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Old 25th March 2013, 10:29 PM   #861
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Originally Posted by xrk971 View Post
Glad I accidentally got the correct ones. It was not clear from the instructions from Nelson Pass which ones to get. So you don't want quiescent current too low then? Maybe someone can explain what the requirement is and how you figured that out.
Q1 needs to have a Gate voltage of between about 4 to 4.5V to idle at the right current and set the idle voltage at it's Drain to 10V. International Rectifier parts are very consistant, other manufacturers could vary. With R9 at 1K, this means Q4's current needs to be 4 to 4.5mA. At that current, Q4's Gate to Source voltage can't be more than (minus) 4.5V or Q4's gate voltage would need to be below ground (and thus P1 could net be set). The 2SK170 has very high Yfs (transconductance) and the Gate cutoff voltage (Vgs(off)) is spec'd at -1.5V so there is no danger of that unless a different part is used.

If Q4's Idss is less than 4.5mA, it's Gate to Source voltage would go positive before there is enough Gate voltage at Q1. So a Toshiba GR grade part (Idss 2.0 to 6.5mA) might not work in the circuit as designed. An increase in R9 would permit the output bias voltage to be set, but a slight increase in high frequency distortion would result. Conversely, you could decrease R9 if you had a high Idss part and the result would be a smoother high end. The concern would be power dissipation of Q4 would increase. If you have a fist full of GR grade parts, you could run 2 or 3 in parallel.
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Old 25th March 2013, 11:34 PM   #862
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When I built the ACA, I used GR parts being a newb, did not know any better. No issues, sounds great, listen to almost everyday in my office. It seems ACA is a very forgiving design.
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Old 25th March 2013, 11:51 PM   #863
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Default 2sk170v

Quote:
Originally Posted by Loudthud View Post
Q1 . Conversely, you could decrease R9 if you had a high Idss part and the result would be a smoother high end. The concern would be power dissipation of Q4 would increase. If you have a fist full of GR grade parts, you could run 2 or 3 in parallel.
I have 2sk170v and 2sj74v parts. are they usable here if i decrease R9 ? would it wise to use a trim pot for R9?
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Old 26th March 2013, 01:17 AM   #864
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I have 2sk170v and 2sj74v parts. are they usable here if i decrease R9 ? would it wise to use a trim pot for R9?
The -V parts will work because of the high Yfs and low Vgs(off) of the 2SK170. I experiment with things like R9 just to see if I can hear or measure the difference. Most times I can't hear it, unless the feedback is disconnected.
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Old 27th March 2013, 01:57 AM   #865
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Does anyone know the status of those beautiful cases for the ACA being built by Hifi2000 that Jason showed us pictures of on March 11? I am really looking forward to them.
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Old 27th March 2013, 05:30 AM   #866
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Does anyone know the status of those beautiful cases for the ACA being built by Hifi2000 that Jason showed us pictures of on March 11? I am really looking forward to them.
Sorry we haven't offered them for sale yet! They got put on hold for a bit while we were working on new modifications for the next batch of 4U and 5U chassis and new bigger, badder, better and F5-T compatible PSU boards. Which are going to be pretty awesome. We're investing in some custom tooling which is going to take the configuration possibilities and space optimization within these chassis to the next level for our DIY construction / configuration needs. We've spend hundreds of hours on that, really. Can't wait to show you some examples.

Being sea shipping that has a very long lead time so we wanted to get that locked and loaded. We really should have a blog for the store or an update thread so we can better keep everyone up to date with what we're doing. Rest assured we've been busy!

That now under control, I spent all day yesterday on the ACA front panels - we're seeing if we can trick up the front panel a bit while keeping it affordable. We're planning on a presale options survey within a week and presale the week after with direct (and affordable) air shipping right after (we've negotiated a special rate with an Italian shipping service specifically for these boxes).

Last edited by Jason; 27th March 2013 at 05:54 AM.
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Old 27th March 2013, 06:42 AM   #867
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We are still working on them, but they are very close. The thing is we're working on a lot of different things so sometimes they creep along. The ACA kits are 95% ready, the chassis should be ready soon too - really! And they look better than the ones you saw.
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Old 27th March 2013, 09:55 AM   #868
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Thanks Jason and Variac for all your hard work!

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Old 27th March 2013, 10:55 AM   #869
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Cool. How will the back panel be handled? Will the ACA chassis accommodate dual laptop PSU plugs? Would the chassis also accommodate mod for a psu board and transformer?
I bought the 4U deluxe for my F5 - its awesome!
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Old 27th March 2013, 05:52 PM   #870
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The back panel is sheet metal and the holes are symmetrical. so it can be flipped to make mirror image monoblocks. Is that your question? The chassis's are small monoblocks, ie one channel each, just like the original Amp Camp Amp chassis' so each has one power supply connection. They are not big so if you want a conventional linear power supply you would buy one of the smallish chassis' without heatsinks that we offer, for the transformers and diodes and caps. The front panel on those will match the 10mm thick monoblocks. (Great new PSU PCB coming incidentally) Then have a detachable cord to each monoblock, or heck, have two separate power supplies, in one box or two!

This would maintain the advantage of the ACA with the Laptop supplies- individual supplies for each channel, located away from the amp circuitry.

Last edited by Variac; 27th March 2013 at 06:22 PM.
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