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Old 20th February 2012, 12:41 PM   #2651
tvrgeek is offline tvrgeek  United States
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Thorsten,
You bring up a good point I don't understand. We have a world where just about everything produces 1 to 1.5 volts output. All our amps product max output with about the same. Why so much gain in preamps? It seems like all we are doing is amplifying it just to burn it off in a pot to amplify it again. Instead of a gain of 10, why not a gain of 4? Use all that extra for something constructive like negative feedback? What good is a preamp that swings 10 volts? Would one that swings 4 very well be better? When I modeled the Rotel RA 840, I noticed it produced maximum output at 150mv in. So I am throwing away a gain of ten in the volume control which raises the source impedance on the input stage etc. I like your idea of selectable feedback. I might extend that to a step of 2.

I have seen old school dissection microscopes of about 20 power at auction. If I had space, that would help.

So, with proper analog and power, do you consider the DCX digital section to be suitable for high end? I was quite surprised how big a difference a decent external TI or Wolfson DAC made on my CD players. None of mine would be considered high end, but my "Muse"/NAD is at least as good as my Rotel 1070 and way ahead of any of my other CD's with internal DAC.
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Old 20th February 2012, 01:14 PM   #2652
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by tvrgeek View Post
You bring up a good point I don't understand. We have a world where just about everything produces 1 to 1.5 volts output. All our amps product max output with about the same. Why so much gain in preamps?
Tradition.

There are many good reasons why a lot of Poweramp's sound better with an active preamp (maybe better to be precise - one with low output impedance), and if gain, it seems people are convinced it gain's much (pun intended).

There is also the 12 O'Clock Dynamics brigade, they claim that if you have to turn the volume above 12 O'Clock the system has "bad dynamics"...

Quote:
Originally Posted by tvrgeek View Post
It seems like all we are doing is amplifying it just to burn it off in a pot to amplify it again. Instead of a gain of 10, why not a gain of 4?
Why stop there? Why not unity?

In my experience using "average efficiency" speakers (so 87dB/2.83V/1m) gives and "industry standard" output levels of 2V at full scale gives normal "audiophile" listening levels with around 16dB overall gain and "realistic levels" at around 26dB overall gain.

Seeing any gain actually used is rare.

Quote:
Originally Posted by tvrgeek View Post
Use all that extra for something constructive like negative feedback?
I would suggest use all that extra for something de-constructive, like removal of gainstages and negative feedback leaving a simple buffer with ideally J-Fet input (to not load the wiper of the attenuator) and either direct output (Z-Load >= 10KOhm) or a beefy diamond Buffer Z-Load < 10KOhm.

Quote:
Originally Posted by tvrgeek View Post
What good is a preamp that swings 10 volts?
Yes, that is a bit on the low side, i prefer around 30 -50V RMS before hard clipping... :-P

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Originally Posted by tvrgeek View Post
I have seen old school dissection microscopes of about 20 power at auction. If I had space, that would help.
USB based high magnification digital cameras are now common, cheap and tiny. I can still work on SMD down to 0603 size with naked eyes. It is the shaking of the hands that becomes a bit of a problem (not that my hands shake when you look at them, they seem rock steady.

Just the fine motor skills ain't what they used to be when I used to (occasionally) shoot the ace out of the card at 500m with a Dragunov on a tripod. Now postioning a chip cap 0.06in * 0.03in cap with tweezers to a lot less less than 1/10th of dimensions becomes a challenge.

Quote:
Originally Posted by tvrgeek View Post
So, with proper analog and power, do you consider the DCX digital section to be suitable for high end?
I am a little weary of Behringers DSP programming, but the answer is "probably". Certainly there is a lot of flexibility in the Behringer that you cannot get in analogue domain at all or just with incredibly complex stuff, at the press of a button and twist of a knob.

The ADC's and DAC's are Delta Sigma and not really my favourites, but I heard them in quite expensive recording gear where they did okay.

I'd probably go digital in to cut out the A2D.

Ciao T
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Old 20th February 2012, 05:53 PM   #2653
tvrgeek is offline tvrgeek  United States
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USB camera. Great idea. I'll look into that. So far my hands seem steady enough. I am sure it is only time.

While waiting on my Digi-Key order, I went back to pick up where I left off on my last set of speakers. A Seas 27TBFC/Zaph budget tower set for my Nephew. Got the first prototype crossover mocked up on my terminal board so time to build the second one and do final voicing. Gasp do the inductors keep going up. A pretty simple 4th LR effective crossover, BSC, and a couple of notches, costs more than an electronic crossover! This is using Jensen coils and dayton caps. Nothing exotic. I went looking for bulk wire and it costs more than pre-made coils. One more reason to quit trying to make metal cone mids to behave. If I get back to paper, I can push the crossover a bit higher and save half on the crossover. Spend that on a better driver in the first place.

At least fumbling around on the WEB I found my old wire supplier when I worked on my cars. KJ Co. 18 ga. 19 strand GXL or TXL wire looks to be a pretty decent generic hook-up wire. (Not for tube B+, you need thicker insulation) What I can get around town is more like 7 to 12 strand with cheap vinyl insulation. KJ sells it by the foot in dozens of colors. They were quite decent to deal with. Now I need to get a spool of something more suitable for internal signal hookup. I have just about used up my supply of "free" interconnect cables. I had some fantastic HP spec mini coax but used it up. If anyone knows some Beldon numbers they have had good results with, it would be appreciated. Even with wire, it is getting expensive to guess wrong. I have a 50' s-video cable I longer use, maybe I'll just start using that. It is just a pair of miniature 75 Ohm coaxes.
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Old 20th February 2012, 08:27 PM   #2654
dvv is offline dvv  Serbia
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On a different subject, here's what's nagging me: at what level of measured performance would you say that you're in no trouble? When is enough - enough?

For many years, 0.1% was considered to be "the magic line", menaing that once you get your THD, IM, etc values below 0.1%, and assuming your are not going wild with NFB, you're in the clear.

Perosnally, I prefer that "magic line" to be more like 0.05% into 4 Ohms, because they are more difficult, so I figure if an amp can cope nicely with 4 Ohms, it should be a clean one (but still not necessarily great sounding, but that's several other matters).

I've never investigated this in any organized way, but experience teaches me that this can be obtained with no more than 26 dB of overall NFB and without breaking the bank, and it should sound relatively clean and trouble free.
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Old 20th February 2012, 09:08 PM   #2655
tvrgeek is offline tvrgeek  United States
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Well, the DH-120 is rated .006 THD at 20K, IM less than .005 at 60W into 8 ohms, DF of over 100 @ 20K It fails my wife's golden ears. I confirmed less than .003 at 4KHz, 1W.

The Rotel 840 is rated .03 at any level to rated 40W, for THD and IM at 20K. DF ( no freq specified) is 30, but I measured 40 at 1K. It passes her test.

I am sure there are numbers that would achieve what you ask, but THD and IM would not seem to be them. I believe this is the very subject of this thread, if I would quit hijacking it to take advantage of the expertise available to understand amp design.
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Old 20th February 2012, 09:22 PM   #2656
GloBug is offline GloBug  Canada
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Single frequency ratings are almost meaningless if your comparing it to a complex musical signal.

They are just a spec that tells you how much distortion your getting if you were actually to listen to a 1K sine wave all night.
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Old 20th February 2012, 09:36 PM   #2657
jcx is offline jcx  United States
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the GedLee metric has some perceptual validation - heavily weights distortion at low levels

so it seems a good way to use THD, IMD is to see if they continually decrease with decreasing signal level, down into the noise - harmonic spectrum envelope shouldn't shift to higher orders as level goes down either

THD vs power is a pretty conventional plot for amp distortion – just pay more attention to the left end of the plot

the poor reputation of "conventional" distortion measurements probably comes from marketing depts searching for impressive #, touting super low THD at higher powers when stuck with an amp that has annoying levels of crossover distortion


how does your wife rate psychoacoutic compression – even 320k/-V0 is “tossing out” 75% of RedBook CD Shannon-Hartley “Channel Capacity” information

the compression algorithms only use ~6-7 bits mantissa per critical band
could be interpreted as humans seldom hearing anything much below 1% - possibly including “distortion” – that tracks signal level, fit our auditory processing "filters" structure

Last edited by jcx; 20th February 2012 at 09:42 PM.
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Old 21st February 2012, 04:33 AM   #2658
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by dvv View Post
On a different subject, here's what's nagging me: at what level of measured performance would you say that you're in no trouble? When is enough - enough?
Distortion audibility is a complex and multidimensional topic, SPL and Frequency feature heavily. THD is completely meaningless WRT distortion audibility.

So, if you want to be absolutely sure no distortion can be heard at any SPL and any frequency AND we insist on using THD as metric we need to set a minimum specification of better than around -20dB SPL for any harmonic at any SPL genertaed up to 120dB or so, so all distortion products would need be below -140dB at full power.

Clearly, that way doth madness lieth and the Map is marked in medieval script "than way be monsters"...

Ciao T
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Old 21st February 2012, 05:23 AM   #2659
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Well said, but why -20 dB?
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Old 21st February 2012, 07:21 AM   #2660
dvv is offline dvv  Serbia
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@jcx

If I understood you properly, you are referring to the phenomenon knows as "distortion comeback", which causes distortion to rise, sometimes dramatically, as power output decreases below say 1W?

Right or wrong, I always thought that to be a typical class B phenomenon, which occurs when the idling (bias) current of the output stage is set at very low values, say below 20 mA (no hard value, just an example). The Japanese used to do that quite a lot in the early 70ies.

I wondered about it - why do highly regarded say Sansui models, with nominally better specs, almost always sound worse than say Grundig (for those who don't know them, a once vast German popular electronics brand, catering mosty in the El Cheapo sector, later bougth out by Philips), which had a whole lot less power and worse specs? As far as I could determine, it was due to three factors: 1) The boys in Grundig knew a whole lot more than they were credited for, 2) their amps were initially designed to drive 2 pairs of 4 Ohm speakers in parallel, so despite their much lower nominal power outputs, they were initally way more load tolerant than the Jap fare, and 3) their quisecent current levels never went below 60 mA, and were often at 70-80 mA. Thorsten may have something to add to this list.

Anyway, I took my cues from Otala's work, and he postulated that the bias level should be set so as to be at around -17 dB of nominal power output. Well, that become quite a bit as your power level rises, but I still set my amps at 100...130 mA bias.

I've tried this on all production amps I own (from Sansui, Marantz and Harman/Kardon) and have found that by about 130 mA of quiescent current, differences in sound stop. You may be able to lift that up even more, depending on your heat sink real estate, but there is no additional gain to be had in terms of sound quality.

The paradox is that if you run an amp designed for say 70 mA at say 130 mA, your nominal THD specs deteriorate somewhat, but you do get a sweeter sound and you do all but eliminate any distortion comeback. At 2.83V/8 Ohms, equivalent to 1 Watt into 8 Ohms, classic forms of distortion tend to fall below the 0.001% mark, which is the limit of reliability of my equipment. The sound becomes audiably warmer and sometimes a little "rounded off", but it can also cause some loss of clarity, the overall becomes better but at the expense of some finer detail. This was particularly noticeable on my H/K 680 integrated (from 1999), while the older 6550 model was the happiest at its factory set value of 70 mA.

Not really surprised - increasing bias values makes them work at levels which they were not designed for, so anything can happen. But in general, Japanese products seem to benefit much from this, so long as you don't overdo it. In their case, you know you're overdoing it when their heat sinks become uncomfortably hot to touch (for models made after say 1975, before that time they weren't skimping on heat sinks) at even low nominal power levels.

That taught me an important lesson - there's no such thing as too much heat sinking.
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