Power Amp Bias setting

Modvlar

Hiya,

Well, I got me my haflers (9180's), and think they are engineered incredibly well, except for one single stupid cheap part... the variable pots for the bias setting.

A - I'd like to replace them at some point with precision pots somehow, but doubt the solder holes will match anything.

B - Any tips on setting the bias? The manual says 200ma is the recommended setting. The value seems about right since there are 2 FETs per rail. The cheapo pots actively float between 180-210 when I've reset them (hate that stupid paint the put over them so makes it a pain to change them).

Not sure if it's related, but being an amp, there is a tiny microscopic buzz in one of the channels. Might just be my brain, but felt when I adjusted the bias to the 190ma level, it was reduced somewhat... Am I going crazy? I think I'm just spoiled from using the old yamaha M-series amps with 127db S/N... Not a single peep at full. Annoying.... but you have to put your ear directly to the tweeter to notice it. Maybe add some kind of capacitor somewhere for extra filtering?

Anyway, great amps, may even bypass the (crunchy) input gain pots for a much shorter signal path. Way too easy to mod. I use an external box for balance/levels anyway. DIYaudio seemes to have disappeared today...

Any ideas welcome.

heiney9

I was going to suggest DIYAUDIO.COM as they would have the best most practical knowledge. I've gotten lots of answers and help from over there. Hafler has a cult like following and I see them discussed quite a bit. Perhaps someone here might be able to help, but DIYAUDIO is your best bet. The site is still there as I was just there. Do a search over there. Those guys are good and usually respond pretty quickly

On a related note this same weakness is also present in the 1st generation Nakamichi PA-5 and PA-7 amps. Poor biasing pots..........a newer more reliable pot is the simple fix. I suspect the same is possible for the Hafler.

H9

Modvlar

Yup, DIYaudio is back, probably just site maintenance. Seem to have more info on the older units... but I'll be hunting in other categories too.

Found a great link: http://www.tnt-audio.com/clinica/bias_e.html
; that expains things in good detail. Just wondering if anyone here prefers to run hotter, with any noticable sonic differences?

heiney9

Modvlar wrote:

Yup, DIYaudio is back, probably just site maintenance. Seem to have more info on the older units... but I'll be hunting in other categories too.

Found a great link: http://www.tnt-audio.com/clinica/bias_e.html
; that expains things in good detail. Just wondering if anyone here prefers to run hotter, with any noticable sonic differences?

I think for most robust amp designs it's ok to bias it to run longer in Class A. Seen many discussions on the subject, but there is a point of diminishing returns and in some cases it's just a bad idea. It's hard to generalize; take each amp/situation and evaluate in alone. Plus it helps to have measuring equipment and a working knowledge of said amp design. If not, you're better off leaving it to factory spec. It takes some knowledge and practical experience to get it right. That's where the DIYAUDIO guys come in.

That is a great helpful site and just pose your question about your model Hafler and see what shakes out. Most of the people over there know their stuff.

Good luck

H9

Modvlar

Yup Yup, Posted on DIY... will wait and see.

Seems the 100ma per transistor (pair) rule seems to hold true. May just stick to it. Sounds insanely clear anyway at that setting.

Saw a lot of opinions against bridging power amps; was to be my big plan for monoblocking.... Not that I listen to music anywhere near mega wattage. Just wanted a bit more power than needed for the safety of my monitor 10s....

Always paranoid, never satisfied.