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The YARRA Preamplifier/HPA for Melbourne DB Group Buy
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Old 3rd January 2021, 04:33 PM   #1341
STUNTfingers is offline STUNTfingers  United Kingdom
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Thanks for confirming this. I have now managed to get the parts off. Will add nylon washers and non conductive thermal pads as per your advice.

I think I’m going to reorder some new transistors as I feel I’ve subjected them to too much heat as getting them off was a real challenge. This time round I will forego using Loctite and will nut and bolt them instead so removal will be easier should I screw up again. I was concerned that I’d end up lifting a PCB trace but is seems those boards are very well made and very rugged, kudos on a quality product.
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Old 3rd January 2021, 04:59 PM   #1342
Vunce is offline Vunce  United States
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The YARRA Preamplifier/HPA for Melbourne DB Group Buy
Hi STUNTfingers,

Great to hear it was basically an easy fix, tracking down a solder bridge somewhere can get very frustrating!
(My first Yarra had one)
Please make sure to use a nylon/plastic shoulder washer to attach the transistors to the heatsink, not a flat washer. There could still be a chance the threads on the fasteners can touch the metal tab and short.
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Old 3rd January 2021, 06:34 PM   #1343
xrk971 is offline xrk971  United States
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The YARRA Preamplifier/HPA for Melbourne DB Group Buy
Your BJTs are probably fine because the short went from collector to ground and not through the junction. But they are inexpensive and may be wise to replace. A good practice is to always use a conductivity meter (beep mode) to check the tab of a transistor or mosfet to make sure it’s not conducting to the heatsink. Rarely is this ever done on purpose. Sometime a small metal burr in the threaded hole can poke through the silicone sheet and short out a transistor. This happened to me once and blew the BJT and the driver transistor. It was a mess to replace.
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Old 3rd January 2021, 07:33 PM   #1344
STUNTfingers is offline STUNTfingers  United Kingdom
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Thanks X and Vunce, much useful advice as always!

I've just ordered some nylon shoulder washers from eBay as well as some hardware to fix them in place.

Re checking conductivity I will certainly do this in the future, today was a good lesson. I've spent the best part of 8 hrs checking the board looking for the fault, learning about NPN vs PNP and re reading this thread looking for clues. As a complete amateur when it comes to board stuffing and building something like this it all seemed to be going too well...
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Old 13th February 2021, 03:09 PM   #1345
aljordan is offline aljordan  United States
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Hello,

I have a Yarra with Melbourne boards, and I want to try a more transparent volume control than what I have in there. I have a spare Gold Point stepped attenuator but it has an input impedance of 100k. Would such a high input impedance have any drawbacks in this circuit?

Thanks,
Alan
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Old 13th February 2021, 04:09 PM   #1346
xrk971 is offline xrk971  United States
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A 100k pot can work fine - it may be more susceptible to noise pickup. But that’s dependent on your specific setup.
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Old 1st March 2021, 03:53 PM   #1347
poseidonsvoice is offline poseidonsvoice  United States
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The YARRA Preamplifier/HPA for Melbourne DB Group Buy
X, Hugh, others,

Referencing to posts 1187-1190 in this thread, I see that the Hakuin has an overall gain of about 11-12 dB. My plan is to use the 2SK170BL/2SA1837 version but I would like to see if overall gain of the board can be lowered to 5-6dB to use in my M2X amplifier.

Thanks,
Anand.
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Old 1st March 2021, 05:43 PM   #1348
Mark Johnson is offline Mark Johnson  United States
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You can certainly install two new components, a series RC circuit, between JFET drain and positive supply. This will lower the gain to any value you like without changing any bias voltages or DC currents. {why? because no DC current flows through a series RC circuit. why? because of the C!} Whether this tweak changes something else that you find sonically significant, and whether such a change is beneficial or detrimental, is not known to me. Try a quick experiment to find out!



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Old 8th April 2021, 02:40 AM   #1349
TboneAK is offline TboneAK  United States
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The YARRA Preamplifier/HPA for Melbourne DB Group Buy
Default Volume pot boards

Hi all,
I'm working on another project in which I'm using Alps RK097 pots. I want to use the board that comes with the Yarra kits. I have two, but need one more.
The board I need is labeled, R211, where the pot mounts. Now if someone had 4, I'll take them too, because I mistakenly double ordered the pots

PM me if anyone has one or more and would be willing to part with them.
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