[kinnon buchan]

with regards to the hints and tips given by David on upgrading the mos 125 ,I would advise all builders to remove c5 and the zenor zd1 , or if you fit a wire link across the zd1 it will bypass the cap and let you compare the amp modded and unmodded but beleive me you wont remove it .
another trick is to fit a 100k resistor across pins 2 and 6 of the opamp, again this will also be a no going back mod ,the 125 now outpreforms a bi-amped audio lab set up as well as my other two elektor amps which cost about three times as much to build .as well as an nva 80 and a sugden 21a.
keep up the good designs david

[Raj]

Hi Kinnon,

What quiescent current are you running your amp at? I've installed a 120mm fan at the back of mine and dropped the voltage of it to 7.5 volts, it is quite audible, but I've set the current to 560ma, it sounds awesome! I'll be purchasing the papst fans whihc are 80mm and only make 12db of noise!!!!

I had thought about doing away with the input caps also, and asked David about this, although he did say that it's a nervous subject as you have to check the ofset at the output, and if too high could be problems for the speakers, I havn't tried this, given that I do sometimes change sources.

Is your 125 the audiophile version, and what power supplies are you using?

I do agree with you about the sound though I have the mos250, and I've not heard anything even come remotely close to what this thing can do.

Thanks
Raj

[kinnon buchan]

Hi Kinnon,

What quiescent current are you running your amp at? I've installed a 120mm fan at the back of mine and dropped the voltage of it to 7.5 volts, it is quite audible, but I've set the current to 560ma, it sounds awesome! I'll be purchasing the papst fans whihc are 80mm and only make 12db of noise!!!!

I had thought about doing away with the input caps also, and asked David about this, although he did say that it's a nervous subject as you have to check the ofset at the output, and if too high could be problems for the speakers, I havn't tried this, given that I do sometimes change sources.

Is your 125 the audiophile version, and what power supplies are you using?

I do agree with you about the sound though I have the mos250, and I've not heard anything even come remotely close to what this thing can do.

Thanks
Raj

raj,
I have removed the input caps but i did check the buffer pre-amp- dac /dti that supplies this and it already has decoupling caps fitted so it was safe.
I am running the amp at about 200ma as I have electronic sympathy but I know that the third order distortion would be lower if you upped the input current . the amp is the audiophile one and it has a few other tricks done that i have learnt over the years of building electronics

[Raj]

Hi,

That's the reverse of what I've done, as I've removed the input caps from the buffer amp instead, and it was definatley a worthwhile upgrade. I'd of thought that removing the caps from either the amp or pre would have the same result?

Is there anything elese you could share on upgrades etc?

Thanks
Raj

[kinnon buchan]

Hi,

That's the reverse of what I've done, as I've removed the input caps from the buffer amp instead, and it was definatley a worthwhile upgrade. I'd of thought that removing the caps from either the amp or pre would have the same result?

Is there anything elese you could share on upgrades etc?

Thanks
Raj

the other good mod is to get some copper or brass shim and make a small box to cover the op-amps on the amp and the pre-amp boards and connect the box to ground to shield the ic ,this gives a better cleaner and more open sound
 KINNON

[DSR]

Hi   Ive got 2x all Vishay/ Black Gate/Elna Silmec
 version of  Mos 100  Teflon/silver wired with seperate
 15 volt power supplies etc    Bypassed with polyprop/silver Mica
 etc  Question   what is the easiest way to apply the 100k resister
 across pins 2/6 as I am reluctant to solder onto the chip as I would also
 want to use the metal shield idea. Suppose it will have to be a Vishay ?Will removing the 220 mf Silmec cap/wiring  accross make enough of a difference.  Dave[]

[davidw]

I find that the easiest way to add the 100k resistor is by wiring it across pins 2 & 6 of the opamp on the solder side of the pcb. Of course this means that you have to unscrew the pcb from the case and turn it over. You could also wire the resistor between the leads of any components joined to pins 2 and 6 on the component side of the pcb but this means long straggly wires, which I wouldn't recommend. And lastly, the best capacitor is no capacitor! The only reason for keeping the 220uF capacitor is to minimize the output offset voltage. This is around 1mV with the cap and 6mV without. Even 6mA is lower than that of most "commercial" amps.

[DSR]

MUST ADMIT THE REMOVAL OF THAT CAP C5 MADE A SIGNIFICANT DIFFERENCE.
  .MINE WAS A SILMEC..WILL TRY THE 100K RESISTER NEXT PLAYAROUND...BLACK GATES IN MOST POSITIONS EXCEPT WERE UNABLE TO GET 63 V VERSIONS...... QUESTION FOR DAVID..C16/C19....ANY THEORETICAL BENEFIT OF UPPING TO 220mf.....Leaving  C2/C7 at 100mf...Supplier hasnt got 100mf.....Another oldie is the still widely held view that op amps need 100nf as close as possible to ic,its also in some spec sheets...In my case using up space ZD1/2 vacated when seperate power supplys introduced...Dave