Great thank you for pointing this article, really helpful to understand the circuit.
Good!
Nelson is a valuable source of information when we talk about First Watt amplifiers.
Someday J2 production ceases and we will have official info to play.
And even expansive JFETs wil not stop us.
I have a 2k trim pot in the place of r33 on your schematic. It is set at 1k1 in my simulation and everything is looking fine. Is that what you were talking about? Anything else I should take a look at?
Hubert
Attachments
I'm about to populate the EUVL designed PCB's from post #1393.
I'm going with IRFP240's for Q4. Semisouths are from Papa via CodyT (Thanks again gents!!!!)
I plan to start with sockets in R33 position to adjust if needed.
What I can't seem to find is how to deal with Rdg...
What should the initial setting of the Rdg pot be when building? I got a 500R pot.
Can anyone help help me understand this better?
"Rdg - Adjust DC Offset ~8.5mA"
DC Offset at speaker terminals after amp is warmed up?
Best way to measure 8.5mA in circuit?
I'm going with IRFP240's for Q4. Semisouths are from Papa via CodyT (Thanks again gents!!!!)
I plan to start with sockets in R33 position to adjust if needed.
What I can't seem to find is how to deal with Rdg...
What should the initial setting of the Rdg pot be when building? I got a 500R pot.
Can anyone help help me understand this better?
"Rdg - Adjust DC Offset ~8.5mA"
DC Offset at speaker terminals after amp is warmed up?
Best way to measure 8.5mA in circuit?
> "Rdg - Adjust DC Offset ~8.5mA"
This means that you can work out the rough value of Rdg off board by measuring the current through the current source.
For example with a multimeter.
> DC Offset at speaker terminals after amp is warmed up?
Yes, after presetting the Rdg as described above.
> Best way to measure 8.5mA in circuit?
If you have to do this in circuit, you should follow the schematics and use 300R for R33.
Then, without any of the power devices installed, short both inputs to Gnd.
Measure the voltage across R33 and divide that by 300R to get current.
But this is just a rough set-up to get you roughly there.
The exact setting varies with the Vgs of your power devices.
You still need to fine trim after fitting in the power devices and warm up to steady state.
Patrick
This means that you can work out the rough value of Rdg off board by measuring the current through the current source.
For example with a multimeter.
> DC Offset at speaker terminals after amp is warmed up?
Yes, after presetting the Rdg as described above.
> Best way to measure 8.5mA in circuit?
If you have to do this in circuit, you should follow the schematics and use 300R for R33.
Then, without any of the power devices installed, short both inputs to Gnd.
Measure the voltage across R33 and divide that by 300R to get current.
But this is just a rough set-up to get you roughly there.
The exact setting varies with the Vgs of your power devices.
You still need to fine trim after fitting in the power devices and warm up to steady state.
Patrick
I'm building the same amp, except I'm using 2SK3497 for CCS.
For Rdg, I adjust the voltage across 300 ohm R33 to 2.55 vdc with inputs IN+ and IN- to ground.
Then with the power devices installed, Rdg adjust for DC offset across speaker terminals. as a fine adjust after thermal stability.
I noticed in the schematic the gate resistors for the power devices also have a designation that seems to be something the builder should adjust. Is this to tune the output devices to equal current flow?
For Rdg, I adjust the voltage across 300 ohm R33 to 2.55 vdc with inputs IN+ and IN- to ground.
Then with the power devices installed, Rdg adjust for DC offset across speaker terminals. as a fine adjust after thermal stability.
I noticed in the schematic the gate resistors for the power devices also have a designation that seems to be something the builder should adjust. Is this to tune the output devices to equal current flow?
@elwood625, I won't comment about the gate resistors here but generally the gate resistors are there to prevent oscillation. To tune the output devices to achieve equal current flow, you might refer to the UDNeSS Revisions pdf which speaks to measuring current sharing and adjusting for equal sharing:
Post 89
The respective current contribution of Q4 and Q5 were then measured; and were found to deviate from to 1:1 as intended. As a result, the values of R11~R14 were adjusted to be 0R34:0R56. This can be verified by measuring the AC voltage across R15. When the output is 8V p-p into 8R, the AC voltage across R15 should be 250mV p-p. These are now set to be the new default values, but might still vary in real life for different MOSFET batches with different transconductances. The ratio does not have to be exact, and 10% variation will have little impact on resulting distortion spectrum.
Post 89
The respective current contribution of Q4 and Q5 were then measured; and were found to deviate from to 1:1 as intended. As a result, the values of R11~R14 were adjusted to be 0R34:0R56. This can be verified by measuring the AC voltage across R15. When the output is 8V p-p into 8R, the AC voltage across R15 should be 250mV p-p. These are now set to be the new default values, but might still vary in real life for different MOSFET batches with different transconductances. The ratio does not have to be exact, and 10% variation will have little impact on resulting distortion spectrum.
Gate resistors are fixed, no adjustments.
Increasing R6 will allow you to increase bias of the current source, vice versa.
So R6 & Rdg are the only two adjustments you need to make.
Unless of course you wish to play with current sharing & distortion spectrum.
FirstWatt J2
Patrick
Increasing R6 will allow you to increase bias of the current source, vice versa.
So R6 & Rdg are the only two adjustments you need to make.
Unless of course you wish to play with current sharing & distortion spectrum.
FirstWatt J2
Patrick
Hi Patrick -
Making a change to R6, will this change the current flow through both devices or just Q4?
To measure the current through each device:
For Q4, measure the voltage drop across R11-R12 to calculate current?
For Q5, measure the voltage drop across R15-R16 to calculate current?
Bill
Making a change to R6, will this change the current flow through both devices or just Q4?
To measure the current through each device:
For Q4, measure the voltage drop across R11-R12 to calculate current?
For Q5, measure the voltage drop across R15-R16 to calculate current?
Bill
Only Q4. So if you change R6, youneed to readjust DC offset.
> So measure the current through each device:
> For Q4, measure the voltage drop across R11-R12 to calculate current?
> For Q5, measure the voltage drop across R15-R16 to calculate current?
Yes, but you only need to check for either Q4 or Q5.
Once your DC offset is zero, Id Q5 automatically equals Q4.
Patrick
> So measure the current through each device:
> For Q4, measure the voltage drop across R11-R12 to calculate current?
> For Q5, measure the voltage drop across R15-R16 to calculate current?
Yes, but you only need to check for either Q4 or Q5.
Once your DC offset is zero, Id Q5 automatically equals Q4.
Patrick