point where the
wires from the XLR enter the PCB?

The HV-3 had the pin one ground at the PCB about 4 inches inside the
case.... I had trouble with it before modification in a venue with bad light
dimmer noise. I have previously had problems with an Ampex mixer picking
up an FM broadcast station about a mile from the transmitter prior to it
being modified in a similar fashion.

Regards:
Eric

Thanks again for your input!
Roger Foote

Man, this thing's looking better and better...may I suggest putting the
ferrite beads on the wire right at the XLR?


R. Foote wrote:
"Eric K. Weber" wrote in message ...

For the AC power part of the issue have a look at.....
http://www.corcom.com/catalog/filters/default.htm


Aren't these for 120vac service? The preamp has an external
transformer with a secondary voltage of 18-20 vac. Would a filter be a
good thing at these voltages?


As for the Microphone input issue..... my solution on a Millennia Media HV-3
was to connect the pin 1 ground to a lug on one of the XLR mounting bolts
with a very short piece of wire..... then run shielded cable from the XLR
to the PCB with the shield connected at the XLR pin 1 end and floating on
the PCB end. You should be able to try this with minimal change to
your existing unit. The shields on the input will not be part of your star
grounding scheme but I believe you will not find any negative effects.


I see what you mean. I guess I was hesitant to think about changing
the grounding, but that sounds like a very logical solution. Consider
it done!


If you are in a very strong RF environment it would also be good to put a
couple of ferrite beads or RF chokes in series with the signal leads at the
XLR.... then add RF bypass capacitors on the input of the PCB.


I have ferrite beads on the inputs and outputs of the preamp.

http://www.mixposse.com/attach/CB01.jpg
http://www.mixposse.com/attach/MP01.jpg

What value of cap would you use for rf bypass at the point where the
wires from the XLR enter the PCB?


The HV-3 had the pin one ground at the PCB about 4 inches inside the
case.... I had trouble with it before modification in a venue with bad light
dimmer noise. I have previously had problems with an Ampex mixer picking
up an FM broadcast station about a mile from the transmitter prior to it
being modified in a similar fashion.

Regards:
Eric


Thanks again for your input!
Roger Foote

S O'Neill wrote in message ...
Man, this thing's looking better and better...may I suggest putting the
ferrite beads on the wire right at the XLR?


Thanks Steve,

Yes you may suggest that! I'll pick up a bag from Digikey on my next
order. The ones I have right now are the ones that are bonded to wire,
they look sorta like a resistor. (you can see them in the pics of the
PCB's.)

I ditched the antennae last time I had it open... Thanks Eric! (you
guys are making me wear out the screws!) Keep it coming!

Roger Foote

Aren't these for 120vac service? The preamp has an external
transformer with a secondary voltage of 18-20 vac. Would a filter be a
good thing at these voltages?

Yes it would still be a good thing, didn't look through all of corcom's to
see if they have lower voltage versions... If not use a connector at the
chassis entry with
..01uf bypasses to ground.

On the audio in line .001uf surface mount capacitors to ground should do....
they should be after the ferrite beads for best effect. Your grounding
change should have taken care of most of the possible RF problems.


Rgds:
Eric



As for the Microphone input issue..... my solution on a Millennia Media
HV-3
was to connect the pin 1 ground to a lug on one of the XLR mounting
bolts
with a very short piece of wire..... then run shielded cable from the
XLR
to the PCB with the shield connected at the XLR pin 1 end and floating
on
the PCB end. You should be able to try this with minimal change to
your existing unit. The shields on the input will not be part of your
star
grounding scheme but I believe you will not find any negative effects.

I see what you mean. I guess I was hesitant to think about changing
the grounding, but that sounds like a very logical solution. Consider
it done!

If you are in a very strong RF environment it would also be good to put
a
couple of ferrite beads or RF chokes in series with the signal leads at
the
XLR.... then add RF bypass capacitors on the input of the PCB.

I have ferrite beads on the inputs and outputs of the preamp.

http://www.mixposse.com/attach/CB01.jpg
http://www.mixposse.com/attach/MP01.jpg

What value of cap would you use for rf bypass at the point where the
wires from the XLR enter the PCB?

The HV-3 had the pin one ground at the PCB about 4 inches inside the
case.... I had trouble with it before modification in a venue with bad
light
dimmer noise. I have previously had problems with an Ampex mixer
picking
up an FM broadcast station about a mile from the transmitter prior to it
being modified in a similar fashion.

Regards:
Eric

Thanks again for your input!
Roger Foote

Eric K. Weber wrote in message
...

Aren't these for 120vac service? The preamp has an external
transformer with a secondary voltage of 18-20 vac. Would a filter be a
good thing at these voltages?

Yes it would still be a good thing, didn't look through all of corcom's to
see if they have lower voltage versions

Is there any reason a Corcom filter designed for 120vac wouldn't work on
lower voltages? Higher voltages would be a problem, but lower voltages
shouldn't. Unless I'm missing something about their design.

Peace,
Paul

"Paul Stamler" wrote in message ...

Is there any reason a Corcom filter designed for 120vac wouldn't work on
lower voltages? Higher voltages would be a problem, but lower voltages
shouldn't. Unless I'm missing something about their design.

Peace,
Paul

My question exactly, although I am going to put the filter module in
the case with the power transformer on the 120vac side and bypass the
secondary inside the preamp case with the .01uf bypass.

Roger Foote

I have not looked at their full catalog to see if they have models which
don't use a standard 120VAC DIN power connector.... the 120VAC ones would
work fine, but I would not use one with a connector that would encourage
accidental application of 120VAC...

Rgds:
Eric

"R. Foote" wrote in message
om...
"Paul Stamler" wrote in message
...

Is there any reason a Corcom filter designed for 120vac wouldn't work on
lower voltages? Higher voltages would be a problem, but lower voltages
shouldn't. Unless I'm missing something about their design.

Peace,
Paul

My question exactly, although I am going to put the filter module in
the case with the power transformer on the 120vac side and bypass the
secondary inside the preamp case with the .01uf bypass.

Roger Foote

Just had a look they have several DC versions including the following with
connectors...

http://www.corcom.com/catalog/DCFilt...es/default.htm

Rgds:
Eric

Eric K. Weber wrote:
I have not looked at their full catalog to see if they have models which
don't use a standard 120VAC DIN power connector.... the 120VAC ones would
work fine, but I would not use one with a connector that would encourage
accidental application of 120VAC...

They have a bunch of them that have solder leads rather than an IEC
entry.

But, on a DC power line, there are other alternatives. Even a big pi
filter may be an improvement.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

(Scott Dorsey) wrote in message ...
Eric K. Weber wrote:
I have not looked at their full catalog to see if they have models which
don't use a standard 120VAC DIN power connector.... the 120VAC ones would
work fine, but I would not use one with a connector that would encourage
accidental application of 120VAC...

They have a bunch of them that have solder leads rather than an IEC
entry.

But, on a DC power line, there are other alternatives. Even a big pi
filter may be an improvement.
--scott

This is AC we're talking about... The secondary of my transformer is
at 16-20 VAC. I will use a corcom filter on the 120VAC side but use a
..01 bypass on the single secondary.

Also, maybe you guys coild explain a little more about rf bypass on
the inputs. Should I put the .001 uf caps right at the XLR input
connector? Then the beads on the wire? You wouldn't put the bypass on
the PCB, would you?

Thanks again

Roger

R. Foote wrote:
(Scott Dorsey) wrote in message ...
Eric K. Weber wrote:
I have not looked at their full catalog to see if they have models which
don't use a standard 120VAC DIN power connector.... the 120VAC ones would
work fine, but I would not use one with a connector that would encourage
accidental application of 120VAC...

They have a bunch of them that have solder leads rather than an IEC
entry.

But, on a DC power line, there are other alternatives. Even a big pi
filter may be an improvement.

This is AC we're talking about... The secondary of my transformer is
at 16-20 VAC. I will use a corcom filter on the 120VAC side but use a
.01 bypass on the single secondary.

That's a start. You might consider using a transformer that is specifically
designed for high RF rejection and good isolation between primary and
secondary, like a medical-grade transformer.

Also, maybe you guys coild explain a little more about rf bypass on
the inputs. Should I put the .001 uf caps right at the XLR input
connector? Then the beads on the wire? You wouldn't put the bypass on
the PCB, would you?

Sure, you could put them on the PCB, though you'd have to be careful about
layout. But the first line of defense for RF rejection is good grounding,
and the second line is a good quality power transformer.

When in doubt, borrow a VHF walkie-talkie and test it out. Two or three
watts on 2M or on an itinerant business-band frequency, right near the case,
will tell you a lot about RF rejection. Then plug it into a circuit with
a touch lamp right next to it.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."