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Mystery amp schematic - assistance/comments/guesses/insights welcomed

Discussion in 'd.i.y.' started by bugbear, Aug 19, 2022.

  1. bugbear

    bugbear pfm Member

    Well, I've traced the power amp section of my Badger 30 Watt amp, at least as well a I can.

    I've got pretty good photos of the component and track sides, and I've got them nicely lined up, which is "helpful".

    [​IMG]Badgering by plybench, on Flickr

    (I've got more of the components labelled since I uploaded that image).

    It was more difficult than I hoped to trace the circuit, mainly because some aspects of it were quite unexpected. So here's the schematic, at least as accurate as I can get it. Faults are likely.

    [​IMG]badger by plybench, on Flickr

    It appears to follow the basic ideas shown on this website:

    https://www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_7

    In particular, figure 9 (quasi-complementary output stage), and figure 19 (PNP long tailed pair differential input stage).

    However, the actual circuit has several quirks (by which I mean things I have never seen before and/or don't understand)

    Moving from right to left:

    1) The diode bias chain has 2 (not 3) diodes. This isn't enough. Did they want cross over distortion?
    2) The feedback resistor/divider chain is sandwiched between 2 47µF capacitors, connecting +v power to earth. Is this some kind of frequency filtering?
    3) is the diode-180Ω-47µF on the left a power conditioner for the LTP?
    4) The filters (capacitor circuits) on the input stage are connected to earth; but the LTP and subsequent driver transistor connect to right down to -Ve power, albeit via a 100Ω resistor.
     
  2. S-Man

    S-Man Kinkless Tetrode Admirer

    Possible errors:
    Emitter of the VAS should go the neg rail (along with the 100R)
    There ought to be a 100R resistor in the BRR61 emitter circuit. A Baxendal diode might help as well.

    Replies:
    1) You will have to measure all the voltages to check, but seems like it's class B.
    2) No. The top one is a bootstrap cap to provide a CCS for the VAS. The lower one is the feedback cap for unity gain at DC.
    3) Yes, basic filter for the 12K that sets the tail current.
    4) Seems OK

    This is a very! basic amp.
     
  3. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    A crude generic amp similar to Naim.
     
    narabdela and Paul McDonough like this.
  4. bugbear

    bugbear pfm Member

    Thanks for that.

    Re: your Baxendal comment; the board is drilled and tracked for 2 components in series from between the 2 diodes in the bias chain to the base of the BFR61. There are no components fitted to these tracks.

    At the risk of revealing my ignorance, which device are you calling "VAS" ?

    (and I should have given the active devices names as well as part nos)
     
  5. PigletsDad

    PigletsDad My intelligence test came back negative.

    VAS Voltage Amplifier stage - the final voltage gain stage. The BFT82 in the lower middle. The RCR network at the top is bootstrap load for the gain stage - the 47uF cap keeps the top of the 2K7 resistor a constant voltage above the output (for higher frequencies, any how), so the incremental impedance of becomes high, increasing the gain.

    Using two diodes makes it mildly underbiased, but thermally stable with no adjustments. Probably sounds bright at low levels.

    The diode / cap network at the left gets the input stage running early on in startup, to reduce the turn on thump - I have been simulating exactly that in the Keeping Amused thread.
     
  6. bugbear

    bugbear pfm Member

    Full class B might explain how a 30watt amp gets by with such miniscule heatsinks!!

    I've re-checked my scans of the board. Working upwards from the 0v track, there's a 100Ω resistor, and then the 100Ω resistor connects to:

    680Ω resistor
    Collector of feedback side transistor
    emitter of VAS

    (to be explicit, all wire crossings in the diagram are connected, where they're not I've drawn a little "skip")


    Starting to sound like a well thought out thing, with "hi-fi defects" becoming "guitar features"!
     
  7. bugbear

    bugbear pfm Member

    Amp back together (outside case). Hums a lot, spits a lot, I'm trying to clean the tracks of the potentiometer.

    Poking around with my toy oscilloscope, I can see 300mV of ripple on 1 PSU cap, and 200mV on the other, with no input (or output), DC PSU level is around 35V

    Is that normal/OK ? I don't have a LCR meter to directly check the caps.
     
  8. Paul R

    Paul R pfm Member

    I tried to do the maths but head's a bit fuzzy...

    Given a draw of 10mA over a time of 1/100s (because 'full bridge rectifier') the cap would drop about 0.1v between charges. So your 200mv implies 20mA. Which seems plausible. The drop is a small fraction of the initial voltage, so assumptions etc.
     
  9. bugbear

    bugbear pfm Member

    Thanks for that. Maths is the answer. Who'da thunk it.

    Further probing with the toy scope shows that the pre-amp/tone section is feeding up to 36mV of AC signal into the power amp ... with NO INPUT signal applied to the pre-amp.

    I guess that's where the unpleasant noise (a sort of gritty mains hum) is coming from.
     
    a.palfreyman likes this.
  10. martin clark

    martin clark pinko bodger

    Yebbut it's a guitar amp: that sort of thing (amp hum, pops, squeaks, glitches ) ...makes you sound serious.

    As I think I said in your other thread about this instrument amps play by other rules.
    I have a couple of little Roland practice amps - one for guitar (for the Yamaha I've had 30years, used with a soundhole mag pickup) , and one Microcube bass (used with either fender Precision clone, or NS Wav4 upright)

    I run them both on 6x AAs, because, they have that facility (class d - a set lasts ages!): result is wonderfully silent, but also as a result , more than a bit exposed making it obvious how badly I 'play' (play being the operative word)

    Use the supplied 9v adaptors however and it's all overlaid with a little fizzy, fuzzy, a little hum - but I sound strapped and ready for action.

    S:NR can be overrated ;)
     
  11. bugbear

    bugbear pfm Member

    Yebbut - this is LOUD, worse than a Strat single coil. And nastily edgy. I think something's sneaking in somewhere - there's no shielding anywhere, and all the signal wiring is just single strand, single core.

    I'll at least semi-trace the pre amp, and see if I can spot where the "bad" signal starts.

    Much easier than tracing a fault in a power-amp with negative feedback...
     
  12. martin clark

    martin clark pinko bodger

    Just like any system - look at the front end / input stage first, for wiring (0v routing) issues etc.

    - anything wrong there, is then subject to the full gain of the PA stage. TBH its unlikely the PA stage is the main issue.
     
  13. bugbear

    bugbear pfm Member

    Agreed - my toy scope shows 36mV LEAVING the pre/tone section (with 0V going in...)

    My rough viewing of the circuit makes me think it's a "mixer" model, with the various tone circuits all merging onto an output busbar via resistors.

    The power to the pre/tone section seems to be cleaner than the power amp - since it goes via a secondary clean up circuit. Ripple = (only) 4mV.

    I need to trace the circuit a little so I know what/where to measure!
     
  14. bugbear

    bugbear pfm Member

    Component identification question. I think the little white rectangular "boxes" in the photo (top post) are capacitors.

    1 (the largest) is labelled only on its upper face: 1K100

    3 are labelled on upper face: 1K250, and on a side face:
    MMK0
    EVOX
    8243

    2 are labelled on upper face: .01M630, side face
    MMK0
    EVOX
    8230

    and finally: .22M100
    MMK0
    EVOX
    8223

    Can any ID these please?
     
  15. martin clark

    martin clark pinko bodger

    1nF/250v; 10nF/630v;and 0.22uF/100v respectively. These are polyester caps - good parts.
     
  16. bugbear

    bugbear pfm Member

    Hmm. Googling with no understanding gives:


    1K100 = 1µF, 10% tolerance, 100V
    1K250 = 1µF, 10% tolerance, 250V
    .01M630 = 0.01 µF (10nF), 20% tolerance, 630V
    .22M100 = 0.22 µF (220nF), 20% tolerance, 100V

    High voltages for tone controls!
     
  17. bugbear

    bugbear pfm Member

    Ouch. Tracing the tone circuitry is proving to be slow and painful. It turns out what little electronic knowledge I have is VERY specific to class A/B power amplifiers.
     
  18. PigletsDad

    PigletsDad My intelligence test came back negative.

    The bunch of op-amps along the bottom row are the pre-amp circuit. A simple test - try shorting the input, rather than leaving it floating. If the problem is mains pick-up on the input wire, that may drop the level a lot. If that were the problem, replacing the input signal wiring with a bit of coax would fix it.
     
  19. bugbear

    bugbear pfm Member

    The input sockets (which are the switched type) are rather nicely wired, so that if no lead is plugged in, the input wire is shorted to earth!

    I have now (for my own gratification) closed the box up, so I can "have a play". The sound is very good - the case is heavy, and the speaker is a 60W/10" McKenzie, so that helps. The tone controls are NOT what you'd find on a hifi. They are more like synthesizer filter controls - they are intended to heavily modify the sound to suit taste and style, whereas hifi tone controls are intended to correct minor faults in flatness of response (or faults with the input flatness, if you go back early enough). I very much like the parametric mid-range controls, which allow the mid-range to be a hump or a dip of any size, and to be moved up down the tone range. Nice.

    The hum/sizzle is still there, much much lower level than anything the guitar puts out. I do find myself leaving the gain on the amp down and the output of the guitar up.

    Here it is in the afternoon sun (click for full size)

    [​IMG]badger downbeat by plybench, on Flickr
     
    chiily and a.palfreyman like this.
  20. bugbear

    bugbear pfm Member

    Last edited: Sep 6, 2022

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