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My first DIY amplifier 20 years a go
My first DIY amplifier 20 years a go
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Old 2nd August 2013, 10:21 AM   #451
ashok is offline ashok
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My first DIY amplifier 20 years a go
As I understood it , as the output power device draws more current the base current also increases. This is an additional current passing through the 22 ohms and hence reduces the voltage available from base to base of the output transistors ( = KxVbe of bias spreader - I( through 22 ohms) x 22 ) and hence the ability to reduce some current in the output stage as it's quiescent current tries to increase.
The 22 ohms does not sense temperature but reduces available dc bias voltage to the output devices as their current increases.
Bob has explained this in the book.
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Old 2nd August 2013, 10:32 AM   #452
AndrewT is offline AndrewT  Scotland
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You appear from your response to be not understanding what Cordell shows.

Try the plugboard experiment.
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Old 3rd August 2013, 10:36 AM   #453
ashok is offline ashok
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Quote:
Originally Posted by gaborbela View Post
.......... the CSS front end like LC.
I'm sure that would work much better than the noisy zeners........
The zener noise is not a problem. It is attenuated by the large resistor and the nfb capacitor. Only very low frequencies down to dc will not be attenuated much. But in actual use the zener implementation is not noisy to the ear.
The amp sounds very good. Raising the bias to 100mA appears to make the mid range sound a bit smoother. Not sure if that is real . Will have to test again at lower bias. Getting tired of changing the bias so many times !
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Old 6th August 2013, 05:00 PM   #454
ashok is offline ashok
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I don't see many posts on this thread. Looks like very few people are actually trying out this amp.

I forgot to mention that a regulated supply does help a lot. It keeps the voltages and current quite stable. However this can also be achieved by zener stabilisation of the supply to the small signal section of the amp. Providing a regulated supply for that stage would be ideal. Something like Ilimzin provided for his implementation of the LM7294 amp. Alternatively you could use separate taps on a transformer ( not everyone could do that easily ) .

However this amp with the TIP142/147 might not be safe with 100mA bias and +/-40 V supplies. At high volume and rising heat sink temp the power devices might hit the SOAR limit causing blue smoke. About +/-35 V seems to be OK. But at 100mA the heatsink must be much larger than what I have used. You might still need to monitor it till you are satisfied that it's playing within the safety limit under all conditions.

Considering how the amp sounds the LatFet is the safest bet but a discrete Darlington should be explored. That will be able to handle higher voltages and a 100 mA bias current and be thermally more stable.
Since it sounds so good it deserves further experimentation.

There is scope for lots of experimentation.
Now I need to try out the P3A to see how it compares !
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Last edited by ashok; 6th August 2013 at 05:02 PM.
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Old 6th August 2013, 05:23 PM   #455
Bigun is offline Bigun  Canada
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My first DIY amplifier 20 years a go
I'm still following you with interest ! - I have a bunch of Sanken Darlingtons that might be well suited. However, I'm busy with another project which is better suited to the +/-50V power supply I have.

Most of all I'm interested in your experiments at stabilizing the bias. The topology does not favour stable bias and integrated Darlington devices are potentially more difficult to stabilize than a discrete so it's a difficult combination. The LatFET makes for such a good (thermal) solution ( e.g. in the VSSA) that I don't know yet if this Darlington output version is the right approach. I will continue to watch with interest.
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Old 8th August 2013, 07:07 AM   #456
ashok is offline ashok
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Hi Bigun,
As you also found out the LatFet version is stable.
The Darlington version becomes unstable at high bias ( 100mA and above) and if the heat sink is small. However with the modified bias spreader ( with the extra resistor in the collector of the bias Tr ) it is stable.
But there is still one catch. The dc bias does go up with high junction temp ( while playing loud music ) and could go up enough to kill the power transistors if operating on the borders of the SOA curve. Removing the music signal brings the bias down very quickly.So with +/-40 V and 100mA or more isn't exactly a safe thing to do. Certainly not with a small heatsink like mine but needs to be tested with a large sink. I'd say the TIP 142/147 might be safe if used with a max supply of +/- 36 V and bias current less than 100 mA. It sounds very good at 50 mA too.

To get to higher voltages and current you need to use Lat Fet's or discrete Darlingtons ! The discrete Darlington is really worth a try.
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Last edited by ashok; 8th August 2013 at 07:11 AM.
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Old 8th August 2013, 02:20 PM   #457
Bigun is offline Bigun  Canada
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I currently need 3 channels from a +/-54V supply and I have decided it is safer to go a different route for that, using a pair of MOSFETs. Once that project is finished I will return, and see what to do with my Darlingtons as they look very nice in their TO-3 packages.
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Old 9th August 2013, 12:10 AM   #458
gaborbela is offline gaborbela  Canada
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These is in my mind to compare with my old darlington.
Please take a look, opinion or and advise welcome on these circuit.
For the BD139 I will test a darlindton device after I power up the amp.
I do like the front of these amplifier all do several thing is different, have to be tested which is closer to my taste - like the feedback


Greetings G
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Old 9th August 2013, 09:48 AM   #459
ashok is offline ashok
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Hi G,
With the CCS the circuit is slightly better with respect to the output offset voltage and output bias current over a range of supply voltages.
However the PSRR appears poorer if you don't have a capacitance multiplier on the input stage supply line.

Remember to add the 22 ohms on the collector side of the bias spreader transistor. Note change of base connection on that side for the power transistor. Makes things more stable.
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Old 14th August 2013, 03:39 PM   #460
ashok is offline ashok
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Times up ! I'm off again on the 16th and couldn't build the stereo board or the P3A ! Time really appears to be in short supply.
Blew the Darlingtons by using it at +/- 42 V and over 350 mA bias current. Didn't really do it intentionally but happened due to a set of goofs while fiddling with the board. But it did run for quite a while in that condition ! Not bad !

The stereo board will most likely use LatFet's . I have to choose between LatFet and Bipolar. Since I need a new power amp for my new coaxial OB speaker , the LatFet would be ideal. Bass will come from a sub.
Cheers.
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