Mic Pre-Amp

[CeaSaR]

The idea behind the placing the switch across the initial inputs was to allow 1 mic to feed both
sections of the Pre-Amp enabling it to record 2 tracks (L + R), but with different bass contours
and or levels, allowing the person sitting at the computer to sample the individual recording to
see what sounds better. Should the solution require a buffer amp at this time to correct the
problem, I'll resort to the old method of testing, i.e., set the mic up in front of the speaker cabinet
and maually change the plug while playing a pre-recorded track. OR, the real old way of having the
track played twice live as above.

CeaSaR

[Bigglez]

The idea behind the placing the switch across the initial inputs was to allow 1 mic to feed both
sections of the Pre-Amp

This is a common feature on audio boards. A pan pot is better
(than a switch) as it allows positioning of the mono image in the
stereo field.

Strapping channel inputs is not a good idea for several reasons,
even though it might work for you. Audio boards have busses and
grouping or sub-groups that allow various inputs channels to be
combined after the input circuits. This avoids distortion of the levels
or filtering applied to each channel.

More info here.

[Bob Scott]

It seems that the left channel (R6 = 100 Kohm) is
rather "gainey", and picks up too much from around the area. I will probably put that R6 back to
10 Kohm. Otherwise this has been successful.

Hi Ceasar. So you built the 1 transistor model. R6 at 100K doesn't load down the output of the preamp as much as the 10K, so at 100K the preamp will have about double the gain, or 6Db more. "gainey" is a good description.

Now to the single problem. The "Stereo/Mono" selector switch does not work. When only 1 microphone is plugged in (either side) and set to mono, only the side with the mic plugged in produces any output.
I have verified that the switch is between the positive (Tip) only....

If the your description of your wiring is right AND you followed that schematic exactly, it should work. Check the wiring of the switch if you used a 3 terminal DPST toggle switch. Make sure that the "common" terminal is wired. Using an ohmeter, confirm that there is continuity when the switch is closed. Other than that, I can't see any way that flicking that switch would cause one channel to stop working.

[CeaSaR]

Thanks for the replies, guys.

Bigglez,

Good link. I have had the fortune of having a number of people around me / my extended family who
have had or still have a life in sound recording /reproduction (inlaws - musicians in bands and also PA
work, friends of family - runs an electronics business and does sound for well known bands (mostly
country) and at large fairs around the country, and a coworker that is in several bands and has a
recording studio). I have not had the need to get into the recording side until now (see very first post).
Since this is a super basic setup (Mic - preamp - computer soundcard), I thought that a handy option
would be to send the signal from 1 mic through the left and right channels of the preamp to allow for
different recorded sounds (ie - less bass / more bass on the L or R channel) so that my son could chose
the best sounding version of the exact same piece. Of course, if he had matched mics, that wouldn't
be necesary. But, he doesn't. He has 3 to chose from, a Vtech (no idea what model, 1/4" plug), a Phillips
(bought at the local WalMart in the electronics aisle, 1/4" plug) and a little mic from a stereo made in the
?80's? with a 1/8" plug. So, 3 different mics with 3 different sets of characteristics able to be plugged into
a stereo pre-amp that can be set to 2 different bass curves on either side. Perhaps you can now see why
this could be an important feature. He can set the pan L/R in the software if he so chooses. A real mixer
is probably a few years down the road, if he stays with it.

Again, thank you for your insight and help.

Bob,

Yes, I started with design #1 on my list. Eventually I will get around to the other 2.

I knew that the side with the 100 Kohm out would have much more output, having run the simulations on it,
but I wasn't sure how that would translate into the real world. That's what testing is for.

As for the switch...
I verified that the switch is wired between the tips of the jacks, there is no continuity from the tips to ground,
no continuity between tips when the SPST (2 connections only on the switch) is open and continuity when the
switch is closed. I haven't taken the time yet to actually measure the resistance across the switch when closed,
but I will do that soon. There is one possibility that will have to be explored, that I have one side wired backward.
I'll have to investigate this, but if it was, it would short out the mics and no sound would come through at all
when the switch was closed.

Perplexing...

Thanks for the help.

For everyone.

Okay, now that that project is mostly done, what would you do for a good quality pre-amp using opamps? The only
other criteria for this one would be both 1/4" and XLR input/output with the ability of supplying phantom power over
the XLR connections. This one would probably be powered from 110V AC.

Have at, folks!

CeaSaR

"Keep the conversation going, it is... fascinating."

[Bob Scott]

Okay, now that that project is mostly done, what would you do for a good quality pre-amp using opamps? The only other criteria for this one would be both 1/4" and XLR input/output with the ability of supplying phantom power over the XLR connections. This one would probably be powered from 110V AC.
CeaSaR

You have to make a decision as to how high quality you want to get. You can build a preamp around the readily available and cheap FET input TL071 or TL072 or TL074 series, or a lower noise op-amp. I looked up an old low noise data sheet for a preamp - LM833. Perusing National's web site led me to the newer LM4562.

http://www.national.com/ds/LM/LM4562.pdf

Check out all the audio circuitry app notes in this datasheet.

To get the lowest noise possible you need to drive it with an impedance that matches the noise figures. R=V/I. Divide the "Equivalent Input Noise Density figure of 2.7nV / √Hz by the input "Equivalent Input Current Density" figure of 1.6pA / √Hz to get the optimum driving impedance of 1,687 ohms. Someone correct me if this is not exactly right.

Do to this (keep noise as low as possible) you need a matching transformer on the input. The center tap of the primary winding of the input transformer is normally used to inject your phantom power down the XLR inputs.

[Bigglez]

Okay, now that that project is mostly done, what would you do for a good quality pre-amp using opamps? The only
other criteria for this one would be both 1/4" and XLR input/output with the ability of supplying phantom power over
the XLR connections. This one would probably be powered from 110V AC.

Would you consider starting a new thread? Perhaps copying the
quoted text above (or some from your very first post on this
topic)?

I didn't follow this thread in real time and got thoroughly lost in
the convoluted references.

[Bigglez]

As for the switch...
I verified that the switch is wired between the tips of the jacks, there is no continuity from the tips to ground,
no continuity between tips when the SPST (2 connections only on the switch) is open and continuity when the
switch is closed. I haven't taken the time yet to actually measure the resistance across the switch when closed,
but I will do that soon. There is one possibility that will have to be explored, that I have one side wired backward.
I'll have to investigate this, but if it was, it would short out the mics and no sound would come through at all
when the switch was closed.

I understand your need for the switch, and that you are working
with three different microphones.

If the switch shunts the two mic inputs you may have current
flowing backwards through the mic's low DC resistance.

The switch would work better with capacitor isolation to block
the DC path. Try adding a large value cap in series with the
switch (10uF to 100uF electrolytics).

Adding two identical caps in series, but connecting one pos to
the other pos will give a bi-polar cap of half the value, and no
worries about polarity.

[Bob Scott]

Ceasar,

I perused the net and found some schematics in a Walt Jung chapter of applications. It is also in Analog Devices' library:

http://www.analog.com/library/analogDia ... inal_I.pdf

In a schematic in the above link, they mention an input transformer made my Jensen Transformer. I went to the Jensen website and found more schematics:

http://www.jensen-transformers.com/apps_sc.html

[CeaSaR]

As promised, I have measured the resistance across the switch, 0.6 ohm. Also, both inputs
go to the - (negative) lead of the 10 uF electrolytic capacitor in each side of the pre-amp(s),
thereby minimizing DC bias from the microphones. I will try to get a full schematic and
(a) picture(s) of the temporary setup by this weekend (Feb. 20-22, 2009).

Thanks again.

CeaSaR

[Bigglez]

As promised, I have measured the resistance across the switch, 0.6 ohm. Also, both inputs
go to the - (negative) lead of the 10 uF electrolytic capacitor in each side of the pre-amp(s),
thereby minimizing DC bias from the microphones.

A schematic would be helpful...
Connecting two microphones together in parallel will place
one as the load on the other. If they have different internal
circuits there is a good chance that one will 'short out' the
other. This is the case regardless of a DC path or capacitor
coupling of the signals. For this reason (and to get repeatable
and consistent performance) each input of an audio mixer
should have its own amplifier channel to isolate the inputs
from each other.

[CeaSaR]

Ok, heres the schematic:



Keep in mind that this is the pre-amp in testing phase complete with
different value output resistors and mono/stereo switch on the input.

I have already come to the conclusion that R15 should equal R16 at
10k ohms because the levels are just too overpowering with the current
value of 100k ohms. I am also thinking of abandoning the mono/stereo
switch on the input if I cannot figure out why it doesn't work as intended
(to be able to share 1 (one) microphone with both sides of the preamp,
so that 2 different takes can be recorded from a single pass). As it stands,
it doesn't matter which side the mic is plugged into, it only records on that
side; the signal does not split between them. And, only 1 mic is plugged in
at a time. A previous post states that I have verified the wiring and that
the switch connects the tips of the jacks, and has continuity when the
switch is closed.

Also note that the LED indicator light is on it's own circuit. That is to keep
it separate from the pre-amp so that it has no way whatsoever of creating
any interference or producing any drag on the power supply.

Separation, separation, separation.

That's about it for now, pics to come later.

CeaSaR

[CeaSaR]

Finally, I have a picture of the pre-amp as temporarily secured:



Once testing is completed and satisfactory, it will be packaged
up nicely in a box of my son's choice.

Please note, the schematic shown in my previous post is drawn
exactly as the pre-amp is wired. Therefore it should be very easy
to trace the picture with the schematic .

CeaSaR

[MrAl]

Hi,

I test circuits that way too sometimes, except i use hot glue instead of duct tape

[CeaSaR]

Ya aint a man if ya caint use duct tape!

Or as Red Green said, you can fix anything with baling wire and duct tape!

CeaSaR

[Bob Scott]

Ya aint a man if ya caint use duct tape!

Or as Red Green said, you can fix anything with baling wire and duct tape!

CeaSaR

Not being a country folk, In town I used a lot of coat hanger wire, especially to hold up loose mufflers, tailpipes, etc. Binder twine was a popular fix-all.

Why do balers use wire? It could hurt some poor animal.