ACP+ Preamp/HPA for Yarra Preamp

[Bones13]

Thanks, I have some of those, so I’m good. Just wondered if you had an obscure source for the Fisher stuff.

[xrk971]

Regarding the Fischer heatsinks: Just need to know someone in Germany.

The ACP+ as designed is really for driving headphones. So the bias current is a rather high 150mA. It runs too hot with these tiny heatsinks.

dBel84 did the following changes to drop the current and it runs at 40C.

R111 = 3K
R115 = 3R2
R117 = 10R

I seem to recall these settings were for about 70mA bias current. Plenty as a preamp and even for higher impedance headphones.

[Bones13]

Any way you could offer the lower bias option before the SMT work gets done?

And what would you recommend to do the same for the Hakuin board?

[xrk971]

We were already thinking of lowering the bias for the SMT prepop’d boards. The Hakuin is already set at a lower bias and heat is not an issue.

[Bones13]

TYVM!

[xrk971]

GB update:

MShipmanPE / SMT Prepopulated / 2 boards/ USA *Pre-Order Placed*
Carl_Huff / bare PCB / 2 boards / USA
mnichols30030 / bare PCB / 2 boards/ USA
Thompsontechs / bare PCB / 2 boards / USA God Bless it in these trying times!
Aljordan / SMT Prepopulated / 2 boards / USA *Pre-order Placed*
Pwagner / SMT Prepopulated (if enough interest) / 2 boards / USA
jwjarch / bare PCB / 2 boards / USA * Order Placed*
meanie / SMT Prepopulated / 2 boards / SGP
diy40 / SMT Prepopulated / 2 boards / USA
Itsmee / SMT Prepopulated / 2 boards / UK *Pre-order Placed*
Bones13 / SMT Prepopulated / 2 boards / USA *Pre-order Placed*
Afoor / bare PCB / 2 boards / USA *order placed*

Thank you Mshipman!

[xrk971]

GB status update:

MShipmanPE / SMT Prepopulated / 2 boards/ USA *Pre-Order Placed*
Carl_Huff / bare PCB / 2 boards / USA
mnichols30030 / bare PCB / 2 boards/ USA
Thompsontechs / bare PCB / 2 boards / USA God Bless it in these trying times!
Aljordan / SMT Prepopulated / 2 boards / USA *Pre-order Placed*
Pwagner / SMT Prepopulated (if enough interest) / 2 boards / USA *Pre-Order Placed*
jwjarch / bare PCB / 2 boards / USA * Order Placed*
meanie / SMT Prepopulated / 2 boards / SGP
diy40 / SMT Prepopulated / 2 boards / USA
Itsmee / SMT Prepopulated / 2 boards / UK *Pre-order Placed*
Bones13 / SMT Prepopulated / 2 boards / USA *Pre-order Placed*
Afoor / bare PCB / 2 boards / USA *order placed*

[xrk971]

Hi Folks,
We are going to test out the lower bias current configuration before committing to get the BOM for all the prepopulated SMT boards. So this will take another week or so then we will be in production. I have the PCBs and solder stencil already. Just need to order the MELF resistors and the opto and BJT. We are moving forward with this. Please place your pre-order soon if you are interested in the prepop'd board as the window is closing soon.
Thanks,
X

[xrk971]

Hi Folks,
We have just tested the ACP+ with the new reduced bias current settings. The current is now 75mA vs 125mA and temps are down to 56C at the heatsinks. Even at 75mA you can drive moderate impedance headphones quite nicely. As a preamp though, it can drive any input of an amp. We are noticing that it will take some fiddling with various values of trim resistors to set the bias current of the JFETs precisely. There are extra large pads there so you can add in parallel, values that can fine tune the bias.

[Bones13]

Thanks for the update!

I know everyone else here knows how, but, could you outline the bias setting? Where to measure, and the desired value. Is it twitchy enough that I should dial in with a trimmer resistor, and order a specific resistor value for each board? Or would a few smaller values (5R, 10R, etc) handle the range to add?

Ok, looked at the page on FirstWatt.com - 10 milliamperes across the R104/R105 parallel resistors.

If perfection is wanted, I would think leaving off the 200R, preset the trimmer pot to 200R, the adjust for desired bias, measure pot, and then order as close a resistor as possible.

Or is that too much work for obtaining a good result?