1 channel of MiniA working - measurments

[Stabist]

As I understood: it is better to replace with pot only R11

[Stabist]

I can't send a longer reply???

[Blues]

Originally posted by Stabist
As I understood: it is better to replace with pot only R11

If you are concerned with the signal path, yes R11 trimpot is a better choice.

[jleaman]

Here is my mini a up and running.. Both channels are running but here is a pic of one..


http://jleaman.ath.cx/dac/IMG_2314.JPG

Feel free to see my photo's ( taken at night )

http://jleaman.ath.cx/dac/

[Stabist]

Zdravo chip,
First about nick - yep it does - I own Stabi2/Stogi Ref combination and I quite like it ... And so this was an idea for nick ...
AHA - now the things become cleare a bit ...
Altough - shouldn't the math for that additional parallel resistance be like - I want to have 25,6mA - and I have 5,5V drop - so resistance is 215R - and yes - by adding 6k8 in parallel to 221R - you get something like that ...
As said before - the problem with mosfet matching is:
- those were purchased as allready matched - and I expected them to be so ... But OK - if not so - I'll just have to deal with them on my own ... Fortunately recently I purchased 50pcs (whole lot) of them - so there must be enough pairs for my all futur Aleph related projects in that lot
The thing that worries me more is - I also bought 14 "matched" output transistors - 12 for A30 and 2 for reserve ... BUT if they are also so badly matched as those 9610 - then I guess I'll have problems with building my A30 with them ( And to buy 25 or 50pcs of 240 is much more expencive than with 9610 ...
Well we'll see ...
Pozdrav

[Stabist]

Finnaly - from home PC it works normaly - from PC at the work I couldn't post more than few words long mesage?!?!?

[chip_mk]

Originally posted by Blues
chip_mk, if I got your explanation right...Stabist can also either tune his Aleph DC offset by replacing R11 or R14 with a trimpot say 500ohms. He can either tune with R11 so as he can have double the current through R14 or he can tune R14 to have half of the total current through R11. He can best tune either way by monitoring DC offset at the output.

Hi Blues

When equal current flows through Q1 and Q2 the pair works in its most linear part of the transfer curve and has highest amplification (transconductanse). However, this doesn't necessarily coincide with zero DC offset at the output. You can have it both only if the pair is well matched. Otherwise you either have well set input stage but some DC offset or zero DC offset but somewhat reduced performance of the input stage (or you make a compromise). That's why the matching is so important (although NP reported that in some cases not so well balanced input stage sounds good).

My suggestion was to use R11 for trimming in order to get optimal polarization of the input diff pair (symmetric current flow) and/or output DC offset. If you want to trim R11 use a serial connection of fixed resistor and a trim pot to provide +/- 20% trimming.

BR
Chip

[chip_mk]

Originally posted by Stabist
As I understood: it is better to replace with pot only R11

Hi Stabist

My point is that in most cases DC offset can be adjusted by slight trim of R11 (+/- 20%). Of course, matching of Q1 and Q2 should be as good as possible. I don't recommend simple replacement of R11 with trim pot since accidental turn to 0 ohm can damage the amp. Instead you can use serial combination of 180E resistor + trim pot of 100E.

However, since you have provided measured values (24.8mA via Q3 and 12.8mA via Q1) I was able to calculate that instead messing with trim pot you can achieve decent correction simply by putting 6k8 resistor in parallel to the existing R11 (which is 220E).

BR

Chip

[Stabist]

Hi Chip - I refered to idea of Blues, that should both (R11 and R14) have the option to be variable - and as understod your good explanation - I think it is not necessary to to do that with R14 at all ... Because changing R14 has bigger "side effects" than changing value of R11 ...

[fcel]

Chip,
Can you recommend a trim pot (in series with R11)? A typical trim pot resistance value that would work with any of the Aleph amps. What I meant is can you provide a PART NUMBER and a VENDOR NAME? I would like to put this trim pot on my order list the next time I'm ordering something.