JLH 10 Watt class A amplifier

It is like coffee;-)
Like Arabica or/and Robusta. Different flavour, savour, character - independently from cultivation, region, climate ... year;-)))

Yeah, like coffee drinkers. They tried everything and still couldn't decide. If only they knew that they didn't like coffee at all. :)

Build every amplifiers and couldn't decide either. If only they knew that they didn't like music that much.
 
Made an attempt to do a linear PSU. It's a BD135+2N2955 + 500VA 25V. A 16V zener and LED as voltage reference for now, LED says it's on mostly. I use 680R with 910R gain adjustment ( { [16 + 1.7] -0.7 } x {1 + [680/910] } ) = 29V. The transformer is a damaged 500VA 0-25 0-25 which will have to be replaced when test done as the live wire is exposed without any cheap cure except potting. As the regulator is low drop out I was able to go to 207 Vrms on the variac. My house is at 249 Vrms today. I have about 8V DC to burn off at 2.4 A or 20 watts. I will draw it when happy. At the moment noise is - 100 dBV and - 90 dBV hum. That is not a bad start. An LD1084 can give -108 dBV and - 100 dBV hum. It looks a good option also. Both 12+12 = 24V and 25V transformer types should work. As the 500VA is giving tight regulation doubtless any > 120 VA type will do. Looks a good happy accident that all might use this idea. Will work for USA also if split primary types.

I will persist with NPN-PNP output pair as it gives low loss and gain adjustment. No oscillation seen. One idea I could use is an LM317 just as Vref. If I used 240R + 4300 + 47 uF = 18.9 gain x 1.25 = 23.7V. 23.7- 0.7V Vbe BD135 = 23V. If using 820R and 240 R it would give an output gain for the NPN-PNP pair of 1.3 = 30V. Keeping output pair gain to 1.3 should be optimum and reduce noise. The big advantage is LM317 used with care gives 6uV noise ( I think it claims 20 uV ). It is also very stable as a pure voltage reference when without any feedback from the load. Setting it 7V DC below 30V DC means it can not drop out. This is vastly better than a LM317 + 2N2955 pass transistor giving 2.5 V loss ( say 3 V for safety ) and some oscillation if typical. Same parts, better results. Drop out is as low as 0.6V before hum is noticed wih my version. LD1084 about 1.4V.

If I wind many layers of insulating tap on the transformer I might save it. The wire snapped just where it enters the lower level. I don't seem to be able to pull it out. I have a 2 mm stump. I am very lucky it's not close the the secondary.

Lavazza of Turin use Robusta and Arabica.
 
Take a switch:
just your mid-woofer without any crossover (but a clamped build-in) / your 2/3/4-way-speaker, complete. And a good amp: say any simple SE;-))))

We call that: COLORATION :) (and where is the bass?)

It's like having a prize already on your hand, though. You need to give it up in order to win bigger prize, but the chance of re-winning is very small.
 
Just finished a kit. Set DC voltage to 50% of supply voltage. Inserted multi meter between B+ wire and B+ post on Amp board. Meter with probe in 10 amp socket and set to DC amperage. I have cranked on second pot with little or no change in amperage. I re-attached the B+ to the amp board and the output transistors started to heat up immediately. I turned it off. Evidently my reducing resistance did increase current to the devices. What am I doing wrong with my measuring process? Thanks
 
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It's only a concept for now. I have a hunch this is a good option.
 
Just finished a kit. Set DC voltage to 50% of supply voltage. Inserted multi meter between B+ wire and B+ post on Amp board. Meter with probe in 10 amp socket and set to DC amperage. I have cranked on second pot with little or no change in amperage. I re-attached the B+ to the amp board and the output transistors started to heat up immediately. I turned it off. Evidently my reducing resistance did increase current to the devices. What am I doing wrong with my measuring process? Thanks

That's why I like the 1969 version. Nail voltage of PSU. Do what JLH says and it works. However, as soon as voltage changes so does current. As getting the PSU sorted also sorts hum and noise it's worth taking trouble. I am sure someone who built the kit can help. As much as I dislike SMPS ( switch mode ) they usually nail the voltage.
 
with little or no change in amperage.

Strange. What was the amperage? If zero, probably your positive probe is attached to low current socket (usually there are 3 sockets incl. ground).

I don't know your circuit and I have never done such thing but may be I prefer reading current through a (series) resistor. Two parallel 1R/5W will give 0.5R/10W. Read the voltage across the resistor. I=V/R. I=V/0.5. I=2*V
 
Just finished a kit. Set DC voltage to 50% of supply voltage. Inserted multi meter between B+ wire and B+ post on Amp board. Meter with probe in 10 amp socket and set to DC amperage. I have cranked on second pot with little or no change in amperage. I re-attached the B+ to the amp board and the output transistors started to heat up immediately. I turned it off. Evidently my reducing resistance did increase current to the devices. What am I doing wrong with my measuring process? Thanks
are you control the isolation of the transistors compared to the dissipator
 
Strange. What was the amperage? If zero, probably your positive probe is attached to low current socket (usually there are 3 sockets incl. ground).

I don't know your circuit and I have never done such thing but may be I prefer reading current through a (series) resistor. Two parallel 1R/5W will give 0.5R/10W. Read the voltage across the resistor. I=V/R. I=V/0.5. I=2*V

It was bouncing between 00.0 and 00.1 no matter how much I changed the resistance with the multi turn pot. The output devices are two pin old style transistors marked 2N3055. I don't think the amp board is getting any power with the meter in series with the B+ That is why I am not seeing any change.
 
It was bouncing between 00.0 and 00.1 no matter how much I changed the resistance with the multi turn pot. The output devices are two pin old style transistors marked 2N3055. I don't think the amp board is getting any power with the meter in series with the B+ That is why I am not seeing any change.

Putting a meter in series is the normal way to measure current. As I predict, you got zero ampere. Have you checked that your meter might have 3 sockets?

Socket#1 is used by black probe (ground).
Socket#2 is used by red probe and low current measurement.
Socket#3 is used by red probe if you want to measure high current only.

So you have to move the red probe to the third socket.
 
Putting a meter in series is the normal way to measure current. As I predict, you got zero ampere. Have you checked that your meter might have 3 sockets?

Socket#1 is used by black probe (ground).
Socket#2 is used by red probe and low current measurement.
Socket#3 is used by red probe if you want to measure high current only.

So you have to move the red probe to the third socket.

Yes. The low current is 300mA and the high current is marked 10A
 
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I would test the meter before going any further. Make sure it is set for DC current and not AC.

If you have no suitable resistors to use for a test then try connecting an incandescent mains voltage bulb across the DC supply via the meter. That will pull enough current to get a reading of some sort. A linear halogen bulb would be ideal.
 
It was bouncing between 00.0 and 00.1 no matter how much I changed the resistance with the multi turn pot. The output devices are two pin old style transistors marked 2N3055. I don't think the amp board is getting any power with the meter in series with the B+ That is why I am not seeing any change.
the metal case of To3 devices is the third pin.
Transistors do not come in 2pin versions. That's what the "2" of 2N3055 tells you.
The "2" = 3 leadouts.
A diode is marked 1Nxxx where the "1" = 2 leadouts.