Mic Pre-Amp

[Bob Scott]

Thank you again Mr. Hayes and MrAl!

I had my first experience with slew rate limiting a long time ago whilst using the uA725 as a phono pre-amp, in a time before JFET inputs were developed, and op-amp latch-up was still a problem. The 725 made a fine high quality instrumentation amp at the time, but slewing audio was not its forte.

Pls ignore this line. Test µ Ω ω

[CeaSaR]

Hey Bob,

What did you do to get the symbols in your test line? I've been wondering
how to do that (almost) forever.

As an update, I didn't have the time I thought I'd have over the holiday
break, so it's still not finished. However, the board is stuffed and soldered,
but the offboard components are not connected yet. Maybe by this
weekend. If only I didn't have to work... I better not jinx myself.

CeaSaR

[Bob Scott]

ΔµΩ±÷Πφ×ΔΘΛ§Γ≤≥ΣΦΨαβχτγδεζηψ∑θλξÏ

[jwax]

Here's a start at some "Alt" symbols:
http://www.tedmontgomery.com/tutorial/altchrc.html
Others: ∞ Alt 236
√ Alt 251
Ω Alt 234
² Alt 253
♥ Alt 259 (for your honey!) ☺

[Bigglez]

Here's a start at some "Alt" symbols:
http://www.tedmontgomery.com/tutorial/altchrc.html
Others: ∞ Alt 236
√ Alt 251
Ω Alt 234
² Alt 253
♥ Alt 259

Don't be surprised to find these Nix' d by various
reformatters on the www and in text toolchains.

If I had a dollar for everytime the omega symbol
was transcribed to an uppercase 'W'....

After a while it's tedious to read about 50W RF
circuits, and 75W video cable drivers.

[MrAl]

Hi again,


Here i'll update the slewing equation to be more accurate to what
it really is in real life, because it also includes an amplitude, as
well as get to try out some of Bob's neat characters

"ΔµΩ±÷Πφ×ΔΘΛ§Γ≤≥ΣΦΨαβχτγδεζηψ∑θλξÏ

[Bob Scott]

MrAl:

Mag=(1+ω²τ²)/(1+ω²α²τ²)

I understand your simple calculus quite well. Times and styles must have changed though. Have you seen the math in Wikipedia? Looks like calculus from mars!

BTW, what are α and τ ?

Actually, it's easier if I set the font so small it goes away.

[MrAl]

MrAl:

I understand your simple calculus quite well. Times and styles must have changed though. Have you seen the math in Wikipedia? Looks like calculus from mars!

Hi Bob,

Really, where?

[MrAl]

Hello again,


Here are a few frequency plots showing different compensation
methods for a given amplifier.
Note that when we get to choose the compensation scheme we
have more control over how we shape the response which means
we can get a higher frequency response sometimes.

[MrAl]

I also found this link:

http://www.consultrsr.com/resources/pstats/bwidth.htm

which has a nice drawing of the slew rate and the sine wave.

Also, the simplest relationship i think is this:

Vp=vps/(2*pi*f)

where

Vp is the max sine wave peak voltage that can pass unchanged, and
vps is the volts per second slew rate of the amplifier, and
f is the frequency.
Thus, for a 0.5 volts per microsecond slew rate and f=1MHz,
the maximum peak volts would have to be limited to about
0.08 volts.

Of course the decrease due to the bandwidth of the amp also
has to be applied, so if it is 3db down at f then that will also
decrease the sine output.

[Bob Scott]

Thanks MrAl. I really, really like your graphs.

I will mull this input over later today and see how it reconciles with Mr. Hayes idea that the stage driving the compensation cap goes into current limiting.

[Bob Scott]

I actually did this with an Ampeg amp, internally. It had two
channels so i wired one channel output into the input of the other.
I got very high gain and some 'distortion' for those rock moments.
It worked that way for a long time, until at one concert it overheated
and went into thermal shutdown in the middle of some Led Zepplin
song. Strange though that was the only time that every happened.
It was a tube amp BTW.

Have you ever heard of "Randy Bachman's Herzog"?

In his experiments Bachman began to hook his amps together in series, one to the other.
"It gives you a beautiful sustain."
The problem was it also set the amps on fire.
"I would actually burn up the amps, burst into flame on stage."

Then he got together with Gar Gillis of Garnet amps in Winnipeg and developed the Herzog.

http://www.garnetamps.com/zog_story.htm

[MrAl]

Hi Bob,

I took a look at that article, and yes, that is the same sound i got i guess.
It was mainly for hard rock though.
The neat thing about it was that after wiring two amps in series,
that gave me two volume controls, two treble controls, and two bass
controls, for the same amp, so that gave me a lot of ways of adjusting the
sound.
And yes the sustain was outrageous, which was a welcome byproduct back
then going first for the increased gain.
Those tube numbers sound familiar too The 6V6 might have
been the tubes my amp used (it was an Ampeg head).
It must have been built better than the ones they were using because
it would last for hours, only giving out on that one occasion.


In the following
α is greek 'alpha'
Ï„ is greek 'tau'

For G=(1+τs)/(1+ατs)=(1/α)*(s+z)/(s+p)
z=1/Ï„ and
p=1/ατ
and
Ï„ is R*C of the lag compensator.

[CeaSaR]

Gentlemen,

I have finally finished the prototype for the stereo Mic Pre-Amp. I know, it's about time! For a
refresher about the design, check out the first photo in my post at the top of page 6 in this
thread. The current differences are:

1. The actual version is a stereo unit, so just duplicate the schematic and place side by side.
2. R5 is a 10 Kohm VR, as Radio Shack doesn't stock 25 Kohm VR's.
3. R7 is currently replaced by a SPDT switch (no stock at RS again).
4. R6 is 10 Kohm on the right channel and 100 Kohm on the left channel, to experiment with
optimum output.
5. Looking at the 3rd picture in the above mentioned post (Titled: Bigglez' w/ Bob's mods on
page 4), the combo of R4/C2is put into place with R4 at 1 Kohm and C2 at 220 uF.
6. Input jacks are 1/4" and 1/8" (wired in parallel on each side, 4 jacks total).
7. Output jacks are Phono (RCA salvaged from an old stereo) and 1/4" (also wired in parallel
on each side, for a total of 4 jacks).
8. A SPST switch wired between the positive (Tip) terminals of the inputs to form a
"Stereo/Mono" selector switch.
9. The power switch is a DPST wired with one 9V clip on one side into the common power for
the Pre-Amp and the second side wired to another 9V clip and a 1 Kohm resistor and red
LED to indicate power on. Yes they are separate, but if one is on, the other will be on,
and vice-versa. This prevents the LED from affecting the Pre-Amp.

A quick check of the circuit shows that all but 1 of the features designed in work (a very
pleasant surprise!). Both channels effortlessly amplify the microphones more than enough
to overpower the soundcard inputs. The "bass contour" switch (in place of R7) makes a noticeable
difference in the lower frequency response. 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.

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, when the switch is closed there is
continuity, and ground is definitely common to both sides (all done with the continuity test on my DMM).
I have made sure that the volume controls on both sides were set to maximum, therefore they had 0 ohms
resistance going into the Pre-Amp (and max resistance going to ground). I'm not quite sure why this works
in this manner. Could there be enough resistance in the switch to prevent the signal from being shared by
both sides of the Pre-Amp? If I recall, the continuity test shows positive if there is less than 200 ohms
between the probe tips. I'll check out the actual resistance later.

Any and all comments welcome. And thank you to all who have helped so far.

CeaSaR

[Bigglez]

I have finally finished the prototype for the stereo Mic Pre-Amp. I know, it's about time!

Congrats! I had forgotten about your project, I stopped
reading the thread when it only had one or two pages.

8. A SPST switch wired between the positive (Tip) terminals of the inputs to form a
"Stereo/Mono" selector switch.

Why not merge (mix) the signals after the pre-amps?
You could add a "pan-pot" to allow placement of the
vocals (from the mic) anywhere in the stereo field.

For mono (a single mic) you could place it equally in
both stereo channels.

The "Stereo/Mono" selector switch does not work.
I have verified that the switch is between the positive (Tip) only, when the switch is closed there is
continuity,

Hmm. Perhaps the switch is upsetting the DC bias?
A better solution would be to combine the pre-amp channels
after DC blocking caps at the outputs. As the circuit is
set up for a 10k driving impedance you can mix the two
channels with resistors, or a "pan-pot" as noted earlier.

In pro mixing boards the inputs are fed through 'channels'
that apply gain and frequency shaping and ultimately through
a pot (more likely a slider) on to one or several busses.
The busses combine the various sources (inputs) and may
be mixed down to one (mono) or two (stereo) for a live hall
with PA system. In a multi-track studio the inputs are
groups on busses that each feed a track on the tape.
(There may be 2,4, 8, 16, or 24 of these).

Adding a mono-stereo switch and/or a pan-pot on the
output side is fairly easy.