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Old 28th December 2008, 07:41 AM   #1
apr is offline apr  Mexico
diyAudio Member
Join Date: Dec 2008
Default Help to understand this schem please!

greetings everybody:

i m just a begginer and i have made simple and little projects, with batteries, but i will try to make this amp:

i found it in an old thread posted by scorp8669 (because im a new user, i couldn't send him a mail):


i just have one question: i know where to connect de 60V rail and the output of the preamp to the input of the poweramp, but Where do i connect the negative rail?? its on the right of the volume pot and under the preamp output.

i hope you can help and understand me and thanks alot!.

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Old 28th December 2008, 01:59 PM   #2
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Join Date: Jul 2005
Location: North Derbyshire
It connects to all the chassis points on the power amplifier diagram.

However, it's a really poor and nasty circuit, not something I'd recommend building.
Nigel Goodwin
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Old 29th December 2008, 11:45 AM   #3
balaboo is offline balaboo  United States
diyAudio Member
Join Date: Jun 2004
Location: South Carolina
Actually it is a very good circuit for a musical instrument amplifier.
I tried simulating the complete amplifier circuit and was surprised to see such a high overload capability. The distortion characteristic is also mostly distributed harmonics, a good balance of even and odd.
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Old 29th December 2008, 04:19 PM   #4
teemuk is offline teemuk  Finland
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Join Date: Nov 2004
Location: Suomi, Finland
I agree with Nigel, "it's a really poor and nasty circuit" – that unless you want a nice project for learning the basics of noding and layout of transistor power amplifiers. And what I'm meaning is a "project" with what you're willing to struggle weeks - possibly even months - in order to get it work right - and all the while learning in the process.

On the other hand, iIf you want to build something simple that you get working quickly and with moderate ease I would ditch this circuit and never look back.

Whatever it will be, you likely will be disappointed to this amplifier in the long run.

I’ve built about two or three different versions of the guitar model of this circuit (the only difference being a different tone control circuit in the preamplifier). That circuit was actually my initiation to world of power amps. Unfortunately, as is, the circuit doesn’t work properly - regardless what any simulation says. …And if it’s a decent simulation it should really reveal quite a few flaws in the design.

There are some reasons why the circuit does not work properly. The biggest issues lie in the bias circuit (insufficient gain of the VBE multiplier transistor), stability (or the lack of it, although to be fair, the topology is quite stable by itself), the volume control arrangement versus inverting amp topology (it won’t control the gain in a logarithmic manner and the control will lack sensitivity), and in providing sufficient drive for the output transistors.

Even if you get those things right, you still need to make a proper layout and noding arrangement that doesn’t hum. (It’s even a bigger issue in this particular circuit because it is basically a single-ended one and has a very poor power supply reduction ratio). I used to struggle a lot with this – much more than with the apparent design flaws. After about a year of studying those topics (e.g. D. Self's book about power amps is a good reference) I got it right in the second version that I built (the nice thing was that I was able to recycle most of the expensive parts). These proper noding and layout techniques have been discussed extensively in literature and in this forum. One could write a book about the subject so I don’t really care to go into that…

What you see in the schematic is the absolute neccessity the amp needs in order to work. You'll likely find yourself adding plenty of more things, re-doing the layout etc. many, many times before you have something you can be content with.

Still, even a modified version of this circuit that worked way better wasn’t anything I wanted to keep. IMO, the preamplifier is useless (unless you happen to love the horrible 1960’s transistor sound and complete lack of features).

No. It doesn't overdrive nicely. And the output stage will die very easily. Very easily. Should be needless to remind, but the circuit needs primary side fusing as well.

I know, it’s a simple design – but that’s also one of its biggest flaws. Why not take a look at some decent bass amplifier design for inspiration and to see how it should be done. This is a perfect example of the type of circuits that gave transistor amplifiers their poor reputation.

But as said, as a learning project it can be a nice one.
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Old 30th December 2008, 12:01 PM   #5
balaboo is offline balaboo  United States
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I have a lot of respect for you ( have your book from :-)),
but c'mon, the guy said he is a beginner, and if he wants to try a simple discrete circuit as opposed to a pair of TLO72 opamps followed by a LM3886, it will be a good project.

I actually built the preamp out of curiosity, and agree, it clips terribly !
But it does have a pretty good overload margin, and for a bass should be alright.
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Old 30th December 2008, 06:25 PM   #6
teemuk is offline teemuk  Finland
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I think there never really was any disagreement about those things.

What I was just implying is that this circuit may look simple and easy one to build but it actually isn’t. It has issues that must be addressed and building this amp will require a good amount of know-how about building discrete amps.

Even after getting this thing to work it’s still nothing but an awfully simple amplifier (in the bad sense of that word). I mean, just look at it realistically: It’s just one clean gain stage with headroom (only a tiny bit more than what one could get with an ordinary OpAmp powered from +-15V rails to be exact), a very crude tone control, unity gain buffer and a volume control. Almost exactly the same thing could be wrapped around one dual Opamp (eliminating a big bunch of those discrete components) and something like a LM3886 would be a hundred times better choice for the power amp. Even if you want to do without IC’s you can still find a better power amp design from almost any schematic of a SS amp of same wattage from the 1980’s or 1990’s.

The way I see it, the only real merit of this circuit is its educative use: Build it to learn about building discrete circuits. In anything else its alternatives beat it. It isn’t a simple project, it doesn’t sound marvellous and doesn’t have a very wide spectrum of different tones, it isn’t very reliable, and it doesn’t have nearly any features one could find usable in bass amps (e.g. good EQ, compressor, FX loop, some decent bass crunch effect for “dirtier” tones and all things like that.) Unless you’re a die-hard fan of “keep-it-simple” this thing will wound up as a door stop or getting gutted (to support the next similar project) as soon as the ecstasy of getting it working and emitting some sound passes.

Now, do you really want to sacrifice your time to build something like that? Or would concerning the alternatives (which there are plenty) be more worthwhile… With just a little bit of more hunting one could find circuits that would have a lot more use and that would be way better in pretty much all regards.
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Old 31st December 2008, 12:29 AM   #7
apr is offline apr  Mexico
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Join Date: Dec 2008
hello everybody:

thanks a lot for your sincere answers, i only want an amplifier to test some stompboxes, i have built the little gem mkii, but now i need to try other sounds with more volume!, if anybody have a simple amp suggestion (30watt or more) , it 'll be welcome!

the real page where this and others schematics are:

Red circuit desing

well thanks again for your quick answers!

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Old 31st December 2008, 01:48 AM   #8
Join Date: Jan 2008
Blog Entries: 2
hi alejandro

Can you get components easily? If you can get components without difficulty then look at the datasheets for the National Semiconductor Overture series chips, LM3886 etc.

These chips are easy to apply, and the circuits shown in the datasheets will work. You can run a guitar straight into the basic circuit and get an output.

You will probably want to add a preamp and tone stack. Look at Rod Elliot's pages for a more-than-adequate preamp, you will find lots of good stuff there, I haven't built his circuit, but I'd bet that he has... steer clear of designs from sites without at least some kind of description of how the circuit works and preferably how to switch it on the first time and how to test it. Many of them have not been built, and some not even simulated. You'll find PSU details on the Elliott site too.

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Old 1st January 2009, 11:45 AM   #9
apr is offline apr  Mexico
diyAudio Member
Join Date: Dec 2008
hello again

wakibaki, first thanks alot man, i think here is easy to find that kind of chips, i found another components (components for stompboxes and some weird components for school) without problem, so i ll gonna check the datasheets, could you please tell me about a chip with more watt rate than the 3886 (i read 3886= 38w at 8 ohms and 68w at 4 ohms)? just curious, i dont want to scare my cat.

thanks alot and happy new year and good luck for everyone in the world, this year will be a very hard one.
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Old 1st January 2009, 04:36 PM   #10
Minion is offline Minion  Canada
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Join Date: May 2006
Location: Cowican Bay , vancouver island
I built this curcuit a long time ago and couldn"t get it to work had really bad crossover distortion and the transistors seemed to want to go into thermal runaway so I had to use a heatsink that was way bigger than it should have been.... I agree a really nasty curcuit....

If you want more power than a LM3886 then try a TDA7293 , it is a 100w chip and sounds really good in guitar amps....

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