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Old 25th May 2004, 08:35 PM   #1
argo is offline argo  Estonia
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Default BUZ-amp

I once pulled a pair of complementary BUZ900/905 lateral MOSFETs from a scrapped subwoofer amplifier and still havenít found a good circuit to put them into. I have looked at some of the Jean-Marc PlantefŤve designs but personaly would prefer to use BJT input to JFETs. So as I am not good at all to make a design of my own, I did some copy and paste from circuits of others. Here is what I ended up. So to all circuit wizards -please help me out - is that feasible design or should better I forget it?

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Old 25th May 2004, 08:57 PM   #2
Geoff is offline Geoff  United Kingdom
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Hi Argo

Sorry, I'm not qualified to comment on the circuit you posted but how about using the BUZ in a JLH MOSFET design. There have been several published over the years that might be suitable. email me if you are interested and I will send you the schematics.

ps Did you complete the JLH Class-AB? If so I would be interested in the results as I have another constructor who is considering building this circuit.

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Old 25th May 2004, 09:04 PM   #3
ChocoHolic is offline ChocoHolic  Germany
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I never tried a similar design, but for me it looks promising.
Could result in a fast but at the same time stable design.
The output stage seems somehow close to a ZEN....
If you find the right bias... promising....

Hm, bias might be critical in reality.
In fact I am not sure if your DC bias of the output stage
will show a satisfying temperature behaviour.

If you observe issues with oscillation/ringing, then my proposals would be:
1. Make the slowest pole more dominant and more linear.
==> caps from gate to drain of the Mosfets.
2. Shift phase of the feedback signal towards earlier.
==> cap parallel to the feedback resistor.

Bye and looking forward to your results (if you go for this amp)
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Old 25th May 2004, 10:07 PM   #4
argo is offline argo  Estonia
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how good to see you here again.
I believe you are not talking about the JHL MOSFET designs published here by some forum members not long time ago. Tried these but with no great results (No offence to the authors of these experimental circuits, just couldnít get mine working properly). If you are talking about the designs made by John Linsley Hood, of course I am more than interested to look at them.

No , unfortunately I didnít complete the JLH Class-AB. Restless soul as I am, got carried away with various other building plans just before firing it up. Now I have cannibalized the amp boards for components and used the heatsinks meant to them to build a voltage regulators by the schematic you once sent me. I donít know if I will return to build it again some time soon. Iíd like to finish an amp with these BUZes before. I need to learn finishing things .


Thanks for encouraging words and hints .

Does anybody see some odd component values there. Cheers,


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Old 26th May 2004, 03:13 AM   #5
fab is online now fab  Canada
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I will try to describe your design: Complementary symetric differential cascode input stage with passive current sources and no "standard" VAS (or VAS formed by the output stage).
I believe that this simple circuit topology may be better for a preamp (class A) than a power amp. I will explain why. It is true that power mosfet does not require a high drive current compared to power BJT but you would need a high diff input current (thus medium power transistors) which in turns will load too much the input stage by the mosfets. Also, unless they are matched very closely, the bias of the mosfets will be different and imply a different current for positive and negative diff input stage which is unusual.

Also, R21 and R20 should be much higher (R21 is usually closed to R1). The overall open loop gain is only about 16 to 30 depending on the transconductance of the mosfet (usually less than 2, thus mosfet stage gain 2 * 8 =16) and you have a "desired" closed loop gain of 11. In practice, because of the low open loop gain, the closed loop gain would be about 6.5 to 8. The amp output resistance would be too high which in turns creates a low damping factor, and so on... Unless your intent is to perform a "non overal feedback" design or extremely low feedback design, you need more open loop gain. Again for a preamp since the output load would be something like at least 10K ohms then the open loop gain would be about
Sorry if I may sound a little pessimistic about your design but I just want to help you and no more. Good luck!


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Old 26th May 2004, 10:40 AM   #6
argo is offline argo  Estonia
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Thank you for fare warning. Better safe than sorry . I have built too many non-working circuits by now. But I didnít quite got it - do you suggest to forget about this topology or is there a chance to modify it for better performance? Or do you perhaps have a suggestion for another nice input stage ?

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Old 26th May 2004, 04:34 PM   #7
argo is offline argo  Estonia
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Is this any better ?
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Old 26th May 2004, 06:36 PM   #8
Herrmann is offline Herrmann  Denmark
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You might consider th MJ-amp described here: http://www.homestead.com/whaan/files/page5.html
I have not heard the amp, but it looks like a fairly easy and nice project, and it shouldn't be too expensive.

Torben Herrmann
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Old 26th May 2004, 08:45 PM   #9
ChocoHolic is offline ChocoHolic  Germany
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Hi Argo!
With the second schematic you will only decouple the differential from the MosFets.
But you will still have the low gain and a high comparably high output stage.
If you go for such much number of stages, you should probably
go for a more standard design.
You could use Q9 and Q10 for a traditional VAS.
By this you will not need anymore gain from the output stage
and you could connect Q11 and Q12 as a normal voltage follower.
...entire circuit would become pretty much standard by this....
But if you are not experienced in own designs and all the related calculation and background for frequency compensation you will probably be more lucky if you pick tested and proved design and only use your BUZ for the output.

In fact your first design was somehow charming for me, because it had the following one straight line.
A 2-stage-amp instead of the typicall 3-stage-design.
Low feedback.
Very fast cascode differential input . Dominant pole given by the
MosFets reverse capacitance and the driving resistance....
Also the low resistances in the feedback matched perfectly to this
approach, as this would shift another parasitic pole to very high frequencies.....
Alltogether charming and worth a trial, even if we consider
all the obvious drawbacks which FAB listed.
The ZEN amps have proven that such a low gain+high output impedance design can sound pretty fine.
Your design would combine a ZEN with some more gain and
a DC-coupled signal path... again charming... at least for me
I am not sure if all your values would be really fortunate....
...cannot answer this just by a short look.....

Most new designs require a lot of work, fine tuning, fundamental know how and PATIENCE !!!
I must admit that when I was trying my first own amplifier designs (about 20 years back ? ) it took me two years to learn some basics until my amps started to work acceptable.... not always fun and really catastrophic for my performance in school... but a passion was born... Well, when I started to work I "forgot" this passion... Now, last winter, I found this forum (accidentially ).... and the fire comes back.....
I am finding a lot of inspiration here, and sometimes ideas
which are new or charming for me.
What did I learn? Nothing!!! I know that it is silly to go
for a own design, if you want a fast success.
But my passion does not wake up, if somebody gives me a prooved design to build....
Even my NAD Receiver did not "survive" more than one day without modification......simply cannot keep my fingers off...

Do you feel the same? What is your intension?
Curiosity? Then blow off your sleep for the next years.
Setting up your DIY amp successful? Then pick a proven design.

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Old 26th May 2004, 10:04 PM   #10
argo is offline argo  Estonia
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Hi Marcus,
Thanks again for encouragement. While I really appreciate Fabs effort to talk sense to me - what the heck - I have built quite few circuits, which after much hassle and time spent, have given me nothing but a disappointment because of the sound I heard from them after fire up, that one more failure cannot kill my interest to this hobby. So I will build it according to the first schematic, thanks to your enthusiasm, you have also stirred up in me by now.
As I told itís not my schematic. I copied a most part of it from here http://member.newsguy.com/~stigerik/..._power_amp.htm and the rest I added from the schematic from here http://home.tiscalinet.ch/cooltune/P...Megalith01.htm . Yes I am curious what it comes out. If it turns out bad, I will always have a safe wonderful sounding circuit - JLH class A with Geoffís updates. I could be happy with JLH amp, enjoy it for the rest of my life and have other interests and not build a thing ever but this is not the case. The thing, what drives me, seems not to be a listening of the good equipment but rather building and experimenting with it.

Thanks Torben for the link. I have seen that cicuit before and I like it very much too.
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