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Frantically Seeking VCA

Discussion in 'Electronic Design' started by Norm Dresner, Jun 27, 2004.

  1. Norm Dresner

    Norm Dresner Guest

    Several (but not too many) months ago I saw a schematic for a voltage
    controlled amplifier (VCA) circuit that claimed to have very low distortion
    in the audio range (~.01% or less). It's my vague recollection that it was
    in a magazine but it may have been on a web-site too. It's also my very
    vague recollection that it might have been by Bob Pease of NS.

    All pointers, hints, suggestions, recollections, URLS, etc graciously
    accepted.

    TIA
    Norm
     
  2. Joerg

    Joerg Guest

    Hi Norm,

    Sorry, don't know about this one but Bob Pease responds to email.

    There may be one interesting device to look at: Analog Devices AD603.
    It's for RF but might be useful at audio. This chips does 40dB range
    which can be controlled in a dB-linear fashion via an input voltage. The
    downside for audio is that you probably need a follower up front as it
    has a 100 ohms input impedance.

    I designed lots of amps with this chip and it always did what I wanted
    it to.

    Regards, Joerg
     
  3. Max Hauser

    Max Hauser Guest

    There are not really very many ways, fundamentally, to do it at that level
    of linearity.

    If the actual gains you want are switched rather than fully continuous, that
    helps a lot.

    Linear Technology has a continuous "video fader" product and some digitally
    set CMOS PGAs of good linearity.
     
  4. Mike Diack

    Mike Diack Guest

    www.thatcorp.com
     
  5. Ben Bradley

    Ben Bradley Guest

    Well, it certainly wasn't a CA3080 or CA3280 based circuit, was it?
    :)

    Was it basically some chip, or did it involve discrete components
    (other than the usual resistors and caps, and maybe an opamp on the
    input and output?)
    That's all of course "pretty vague" and I haven't seen anything
    like that lately...
    There's the chips by THAT corp, http://thatcorp.com/ which seems t
    make some high-end products in this area, and also Analog Devices
    (http://analog.com) still makes some VCA's they got with the SSM line.
    I think at least some of these should have specs like that.
     
  6. Norm Dresner

    Norm Dresner Guest

    It's my recollection that it involved an Op Amp or two and a bunch of
    discrete transistors and the usual passive stuff.
     
  7. For the 0.01% or less distortion level, the variable transconductance
    scheme with BJTs isn't going to be very successful. The 'thatcorp'
    technology (or the approx equvalent SSM stuff from AD) can come close
    to 0.01%, perhaps exceeding it. (It is probably possible
    to cancel the distortion for the simple variable gm schemes, but it would
    be difficult to cancel the distortion as well as a thatcorp device
    might achieve.) The Analog Devices SSM2018 might/might-not be
    readily available... The Thatcorp THAT2180 is good for audio
    applications, but might not meet your 0.01% in real world
    conditions, even though it's specs are pretty good. (Most all of the
    variable gain BJT devices utilize the log characteristics of the
    device, but there is a difference in peformance based upon circuit
    topology.)

    Likely, the most successful scheme for incredibly low distortion
    over a very wide bandwidth would be a switched resistor thing (similar
    to a D/A), and Analog Devices has a device targeted to IF AGC applications,
    but I don't know if it will work well for audio (with low 1/F noise.) There
    might be some problems in audio if the AGC device doesn't have fine
    enough gain changes. (AD8334 is an example of the 'switched resistor'
    scheme -- I don't know the technical name for it.)

    My guess is that the most likely successful device for audio would
    be a selected (high quality) thatcorp component. In medium production
    quantities, it is about $5.00, and a discrete implementation with
    such high performance would be 'challenging' :). Even though Analog
    Devices also has similar component families, their availability has
    been questionable.

    John
     
  8. Given a few OpAmps are used in conjunction with 'discrete' BJTs,
    you'd likely need to use well matched pairs (e.g. Analog Devices
    MAT01, MAT03, MAT04 or ThatCorp matched transistors), and it would
    take some serious design, debugging and verification to replicate
    the performance of the Thatcorp VGA devices.

    If possible, it is higher quality to do the signal processing with
    a DSP or other kind of CPU (assuming that you have the signal in
    a digitized form, and you aren't designing a gain control element
    for guaranteeing signal levels before A/D conversion.) Of course,
    if you are designing a last chance peak limiter before an A/D conversion
    (trading some peak limiting AGC aciton for the more obnoxious clipping),
    then there is little that can be done in software.

    In my experiments, I have found that the flexibility of doing audio
    signal processing in software can help to better maintain quality,
    to provide more degrees of freedom, and provide more precise performance...

    John
     
  9. Paul Burke

    Paul Burke Guest

    The old PMI apps book had a design based on the MAT-04 and OP27. I used
    it years ago, and it worked very well. Maybe AD has the design lying
    around somewhere.

    Paul Burke
     
  10. Norm Dresner

    Norm Dresner Guest

    I've already e-mailed THAT asking about a small-quantity buy since their
    2180 looks quite promising.

    Norm
     
  11. Norm Dresner

    Norm Dresner Guest

    Now that's going to take some serious digging to find the book but it should
    be interesting.
     
  12. Norm Dresner

    Norm Dresner Guest

    Hey, I found it. In the 1990 PMI Analog Databook, p 9-41. It _only_ uses a
    MAT-04, an OP-27, and a pair of OP-41's. Oh, yeah, and 1% metal film
    resistors and a 100 uF non-polarized tantalum cap. The writeup says it
    shoudl have "a distortion level of 0.03% over a wide range of control
    voltages."

    The THAT2180 is looking quite good.

    Thanks

    Norm
     
  13. Norm Dresner

    Norm Dresner Guest

    Just another thought: The MAT-04 is four NPN transistors in a single
    package. Does anyone know about the matching for an MPQ2222? It's got to
    be cheaper and more readily available. Not that I'd seriously do this
    much work to replace a ~$5-10 IC, but there's the curiosity in my
    engineering side that's pushing thru.

    Norm
     

  14. I talked with them recently; their minimum buy is one tube, which is either
    25 DIPs or around 100 (if I remember correctly, which I probably don't)
    SMD's.

    They do say that "there may be some unauthorized folks selling smaller
    quantities." I have been considering ordering a tube of these myself, for a
    project I'm cooking up, so perhaps I might become one of those unauthorized
    folks if you need :)

    However, I was thinking of ordering the B grade, which is .02%, not the more
    expensive .01% grade. And I'd be getting DIPs, to prototype with. If you
    need A-grade SMD's, plan on spending a chunk of money...
     
  15. Norm Dresner

    Norm Dresner Guest

    I got pretty much the same e-mail from them explaining that 1 tube of 25 was
    the minimum. But it appears that they're available in 2's and 5's from
    http://www.profusionplc.com/cgi-bin/gex/pcatdtl?ipartno=THAT2180LB

    They're in the UK. Since my son lives there [but is here visiting right
    now] I'll probably give him a few bucks to buy some parts and ship them back
    to me. The A-version is seeing for 10.6GBP in 2's and 8.5GBP in 5's while
    the B-version is 6.0GBP for 2's and 4.9GBP for 5's.

    Norm
     
  16. John Dyson

    John Dyson Guest

    I don't think that you'll see the desired matching from devices like the
    MPQ2222. There is no specficiation for matching, and the component
    might even be built from multiple chips. Even if it is built from a single
    chip, the matching could be +-25mv or worse, which is of little use for
    you.

    This 'Gain Control' amplifier thing is ugly, no matter how it is done. It
    is fortunate that matching and component characteristics nowadays can
    be impressive.

    When looking at old tube designs (or even old FET designs), they tend to
    be quite ugly, and amazing that they had performed as well as they did.
    Early on, I had even seen the use of the dynamic resistance of diodes (and
    using ad-hoc techinques to remove the thump.)

    If absolute precision isn't needed, and super fast attack/release times
    aren't needed, and some drift is allowed, then the Lamp(LED) along
    with resistive photocell scheme can be quite nice.

    The technology today can be very repeatable and very precise. Such
    approaches (like the That stuff, or some of the SSM stuff from AD) do
    as well as someone can build.

    Except for input AGC (before the 18-20bit A/D converter), I prefer
    using DSP techniques for gain control. For before the A/D converter, then
    all I'd tend to use would be a peak limiter which would avoid harsh
    distortion
    on peaks. With the large bit depth, then the recording can be done with
    plenty of headroom.

    Nowadays, my approach would be (in order of preference, but also with
    prejudice): (For audio)

    1) DSP!!!
    2) Gilbert or That gain control element.
    2A) Switched resistor scheme??? ( I don't know how well it might work.)
    3) Photo resistor
    4) FET
    5) Dynamic resistance or simple variable gm method.

    The photo resistor can be done with very low distortion, but might
    have more limited lifetime and more drift. The FET is mostly just
    useful for peak limiting, but matching across channels across a wide
    gain control range isn't very good.

    A WORST approach is to use the dynamic reisstance of a diode or simple
    gm change for a transistor. The linearity will tend to suck badly (without
    some compensation.) By the time that one would apply excellent distortion
    compensation, then the Gilbert or That gain control cell would be best.

    John
     
  17. Ian Buckner

    Ian Buckner Guest

    I've seen an AGC loop/variable gain done with an LED illuminating a
    matched pair of photoresistors. One of them was connected in a DC
    (no signal) loop, used to set the LED current to force that resistance
    to the required value. The match between the resistors provides a
    reasonably well controlled and very linear gain change in the signal
    path. Obviously, this only works well over the range of resistance
    where the pair are well matched. You also have to be wary of the
    response time variations of LDRs at different illumination levels.

    Regards
    Ian
     

  18. That's right, I'd forgotten about ProfusionPLC.

    No need to enlist your son; Profusion will ship directly to the US, and they
    are responsive to web and email orders. ISTR that the last time I ordered
    from them (some power audio MOSFETs) there was actually NO shipping cost (!)
    and the goods arrived faster than they generally do within the US. I was a
    very satisfied customer.
     
  19. ddwyer

    ddwyer Guest

    There is a led driven photoresistor inthe Farnell catalog.
    The variable gain amp can be considered as a multiplier with an
    amplifier, the LTC part is expensive, an op amp + led/photoresistor is
    low cost.
    Precision is not required low distortion is usually the critical
    parameter.
    Separate the level detection fronm the variable gain element solves the
    proble for many applications.
     
  20. It is true that in many applications, 'precision' isn't incredibly
    important, and distortion does tend to be much more important, however
    there are some gain control applications where sub-dB tracking between
    channels is important. Tracking tends to be easier over narrow gain
    control ranges (e.g. 10-20dB) vs. very wide ranges that might have been
    useful for DBX noise reduction type applications. The Gilbert/That
    type approaches allow for excellent tracking.

    When using gain control elements that have 'excellent' tracking, then
    there might be more topologies for practical and useful designs. For
    example, (using a FET gain control), the feedback topologies are easier
    to deal with than the feedforward designs. For the very repeatable
    Gilbert or 'That' designs, the feedfoward topologies become less
    troublesome. (Feedforward topologies don't provide the self-correcting
    abilities.)

    I still strongly suggest using DSP techniques if possible. There
    are numerous opportunities for mitigating the effects of gain modulation
    and much easier implementation of dynamic attack/decay times.

    One of my experimental methods uses a running FFT with an appropriate
    windowing function. (This is very easy to do, but there are some design
    considerations that aren't initially obvious, or there will be alot
    of glitches.) The FFT method can do some amazing gain control, and a
    small amount of compression (or expansion) can result in near optimal
    results. Using the FFT approach is like having a 32 to 1024 band
    compressor (or expander.)

    John
     
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