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.
Attachments
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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
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
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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/diya...-amp-1-pictorial-build-guide.html#post3194595
It will be really useful for those of you just starting out especially.
http://www.diyaudio.com/forums/diya...-amp-1-pictorial-build-guide.html#post3194595
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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...)
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. ?
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.
Do you mean the article?
It's here:
http://www.diyaudio.com/forums/diyaudio-com-articles/158899-arpeggio-loudspeaker.html
It's here:
http://www.diyaudio.com/forums/diyaudio-com-articles/158899-arpeggio-loudspeaker.html
Speaker impedance question
Hi
I ordered the kit from the store and am looking fwd to building what will hopefully turn out to be my first real go at a solid state amp. Both sets of my speakers are vintage and 12 to 15 ohm impedance. I did manage to build a 3.5w diytube Getsetgo amp a couple of years back that drives them just fine so I'm hoping the 5w ampcamp will be more than enough. Should I look to change any components to drive these speakers? and/or would it be better to consider upping power supply voltage? (I did read the original JLH article and while a lot of it went over my head it suggested a higher power supply voltage for that amp into a 15r load). Although I like to solder
my knowledge is more basic than I would like.
Mike
Ps great that you guys have put together a high quality beginner level project.
Hi
I ordered the kit from the store and am looking fwd to building what will hopefully turn out to be my first real go at a solid state amp. Both sets of my speakers are vintage and 12 to 15 ohm impedance. I did manage to build a 3.5w diytube Getsetgo amp a couple of years back that drives them just fine so I'm hoping the 5w ampcamp will be more than enough. Should I look to change any components to drive these speakers? and/or would it be better to consider upping power supply voltage? (I did read the original JLH article and while a lot of it went over my head it suggested a higher power supply voltage for that amp into a 15r load). Although I like to solder
my knowledge is more basic than I would like.Mike
Ps great that you guys have put together a high quality beginner level project.
I took some measurements today.
My math tells me the output resistance of the ACA is around 2.4 ohms.
I'd like some confirmation, either from Mr. Pass or someone else who has taken measurements that this is a reasonable number.
This is how I calc'ed 2.4 ohms:
Connected a frequency generator to the input, 1kHz and 1V (should be arbitrary.)
Connected a scope to the output.
Measured the output voltage with no load, 6.4V
Measured the output voltage with a 10 ohm load, 5.2V
Measured the output voltage with a 5 ohm load, 4.3V
I then used the equation on the previous page to find that the measurement with the 10 ohm resistor gives an output impedance of 2.31 ohms, while the 5 ohm measurement gives an output impedance of 2.44 ohms. They were 5% resistors, the calculations are within this tolerance. I measured the resistors with a multimeter, but I generally don't trust readings down that low.
Best,
Nelson
My math tells me the output resistance of the ACA is around 2.4 ohms.
I'd like some confirmation, either from Mr. Pass or someone else who has taken measurements that this is a reasonable number.
This is how I calc'ed 2.4 ohms:
Connected a frequency generator to the input, 1kHz and 1V (should be arbitrary.)
Connected a scope to the output.
Measured the output voltage with no load, 6.4V
Measured the output voltage with a 10 ohm load, 5.2V
Measured the output voltage with a 5 ohm load, 4.3V
I then used the equation on the previous page to find that the measurement with the 10 ohm resistor gives an output impedance of 2.31 ohms, while the 5 ohm measurement gives an output impedance of 2.44 ohms. They were 5% resistors, the calculations are within this tolerance. I measured the resistors with a multimeter, but I generally don't trust readings down that low.
Best,
Nelson
Ah ha, I missed that post, thanks for pointing it out. It seems then that matching the output impedance to a speaker like the arpeggio is a bit more complicated than I previously believed. I'll have to put some more thought into this one.
The complete kit just puts three different items in your cart:
the parts kit
the pair of heatsinks and
2 PSU's
Just the PSU's (power supplies) are out of stock. If you click on the kit you get the parts kit and the heatsinks in your cart, and no PSU's, but you aren't charged for the PSU's...
I'd suggest getting what you can and then ordering the PSU's later when they're back in stock. They'll be back in stock soon, but the parts kits are running low but itwill be 2 months before we make up more ...
That was my mistake - somewhere last night in between moving all the categories around I forgot to re-add the bundle. It's fixed now
But as Mark says, we're out of PSUs for the time being. We'll have more PSUs next week.