I am a bit of irritated...
One of my Aleph 4 mono blocks had before a quite high DC-offset, 100mV – in spite of bought “good matched” input stage 9610-FETs. So I matched new FETs by myself – found perfect pairs with just some mV difference as well at 10 and 20mA. Expectantly I soldered them in and: starting cool with less than 10mV still 50mV offset after warming up! How is that possible? All the voltages in the amp fit to the schematic values, it works fine, both Fets are of almost equal temperature, 0,1% tolerance Resistors everywhere...
Has anyone a tip what the reason could be?
Klaus
One of my Aleph 4 mono blocks had before a quite high DC-offset, 100mV – in spite of bought “good matched” input stage 9610-FETs. So I matched new FETs by myself – found perfect pairs with just some mV difference as well at 10 and 20mA. Expectantly I soldered them in and: starting cool with less than 10mV still 50mV offset after warming up! How is that possible? All the voltages in the amp fit to the schematic values, it works fine, both Fets are of almost equal temperature, 0,1% tolerance Resistors everywhere...
Has anyone a tip what the reason could be?
Klaus
how to get 60°C?
But how?
At matching they get 10 or 20mA and dissipate 4V = 0,04 or 0,08W. Even w/o cooler just a warm up of some degrees from ambient – never the real temp of 60°C like at later operation in the block. So how to warm up the FETs? Of cause I waited until they stabilized at matching.
But how?
At matching they get 10 or 20mA and dissipate 4V = 0,04 or 0,08W. Even w/o cooler just a warm up of some degrees from ambient – never the real temp of 60°C like at later operation in the block. So how to warm up the FETs? Of cause I waited until they stabilized at matching.
AudioFreak, check this page - translate it with babelfish http://www.gmx.cz/ti_aleph.htm
and post what do you think
and post what do you think
Your probably better to use a PI filter in the power supply than to try and get almost perfectly matched pairs ... the reality is that they will creep apart a little regardless of what you do. If you want to get rid of hum, you need to remove as much of the ripple from the DC rails as possible.
Klaus said:
Hi Klaus,
I would experiment with R3 (221R) as this resistor defines the
gatevoltage of the outputfet. If outputQ needs to have e.g.
higher Vgs than diff pair delivers then the diffpair gets out of balance thus creating outputoffset:
Replace R3 with a 200R in series with 100R and a 100R trimmer
parallel to that 100R thus creating a range 200 - 250R.
Trim for lowest possible outputoffset when warm(!) and replace
R3 with the adjusted value.
Uli
czech
i dont know if babel fish knows czech
but this guy there said that he has 0,8% thd insted of 0,2% measured in stereophile and he replaced frontend mosfets with bipolars and it reduced thd to 0,08%
and in my interpretation concluded that aleph is not as good as it is beeing said
and loses with his thule audio because of dc offset and thd
i am rather sceptical about this opinion
and for sure aleph will be far superior than my current integrated taf - 555 es sony
i dont know if babel fish knows czech
but this guy there said that he has 0,8% thd insted of 0,2% measured in stereophile and he replaced frontend mosfets with bipolars and it reduced thd to 0,08%
and in my interpretation concluded that aleph is not as good as it is beeing said
and loses with his thule audio because of dc offset and thd
i am rather sceptical about this opinion
and for sure aleph will be far superior than my current integrated taf - 555 es sony
Well, I'd say that I'd tend to take that guys opinion with little if any authority. It is true that you can get lower thd with the right pair of BJT's but THD is not a measure of musicality unfortunately we only have our ears for that. You've also got to realise that a diy project will not measure and sound the same as the original product ... better or worse ... well that depends on how much tweaking you do.
Re: Re: DC-offset in spite of perfect matched FETs!?
Thanks, Uli, I´ll also test it in this way. But before I will match "warmed up" FETS - I will insulate them thermally so that they should warm up to heat even with the small power dissipation. Hopefully one of both possibilities will work...
Why "xsnailx" - not even telling a name - is trying to run down Aleph Amps in this strange way will stay his own secret, I think. If he ever has heared an Aleph - I don´t think so! Talkers...
uli said:
Hi Klaus,
I would experiment with R3 (221R) as this resistor defines the
gatevoltage of the outputfet. If outputQ needs to have e.g.
higher Vgs than diff pair delivers then the diffpair gets out of balance thus creating outputoffset:
Replace R3 with a 200R in series with 100R and a 100R trimmer
parallel to that 100R thus creating a range 200 - 250R.
Trim for lowest possible outputoffset when warm(!) and replace
R3 with the adjusted value.
Uli
Thanks, Uli, I´ll also test it in this way. But before I will match "warmed up" FETS - I will insulate them thermally so that they should warm up to heat even with the small power dissipation. Hopefully one of both possibilities will work...
Why "xsnailx" - not even telling a name - is trying to run down Aleph Amps in this strange way will stay his own secret, I think. If he ever has heared an Aleph - I don´t think so! Talkers...
Re: Re: hum at DC-offset
It is acceptable, but it would be nice to get rid of. With increasing offset at warming up the hum becomes louder - so it is causual. I have headred originally Pass Amps with also a very quiet hum, really no issue to care about - but for DIY perfectionalists...
jh6you said:
It is acceptable, but it would be nice to get rid of. With increasing offset at warming up the hum becomes louder - so it is causual. I have headred originally Pass Amps with also a very quiet hum, really no issue to care about - but for DIY perfectionalists...
I also like to pursuit perfection and, Yeah, I am with you on the pursuit of the ¡°nice to have.¡± By the way, I wonder whether the hum is really due to the DC offset. If the hum is at low level and if you do not mind a bit less tight bass, I would like to recommend a try to increase R29 to about 750. If you still hear the hum, I think the PI filter AudioFreak has mentioned would be a help.
JH
JH
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