1. Things you need to know about the new ‘Conversations’ PM system:

    a) DO NOT REPLY TO THE NOTIFICATION EMAIL! I get them, not the intended recipient. I get a lot of them and I do not want them! It is just a notification, log into the site and reply from there.

    b) To delete old conversations use the ‘Leave conversation’ option. This is just delete by another name.
    Dismiss Notice

A speaking schematic-McBride

Discussion in 'd.i.y.' started by Retro, Jan 14, 2008.

  1. Retro

    Retro pfm Member

    Hi All,

    I've been using Mcbride 135 clones for about two years and I am delighted with them. I have a reasonable ammount of technical knowledge, I built them and got them running, but I can't say I really understand how the amp circuit works.
    Is there anyone of this forum who would be prepared to talk us all through the schematic, block by block or per transistor say, so we can all appreciate what the circuit does. I have attached a link to Neil's PDF schematic below.


  2. stevec67

    stevec67 pfm Member

    Your link doesn't work. I've found it anyway, and I can work out some of the basics.

    You've got a pair of Darlington paired transistors, in classical fashion, driving the outputs. (Q9-Q11 and Q10-12). One pair drives the positive bits of the wave, the other pair the negative bits of the wave. As you can see, the whole circuit is divided into a "negative half" and a "positive half", hence the 38V-0-38V power rails. Darlington pairs are characterised by one small transistor controlling/switching base voltage and current to a larger power transistor, usually these last are in a steel can on a heatsink.

    The 2 halves are set up with different value resistors (R16 cf.R18) so I imagine this means there will be a difference in gain between the 2 halves, by design. This will be corrected by the network around Q6 which has a feedback loop and a pot, presumably this is for setting up bias so both halves have the same gain and hence volume. This will I imagine be necessary to account for batch variation in trannies. Q4 and Q5 control the base voltage to the 2 darlington pairs. You can see one is a PNP and one an NPN to account for the difference in polarity, as for the Darlingtons.

    Q1,2 and 3 are the input transistors and I'm not sure how that part of the circuit works, sorry. I can see what happens when the IP goes +ve but not -ve. Someone more knowledgeable will be along soon.

    Caveat:I'm not a hifi designer and this is really basic stuff dimly remembered from years ago so if any of it is rubbish, let me know (politely!). Also if you knew all of this already and more, and you are thinking "why is this idiot telling me how to suck eggs?" feel free to ignore this post.
  3. LesW

    LesW Retired at user request

    You're doing great Steve, all credit to you.

    The input transistors Q1, Q2 are in what is known as a 'long tail pair' or differential amplifier configuration. They're supplied with a constant current source Q3 to ensure immunity from any perturbations (don't you just love that word) appearing on the supply rails. Q1 is the amplifying element here because Q2 acts as a brake or throttle control for Q1 by virtue of the feedback from the output transistors applied via C7, R9, R10.

    Simply put, the more Q2 amplifies the feedback, the less Q1 amplifies the incoming signal.

    OK so far Graham..??
  4. Paul R

    Paul R pfm Member

    McBride schematic here.

  5. Paul R

    Paul R pfm Member

    I would start with the knowledge that

    A diode in its working state always has about 0.6v between its anode and cathode.

    A transistor in its working state always has about 0.6v between its base and emitter.

    The current flowing into/out of the base is very much less than that flowing into/out of the collector.

    Ohms law is V=IR, I=V/R.

    This pretty much allows you to work out the currents/voltages at many places in the circuit before you start applying signal. First base, as it were. You can work out how Q3 and Q4 work as more or less constant current sinks/sources. What the current flowing through R4 must be, what Q6 does and consequently how the voltage between the emitter of Q11 and the collector of Q12 is controlled, which leads you to the bias current.

    I strongly suggest investing in a book, Horowitz and Hill might be old but it's excellent. A bit expensive though, with a hint of a third edition, perhaps others have more economic suggestions?

  6. trancera

    trancera pfm Member

    Something in between complete numpties r us and that book would be great, I have tried another myself and it was too simplistic. One of my colleagues does refer to the Hor/Hill as the bible. I think it is / has been a standard Uni text for years.
  7. david ellwood

    david ellwood Kirabosi Kognoscente

    horowicz and hill is indeed the bible of electronics.

    for amplifier research i would recommend doug selfs audio power amplifier book.

    Although biased and full of stuff I would disagree with it is a good place to start and explains many of the basics.
  8. PigletsDad

    PigletsDad pfm Member

    Another vote for Horowicz and Hill. It isn't too advanced, but covers all the stuff that matters. The final couple of chapters about digital systems are completely out of date, so ignore those.
  9. AKprentice

    AKprentice pfm Member

  10. Si-P

    Si-P pfm Member

    Horowitz and Hill is still the best written text on the subject, as far as I am aware.
    Extra reading (taken with a pinch in places) can be found by Doug Self and John Linsley Hood.
    Those last 2 authors are not strictly objective in their writing though!
  11. LPSpinner

    LPSpinner pfm Member

    Hi guys:

    The McBride 135 clones is a pretty common circuit and I think to call it a naim 135 seems to suggest that it is a naim design when we all know that naim just lifted the circuit out of an applications handbook.

    For what it is worth though I have three questions regarding the “McBride 135 clone” circuit.

    1: Why are people still using a quasi-complementary output stages. Modern PNP power transistors are just as capable and robust as their NPN complimentary partner so why avoid using them? A fully symmetrical emitter follower output should provide the lowest possible distortion, lowest output impedance and you get a much more cleanly defined and symmetrical crossover region which makes biasing the output stage much more predictable.

    2: Why is the differential input pair upside down? Modern convention normally puts the curent source on the positive rail and the tails toward the negative rail. Just curious on this one … I’m not trying to be a smart @rse; really.

    3: Modern design practices these days would also place curent mirrors on the differential input pair tails. I think this would make a big difference as it’s the job of the input pair to integrate (I know - bad choice of words) the input signal with the negative feed back loop.

    What are your thoughts?

    Also, has any one built the Doug Self designed amplifier circuit described in has amplifier discourse on the main sources of amplifier distortion? I know he was intending to release PCB’s to the DIY community but I haven’t seen any thing yet.

  12. MartinC

    MartinC pfm Member

    LP Spinner,

    Can't answer most of your questions (1-3) but I did build a amp kit sold by Randy Sloane that was I believe designed around Doug Self's principles. The amp performed well and replaced by 135 clones. Rany used to sell kits but unfortunately he and his designs got snapped up by a commercial amp company and therefore the DIY kits went off the market.

  13. Si-P

    Si-P pfm Member

    I know that Julian Vereker (founder and md of naim until he died) said that a pair of complimentary NPN and PNP transisters were only as similar to each other as a man and woman of roughly the same height and build.

    What he was trying to say is that npn transistors and pnp transistors have different characteristics and so each half of the waveform ends up being amplified differently. Keeping the output devices the same type means both halves of the waveform are amplified in the same way and therefore sounds better. At least, that's why naim never changed it, i do believe. As for mcbride I haven't a clue.

    For q's 2 and 3 I don't know. I don't think it really matters but you could try the turning the ltp upside down and adding mirrors. I don't know if it'll sound a lot different though! I'm sure there'll be someone along who knows!
  14. LPSpinner

    LPSpinner pfm Member

    The Problem with the Quasi-complementary is that the positive half is acting as emitter follower output stage and the negative half is acting as if it was a “Sziklai pair”. Both type of output stages offer very different transfer functions and would result in greater non-symmetry than any variation in NPN and a complementary PNP. Also modern complementary pairs are actually very well matched. To my eyes, for a modern circuit I would definitely be looking at a fully symmetrical emitter follower stage.

    Looking at Douglas Self’s papers he goes to great lengths to prove via spice circuit simulation that adding current mirrors as well as using a current source to a differential input pair you reduce the second harmonic distortion to very low levels. Adding curent mirrors will also help to minimise any DC offset at the output stage as well. It is important to remember that the differential input pair is actually outside the global feed back loop so it must introduce a minimum of distortion of it own since it cannot be corrected by the NFB loop.

    Another observation I have come across while I have been comparing amplifier designs. Many of the better amplifiers actually use a second differential pair for the VAS stage, has anybody got any Ideas on that one… Just curious.

  15. Si-P

    Si-P pfm Member

    Good points LPSpinner. Obviously I wasn't saying Mr V was right or wrong. I was just trying to answer the question of why it's like that and naim stuck with it.

    As for current mirrors, yes you are right. Sorry, I didn't think about that question properly before answering. What effect would turning the lpt upside down have, do you think? I'm completely blank on that one!
  16. PigletsDad

    PigletsDad pfm Member

    There are several reasons for doing this. From my point of view as a designer, the big advantage is the improvement in Power Supply Rejection Ratio(PSRR) that it gives - rather than just measuring the voltage across a resistor in one leg of the LTP, you put equal resistors in both legs, and measure the difference. This cancels lots effects, like the change in transistor current with collector voltage (the Early effect), as it is the same in both transistors.

    As well as PSRR it improves the Common Mode Rejection Ratio(CMRR), hence reducing CM related distortions. It also improves the VAS linearity, and cancels out most of the assymetry in the second stage response.

    A common choice when using a differential second stage is to use a current mirror on the opposite supply rail. This has the effect of doubling the available AC current drive for a given standing current in the VAS. This is sometimes called the "Hitachi circuit", as it was published in a Hitachi application note when they first started selling power MOSFETS, and lots and lots of amps used that circuit in the 1980s.
  17. PigletsDad

    PigletsDad pfm Member

    The question of polarity for the LTP is kind of minor. Some circuits use a fully complementary design with both NPN and PNP pairs.

    Once upon a time, in the early/mid 1960s, PNP devices were significantly worse than NPN. This is when the Mullard book that JV got his circuits out of was published.

    Some use NPN input devices, some PNP, and frankly the differences are kind of small. NPN devices tend to have slightly lower capacitances; PNP may have slightly lower voltage noise. The Early voltage in NPN transistors is higher than for PNP in most cases.

    Naim clearly think that PNP devices are the work of the devil and try to use NPN as much as they can, probably based on heritage from the situation in the 1960s! :)
  18. Geesixty

    Geesixty Well-Known Member

    I find a good in-between book to be 'A practical introduction to electronic circuits' by Martin Hartley Jones, a kind of miniature, summarised version of Hor/Hill. I bought a copy of Self's book on PA design last year but find it heavy going in places for someone like me - an 'enthusiast' (numpty?) rather than engineer.
  19. LPSpinner

    LPSpinner pfm Member

    Thanks for the response Pigletsdad, (and the other guys as well) the circuits I saw using the differential VAS were at the higher end of the published amplifier kit designs. One notable design was the Tilbrook ETI 6800 series (I’m showing my Age and my antipodean heritage now), so it all makes sense. Now that you remind me I do remember Doug Self describing it one of his articles on amplifier design but he dismissed it on the grounds of cost and unnecessary complexity.

    On NPN vs. PNP thing, yes the 60’s and 70’s were a bad time for PNP transistor manufacture. I suppose that’s also why single ended and single rail - capacitor coupled output stages like the Quad 303 ruled the waves back then when a modern symmetrical design offers so much more performance.

    Thanks: LPSPinner.
  20. stevec67

    stevec67 pfm Member

    "I find a good in-between book to be 'A practical introduction to electronic circuits' by Martin Hartley Jones, a kind of miniature, summarised version of Hor/Hill"

    That's the one I remember! It's very good and more accessible than H&H. I had a copy in the 80s when I was studying O Electronics and A Physics, I think it now resides in the loft at my parents', I have been meaning to dig it out for a while and brush up my knowledge. I didn't read the section on amplifiers at the time, I was into logic which seemed easier and featured more heavily in the syllabus. I should now revisit the thing.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice