Question on pre design for Kevin A. (or anyone else)

[Guest]

Archived from groups: rec.audio.pro (More info?)

On Sun, 01 May 2005 12:18:32 -0700, Bob Cain
<arcane@arcanemethods.com> wrote:

>> How many simultaneous channels?
>
>Four.

I've got a four gang Alps pot, audio taper 100K with
a motor on its back and a knob on its front, mounted
on a circuit board with motor driver IC, etc. Yours
if you want it.

Also several Alps 50K six gangs with motor, unmounted.

Chris Hornbeck
"Don't panic."

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Mike Rivers wrote:
> how about digitally controlled
> attenuators? The steps are quantized, but you should be able to get
> ones with fine enough resolution to seem continuous.

Got a PN for any you've heard good things about?


Thanks,

Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Pooh Bear wrote:

> If you can live with 1dB steps TI's PGA2500. Rather a good mic amp in
> its own right. Gain is controlled by an SPI interface.

_Very_ nice part, thanks. I'd like to control it with a
pot, though, and that will take a little thought. Maybe
there's a PIC that can do the job with a single chip.

Definitely a go-to part for a pre/ADC.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Ben Bradley wrote:

> On Sat, 30 Apr 2005 12:52:22 -0700, Bob Cain
> <arcane@arcanemethods.com> wrote:
>
> >
> >What would be the most cost/performance effective way today
> >using electronics to achieve continuous trim for multiple
> >pre channels using a single control and having tight
> >tracking? Ganged, rotary switched resistors not of interest.
>
> There's the (lamp or LED)/CDS photocell combination such as the
> Vactrol brand name or cheap surplus ones such as this:
>
> http/www.goldmine-elec-products.com/prodinfo.asp?number=G15396&variation=&aitem=1&mitem=1
>
> Running a control current through each LED of several such units
> and using the photocell resistance in the preamp feedback circuit for
> each channel would 'work', but I understand CDS photocells don't have
> very consistent light-resistance characteristics from one unit to the
> next (I wonder if they're temperature-sensitive as well). So, use a
> 12-bit A/D to read the control voltage (from a front-panel gain
> potentiometer), and use a microcontroller to generate several outputs,
> each calibrated for its particular LED/photocell combination.
> Calibration is not something one wants to do in production, but
> since you've already put a microprocessor in there, have a path from
> the micro to the pre inputs, so it injects a known level signal, and
> then measure each output in 1dB steps over the whole gain range of
> each preamp, writing each LED current value to FLASH memory (you DID
> pick a chip that can write FLASH under program control, didn't you?).
> Values between 1dB steps (from that 12-bit word from the front-panel
> control) can be interpolated by the processor.
> A pushbutton, perhaps on the back panel, would initiate
> calibration.

Those Vactrols have a horribly non-linear resistance. Matching them is something else and the
resistance has a 'history effect'.

Graham

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Bob Cain wrote:

> Bob Quintal wrote:
> > Bob Cain <arcane@arcanemethods.com> wrote in
> > news:d5269002nk3@enews1.newsguy.com:
> >
> >
> >>
> >>Phil Allison wrote:
> >>
> >>>"Bob Cain"
> >>>
> >>>
> >>>>What would be the most cost/performance effective way today
> >>>>using electronics to achieve continuous trim for multiple pre
> >>>>channels using a single control and having tight tracking?
> >>>>Ganged, rotary switched resistors not of interest.
> >>>>
> >>>
> >>>
> >>>
> >>>** Is this for a mic-preamp ??
> >>
> >>Yes.
> >>
> >>
> >>> Real application or some asinine hypothetical ??
> >>
> >>Real. The only box capable of it today is the Metric Halo
> >>2882 and it's overkill for my app.
> >>
> >>
> >>Bob
> >
> >
> > How many simultaneous channels?
>
> Four.
>
> > You might consider a light
> > dependent resistor in the feedback path of an op-amp, controlled
> > by the brightness of a LED.
>
> Hmmm, that's a new idea! (At least for this app.) Any idea
> how well such devices track? I'll look into it but I'll bet
> that noise will be a signifigant problem; noise from the
> light source as well as from the device itself. Definitely
> worth an investigation, though, because it's so darned simple.

LDRs are horribly non-linear. Hope you don't mind lots of THD.

You'll need to buy hundreds to get 4 that match too.

Graham

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Pooh Bear wrote:

> LDRs are horribly non-linear. Hope you don't mind lots of THD.
>
> You'll need to buy hundreds to get 4 that match too.

Never mind.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Bob Cain wrote:

> What would be the most cost/performance effective way today
> using electronics to achieve continuous trim for multiple
> pre channels using a single control and having tight
> tracking? Ganged, rotary switched resistors not of interest.

Just in case you missed it earlier, provided you can handle a simple
microprocessor control interface - you simply won't beat this.

http/focus.ti.com/docs/prod/folders/print/pga2500.html

Fully Differential Input-to-Output Architecture
Digitally Controlled Gain Using Serial Port Interface:
Gain Range: 10dB through 65dB, 1dB per step
Unity (0dB) Gain Setting via Serial Port or Dedicated Control Pin
Dynamic Performance:
Equivalent Noise with ZS = 150 and Gain = 30dB: –128dBu
Total Harmonic Distortion plus Noise (THD+N) with Gain = 30dB: 0.0004%
Zero Crossing Detection Minimizes Audible Artifacts when Gain Switching
Integrated DC Servo Minimizes Output Offset Voltage
Common-Mode Servo Improves CMRR
Four-Wire Serial Control Port Interface:
Simple Interface to Microprocessor or DSP Serial Ports
Supports Daisy-Chaining of Multiple PGA2500 Devices
Dedicated Input Pin for Selecting Unity Gain
Overload Output Pin Provides Clipping Indication
Four General-Purpose Digital Output Pins
Requires ±5V Power Supplies
Available in an SSOP-28 Package
APPLICATIONS
Microphone Preamplifiers and Mixers
Digital Mixers and Recorders


I currently have an eval board on loan from TI. It looks like an
impressive part. Designed by the Burr-Brown division of course.


Graham

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

> > What would be the most cost/performance effective way today
> > using electronics to achieve continuous trim for multiple
> > pre channels using a single control and having tight
> > tracking? Ganged, rotary switched resistors not of interest.

Exactly how much gain variation do you need? If you only need to tweak it
say 12 dB, say, you can use a quad linear-taper pot; set up series resistors
so the series resistor is 1/3 the value of that pot section (measure and set
exactly). Within that range, while your attenuation-vs.-rotation won't be
perfect in terms of dB-per-unit rotation, it won't be as horrible as with a
pot with wider range. And linear pot sections track one another a good deal
better than log.

Not at all elegant, but if it'll fit the application it's at least
cost-effective.

Peace,
Paul

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Ben Bradley wrote:
> On Sat, 30 Apr 2005 12:52:22 -0700, Bob Cain
> <arcane@arcanemethods.com> wrote:
>
>>
>> What would be the most cost/performance effective way today
>> using electronics to achieve continuous trim for multiple
>> pre channels using a single control and having tight
>> tracking? Ganged, rotary switched resistors not of interest.
>
> There's the (lamp or LED)/CDS photocell combination such as the
> Vactrol brand name or cheap surplus ones such as this:
>
>
http/www.goldmine-elec-products.com/prodinfo.asp?number=G15396&variation=&aitem=1&mitem=1

CdS cells aren't exactly what we'd call linear resistors, by modern
standards. IOW if you pass an audio signal through a voltage divider
that includes a CdS cell, pure signal in gives moderately distorted
signal out.

I did quite a bit of work with CdS cells as amplitude regulators in
audio signal generators some decades ago, and they were the weakest
link, even when their resistance was swamped with good resistors.

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

In article <d54kfu11haj@enews3.newsguy.com> arcane@arcanemethods.com writes:

> > how about digitally controlled
> > attenuators? The steps are quantized, but you should be able to get
> > ones with fine enough resolution to seem continuous.
>
> Got a PN for any you've heard good things about?

Nope. I don't do design. I leave that to qualified professionals like
Kevin. I thought about it, though, when recalling that Dan Kennedy
uses those devices for his equalizer. He'll probably give you a good
steer.

The PGA2500 digitally controlled mic input chip that Graham (I think)
suggested might be a good bet though. This is what Mackie is using in
the mic input card for their new digital console.

--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Pooh Bear wrote:
> Bob Cain wrote:
>
>> What would be the most cost/performance effective way today
>> using electronics to achieve continuous trim for multiple
>> pre channels using a single control and having tight
>> tracking? Ganged, rotary switched resistors not of interest.
>
> Just in case you missed it earlier, provided you can handle a simple
> microprocessor control interface - you simply won't beat this.
>
> http/focus.ti.com/docs/prod/folders/print/pga2500.html
>
> Fully Differential Input-to-Output Architecture
> Digitally Controlled Gain Using Serial Port Interface:
> Gain Range: 10dB through 65dB, 1dB per step
> Unity (0dB) Gain Setting via Serial Port or Dedicated Control Pin
> Dynamic Performance:
> Equivalent Noise with ZS = 150 and Gain = 30dB: -128dBu
> Total Harmonic Distortion plus Noise (THD+N) with Gain = 30dB: 0.0004%
> Zero Crossing Detection Minimizes Audible Artifacts when Gain
> Switching Integrated DC Servo Minimizes Output Offset Voltage
> Common-Mode Servo Improves CMRR
> Four-Wire Serial Control Port Interface:
> Simple Interface to Microprocessor or DSP Serial Ports
> Supports Daisy-Chaining of Multiple PGA2500 Devices
> Dedicated Input Pin for Selecting Unity Gain
> Overload Output Pin Provides Clipping Indication
> Four General-Purpose Digital Output Pins
> Requires ±5V Power Supplies
> Available in an SSOP-28 Package
> APPLICATIONS
> Microphone Preamplifiers and Mixers
> Digital Mixers and Recorders
>
>
> I currently have an eval board on loan from TI. It looks like an
> impressive part. Designed by the Burr-Brown division of course.
>

This is a very impressive part. I didn't know they existed. I find the
distortion going up from 0.0004% to 0.03% as the output signal goes down
from 3V to 30mv at fixed gain a little odd.

From a practical point of view, its designed to be used with an A/D, so
as long as its distortion is consistent with the raise of distortion in
the AD with lower signals, net specs will be ok. However, I don't know
the trade of that dictated such a state of affairs. Most of the internal
amps could be in class A avoiding potential class ab issues.

Kevin Aylward
informationEXTRACT@anasoft.co.uk
http/www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Bob Cain <arcane@arcanemethods.com> wrote:
>
>What would be the most cost/performance effective way today
>using electronics to achieve continuous trim for multiple
>pre channels using a single control and having tight
>tracking? Ganged, rotary switched resistors not of interest.

If it has to sound good, the ganged resistors are definitely the way
to go.

If it doesn't have to sound good, there are a whole lot of CMOS based
digital stepped controls. I'm not sure I would risk putting one into
a feedback loop, but you could try it. Check the Maxim Semiconductor
catalogue out for a bunch of them.

I'm curious what the RNP does for digital gain control. Whatever it is,
it can't be all that bad.

Of course, you could just take the Gordon approach and do away with the
trim completely. You don't get anything approaching continuous gain
control, though.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Joe Sensor <crabcakes@emagic.net> wrote:
>Bob Cain wrote:
>>
>> What would be the most cost/performance effective way today using
>> electronics to achieve continuous trim for multiple pre channels using a
>> single control and having tight tracking? Ganged, rotary switched
>> resistors not of interest.
>
>Bob, what would be the point of this?

5.1 mike arrays are where I'd first want something like that.

I'd forgo the continuous trim, though and take the ganged resistor
route. But I'm obsessed with keeping things out of the signal path.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Ben Bradley <ben_nospam_bradley@frontiernet.net> wrote:
>
> If you want a pure analog solution, there's the idea of the
>front-panel knob adjusting vanes between small lamps or LED's and
>photocells, sort of a multichannel Morley volume pedal behind a front
>panel. But that could have 'issues', it might need manual calibration.

If it's anything like the Lumiten, it will require manual calibration
every five minutes. Hopefully modern LEDs would improve the stability
a lot, though.
--scott


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

 

[mark]

Archived from groups: rec.audio.pro (More info?)

Bob,
you might find some Analog Devices parts useful, such as the AD604
variable gain amplifer or another part in the series or the AD5220
digital pot or another part in that series.

(financial disclosure: I own stock in AD)

Good luck.

Mark

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Scott Dorsey wrote:
> Bob Cain <arcane@arcanemethods.com> wrote:
>>
>> What would be the most cost/performance effective way today
>> using electronics to achieve continuous trim for multiple
>> pre channels using a single control and having tight
>> tracking? Ganged, rotary switched resistors not of interest.
>
> If it has to sound good, the ganged resistors are definitely the way
> to go.

Not really. There are more optimum solutions.

>
> If it doesn't have to sound good, there are a whole lot of CMOS based
> digital stepped controls. I'm not sure I would risk putting one into
> a feedback loop,

This is because you are clueless about analogue design. At least you
know your limitations.

>but you could try it. Check the Maxim Semiconductor
> catalogue out for a bunch of them.

Oh dear...cmos pots can easily perform as an audibly straight piece of
wire with gain in the appropriate circuit.

If resister strings are tapped into the feedback negative point via cmos
switches, the distortion of the switches are essentially zero. The
negative input feedback point is high impedance, so essentially, there
is zero volts dropped across the cmos switch as the current through them
is essentially, zero. The feedback ratio is simply due to real
resisters, which is linear.


Kevin Aylward
informationEXTRACT@anasoft.co.uk
http/www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Bob Cain wrote:

> Pooh Bear wrote:
>
> > If you can live with 1dB steps TI's PGA2500. Rather a good mic amp in
> > its own right. Gain is controlled by an SPI interface.
>
> _Very_ nice part, thanks. I'd like to control it with a
> pot, though, and that will take a little thought. Maybe
> there's a PIC that can do the job with a single chip.
>
> Definitely a go-to part for a pre/ADC.

What I would do is to use a simple microcontroller ( I prefer the 8031
family myself ) and use a rotary encoder - as opposed to a pot - to adjust
gain. Would be dead easy to include a readout of gain on a couple of 7
segment displays too.


Graham

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Pooh Bear wrote:
>
> Bob Cain wrote:
>
>
>>Pooh Bear wrote:
>>
>>
>>>If you can live with 1dB steps TI's PGA2500. Rather a good mic amp in
>>>its own right. Gain is controlled by an SPI interface.
>>
>>_Very_ nice part, thanks. I'd like to control it with a
>>pot, though, and that will take a little thought. Maybe
>>there's a PIC that can do the job with a single chip.
>>
>>Definitely a go-to part for a pre/ADC.
>
>
> What I would do is to use a simple microcontroller ( I prefer the 8031
> family myself ) and use a rotary encoder - as opposed to a pot - to adjust
> gain. Would be dead easy to include a readout of gain on a couple of 7
> segment displays too.

I like it! I was just looking at the dsPIC but that's
overkill even tho they're cheap. It would be nice, however,
to have some reserve capability and bond with what looks to
be a very interesting part. I assume the 8031 could
generate the required select and serial control stream to
the PGA2500 without any jelly beans.

Are shaft encoders Gray code or base 2? OST, the 8031 could
do away with any possible intermediate states that would
cause glitches.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Bob Cain wrote:

> Pooh Bear wrote:
> >
> > Bob Cain wrote:
> >
> >
> >>Pooh Bear wrote:
> >>
> >>
> >>>If you can live with 1dB steps TI's PGA2500. Rather a good mic amp in
> >>>its own right. Gain is controlled by an SPI interface.
> >>
> >>_Very_ nice part, thanks. I'd like to control it with a
> >>pot, though, and that will take a little thought. Maybe
> >>there's a PIC that can do the job with a single chip.
> >>
> >>Definitely a go-to part for a pre/ADC.
> >
> >
> > What I would do is to use a simple microcontroller ( I prefer the 8031
> > family myself ) and use a rotary encoder - as opposed to a pot - to adjust
> > gain. Would be dead easy to include a readout of gain on a couple of 7
> > segment displays too.
>
> I like it! I was just looking at the dsPIC but that's
> overkill even tho they're cheap. It would be nice, however,
> to have some reserve capability and bond with what looks to
> be a very interesting part. I assume the 8031 could
> generate the required select and serial control stream to
> the PGA2500 without any jelly beans.

Absolutely. The only bits you'd need are a few pull up Rs and the like.
Everything including the LED drivers would fit into an 87C51 ( internal Eprom )
nicely.

> Are shaft encoders Gray code or base 2? OST, the 8031 could
> do away with any possible intermediate states that would
> cause glitches.

The encoders I'm familar with ( the cheap and cheerful consumer types ) have
'quadrature' outputs.

You hook one of them up to a hardware interrupt line and the interrupt reads 0
or 1 on the other output which means clockwise / counterclockwise. You need to
be careful about contact bounce of course.

It would make sense also to have a tiny serial EEprom to store gain data so that
the unit powers up with the same gain in effect as when it was powered off.

Graham

 

[Guest]

Archived from groups: rec.audio.pro (More info?)

Very simple, been doing it for years, in a variety of formats including
PIC and 80x1 implementations.

The TI should do a reasonable job, but it looks like it's sort of
strangled by noise, or at least that's what I see in those graphs of
gain vs noise. Not front end noise, but digital noise getting
into the system.

I know you can do better, but not easier. It'll sound like an SSM-2017
or any other variation of hybrid instrumentation amp done passably well.

Clean, edgy, gutless, and lifeless.






Pooh Bear wrote:
> Bob Cain wrote:
>
>
>>Pooh Bear wrote:
>>
>>>Bob Cain wrote:
>>>
>>>
>>>
>>>>Pooh Bear wrote:
>>>>
>>>>
>>>>
>>>>>If you can live with 1dB steps TI's PGA2500. Rather a good mic amp in
>>>>>its own right. Gain is controlled by an SPI interface.
>>>>
>>>>_Very_ nice part, thanks. I'd like to control it with a
>>>>pot, though, and that will take a little thought. Maybe
>>>>there's a PIC that can do the job with a single chip.
>>>>
>>>>Definitely a go-to part for a pre/ADC.
>>>
>>>
>>>What I would do is to use a simple microcontroller ( I prefer the 8031
>>>family myself ) and use a rotary encoder - as opposed to a pot - to adjust
>>>gain. Would be dead easy to include a readout of gain on a couple of 7
>>>segment displays too.
>>
>>I like it! I was just looking at the dsPIC but that's
>>overkill even tho they're cheap. It would be nice, however,
>>to have some reserve capability and bond with what looks to
>>be a very interesting part. I assume the 8031 could
>>generate the required select and serial control stream to
>>the PGA2500 without any jelly beans.
>
>
> Absolutely. The only bits you'd need are a few pull up Rs and the like.
> Everything including the LED drivers would fit into an 87C51 ( internal Eprom )
> nicely.
>
>
>>Are shaft encoders Gray code or base 2? OST, the 8031 could
>>do away with any possible intermediate states that would
>>cause glitches.
>
>
> The encoders I'm familar with ( the cheap and cheerful consumer types ) have
> 'quadrature' outputs.
>
> You hook one of them up to a hardware interrupt line and the interrupt reads 0
> or 1 on the other output which means clockwise / counterclockwise. You need to
> be careful about contact bounce of course.
>
> It would make sense also to have a tiny serial EEprom to store gain data so that
> the unit powers up with the same gain in effect as when it was powered off.
>
> Graham
>