Hi,
I'm new to this group, was wondering if any of you guru's could help
me with modifying this mixer :

http://www.shure.com/pdf/discontinued/m68.pdf

to act as a 4 channel mic preamp with unbalanced outputs?

I imagine it should be simple but I'm not great at deciphering
circuitry, just soldering...
-James.

"splitlips" wrote in message
oups.com...
Hi,
I'm new to this group, was wondering if any of you guru's could help
me with modifying this mixer :

http://www.shure.com/pdf/discontinued/m68.pdf

to act as a 4 channel mic preamp with unbalanced outputs?

I imagine it should be simple but I'm not great at deciphering
circuitry, just soldering...
-James.


you will be much happier with a couple of dual behringer or blue tube or
similar. however just clip the wire from the center pin of each level pot
and cable out from that point.

Thanks,
Simple enough!
looks like I can also reverse the output transformer for another input
and do 5 channels too without too much hassle.
-James.

"splitlips" wrote in message
ups.com...
Thanks,
Simple enough!

But once you've done it, I bet you run out and buy a Behringer pretty
quickly. The M68 was not one of Shure's cleaner mixers.

looks like I can also reverse the output transformer for another input
and do 5 channels too without too much hassle.

Into what? There are only four sets of preamp electronics. And the output
transformer is probably....well, never mind, there's no place to connect it
anyway.

Peace,
Paul

Paul Stamler wrote:
"splitlips" wrote in message

But once you've done it, I bet you run out and buy a Behringer pretty
quickly. The M68 was not one of Shure's cleaner mixers.

looks like I can also reverse the output transformer for another input
and do 5 channels too without too much hassle.

Into what? There are only four sets of preamp electronics. And the output
transformer is probably....well, never mind, there's no place to connect it
anyway.

The output transformer is different than the input transformers. They are
not the same ratio.

And I will agree that the M68 was... well... kind of nasty-sounding.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:
Paul Stamler wrote:
"splitlips" wrote in message

But once you've done it, I bet you run out and buy a
Behringer pretty
quickly. The M68 was not one of Shure's cleaner mixers.

looks like I can also reverse the output transformer for
another
input and do 5 channels too without too much hassle.

Into what? There are only four sets of preamp
electronics. And the
output transformer is probably....well, never mind,
there's no place
to connect it anyway.

The output transformer is different than the input
transformers.
They are not the same ratio.

And I will agree that the M68 was... well... kind of
nasty-sounding.

How can that be? None of that *nasty* stuff that some
audiophiles hate like ICs or loop feedback! ;-)

The output transformer is different than the input
transformers.
They are not the same ratio.

Well looking at the schematic and parts list it seems the output
transformer is exactly the same as the input only wired in reverse :
T1-T5 : part 90m2150.

Into what? There are only four sets of preamp
electronics. And the
output transformer is probably....well, never mind,
there's no place
to connect it anyway.

The output section of the preamp would be redundant after cutting
before the mixer, and looks just like a duplicate of the preamps so I
could patch in there after reversing the output transformer and putting
in another xlr socket to replace the mic out socket.

And I will agree that the M68 was... well... kind of
nasty-sounding

I'm not expecting it to sound fantastic, It looks pretty easy to
overload as is. I think wiring in pad switches before the transformers
and wiring the pots before the preamp electronics might help. I mean
there's no provision in the stock unit to prevent clipping, only to
lower the volume once it does...

If nothing else I'll have learned a lot so I can't really go wrong.
-James

"splitlips" wrote in message
ups.com...
And I will agree that the M68 was... well... kind of
nasty-sounding

I'm not expecting it to sound fantastic, It looks pretty easy to
overload as is. I think wiring in pad switches before the transformers
and wiring the pots before the preamp electronics might help. I mean
there's no provision in the stock unit to prevent clipping, only to
lower the volume once it does...

If nothing else I'll have learned a lot so I can't really go wrong.

Unless you try to use it on a recording that matters.

Easy to overload is the understatement of the year. The one we used to use
for dances required a 20dB inline pad on each microphone to avoid clipping,
with dynamic mics; for condensers I'd want at least 30dB pads, maybe more.
Wiring the pots before the electronics will, unless I misremember the
schematic, load the transformers wrong and ruin the noise performance.

Besides, even when these circuits weren't overtly clipping, they still
sounded pretty bad.

On the other hand, as you say, you'll have learned a lot. And if you
actually make them sound decent, my hat will be off to you. Just make sure
you have a better preamp on hand to compare.

Peace,
Paul

Scott Dorsey wrote:

I will agree that the M68 was... well... kind of nasty-sounding.


But hard to kill.

The M267/8 weren't quite as nasty sounding.

splitlips wrote:

Hi,
I'm new to this group, was wondering if any of you guru's could help
me with modifying this mixer :

http://www.shure.com/pdf/discontinued/m68.pdf

to act as a 4 channel mic preamp with unbalanced outputs?

I imagine it should be simple but I'm not great at deciphering
circuitry, just soldering...
-James.

Have a look at the numbers in the rectangular boxes near the XLR input
jack, each of the transistor pins, and the input to the master
amplifier stage. If you do the math, you'll see that there is nearly
22dB of gain coming from the input transformer, which means it has a
ratio of more than 1:10. It's hard to make good-sounding transformers
with high ratios, especially when you're not really trying. Then
there's another 21dB coming from the single transistor amplifier stage.
you're now up to about 43dB of gain, which is a reasonable amount for a
mike preamp. Next is the 20k pot for a level control, which is fed by
a 33k resistor. Now the signal hitting the master amplifier is back
down to about the same level it was at before it hit the first
transistor stage. 22dB of gain is probably not enough for many
applications. But this is easy to fix. The output of the preamp is
seeing a voltage divider with the 33k resistor on top, and all five
level pots (4 mikes and one aux input) in parallel on the bottom for
4545 ohms. This gives you an 18dB attenuator, even with the pot turned
all the way up. By disconnecting all the inputs from the master
amplifier, you'll change each one into an 8.5dB attenuator which will
leave you with about 34dB of gain on each channel. You can increase
this by making the 33k resistor smaller, but that will have some
drawbacks (sensitivity to output loading, requiring a larger coupling
capacitor C9).

Don't forget to change the 5000pF cap C14 in the master section to a
1000pF cap, so it will match the other 4 channels.

I suggest removing the impedance switches from all channels - the cheap
switches will only cause problems, and you surely don't want to apply
phantom power to this circuit if those switches are still there.

You'll notice that the entire mixer circuit uses about 5mA of 23V
power, and almost half of that goes to the aux output. So if you're
not using the aux output (because you disconnected all the sources from
it) then you should probably disconnect that circuit from the power
supply. Now you've got 2.8mA of current draw, and you could easily
power this thing from 3 9V batteries that will last a very very long
time and not be susceptible to hum from the mains transformer.

The input transformer secondary is loaded with about 150k ohms, so
putting the 20k level pot ahead of the amplifier circuit would mess up
the transformer loading. You would have to change the pot to perhaps a
250k pot, which would be noisy.

I think you'd be better off increasing the amplifier's headroom
instead. I think you could do this by simply increasing the power
supply voltage. The 35 capacitors have plenty of room for an increase
over the 6.5V hitting them now, and if the transistor really is a
2N5088 then it'll be fine too. I suspect you could increase the power
supply voltage to about 50V without trouble (measured across C22),
though you will have to replace C22 with a higher-voltage part. Same
goes for C20-21, but you could just remove those from the circuit if
you're not using the internal power supply. You have two options for
increasing the power supply voltage: modify the internal power supply
or use an external supply.

You could easily power your 5-channel mixer off of a 48V phantom power
supply, and it'll use less power than a typical modern condenser
microphone. I would apply the external power source to the accessory
28V jack. I would cut off the power cord.

The other option would be to replace the power transformer with a new
one that would give you a 50VDC output. I would use an Amveco 62025
from Digikey ($13.68) and wire the two secondaries in series for 44VAC
output. In this case, I would replace the 1N4002 rectifier diodes with
1N4004 or better (just buy 1N4007, they're the same price and good for
everything). Replace C20 and C21 with 63V parts and C22 with a 50V
part, and you should be set. If you need help wiring up the power
transformer primary, then get help from a qualified technician.

While you're at it, you could bypass R16 with a 270uF/4V electrolytic
capacitor for more gain, and you could replace R12 (the 10k collector
resistor) with a constant current source. At that point, you'll have
about maxed out this circuit. You're still limited to feeding these 5
outputs into relatively high-impedance loads, and changing that would
require adding additional stages. This circuit isn't worth that level
of modification. The transformers will continue to suck. But it'll be
fine at this point for feeding most DAW interfaces and the like.

ulysses