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N Brock
diyAudio Member

Join Date: Oct 2007
Location: Cambridge, MA
Quote:
 Originally Posted by Variac OK, I went through the same thing.There were various "seat of the pants" estimates of how to achieve around 6.5 ohm output impedance. Then a friend N-Brock used the lovely bunch of formulas Morgan Jones gives showing how he calculated his speaker parameters to adjust for the new version of driver, as the old one driver. The result was that the cabinet needs to be about 25% bigger. More than we expected but doable- but I'd like to revisit those calcs before cutting wood.. ......... I need to review how to get the exact amp impedance, but I remember it seemed daunting. Any help?
What you say about the arpeggio is true, with the new driver, box volume went up about 25% using the same source impedance of 6.5 ohms. However, if you fix the box volume, and calculate for an ideal source resistance you get 5.2 ohms.

As I see it, there are two ways to approach this problem. The first way would be to measure the output impedance of the ACA and then tailor the Arpeggio (or cables) to match.
The other way, aka, how I would do it, is to adjust the output impedance of the ACA to be 5.2 ohms and build the Arpeggio as specified in Morgan's article.

It's true that calculating output impedance is difficult, measuring it isn't really that hard. Put a sine wave on your amp's input and measure the output (peak to peak) voltage with no load. If you put a load across the output you get something that looks like the picture below. This is a voltage divider. If you measure the output now, the voltage will follow the equation on the right. When V_out is half what you measured with no load, R_load = R_out. To actually make such a measurement you would need a fairly beefy variable resistor to use as R_load. Measure the resistor when you have half the voltage and that's your output impedance.

If however you wanted to adjust the ACA to a specific output impedance, 5.2 ohms in this case, you could replace R12 with a 250k multiturn pot wired as a variable resistor and get a 5.2 ohm power resistor (or combination of power resistors that measure 5.2 ohms) and adjust R12 until, again, you get a V_out with the load that's half the V_out with no load.

Hope that helps.

Best,
Nelson

EDIT: I apologize, the equation below was incorrect when I first uploaded. It has been fixed.
Attached Images
 DSCF0899.JPG (226.3 KB, 1228 views)

Last edited by Variac; 10th October 2012 at 02:00 AM.

 9th October 2012, 08:26 AM #402 JRKO diyAudio Member     Join Date: Dec 2003 Location: U.K. Boards arrived yesterday and heat sinks half an hour ago Just need to pick out some power supplies and I'm good to go __________________ If I'm not making noise, I'm making something to make noise
 9th October 2012, 03:05 PM #403 Jason   :) diyAudio Administrator     Join Date: Oct 2000 Location: Melbourne Nelson Brock's build guide for the amp camp kit is now available!
 9th October 2012, 04:25 PM #404 LBHajdu   diyAudio Member   Join Date: Feb 2002 Addendum to my review: Oh I forgot to state in my review, the Amp Camp is very quiet with the stock P.S. I had one of the 15W Tripath boards. It had nice sound but it hisssss like crazy which was a deal breaker for long term listening on the 94db\w Fostex speakers. I tried to quite the Tripath with bigger and bigger P.S. caps until I blew the s.m.p.s, I don’t think it was the source of the hisssss in the first place, I suspect it’s the modulator frequency which is spread in the Tripaths case. The 300b is quite with its dc heaters and the 6p13p has some 60Hz buzz with AC heating and indirectly heated cathodes but only at one foot away from the speakers can you hear it. With the Amp Camp you cannot tell that the amp is on, holding your head one foot away from the speakers. If you hold your ear one inch away from the whizzer cone you can tell if the amp is on or off, but the sound is very faint even there. Also let me state that this test was done without any shielding on the amp what so ever. The input leads are alligator clips. I would say you can consider it dead quite. Leve Last edited by LBHajdu; 9th October 2012 at 04:28 PM.
Nelson Pass
The one and only

Join Date: Mar 2001
Quote:
 Originally Posted by LBHajdu that “I can’t believe I built that” feeling has worn off now, where even a tin can connected by a string can beat a 100k system.

 9th October 2012, 10:24 PM #406 Variac   diyAudio Editor     Join Date: Dec 2001 Location: San Francisco, USA San José, Costa Rica My buddy N-Brock (who's named Nelson also in an interesting coincidence) was a principal organizer of Amp Camp, and now took the time to make up this tutorial. It lives in the diyAudio Store under "Support" It will be really useful for those of you just starting out especially. http://www.diyaudio.com/forums/diyau...ml#post3194595 __________________ "The geek shall inherit the earth" 2017 Burning Amp is November 12 in S.F. Thread here on diyAudio: http://www.diyaudio.com/forums/clubs...francisco.html Last edited by Variac; 9th October 2012 at 10:27 PM.
PierreQuiRoule
diyAudio Member

Join Date: Feb 2004
Location: Ottawa
Quote:
 Originally Posted by LBHajdu I’ve just notice something strange Mouser (Mouser Electronics - Electronic Components Distributor), there sort of like another Digikey, has 6,793 units of the 2SK170-GR(F) in stock. They have zero of the 2SK170-BLF in stock but they have 29,600 on-order of which they will receive 8,998 by 8/20/2012 and another 13,600 by 11/9/2012 (at least according to their web site).
Just wanted to thank you. I read your post in July, put in an order for some 2SK170BLs, patiently waited, and... they showed up in the mail today. Very good deal. Mouser has over 13,900 units in stock.

(With apologies if this is off topic...)

Variac
diyAudio Editor

Join Date: Dec 2001
Location: San Francisco, USA San José, Costa Rica
Quote:
 Originally Posted by N-Brock What you say about the arpeggio is true, with the new driver, box volume went up about 25% using the same source impedance of 6.5 ohms. However, if you fix the box volume, and calculate for an ideal source resistance you get 5.2 ohms.... ...If however you wanted to adjust the ACA to a specific output impedance, 5.2 ohms in this case, you could replace R12 with a 250k multiturn pot wired as a variable resistor and get a 5.2 ohm power resistor (or combination of power resistors that measure 5.2 ohms) and adjust R12 until, again, you get a V_out with the load that's half the V_out with no load. Hope that helps. Best, Nelson

I like that plan! we make a bigger box while you get the output impedance, and then in this utopian scheme everyone has Arpeggios.

Umm Mr. Papa Pass: What does messing with the feedback do other that change the output impedance? Is is more or less feedback to bump up the O.I. ?
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"The geek shall inherit the earth"
2017 Burning Amp is November 12 in S.F. Thread here on diyAudio:
http://www.diyaudio.com/forums/clubs...francisco.html

 10th October 2012, 11:39 PM #409 jonparkhurst   diyAudio Member   Join Date: Dec 2010 Location: USA I may have missed this but I think it was stated you could use one power block if you put it in one case. Also, where would I attach the volume attn/Pott if I wanted too do it all together?
fastbike1
diyAudio Member

Join Date: Aug 2009
Location: North Texas, USA
My apologies, but being a bit of a latecomer, can someone point me to the Arpeggios thread/info? A search for Arpeggios turns up too many threads to be helpful.

Quote:
 Originally Posted by Variac I like that plan! we make a bigger box while you get the output impedance, and then in this utopian scheme everyone has Arpeggios. Umm Mr. Papa Pass: What does messing with the feedback do other that change the output impedance? Is is more or less feedback to bump up the O.I. ?
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