Newbie - Constructing a Nelson Pass class A amplifier

Thread Starter

CosmicOrderMachines

Joined Oct 31, 2019
46
I love how rugged this amplifier is with its large bandwidth. I'm a newbie so I've just researched alternative transistors and tried a few in place of the old ones. It hasn't worked. I'm wondering if anyone can provide any insight. I'm so new to electronics I probably missed a connection or misplaced a resistor. Hardly know what those things do, I'm willing to pay a little for someone to help me with my projects if anyone wants to hear me out over PM. I'll list my replacements chosen. Can anyone point me in the right direction or give me a little motivation? Thanks.

~Replacement Parts~

Motorola MPSL01 -- 2N5551

RCA 1A16 -- MJ21193 : MJ21193 : MJE15033 : MJL1302A : MJL1302A

RCA 1A15 -- 2N3442G : 2N5672 : 2N6274 : 2N6275 : 2N6352 : 2N6353

Motorola 2N5877 -- 2N6686 : 2N6687 : 2N6688

https://www.passdiy.com/project/amplifiers/construct-a-class-a-amplifier

View attachment 218562
 

Audioguru again

Joined Oct 21, 2019
2,061
1) The polarity of your 100V battery is upside down.
2) You added a 12 ohms resistor in series with the battery which will get extremely hot and reduce the amplifier output power a lot.
 

Audioguru again

Joined Oct 21, 2019
2,061
Your project is an antique home heater and produces only a low output power.
A modern IC amplifier produces much more output power with a small amount of heat. Its distortion cannot be heard and is difficult to measure its tiny amount.
 

Thread Starter

CosmicOrderMachines

Joined Oct 31, 2019
46
Your project is an antique home heater and produces only a low output power.
A modern IC amplifier produces much more output power with a small amount of heat. Its distortion cannot be heard and is difficult to measure its tiny amount.
The signal I'm amplifying has a high demand of being recreated perfectly. Figure Class A is best for this. Also wanted to experiment between 0 and 500khz.
 

AlbertHall

Joined Jun 4, 2014
10,558
It may be that the reversed voltage has damaged electrolytic capacitors and/or transistors.
Do you have a multimeter and do you know how to test transistors?
 

Ian0

Joined Aug 7, 2020
750
Your project is an antique home heater and produces only a low output power.
A modern IC amplifier produces much more output power with a small amount of heat. Its distortion cannot be heard and is difficult to measure its tiny amount.
A modern discrete amplifier produces even less distortion than a modern IC amplifier!
Hypothetically, Class A could be even better than a modern discrete Class AB. First thing to fix on this design would be the poor balance on the long-tailed pair - a pair of degeneration resistors and a current mirror load would do the trick.
Having said all that, one can learn a lot from building a Class A amp from a well documented magazine article, especially on the topic of heatsinking! It will certainly sound better than a bad Class AB design.
 

Audioguru again

Joined Oct 21, 2019
2,061
On the other website forum that also has this discussion, a link to this this old Nelson Pass amplifier was posted. It was built by Rod Elliot (Elliot Sound Products) who tested it and he found it had the same bad distortion as an old vacuum tubes amplifier. He improved it by adding lots of negative feedback but since it was an audio amplifier it did not reach the 500kHz radio waves frequency you want.
 
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