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Old 27th March 2006, 01:00 AM   #1
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Default hafler 9180 BIAS+Mods

Hi!

I'm not an electronics pro, but have a good handle on the basics (I hope). Wanting to get my hands dirty...

(#1)
Just got me a pair of 9180s. Love the design and build, except for one silly flaw... they used a 5 cent trim pot for the bias settings. I'm going to hunt for some multiturns to replace them. The schematics say to set bias to 200ma... No Problem. But just curious, would there be a noticable improvement making things a bit hotter? Maybe around 240ma... ? Also wondering if things would get too hot if I then used the amp bridged.

(#2)
Would putting a smallish polarized or ceramic disc capacitor in parallel with the huge smoothing capacitors help prevent noise entering the amp stage?

(#3)
When measureing DC at the speaker outputs, with no signal (input grounded), one channel reads 20mv that then reduces to about 11-12mv, while the other is stable at 1 or 2mv). Nothing to fear?

(#4)
Maybe absolutely nothing, but one channel has a tiny tiny buzz (need to place ear near the speaker)... Not a clue here.... Maybe normal? I'm used to the Yamaha M-series amps that are super-dead silent. The buzz is just one of those issues that once to notice them, never go away.


Another small easy thing I will do is bypass the input pots (drastically reducing the signal path, since I use an external passive box for balance/volume anyway.

Anyway, lovely design, great sound, and a layout just yearning for some small tweaks. Will welcome all advice.


Thanks.
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Old 27th March 2006, 05:24 PM   #2
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Hi bluesmoke,
1) There is nothing wrong with single turn pots. I don't recommend multiturn types in the bias circuit. Leaver the bias at the factory setting.

Bridging amplifier outputs does negative things with sound quality some times (many times) for a small increase in volume. Avoid.

2) Try some plastic film type caps here. They may or may not help.

3) Those DC offset values are okay. The drifting may be due to a diff pair that needs to be matched tighter. Not harmful though.

4) Hard to say about the buzz. Some of the electrolytics may be in poor condition. That would be a service type issue, not an upgrade.

Leave the volume pots. Turn down to play with interconnects, turn to max for listening. No problem. Leave it as stock as you can for the time being.

-Chris
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Old 27th March 2006, 08:54 PM   #3
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Hiya,

Thanks!

Just wondering why you don't reccomend the multiturn pots for the bias... (I read this just after picking up a bunch from half way across the city (the closest place there is)). Can become just as flakey? Needs to handle more current?

Will keep the bias to spec. What the hey. Sounds great as it is.

Will gently play with different power caps. As for the buzz issue, it's in no way serious. All the caps on the board look fine visably (no leakage), will take a peek at the solder joints next, just in case.

As for the volume pots. One amp is just fine (not crunchy), but the other one needs them to be replaced... May bypass them for the time being (again using my external volume pot). If I decide not to, back to the stores I go...

Thanks again for the info.
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Old 27th March 2006, 09:20 PM   #4
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Hi bluesmoke,
In multiturn pots, the wiper moves very small distances. They are sometimes less reliable than the larger single turn controls when passing DC current. I normally use contact cleaner on them if they get a little noisy (on the metal parts only). You can't clean a multiturn and the contacts are typically smaller.

I only see multiturn pots used in bias circuits when the design is bad. The value being too sensitive and the range great. Bad design work. Yamaha amps are famous for this trait.

A multiturn pot was designed for precision adjustments in things like instrumentation, control circuits for fine null. A bias control should be non of these things.

-Chris
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Old 28th March 2006, 03:59 AM   #5
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Definitely a good point.

What got me into all of this was:
---> http://www.tnt-audio.com/clinica/bias_e.html

So I guess it's a no-go on the multiturns I bought... Poo. Thought they would be cool for a nice accurate rating.

I know what you are saying about the Yamaha's. Read a lot about the touchy bias. Even when I had to get my Yammy professionally repaired, the first word that popped out was 'bias'.

The haflers are definitely stable in that respect, even with the lousy pots.

Just wondering, would "the source" (Catalog Number: 2729044) mylar capacitors be of any benefit in parallel with the power supply caps. Just for some kind of extra filtering? Again, I'm just shooting ideas. Sadly, they are the only place nearby... I actally have some big blue ones, but the voltage rating is not printed (and need something that can safely handle 75V, don't want to risk it if they are 50V)

Really there is nothing special to do with the amps but listen to them, except for all the noisy pots. Hate trekking downtown for $2 in parts (+$5 for TTC) when I'm also cripppled. Just eager to tinker, but not go overboard.
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Old 28th March 2006, 02:47 PM   #6
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Hi bluesmoke,
I see you live in Southren Canada! We still have some snow in the back yard.

Bypass the power caps with what you have that you are sure is 100V or more in a film cap. You may or may not hear a difference. You will have to look at the part in the package to be sure (the Source).

You can mail order from Electrosonic, Newark or Digikey plus others. Cheaper considering your time in travelling. I miss the days of Dominion Radio and Electrosonic on Young Street. We used to leave high school and pile down into Toronto on the GO train for our parts expeditions. You were sure to find just about everything you needed. Not even Sayal has a complete selection these days (watch out for "pulls" there).

Amplifier output stage bias is a variable thing. Set exactly to one value, if you monitor it you will set it change with temperature and sometimes AC supply voltage. Get it in the ballpark. Some amplifiers are better than others with regard to bias current compensation. In any event, it's not the end of the world.

-Chris
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Old 28th March 2006, 10:38 PM   #7
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We've been snow free, sunny and warm except for a chilly day on the weekend where I saw a few flakes fall.

I almost killed myself going downtown to over buy some more parts (had to treat the wife to lunch too). Got some wild looking philips .47uf 400V film caps cheap (marked Philips HQ), plus some smaller .22uf 250V. Also picked up replacement voume pots (use 'em if ya got 'em; I'll use them). No place I went to has 1K single turn pots!!!! Would have to try trekking up north again to do it. for a 15cent part!!!

I picked up a 'long style' multiturn as an alternative backup at 500ohms. All my readings for bias were around 400ohms, so maybe closing the gap a bit might help a touch with reliability. If I sense any problem, I'll switch back to a single turn.

Definitely bought too much (feel guilty and poor). Even picked up a new Multimeter as Radio shack on sale for $50. I feel it is the same quality as the $10 chinese junk one I'm using, but it can range up to 400ma... so I don't need to swap probe ports. Will donate the other to a friend in need.

Tonight will be the night.... will let you know if it survives....

Do you happen to know where to get Panasonic 50K stereo pots? Someone had sent me one, and love it for a passive volume control. Much better then the cheap tin can type.

Will post again later!
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Old 30th March 2006, 02:22 AM   #8
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Default Complete!

Well... I'm still Alive, and the amps are still alive!

Posted a pic of the caps I used. After crimping the lugs, I flooded the lug with solder for extra security. Can't tell if it did anything audio-wise. Easy enough to remove if needed.

Forced to use the multiturns I bought. They seem to have held the bias seting well . Kept it at spec. No other choice. Hope they last. No one had the dang single-turn parts (or at least more than one piece). This city really is starting to suck. May heed your advice and just mail order from home from now on. The long types would have needed to be mounted on a daughter board... (poo, waste of $6)

Put in some crunch-free volume pots. Think they may have been linear.... don't match the levels of the originals at 30/70% range. No whoop, I just use them to preset any global panning difference anyway.

Lastly, I found that Hafler did such a great job on the black case coatings, that they don't conduct! Attached ground onto the central brace, and front faceplate.

I think that buzz I had was from a cheap cable I was using. Once I grounded the input, nada. Next project will be to make lots of cables.

So I'm happy. Just tested them on my DIY bookshelf speakers, as well as a larger pair. (now feeling a bit deaf, plus feel the need to hear some bass again; hate bookshelves)... will move to my full-range in the days to come. May burn-in the amps on my beaters before connecting to the good stuff. Then will pull out my Yammy (gotta clean the relay contacts, and replace the binding posts); at a much later point in time. Need to recouperate.

Thanks again for the advice. Let me know if you have any other ideas, or advice on this.
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Old 30th March 2006, 02:57 AM   #9
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Hi bluesmoke,
Good job!

Music time. Just go slow and easy on any projects you have. When you rush, that's when problems occur.

As for mods, approach radical changes and "magic" parts with great suspicion.

-Chris
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Old 30th March 2006, 03:15 AM   #10
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Yup, I hear ya.

Definitely time to take a looooong break.

The TTC fare is going up again in April. I'm staying home.

Cheers!
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