High voltage capacitors in audio

[MooseFET]

Keep the capacitor and 100K to ground after the switch. This way the
volume goes to zero if the switch connections go open. Letting the
input of an opamp float is a bad thing to do.
Yes.

BTW: Why only 3 positions?
[...]
Why not leave the volume control where it is?

The 'volume' control is actually a gain control, it doesn't offer
volume 0%-100%. As the author says "The gain control being used to pre-
set the gain so that the pre-amp's gain control is normally used for
setting the listening level.". Gain might start from 100%, maybe less,
but not 0% (silence).

I don't use a preamp, so I have to add a volume potmeter. I don't see
sense in having two potmeters (one volume, one gain) do about the same
thing, so I figure gain could be a 3-position switch which makes a lot
more sense (I could also use a 2 position or 4 or 5... but 3 I think
will be enough).

I think the volume should be in the first stage before the first
opamp. However it would certainly affect the filter around the 1nF in
some way, I'm afraid.

It looks like google lost my reply.

The pot should he on a high amplitude low impedance signal. Putting
it after the first op-amp is the right place to have it.

The circuit can be reduced to a simplified model that looks like this:


In ---- 1M
! --/\/\---
\ 100K ! !
/<---/\/\-----+---!-\ !
100K \ ! >---+-+---- Vout
! GND-----!+/ !
! !
--------------------------


A cute trick is to make a spice model of this, apply 1V DC at the
input and step the pot setting. This way you can make a graph of the
gain vs pot setting. I think you will find that it follows a curve
very like the one you want for a volume control.

You could also plug the math into a spreadsheet.

The math looks a bit like this:

Let X be the pot setting:

Vth(wiper) = Vin * X + Vout * (1-X)
R(wiper) = 100K * X * (1-X)

Vout = -1M / (R(wiper) + 100K)

I'm too lazy to do the subst and solve for Vout but it will increase
much more rapidly at the top of the dial than at the bottom.

 

[martin griffith]

Keep the capacitor and 100K to ground after the switch. This way the
volume goes to zero if the switch connections go open. Letting the
input of an opamp float is a bad thing to do.
Yes.

BTW: Why only 3 positions?
[...]
Why not leave the volume control where it is?

The 'volume' control is actually a gain control, it doesn't offer
volume 0%-100%. As the author says "The gain control being used to pre-
set the gain so that the pre-amp's gain control is normally used for
setting the listening level.". Gain might start from 100%, maybe less,
but not 0% (silence).

I don't use a preamp, so I have to add a volume potmeter. I don't see
sense in having two potmeters (one volume, one gain) do about the same
thing, so I figure gain could be a 3-position switch which makes a lot
more sense (I could also use a 2 position or 4 or 5... but 3 I think
will be enough).

I think the volume should be in the first stage before the first
opamp. However it would certainly affect the filter around the 1nF in
some way, I'm afraid.

It looks like google lost my reply.

The pot should he on a high amplitude low impedance signal. Putting
it after the first op-amp is the right place to have it.

The circuit can be reduced to a simplified model that looks like this:


In ---- 1M
! --/\/\---
\ 100K ! !
/<---/\/\-----+---!-\ !
100K \ ! >---+-+---- Vout
! GND-----!+/ !
! !
--------------------------


A cute trick is to make a spice model of this, apply 1V DC at the
input and step the pot setting. This way you can make a graph of the
gain vs pot setting. I think you will find that it follows a curve
very like the one you want for a volume control.

You could also plug the math into a spreadsheet.

The math looks a bit like this:

Let X be the pot setting:

Vth(wiper) = Vin * X + Vout * (1-X)
R(wiper) = 100K * X * (1-X)

Vout = -1M / (R(wiper) + 100K)

I'm too lazy to do the subst and solve for Vout but it will increase
much more rapidly at the top of the dial than at the bottom.

This is quite nice for a panpot, a bit of positive feedback works
wonders, but I never got around to trying it

http://es.geocities.com/mart_in_medina/PANmodified.jpg


martin

 

[MooseFET]

Keep the capacitor and 100K to ground after the switch. This way the
volume goes to zero if the switch connections go open. Letting the
input of an opamp float is a bad thing to do.
Yes.
BTW: Why only 3 positions?
[...]
Why not leave the volume control where it is?
The 'volume' control is actually a gain control, it doesn't offer
volume 0%-100%. As the author says "The gain control being used to pre-
set the gain so that the pre-amp's gain control is normally used for
setting the listening level.". Gain might start from 100%, maybe less,
but not 0% (silence).
I don't use a preamp, so I have to add a volume potmeter. I don't see
sense in having two potmeters (one volume, one gain) do about the same
thing, so I figure gain could be a 3-position switch which makes a lot
more sense (I could also use a 2 position or 4 or 5... but 3 I think
will be enough).
I think the volume should be in the first stage before the first
opamp. However it would certainly affect the filter around the 1nF in
some way, I'm afraid.

It looks like google lost my reply.

The pot should he on a high amplitude low impedance signal. Putting
it after the first op-amp is the right place to have it.

The circuit can be reduced to a simplified model that looks like this:

In ---- 1M
! --/\/\---
\ 100K ! !
/<---/\/\-----+---!-\ !
100K \ ! >---+-+---- Vout
! GND-----!+/ !
! !
--------------------------

A cute trick is to make a spice model of this, apply 1V DC at the
input and step the pot setting. This way you can make a graph of the
gain vs pot setting. I think you will find that it follows a curve
very like the one you want for a volume control.

You could also plug the math into a spreadsheet.

The math looks a bit like this:

Let X be the pot setting:

Vth(wiper) = Vin * X + Vout * (1-X)
R(wiper) = 100K * X * (1-X)

Vout = -1M / (R(wiper) + 100K)

I'm too lazy to do the subst and solve for Vout but it will increase
much more rapidly at the top of the dial than at the bottom.

This is quite nice for a panpot, a bit of positive feedback works
wonders, but I never got around to trying it

http://es.geocities.com/mart_in_medina/PANmodified.jpg

That looks like too much positive feedback. Are you sure you
transcibed the values correctly?

 

[ectoplasm]

(I also lost my post.. Luckily I know how to trust crappy Google
groops, I saved my text message locally before sending, so here it is
again)

===

Funny you mention spice just now: I went to simulate the whole circuit
(except for the transistor output stage) in PSpice 9.1. I took the
exact circuit with all component values as in the design, but I took
the output of the last opamp as output for the feedback (because I
left out the transistors as I said).

There's nothing above simulation I think... because I found out I was
wrong! The gain potmeter, as is, *does* provide a volume setting
0%-100%.

I put 300mV at the input ('VAC' in PSpice) (over the 220k resistor),
and get the following voltages at the last opamp's output (potmeter %)

0%: 0V
5%: 13mV
10%: 28mV
25%: 82mV
50%: 229mV
75%: 584mV
100%: 2880mV

The bandwidth of the whole thing is about 5Hz to 40kHz (flat).

I hope I'm not wrong! It means I can use the circuit as it is... the
'gain' potmeter is really a volume control. I checked everything
several times so it should be ok. If I still want an extra gain
option, I could use a resistor for 'AOT' with a switch over it.

A cute trick is to make a spice model of this, apply 1V DC at the
input and step the pot setting. This way you can make a graph of the
gain vs pot setting. I think you will find that it follows a curve
very like the one you want for a volume control.

I'm not sure yet how to do this... it would be useful, yes, because
now I manually adjusted the potmeter & generate a graph each time.

Thanks,
E.

 

[martin griffith]

On Jun 28, 9:22 pm, MooseFET <[email protected]> wrote:

Keep the capacitor and 100K to ground after the switch. This way the
volume goes to zero if the switch connections go open. Letting the
input of an opamp float is a bad thing to do.

Yes.

BTW: Why only 3 positions?
[...]
Why not leave the volume control where it is?

The 'volume' control is actually a gain control, it doesn't offer
volume 0%-100%. As the author says "The gain control being used to pre-
set the gain so that the pre-amp's gain control is normally used for
setting the listening level.". Gain might start from 100%, maybe less,
but not 0% (silence).

I don't use a preamp, so I have to add a volume potmeter. I don't see
sense in having two potmeters (one volume, one gain) do about the same
thing, so I figure gain could be a 3-position switch which makes a lot
more sense (I could also use a 2 position or 4 or 5... but 3 I think
will be enough).

I think the volume should be in the first stage before the first
opamp. However it would certainly affect the filter around the 1nF in
some way, I'm afraid.

It looks like google lost my reply.

The pot should he on a high amplitude low impedance signal. Putting
it after the first op-amp is the right place to have it.

The circuit can be reduced to a simplified model that looks like this:

In ---- 1M
! --/\/\---
\ 100K ! !
/<---/\/\-----+---!-\ !
100K \ ! >---+-+---- Vout
! GND-----!+/ !
! !
--------------------------

A cute trick is to make a spice model of this, apply 1V DC at the
input and step the pot setting. This way you can make a graph of the
gain vs pot setting. I think you will find that it follows a curve
very like the one you want for a volume control.

You could also plug the math into a spreadsheet.

The math looks a bit like this:

Let X be the pot setting:

Vth(wiper) = Vin * X + Vout * (1-X)
R(wiper) = 100K * X * (1-X)

Vout = -1M / (R(wiper) + 100K)

I'm too lazy to do the subst and solve for Vout but it will increase
much more rapidly at the top of the dial than at the bottom.

This is quite nice for a panpot, a bit of positive feedback works
wonders, but I never got around to trying it

http://es.geocities.com/mart_in_medina/PANmodified.jpg

That looks like too much positive feedback. Are you sure you
transcibed the values correctly?

martin

Yep, it's screen capture of a service manual (soundcraft), and
apparently patented,sorry, lost the references


martin

 

[MooseFET]

Funny you mention spice just now: I went to simulate the whole circuit
(except for the transistor output stage) in PSpice 9.1. I took the
exact circuit with all component values as in the design, but I took
the output of the last opamp as output for the feedback (because I
left out the transistors as I said).

I use LTSpice. It works quite well for this sort of thing. It has
hacks that were intended to make DC-DC converter stuff work better. I
think they also help to make class B output stages converge.

I put 300mV at the input ('VAC' in PSpice) (over the 220k resistor),
and get the following voltages at the last opamp's output (potmeter %)

0%: 0V
5%: 13mV
10%: 28mV
25%: 82mV
50%: 229mV
75%: 584mV
100%: 2880mV

This is a nice curve for a volume pot. Now you just need to make a
nice little dial for the pot with numbers up to 11 and you will have a
very good headphone amplifier.

I'm not sure yet how to do this... it would be useful, yes, because
now I manually adjusted the potmeter & generate a graph each time.

Manual net list:

****
* A POT is two resistors
R1 Node1 Node2 {100E3*(1-X)}
R2 Node2 Node3 {100E3*X}

*Note resistors can never be zero ohms
*step this from to increments
..step param X 0.1 0.9 0.1

* Do an operating point computation
..op
****

In LTSpice, you can enter this on the schematic.

 

[[email protected]]

Manual net list:

****
* A POT is two resistors
R1 Node1 Node2 {100E3*(1-X)}
R2 Node2 Node3 {100E3*X}

*Note resistors can never be zero ohms
*step this from to increments
.step param X 0.1 0.9 0.1

* Do an operating point computation
.op
****

In LTSpice, you can enter this on the schematic.

Thanks, MooseFET. I am trying to figure it out in PSpice. I didn't
succeed yet with potmeter variations for plotting frequency response,
or for any attribute. Luckily there are numerous tutorials available
for PSpice. It's fun!

 

[ectoplasm]

Just to report back:

I built this headphone amplifier; finished it about a month ago now.
In terms of noise: it is dead silent... and I really mean DEAD SILENT.
It has incredibly good sound, and it goes incredibly, incredibly loud;
and that's nice because I also like loud music styles. There is no
audible distortion. I use Sennheiser HD480-II and Koolsound HD-627
headphones.

Basically, it is this design:
http://sound.westhost.com/project24.htm
(Project 24 from Elliott Sound Products, "Hi-Fi Headphone Amplifier")

I just made a few changes / additions, with the help of everyone in
this thread; thanks! (excluding the few audio-phoolery bashing
trolls.. it's all in *your* minds :-D )
Here are the changes and additions I made:

- Added a bass enhance stage, an extra 5534; so I used 5532's. The
15pF becomes unnecessary. fT is switchable (4 positions), the lowest
is about 200 Hz (-3dB point), about 10dB amplification. Using good
quality Wima MKT10 capacitors.
- Added a 2nd order rumble filter in the input stage, fT under 10Hz.
This is useful when using a record player esp. when bass enhance is
on; DC coupling capacitors 10 and 100uF at input stage became
unnecessary.
- The 470pF at the opamp input is omitted
- Output transistors, I used the Toshiba 2SC5171 (NPN) and 2SA1930
(PNP)
- Additional 100nF capacitors at each opamp IC's supply voltage pins,
to ground (for both pos + neg voltages)
- Attenuation switch, an optional -24dB. Useful with my low
impedance / high dB/mW headphones, you use more volume pot range which
is handy.
- Additional elco's around the 7815/7915 that I saw recommended in one
of these regulators' data sheet