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Old 20th May 2013, 08:33 AM   #2351
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Hi,

that is not so difficult to calculate yourself. If you know the gain of your amp (10x?) and the maximum output voltage Vrms (36?) you would need Vrms devided by the gain Volts at the input.
Finding a DAC that puts out this much could be difficult.

William
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Old 20th May 2013, 12:20 PM   #2352
Eric is offline Eric  United States
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With the following resistors values, the Aleph-X needs 2.5v to hit full output power:

R18 = 10k (input resistor)
R19 = 10k (input to ground)
R16= 100k (feedback)
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Old 20th May 2013, 12:44 PM   #2353
Eric is offline Eric  United States
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Default one step forward, two steps back...

OK, so things at work have slowed down a bit and I have some time to get back to my amps. The jFet input was working relatively well, but I wanted to follow up on Zen's request for voltage measurements across the CSS to see just what was going on.

The first thing that I did was to back the bias current down from 9A to 8A. This seemed to impact my Rel DC offset, bringing it down to about ~10mV or so, which made me really happy (it had been ~125mV). I also noticed the ZTX devices were running a bit warm - I measured them at about 50c which seemed rather high to me...

Then I started measuring across the back to back zeners in the CSS. For a brief moment, I saw about 9.2v on my meter, then my probe slipped and I ended up shorting the leg of the zener (+rail) to the leg of R45. A nice little spark resulted, taking out both zeners as well as the 9610 in the CSS. The output to ground resistors (5 x 390R, 5w bundle) got so hot, one of them de-soldered itself from the rest... schematic here

So, I replaced the Zeners and the 9610. I also used this as an opportunity to rebuild the LTP according to Zen's advice. I remeasured the 2SJ74's to make sure they still matched. I added 10R to the source leg of each of the 74's and cascoded them with a matched (hfe on my meter) set of ZTX550's.

But I missed something. I am still getting full rail voltage - from BOTH rails - across my speaker terminals now.

Any ideas on what to look for now?
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Old 20th May 2013, 12:48 PM   #2354
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Default ps audio aleph-x

so my ps audio perfect wave dac should be drive aleph-x because it have 5.5 volt balanced output but it did not sound right i have using aleph-p and i liked the sound now could any one let me know should i buy the mcintosh d100 for it's 16 volt balanced output ?
please let me know
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Old 20th May 2013, 01:47 PM   #2355
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by Eric View Post
....

But I missed something. I am still getting full rail voltage - from BOTH rails - across my speaker terminals now.

Any ideas on what to look for now?
full rail positive or negative ?

also - give us voltages (ref. that pdf from your post ) :

- across R23 &R25
- source of input Jfets ref. GND
- across source resistors of output mosfets , for each quadrant separately
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Old 20th May 2013, 07:26 PM   #2356
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OK, i made some measurements. Voltage across:

R23 = 14.0v
R25 = 0.0v

Source resistors on:
Q10 = 0.02v
Q11 = 0.0v
Q2 = 0.29v
Q1= 0.25v

JFET source to ground
R25 side = -9.2v
R23 side = -9.1v

Voltages on speaker terminals:
Negative post to ground = 23.6v
Positive post to ground = 20.2v
Across speaker terminals = 43.8v

Power supply rails = +/- 24v
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Old 20th May 2013, 07:34 PM   #2357
Zen Mod is offline Zen Mod  Serbia
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check input jfets and all connections , same as current through LTP CCS

I asked for polarity of output offset (which you didn't wrote ) , but it's clear from R23/R25 voltages - one side is full ookin throttle , while other side is Dodo
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Old 20th May 2013, 08:35 PM   #2358
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Thanks, Zen. Sorry that i overlooked the polarity of dc offset. Speaker terminal offset = +43.8v

JFET Source / Gate / Drain voltages ref to GND
R25 side: -9.2v / -10.0v / -9.2v
R23 side: -9.1v / -2.1v / -2.8v

CCS 9610 voltages ref to GND
Source = +18.2v Gate =+14.2v Drain = -9.11v

Voltages across:
R45 = 14.3v
R49 = 5.75v
R56 = 2.8v

ZTX550 ref to GND
Drain = -9.1v on R25 side and -2.8v on R23 side
Base = -0.8v pins tied together for all 4 ZTX devices
Collector = -10v on R25 side and -24.2v on R23 side
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Old 20th May 2013, 09:05 PM   #2359
Zen Mod is offline Zen Mod  Serbia
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it seems one of input LTP side is problematic

recheck everything , I can't find any sane relation between your values , especially ZTX

there need to be one voltage for all bases , emiters need to be 0V65 more positive , while colectors need to be at proper voltage for opening output mosfets ( 4 to 5 V positive ref. to neg rail)

when I said - polarity of output offset - I meant each output ref. gnd

when one write "ref. something " that always mean black probe parked on that "something"

so , when you write speaker terminal offset +xxx , I dunno where you parked black probe of your DVM , and I really don't care , because our goal is that speaker outputs are near ground potential , but they can't be any sort of reference
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Old 20th May 2013, 10:58 PM   #2360
Eric is offline Eric  United States
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Zen, you are right again! Only after inspection with a magnifying glass did I find a tiny solder bridge across two pins on the input differential. Removing it has worked wonders for my amp!

More evidence to my belief : 99% of the problems one faces are user error....

Bias and offset are now behaving properly.
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