JC-80 eBay PCBs & Power Train

[Wolfsin]

Originally Posted by pinnocchio

I believe it is a DIY disease...!

No question, no cure, no end

Considering ways to keep a component flexible after 'completion' is worse than fanatical -- it is obsessive!

[Wolfsin]

The r-cores arrived and appear to be perfectly suited. I will begin to test them under load tomorrow and measure voltage drops across the various components. The tiny gold banana plug socket pairs arrived as well and are just what I had in mind for allowing the regulator modules to be changed easily. My goal tomorrow will be to assure that the entire power train comes up to voltage under load without causing any of the components or their heatsinks to overheat. This ignores the temperature of the MOSFET / Caddock sinks but we already know they run hot.

Several components for the JC-80 remain in-transit but when they arrive everything will finally be in-house. Wolfsin now knows a source for every component assuming the r-cores will hold the required 30vDC before the cap multipliers.

[Wolfsin]

As might be surmised from the delay, the r-cores are overvoltage. I looked at chokes to reduce the voltage but one's choices come down to 90% (instead of 141% using caps) for large chokes or somewhere in between with lots of noise. Resistors can be substituted for thermistors readily, are cheap, and show less noise in a simulation.

[Wolfsin]

Originally Posted by Wolfsin

or somewhere in between with lots of noise. Resistors can be substituted . . . and show less noise in a simulation.

Even in the steady state there appears to be a lot of jitter in the choke case but the difference in ripple voltage is less overall (by about 10%) using a 25mH choke than a 10r resistor right after the rectifiers. In addition, the effect of a common mode choke at filtering noise across both rails is not being simulated. What I observed may be a function of the simulator itself, as it takes much longer to run, leading me to believe it is using much smaller time intervals.

[pinnocchio]

One of my friend is putting in Stanton's schematic for PSpice simulation to know the impact of bypassing the cap multiplier and to adjust the gain lower than the default 14...

I'm hoping to get results soon..

Ciao!
Do

[jrosser]

I was able to spice the jims audio circuit quite successfully to understand the bias/dissipation and how to adjust the gain.

BUT! I know that the models I have for the semiconductors are poor at best and I should treat any results from the simulation accordingly. If anyone has models they trust for these devices it would be good to share them.

On another matter altogether - does anyone know a source of heatsinks for the cap multiplier mosfets? The pin spacing on the board seems a bit unusual.....

[Wolfsin]

Originally Posted by jrosser

I was able to spice the jims audio circuit quite successfully to understand the bias/dissipation and how to adjust the gain.

Would you be willing to post your spice file? Which component models do you mistrust?

>>On another matter altogether - does anyone know a source of heatsinks for the cap multiplier mosfets?

I have not yet but am still looking. For now I am tapping one hole and using a single 2-56 screw to anchor it. Please post source if you find it.

[pinnocchio]

Originally Posted by jrosser

I was able to spice the jims audio circuit quite successfully to understand the bias/dissipation and how to adjust the gain.

BUT! I know that the models I have for the semiconductors are poor at best and I should treat any results from the simulation accordingly. If anyone has models they trust for these devices it would be good to share them.

On another matter altogether - does anyone know a source of heatsinks for the cap multiplier mosfets? The pin spacing on the board seems a bit unusual.....

I've used the closest pin spacing for this type of heatsink but I grinded them from the outside in afterward. Left a half of the pin thickness and solder! Worked fine! I have some small 1" and 1.25" chinese heatsinks that are made to fit exactly on those holes but are too small for the dissipated heat... Most of Jim's Audio boards use these pin spacing, which are not standard anywhere else in the world it seems...

[Wolfsin]

First note that you will need dual rails at + and - 30 vDC. The preamp bias current is adjustable and is usually set in the 200ma to 300ma range on each rail. Each JC-80 will require as much as 18 watts but with various losses your transformer will need to be at least 25va per channel. I start by mentioning this because sourcing appropriately sized r-core transformers may become challenging and, if the item(s) you receive are undersized and non-returnable, expensive. Transformer(s) comprise roughly a fourth of the cost of the entire project and their characteristics impact some other components.

Many configurations of transformers are supported by the rectifier pcb, including dual secondary, and center tapped. The transformer I settled on was advertised as dual 30VAC secondaries @.5A. In the no load case it measured 37VAC. The resistance of each secondary is 6 ohms. With no more than this data you can use PSU Designer II to simulate the rectifier and capacitor board behavior. I have attached the resistor design, with ten ohm resistors, and the 25mH choke design. I have included higher voltage diodes with the choke because the simulator detects high spikes from it. The voltage across the capacitors must be enough above 30VDC to allow for regulator drop so both of those designs provide roughly nine volts over. Keep in mind that we are simulating only one rail so, especially with center tapped transformers, YMMV.

My recommendation would be that you google PSU Designer and download the simulator so you can understand the effects of changes in component values. Then select candidate transformers and when they arrive simulate again to determine with greater precision the values of resistors or common mode chokes you need for the voltage you desire.

[Wolfsin]

While both resistors and chokes are in transit, Wolfsin looked carefully at the rectifier pcb. He noted that 10x2200uf 'lytics would fit easily, 10x3300uf could be squeezed in, but by careful staggering of 2200 & 6800uf caps it would be possible to place 40,400uf on that same pcb. The simulator shows doing that on a channel with chokes rather than resistors cuts the ripple in HALF not even considering the common mode benefits.

I know ticknpop was planning to simulate the tracking prereg and Do's silent partner is doing same for JC-80, so we might squeeze some juice yet!