diyAudio

diyAudio (http://www.diyaudio.com/forums/)
-   Pass Labs (http://www.diyaudio.com/forums/pass-labs/)
-   -   How to ruin a perfectly good alephX (http://www.diyaudio.com/forums/pass-labs/18526-how-ruin-perfectly-good-alephx.html)

grataku 2nd August 2003 03:24 PM

How to ruin a perfectly good alephX
 
In short:
add input capacitors.

The long story:
after getting the differential probe I readjusted the AC current gain which was kind of off and I got an additional 3 volts RMS out of the amp.
This was all dandy until I pugged the amp into the speakers at which point the DC offset at the speaker was no longer 15mV like I measured it before, but 200 mV. I always use a cap in the input when testing the amp since I never bothered measuring the DC offset of the oscillator. I wasn't worried because earlier in the system with preamp and speakers only a modest 10mV increased was produced. So I figured input caps don't matter.

So a couple of days ago I got a batch of 4.7 N type BG and I put them in. While DC is back under control, the whole tonal balance of the amp is changed for the worse, and it now sounds flat, ie the harmonic content of the instruments is gone (well, gone enough).

I am now reevaluating the whole need for AC current gain...

jarek 2nd August 2003 06:53 PM

Hi,
Did you read this?
http://www.diyaudio.com/forums/showt...519#post105519

Peter Daniel 2nd August 2003 07:11 PM

How to ruin a perfectly good alephX
 
I still can't figure out if you complaining about caps or active current gain?;)

grataku 2nd August 2003 08:32 PM

Re: How to ruin a perfectly good alephX
 
Quote:

Originally posted by Peter Daniel
I still can't figure out if you complaining about caps or active current gain?;)
As a result of my "adjusting" the current gain I run into this DC offset problem, but the caps screwed up the sound so I guess it's a little bit of both.

Jarek,
I don't think manifacturers are smart enough to be able to do what Jcarr claims they are doing (artificially coloring the sound of audiophile parts)

Netlist 2nd August 2003 08:41 PM

Re: How to ruin a perfectly good alephX
 
Quote:

Originally posted by grataku

after getting the differential probe I readjusted the AC current gain which was kind of off and I got an additional 3 volts RMS out of the amp.

Do you mean R12/R34 in Grey's schematic?
I took out the input caps in my 'final' version, readjusted DC and all is fine now and sounds good to me.

/Hugo

jarek 2nd August 2003 09:52 PM

Grataku,
About two weeks ago I changed one of the filtering caps on my PSU from 10 000uF to 20 000uF. It was a shock for my AX and the sound went worse, no musicality etc. After one week, things started going better and after another week I observed first signs of the musicality . Sound is still changing.
You did two significant things with your AX. Changing the current gain, you changed needs of the current in your amp and the PSU must adapt to that. New caps at the input also have big influence on whole amplifier performance. I think, you must be patient and verify your observations after running amp some time.
I know, that what I say is very controversial, but my experiences are just like that. :(

dshortt9 2nd August 2003 10:28 PM

Be patient, caps take time to break in. I have replaced caps in electronic equipment and they do sound flat for a while. BG in particular can take 10's of hours to break in.

grataku 3rd August 2003 02:30 AM

Re: Re: How to ruin a perfectly good alephX
 
Quote:

Originally posted by Netlist

Do you mean R12/R34 in Grey's schematic?
I took out the input caps in my 'final' version, readjusted DC and all is fine now and sounds good to me.

/Hugo


Yep, things are ok in terms of DC on the bench as soon as the pre amp and speakers are put in the offset jumps up to 200mV and over. This is with absolute and relative DC offset adjusted to 0 after the amp is at steady state temperature.

Let's hope this flatness comes from BG break in.

macka 3rd August 2003 05:37 AM

Grataku,

I think I understand what you are saying, but something else could be wrong or out of wack here.

Previously you ran the pre amp direct (without capacitors) to your X Aleph with only 10mv offset?

You have only tweeked the AC current gain, so both sides of the amp have the same AC current right (ie same value for R 20 & R9 or R12 & R34 depending on the schematic).

If this is the case then only VR1 for the CCS of the input pair needs to be tweeked for near zero DC offset. There should be sufficient range if the actual DC bias is also balanced on each side, you may need to reset this also after tweeking the AC current gain and then check the ac current gain for close to 50%.

However, it depends on the conditions under which you calibrate the amp for 50/50 AC current gain and DC offset.

When I set up my AC current gain I fed the ocsillator via the pre amp to get a true balanced input to the X Aleph. The BOSOZ with active ccs is capacitor coupled with BG's, so there is no dc bleed to the X Aleph.

I tweek the AC current gain in this condition and the DC offest to simulate actual operatiing conditions (ie with a 8 ohm load).

So I cannot see why the BG's are necessary when they were not there previously for 10 mv offset?

It is also possible that if the BG's are not a perfect match then the balance of each side of the amp will be uneven. I think Peter Daniel raised this in another thread.

Perhaps try without the caps (bypass them) to realign the current gain and offset and see what happens. Besides 200mv although a little high is not too bad for the price of good sound, anything inder 100mv is for this type of amp is very good.

Hope this helps,

Macka


All times are GMT. The time now is 04:35 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio


Content Relevant URLs by vBSEO 3.3.2