[DaveC[_2_]]

Why not go active and get rid of the caps altogether?

Thanks John. Good simple design.

Can you suggest a good op amp for such an audio application?

Is it possible to use single-supply amps throughout?

Thanks,
Dave

[John Fields]

On Sun, 30 Oct 2011 02:26:06 -0700, DaveC wrote:

Why not go active and get rid of the caps altogether?

Thanks John. Good simple design.

Can you suggest a good op amp for such an audio application?

---
I used an LT1007 in the simulation.

Did you run it?
---

Is it possible to use single-supply amps throughout?

---
Yes, but you'd have to use caps on the inputs and outputs and
reference the opamp + inputs to Vcc/2.

--
JF

[DaveC[_2_]]

Can you suggest a good op amp for such an audio application?

I used an LT1007 in the simulation.

Did you run it?

No tools.

I made the L & R op amps fixed gain and added input level control pots:

http://i43.tinypic.com/2hh3y12.jpg

Comments?

Thanks.

[DaveC[_2_]]

http://i43.tinypic.com/2hh3y12.jpg

Should all pots be log taper?

Thanks.

[John Fields]

On Sun, 30 Oct 2011 21:47:34 -0700, DaveC wrote:

http://i43.tinypic.com/2hh3y12.jpg

Should all pots be log taper?

---
The input pots, OK. The feedback pot, probably not.

Plot the change in output voltage as a function of wiper position to
get a better handle on it.

--
JF

[John Fields]

On Sun, 30 Oct 2011 21:44:33 -0700, DaveC wrote:

Can you suggest a good op amp for such an audio application?

I used an LT1007 in the simulation.

Did you run it?

No tools.
---
They're free:

http://www.linear.com/designtools/software/
---


I made the L & R op amps fixed gain and added input level control pots:

http://i43.tinypic.com/2hh3y12.jpg

Comments?

---
Looks OK.


--
JF

[DaveC[_2_]]

I used an LT1007 in the simulation.

Can't find them on Mouser.

Suggest another one that's good for audio?

How about LM833? ::

http://www.national.com/ds/LM/LM833.pdf

The SR and GBW beat the 1007... ;-)

Thanks.

[John Fields]

On Mon, 31 Oct 2011 00:16:15 -0700, DaveC wrote:

I used an LT1007 in the simulation.

Can't find them on Mouser.

---
You can buy them directly from Linear:

http://www.linear.com/purchase/LT1007
---

Suggest another one that's good for audio?

How about LM833? ::

http://www.national.com/ds/LM/LM833.pdf

The SR and GBW beat the 1007... ;-)

---
Go for it; they're a lot cheaper!

--
JF

[DaveC[_2_]]

Is the type of resistor important (wire wound, etc.)?

Thanks.

[John Fields]

On Mon, 31 Oct 2011 01:11:57 -0700, DaveC wrote:

Is the type of resistor important (wire wound, etc.)?

---
Don't use wirewound.

5% 1/4 watt carbon film is fine.

--
JF

[DaveC[_2_]]

Is it possible to use single-supply amps throughout?

---
Yes, but you'd have to use caps on the inputs and outputs and
reference the opamp + inputs to Vcc/2.
[J. Fields]

Only a single supply (in the amplified speakers) is available to power this
circuit. I can tap this supply for my circuit:

http://i41.tinypic.com/2vlo2t2.jpg

I've added input & output caps. Are these values sound? ;-)

How do I go about getting a 1/2 Vcc ground reference? (See my non-EE
attempt.) What values to use for the divider resistors?

What needs to be reference to the new "ground"? Everything between the input
caps and output caps?

Thanks,
Dave

[Dave Platt]

In article ,
DaveC newsgroups wrote:

Only a single supply (in the amplified speakers) is available to power this
circuit. I can tap this supply for my circuit:

http://i41.tinypic.com/2vlo2t2.jpg

I've added input & output caps. Are these values sound? ;-)

How do I go about getting a 1/2 Vcc ground reference? (See my non-EE
attempt.) What values to use for the divider resistors?

If you want to do it with just resistors, you could make R16 and R17
somewhere around 4k7 each, and add a few uF of bypass capacitance
around one or both resistors. 1k would give a stiffer reference if
you don't mind the additional power consumption.

If you can spare an op-amp section, you can get a better (stiffer) ground
reference than you'll get with just resistors, with lower power
dissipation (I think). Use R16 and R17, and a small cap from the
junction point, to create a Vcc/2 reference, but do not "ground" this
directly to your internal reference point (the rectangular-looking
ground symbol). Instead, feed this to the noninverting input of an
op-amp section, feed the op-amp output back to the inverting input
(i.e. create a unity-gain follower), and use the op amp's output as
your ground reference. In this arrangement R16 and R17 can be
high-value (100k?) as their junction point will be looking into a
high-impedance op amp input.

What needs to be reference to the new "ground"? Everything between the input
caps and output caps?

Pretty much... each of the op amps' noninverting inputs, and the
"bottom ends" of the potentiometers, as you have drawn them. *NOT*
the V- input to the op amp(s), of course.

You might want to add "pop preventer" resistors at the inputs and
outputs... say, 100k to DC ground, from the "outside" end of each of
the DC-blocking capacitors.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

[DaveC[_2_]]

Latest rev:

http://i40.tinypic.com/35m026h.jpg

What needs to be reference to the new "ground"? Everything between the
input
caps and output caps?

Pretty much... each of the op amps' noninverting inputs, and the
"bottom ends" of the potentiometers, as you have drawn them. *NOT*
the V- input to the op amp(s), of course.

You do mean each of the op amps' *inverting* inputs, yes?

You might want to add "pop preventer" resistors at the inputs and
outputs... say, 100k to DC ground, from the "outside" end of each of
the DC-blocking capacitors.

Is this what you mean (see link)?

Thanks.

[John Fields]

On Tue, 8 Nov 2011 21:07:44 -0800, DaveC wrote:

Latest rev:

http://i40.tinypic.com/35m026h.jpg

What needs to be reference to the new "ground"? Everything between the
input
caps and output caps?

Pretty much... each of the op amps' noninverting inputs, and the
"bottom ends" of the potentiometers, as you have drawn them. *NOT*
the V- input to the op amp(s), of course.

You do mean each of the op amps' *inverting* inputs, yes?

---
No.

Your drawing is wrong; signal goes to the inverting (-) inputs and the
Vcc/2 reference goes to the non-inverting (+) inputs.

--
JF

[DaveC[_2_]]

Your drawing is wrong; signal goes to the inverting (-) inputs and the
Vcc/2 reference goes to the non-inverting (+) inputs.
JF

Thanks guys. Fixed:

http://i44.tinypic.com/r1k8qa.jpg

All else looks good?

Are cap values reasonable? I added C8 & C9 out of habit of seeing in other
designs. Values for these?

Thanks.

[isw]

In article
,
DaveC wrote:

http://i40.tinypic.com/35m026h.jpg

Distortion performance might be better if you run the amps in inverting
mode. If you run the audio into the non-inverting inputs, the inverting
input follows and the amp has to operate throughout it's common-mode
range. If you use the amps in inverting mode, both the + and - inputs
will stay very close to ground.

Isaac

[Dave Platt]

In article ,
DaveC newsgroups wrote:

Only a single supply (in the amplified speakers) is available to power this
circuit. I can tap this supply for my circuit:

One cautionary note with regards to this circuit, as drawn: be careful
when you wire up R15. If you use a standard three-terminal
potentiometer, make sure that you wire both the wiper, and one of the
two ends to U3's inverting input. Don't just wire up the wiper!

The reason: pots occasionally go "open" due to dirt or wear. If you
have only the wiper connected, and it goes open, you'll have no
feedback path around U3, and it'll immediately and enthusiastically
slam its output against one of the rails (or both in rapid succession
if there's a signal present). This will let out a really unholy
THWOMP from your subwoofer, and may pop the cone out of the cabinet or
at last shove the voice coil out of the gap. Expensive damage.

With a three-terminal wire-up, the resistance in this part of the
feedback loop will never be more than the bulk value of the pot (i.e.
open wiper == wiper all the way at one end) and this will limit the
maximum subwoofer volume. You can choose the maximum loudness by
setting the value of the pot.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

[Don Pearce[_3_]]

On Sun, 30 Oct 2011 02:26:06 -0700, DaveC wrote:

Why not go active and get rid of the caps altogether?

Thanks John. Good simple design.

Can you suggest a good op amp for such an audio application?

Is it possible to use single-supply amps throughout?

Thanks,
Dave

The best amps are pretty much all dual supply. The big advantage is
simplicity of the circuit - and of course not having the coupling caps
(you need to put another set at the output, by the way).

Generating a negative rail is pretty simple even if you only have a
positive one available. An oscillating op amp, a diode and a capacitor
does the job nicely.

d