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Old 11th February 2008, 05:58 PM   #1
latala is offline latala  United Kingdom
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Default Here is a Challange

Just suppose the pnp transistor had not been perfected/produced . Could any of you design a Dc coupled amplifier using NPN devices only !!!
Target power is 30 watts rms or more in 8 ohms at less than 0.1 % distortion midband
I have produced a no of small class A designs with succes
You can use any technique multiple supplies etc any thing goes but no PNP devices.
It seems to me that most of the designs I have seen on this site are all generic and feel that the challange will stretch our minds a little further
Regards Trev
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Old 12th February 2008, 03:13 AM   #2
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Default Re: Here is a Challange

Quote:
Originally posted by latala
Just suppose the pnp transistor had not been perfected/produced . Could any of you design a Dc coupled amplifier using NPN devices only !!!
Target power is 30 watts rms or more in 8 ohms at less than 0.1 % distortion midband
I have produced a no of small class A designs with succes
You can use any technique multiple supplies etc any thing goes but no PNP devices.
It seems to me that most of the designs I have seen on this site are all generic and feel that the challange will stretch our minds a little further
Regards Trev

I love to break challenges and also the people who set the challenges

I can build with both NPN or N-channel FET as well!!!!!!!

All 100% Single polarity.
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Old 12th February 2008, 03:58 AM   #3
latala is offline latala  United Kingdom
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Go0 on then Trojan

Lets see a concept circuit I dont mind if its fet or bipolar it could even contain vacum tubes any thing but PNP devices .
Please do not thing I am being smart or agressive its just I have not myself been able myself to come up with a dc coupled circuit that would have low and stable offset!
regards Trev
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Old 12th February 2008, 04:55 AM   #4
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Isn't all tube amps using output transformers are all using Nch devices (as there is no P tubes)?
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Old 12th February 2008, 05:21 AM   #5
latala is offline latala  United Kingdom
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Tubes don,t have to use output transformers!
What about the futterman type circuits they don.t use transformers
I desined a hybrid that was dc coupled that used 4 transistors and 4 tubes that was dc coupled and has an offset of 50 mv at the spk terminals.
Three of the transistors were pnp though so no prize for me with that 1
regards Trev
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Old 12th February 2008, 07:50 AM   #6
teemuk is offline teemuk  Finland
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The interstage transformer coupled designs (pre early 70ís) commonly used only PNP transistors because no decent high power NPN ones we available. The transformer performed the required phase inverting that sort of turned the PNP device into NPN (think Sziklai pair). You can easily take one of these designs and convert it to use NPN transistors instead, but I suppose youíll have plenty of problems finding the required iron nowadays.
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Old 12th February 2008, 08:37 AM   #7
Elvee is online now Elvee  Belgium
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Hi

The output part is relatively easy: you just have to use something based on the "virtual complement" concept, introduced by Bengt Olsson (Electronics World, Dec 1994). The idea had been used in the tubes days, but without specific denomination (quite understandably).
The phase splitter is equally easy.
The real problems begin with the voltage shifting: if you want the DC coupling, this means no capacitors, and without PNPs, the only solution is "rigid" level shifters, using zeners or similar devices. In tube op-amps, it was done with gas-filled tubes, but it is not very satisfactory from the PSRR point of view.
Any idea of improvement on this point?
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Old 12th February 2008, 04:33 PM   #8
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The chip amps from National, think LM3886, are all NPN and the DC offset is manageable with no input capacitor or feedback cap. The popular Gainclones people built can be done without caps for more pure sound. I forget what the DC offset is but it is not typically a problem. The chip amp forum can be searched for such.

-SL
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Old 12th February 2008, 05:14 PM   #9
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LM3886 all NPN?? Where did you get that idea? The datasheet shows lots of PNP devices in the "equivalent schematic".
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Old 12th February 2008, 05:58 PM   #10
latala is offline latala  United Kingdom
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The output part is relatively easy: you just have to use something based on the "virtual complement" concept, introduced by Bengt Olsson (Electronics World, Dec 1994). The idea had been used in the tubes days, but without specific denomination (quite understandably).
I vaguely remember reading this article but did not that only refer to the output fets I seam to remember that it used pnp transistor however I am not sure
I agree the main problem is level shifting
Also I stated earlier that I built a hybrid but I used 1 more npn I forgot the bias
We could level shift using opto device ?
Regards Trev
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