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Old 2nd June 2012, 12:34 AM   #231
RNMarsh is online now RNMarsh  United States
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Quote:
Originally Posted by jcx View Post
use indirect IMD test - I had a noise floor of ~ 160 dB just by using ESI JULI@ soundcard DAC/ADC - one DAC channel for each of 10,11 kHz sine, resistively/summing amp added - then looked for the 1 kHz difference product with 30 dB gain amp/filter that rejects the test signal pair

ultra low harmonic distortion of the individual test signals isn't required - only the summing method has to avoid IMD at the level you want to measure the difference - can be easier than ultra low distortion osc/filters

3rd order nonlinearity can be probed with different freq pair 10,21 kHz gives 1 kHz 3rd order diff

the lower IMD difference products are more audible than harmonic distortion - at higher SPL levels masking is less
Another good idea to try that cost less than a full blown new AP. At some point could need thd just for apples to apples comparison purposes.
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Old 2nd June 2012, 12:44 AM   #232
RNMarsh is online now RNMarsh  United States
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Originally Posted by coluke View Post
Get a newer AP
The SYS-2722 analog generator seems to be in the 0.00003% - or better - THD range @ 1 kHz/2 Vrms out:
Attachment 285260
Did you manage to try Bob Cordell's THD magnifier? Still waiting for LA issue describing it (and the one with your amplifier, of course ), but it seems that with a pre-nulling circuit you can easily measure THD well below -120 dBc - I've quickly tried a resolution enhancing setup by Scott Wurcer and Walt Jung from an old AD application note, and it works very well indeed: with both the EMU internal generator and my old HP239A 3rd harmonic from my amp sits at about -115 dBc (@ 1 kHz and 7 kHz, 1 Vrms into 30 ohms).

L.
SYS2722 is about -136...... that might be Ok for awhile. But, then there is that nagging question... how much is curiosity worth for a one time test? Time or money? Hmmmm.
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Old 2nd June 2012, 12:29 PM   #233
coluke is offline coluke  Italy
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Originally Posted by RNMarsh View Post
BTW - what amp do you have? What is the topology?
Basically it's a pretty standard discrete open-loop CF topology- I've attached a simplified schematics (well, actually an over-simplified one - output CFPs are not going to work that way ) and a sample FFT run (@ 1 Vrms into 30 ohms). It works very well - in principle you could further improve performances by selecting bjts, but with 18-20 trannies/channel it could be a real pain...The interesting point is that you can easily convert it to closed-loop if you're interested in a CL vs OL comparison (and I was pretty curious indeed, but heard no appreciable differencies...), but I know it is by no means a simple circuit, nor it was that easy to have it running smoothly (@ about 0.2 ohm Zout, at least).

L.
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File Type: png CMOLAmpLoZ - THD 02.png (23.3 KB, 580 views)

Last edited by coluke; 2nd June 2012 at 12:34 PM.
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Old 2nd June 2012, 03:15 PM   #234
RNMarsh is online now RNMarsh  United States
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Originally Posted by coluke View Post
Basically it's a pretty standard discrete open-loop CF topology- I've attached a simplified schematics (well, actually an over-simplified one - output CFPs are not going to work that way ) and a sample FFT run (@ 1 Vrms into 30 ohms). It works very well - in principle you could further improve performances by selecting bjts, but with 18-20 trannies/channel it could be a real pain...The interesting point is that you can easily convert it to closed-loop if you're interested in a CL vs OL comparison (and I was pretty curious indeed, but heard no appreciable differencies...), but I know it is by no means a simple circuit, nor it was that easy to have it running smoothly (@ about 0.2 ohm Zout, at least).

L.
That's a nice variation on what has become what might be called the better topologies in use today .... current-mode, too. There are a million varieties and this one seems good too. However, as you noted, the complexity is much greater. And, is it dc stable in offset and drift?

For myself, the lowest thd is not the goal. I just want to measure it accurately. It seems, IMHO, that a more elegant solution/circuit is the one which gets the job done best with the fewest of parts and the lowest cost.

An Ic with external power output stage comes close to meeting the goal.... but usually still needs a dc servo. And, the better IC's are not cheap.

Thx,
Richard

Last edited by RNMarsh; 2nd June 2012 at 03:20 PM.
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Old 2nd June 2012, 03:39 PM   #235
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don't understand the expense argument - do you get free shipping? count every minute of hobby time as free even for the boring bits?

have to buy batches of discrete anyway for matching - I count my time doing repetitive, boring measurements/sorting as a cost

TPA6120 dual DSL driver wraped with OPA6142 ultra low distortion fet dual in a multiloop composite amp gives absurdly great active device performance for ~ $5 per channel at Digikey's overinflated singles price, ~ $2/ch in qty

Last edited by jcx; 2nd June 2012 at 03:43 PM.
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Old 2nd June 2012, 03:44 PM   #236
qusp is offline qusp  Australia
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TPA6120 dual DSL driver wraped with OPA1642 (fixed) ultra low distortion fet dual give absurdly great active device performance for ~ $5 per channel at Digikey's overinflated singles price, ~ $2/ch in qty
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Old 2nd June 2012, 03:56 PM   #237
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agreed though, I dont understand it either when people are so focused on saving a few dollars while blowing hundreds of dollars in man hours
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Old 2nd June 2012, 07:12 PM   #238
RNMarsh is online now RNMarsh  United States
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agreed though, I dont understand it either when people are so focused on saving a few dollars while blowing hundreds of dollars in man hours
It is in the context of commercial app. Yes, i spend a lot of time and money on this sort of thing. 'cause i like it. You would be surprised (maybe you wouldnt be) that to make a mass produced item - 2-5 per dollars is way too much. Its the total package i am talking about - how good, small, cheap etc can you make it. Sorry, thats just my own thing.... the challenge to myself to keep bordom at bay during the winter when I cant travel.

But for a single DIY project, $5 worth of IC's per channel can hardly be considered expensive. Certainly is fast and convenient to make for oneself. This circuit is an alternative. yep, a little more time is involved. Before this project I went that route and bought a top rated Headroom headphone amp. It didnt sound as good as what i knew the sound should be That is what got me to try my own discrete design. BTW -- did I mention that it Sounds Great!?

[Besides, I am retired and have lots of time to blow until I'm dead. Meanwhile, sharing the journey/story.] I'm leaving for Alaska today. See you next time in Thailand and Nepal! Enjoy!

Thx,
RNM

Last edited by RNMarsh; 2nd June 2012 at 07:42 PM.
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Old 2nd June 2012, 08:31 PM   #239
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Quote:
Originally Posted by coluke View Post
Basically it's a pretty standard discrete open-loop CF topology- I've attached a simplified schematics (well, actually an over-simplified one - output CFPs are not going to work that way ) and a sample FFT run (@ 1 Vrms into 30 ohms). It works very well - in principle you could further improve performances by selecting bjts, but with 18-20 trannies/channel it could be a real pain...The interesting point is that you can easily convert it to closed-loop if you're interested in a CL vs OL comparison (and I was pretty curious indeed, but heard no appreciable differencies...), but I know it is by no means a simple circuit, nor it was that easy to have it running smoothly (@ about 0.2 ohm Zout, at least).

L.
DC servo would be needed with that, Im also building something similar over the weekend, just curious.
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Old 2nd June 2012, 09:32 PM   #240
coluke is offline coluke  Italy
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Originally Posted by RNMarsh View Post
And, is it dc stable in offset and drift?
Output drift is less than about |5| mV over 2 hours or so @ about 100 mA Iq (fairly stable, and this was somewhat the real challenge with such a low Zout design goal), en plein air and with a far-from-optimum layout: not a precision DC amplifier at all , but seems fine for audio - actually a DC servo coluld be not necessary at all, although I've already tested one, just to be on the safe side.

Quote:
Originally Posted by RNMarsh View Post
It seems, IMHO, that a more elegant solution/circuit is the one which gets the job done best with the fewest of parts and the lowest cost.
Agree - that's why I'm so curious about your amplifier.

L.
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