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 Post subject: Amp distorting
PostPosted: 28 Apr 2021, 02:16 
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Joined: 28 Apr 2021, 02:08
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Hi. Can you please advice on the attached circuit diagram for an amplifier.
The amp works fine for the midrange and treble but distorts on bass notes, and if i raise the volume.
Q3 seems to be driven incorrectly. The circuit appears wrong: the lower output transistor has no bias. The top transistor is not receiving enough voltage (needs a bootstrap possibly), so distortion occurs.
How can any of these problems be addressed, or any more problems you may have noticed.
Many thanks.
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 Post subject: Re: Amp distorting
PostPosted: 28 Apr 2021, 13:30 
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Hi, Not my style of design but based on how you describe it's operation I wonder if the diodes are incorrectly oriented. It appears from the orientation that only th etop transistor gets votlages on the base and perhaps nothing is getting to the lower one. Where did the circuit originate as it might give us some clues on why it is not functioning well.



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Bruce

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 Post subject: Re: Amp distorting
PostPosted: 28 Apr 2021, 22:14 
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You cannot rely on base leakage of Q3 to properly set bias on the bottom transistor. There needs to be a resistor between the Q3 base and ground. This resistor need to be the same value as R3. This will bias the output pair just barely "on" at idle. Also, Bruce is correct, the diodes are reversed in your schematic.

In addition, you need to make sure that BCE of Q1 is exactly 6 volts (i.e. Vcc / 2) or the amp will be unbalanced. I recommend using a capacitor in line with the Q1 drive to block DC and allow the output pair to bias naturally. Also note that by biasing both Q2 and Q3 "on" at idle means the the pair runs class B all the time. Is this what you intended?

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 Post subject: Re: Amp distorting
PostPosted: 29 Apr 2021, 00:42 
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Thanks for the replies, much appreciated.
I have applied the modification concerning the diodes as you stated. Please see the attachment. I am also thinking of dropping the value of R2 from 100k ohms to 300 ohms to help with this diode mod, but have not applied this mod yet, waiting for your approval.
Can you please write on the attachment i included concerning the other mods you mentioned, where they should go, what component values to use. Even if it is rough esimates it will give me a good point to start from and experiment with.


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 Post subject: Re: Amp distorting
PostPosted: 02 May 2021, 00:56 
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Sorry, i meant reduce R3 from 680 ohms to 100 ohms. Any thoughts.
R3 sets the voltage gain of the first stage which is, the input voltage is converted to a current, the resistor multiplies the current to produce a voltage, smaller resistor -> smaller voltage, bigger resistor gives a bigger voltage.
What do you think the pros and cons are of adjusting R3 at this point in the circuit.
Also the speaker negative terminal goes to the ground, what do you think of connecting the speaker negative to the +12 positive rail instead.

Do you think the power supply 12v could be raised, and to what value.

Many thanks.


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 Post subject: Re: Amp distorting
PostPosted: 03 May 2021, 12:31 
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I would leave the driver connected to the mid point. You need to add a resistor from the base of Q3 to ground. It needs to be the same value as R3. The series chain of R3-D2-D1-Rb sets the bias condition for the output stage.

You want the current through this chain to just be a few milliamps or so. You also need to make sure you are not presenting too low an impedance to the driver transistor Q1. So Vcc is 12V. The 4148 is rated at 1.0v at 10mA so subtract 2 times this voltage and divide by 10 mA to get the total resistance in the biasing chain (i.e. (12v-2v)/.01A=1kΩ). Divide this value by 2 and that's the value for R3 and Rb (i.e. the added resistor). This gives 500Ω each. You can stick with the 630Ω without issue.

At his point the output stage is properly biased but is is simply an impedance matcher. It provides no amplification. So now you need to properly configure Q1 as a class A amplifier and capacitively couple it to the output stage.

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 Post subject: Re: Amp distorting
PostPosted: 04 May 2021, 01:30 
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Joined: 28 Apr 2021, 02:08
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Thanks for the reply, much appreciated.

One last point, in the attached circuit diagram the speaker negative terminal goes to the ground, is there any advantage to taking this speaker terminal to the +12v vcc. In quite a few diagrams i have seen designers do this, just wondering if it may be helpful.


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 Post subject: Re: Amp distorting
PostPosted: 05 May 2021, 10:20 
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john121 wrote:
... in the attached circuit diagram the speaker negative terminal goes to the ground, is there any advantage to taking this speaker terminal to the +12v vcc.
There is no good reason for doing this. The speaker should remain connected between the drive point and ground. Otherwise the entire speaker ends up at a positive DC potential. This is just another opportunity for bad things to happen.

Also note that the output capacitor (C2 in your diagram) would have to be reversed if one connected the speaker to the positive rail. Better to leave it as drawn above.

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