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Old 7th February 2012, 10:55 AM   #11
jaycee is offline jaycee  United Kingdom
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Quote:
Originally Posted by sreten View Post
Hi,

Your misguided. You can't build an amplifier just using small signal transistors.
Its stops being simple very quickly when you realise just how rubbish your
simple amplifier is compared to simple IC circuits that have been around
for years, and for good reason, they are simple, and they work well.
You miss a good reason he may be requesting this sort of information - to understand how amplifier circuits WORK. A chip tells you nothing in this regard, it is just a "black box full of tricks".
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Old 7th February 2012, 02:30 PM   #12
Wakh is offline Wakh  Russian Federation
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Quote:
Originally Posted by Elvee View Post
Here is an original proposition.
Despite its simplicity, it even manages to include an auto-bias circuit:
Indeed the original proposal !

Just a little bit of practice, not a very good agreement.
This refers to the thermal instability of the mid-point scheme +6V, as well as the instability of the change in power voltage within +/-10%.

Minimal changes to make everything Ok!:
Resistor R4 = 15 kilohms.
And to add R7 = 75 Ohm emitter in Q4.

Sorry, I dont speak English,
use a google translate.

Last edited by Wakh; 7th February 2012 at 02:55 PM.
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Old 9th February 2012, 02:41 PM   #13
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Wakh View Post

Indeed the original proposal !
Hi,

Yeah .... build it. Try that test into a real speaker. One dead amplifier.
The output devices are about 1/2W each and simply will expire if they
haven't already due to to the standing current and thermal runaway.

Here is a circuit that shouldn't blow up,
and shouldn't hum* (good ripple rejection) :

Click the image to open in full size.

Bolt something to each output device as a heat sink.
IMO you should use 0.47R output emitter resistors.
Should get about 4 watts into 4 ohms.

rgds, sreten.

* too much ........

model it using the free TinaTi circuit emulator
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Last edited by sreten; 9th February 2012 at 03:06 PM.
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Old 9th February 2012, 03:31 PM   #14
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by sreten View Post
Hi,

Yeah .... build it. Try that test into a real speaker. One dead amplifier.
Did you actually try it?
Or is it just an informed opinion?
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Last edited by Elvee; 9th February 2012 at 03:36 PM.
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Old 9th February 2012, 05:42 PM   #15
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If you go back to take another look at my circuit it is basically a 4-transistor architecture but with a bias transistor and parallel outputs. Actually you could use a single pair of Zetex devices but these cost more. You could use a single pair of BC337/BC327 but they suffer a little gain loss at 500 mA, buit you could increase the driver current. The circuit is capable of producing 1W from a 9V supply with an excellent frequency response and very low crossover distortion because of the high speed of the output transistors. In fact this alone makes such a simple circuit sound better than many amps using slow output transistors and even IC amps which have Miller stabilisation capacitors.
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Old 9th February 2012, 08:46 PM   #16
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by john_ellis View Post
If you go back to take another look at my circuit it is basically a 4-transistor architecture but with a bias transistor and parallel outputs. Actually you could use a single pair of Zetex devices but these cost more. You could use a single pair of BC337/BC327 but they suffer a little gain loss at 500 mA, buit you could increase the driver current. The circuit is capable of producing 1W from a 9V supply with an excellent frequency response and very low crossover distortion because of the high speed of the output transistors. In fact this alone makes such a simple circuit sound better than many amps using slow output transistors and even IC amps which have Miller stabilisation capacitors.
Nice classic design, quite robustly dimensioned. I am sure it delivers the goods.
I have built many similar amplifiers in the early seventies, the first ones with Ge transistors, and I was very pleased and amazed at the quality, compared to what was routinely available at the time.

Quote:
Here is a circuit that shouldn't blow up,
and shouldn't hum* (good ripple rejection) :
About that golden standard of an amplifier, I am less sure: to begin with, it manages to achieve a positive PSRR (I mean, it amplifies the supply noise) of +6dB or thereabout, a feat I have never seen before, and it begins to clip at ~150mW output power.

My own humble proposal only has a relatively poor -40dB+ rejection, but at least, it's rejection, not amplification.

Quote:
if they
haven't already due to to the standing current and thermal runaway.
That is an impossibilty with the autobias circuit: in fact, Q1 and Q2 could be replaced by Ge transistors and no other adaptation, and the thing would still not go into thermal runaway, even at 75°C and without any heatsink.
The same cannot be said of other amplifiers....

The circuit I gave is absolutely minimalist, but it does work, and it is incredibly resilient.
As it is, the (audio) quality is not very good, but with an antisaturation diode on the control transistor it can be substantially improved, and if quality is the first objective (not that of the OP), some others mods can be brought, keeping the incredible thermal robustness.

Hint: the total quiescent current is approximately twice the VAS bias curent, and it diminishes when the OP transistor's temperature increases.
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Old 9th February 2012, 09:22 PM   #17
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I think I know where the ripple amplification on the amp posted by sreten comes from. The upper 2N3906 (VAS) base voltage is cleaned up, but there's full ripple on the emitter and hence on B-E voltage. Oops. Ripple should still be reduced by loop gain though (at least the part contributed by input and output Qs).

The output stage on that one is pretty decent though. I'd combine it with a pnp input / npn VAS as used by john_ellis (well, in fact, his amp would also work fine when beefed up with one pair of BD139/140 and 0.47R emitter Rs). VAS V_be is all ground referenced there. So that's why pnp inputs were popular in the olden days.

Last edited by sgrossklass; 9th February 2012 at 09:32 PM.
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Old 10th February 2012, 12:15 AM   #18
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by sgrossklass View Post
VAS V_be is all ground referenced there. So that's why pnp inputs were popular in the olden days.
That, and all the good VAS transistors were NPN.
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Old 10th February 2012, 12:56 AM   #19
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Old 10th February 2012, 05:48 AM   #20
PRR is offline PRR  United States
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> Here is an original proposition.

That's a brain-fork, but it will work.

It will work exceptionally well for the number of parts.

It expects some ratio hFE(Q3):hFE(Q1). The suggested types may give an output idle bias that varies 5:1, which is fuly acceptable here.

Heat (even leakage) have very little effect. Rise of Q3 hFE is cancelled by rise of Q1 hFE. If Q3 Q1 leak real bad but in similar amounts, they cancel.

Vbe of Q3 Q1 Q2 does not matter (which is my mind-fork problem).

Output center-point voltage depends on Vbe and hFE of Q4, and will drop with temperature. Unless Q4 is in the oven it won't go off a happy operating point. If supply voltage changes slightly, center point tends to change in the right direction but not enough. For a large change like 9V, R4 is there for tweaking.

There's no outright short-protection but it may stand many shorts. When output is shorted the bootstrapping fails. If shorted with large input signal you can probably kill it. I see the "power" transistors are 7 cents.... not worth crying over.

NFB is small, especially through crossover... but astonishing to have any NFB at this voltage and current gain and efficiency with just a dozen parts.

My all-time favorite small amp had a couple more parts and many of the same "flaws". I think I had as much listening pleasure from that amp as any other. I had to sit close... but same as I sit to my PC speakers 40+ years later.
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