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Author Topic: music man hd130 opamp  (Read 5380 times)

ilyaa

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music man hd130 opamp
« on: August 04, 2015, 05:59:40 PM »
oh boy - this amp is so confusing! at least starting with the PI onwards....

brief rundown: amp was working OK but had some weird distortion. opened it up and saw one of the caps across the plates had a broken lead - figured there was maybe some HF oscillation or something - replaced that cap - amp was working but had quite a large very high frequency (100KHz) oscillation on the output. after some soul searching i saw someone had put an additional 68k resistor across the existing one (R49) coming back for NFB from the load. not sure why.....removed it and the oscillation was gone! amp worked great and biased up fine - ~25mV across those little emitter resistors on the transistors feeding the tubes.

all good - lots of play for a while - then one day signal dies very low power....?? open it up and see that the voltages look good but the signal looks weird around the LM1458 - the little chip feeding the PI. i replace it with a 5532 and we are back up to power! EXCEPT (THIS IS THE CURRENT ISSUE) now the amp won't bias up right....im getting no less than 80mV across those little emitter resistors....ive tried a couple diff chips - the TL082 got me down to like 60mV but still tooo high. is it possible i need to replace it with an LM1458? thought any of those dual opamps would be about the same.......

other details is there is a very small about 1 Vp-p very high freq oscillation that disappears by the tube plates but is definitely present around that last opamp section....figure that might be related.......this amp is a bit confusing to me - anyone able to demystify??

ive been using this schematic: http://www.music-in.de/pics/hd130a.gif

J M Fahey

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Re: music man hd130 opamp
« Reply #1 on: August 05, 2015, 11:45:05 AM »
1) use an LM1458 which, thanks God, are inexpensive and still available.
"better/improved" Op Amps are not so there.
The amp was designed and compensated with the old one, a new faster one may throw stability out ofn the window.
Just as a stop gap, try adding a 100pF ceramic cap across R48 and R50 , that should take care of the HF oscillation.
2) *after*  you killed it, try to bias again.

Worst case, add, say, 1k across R65 and R68 , wait 5 minutes so it cools down (solder heat will throw everything out of whack)  and try again.
Check again 1/2 or 1 hour later (leaving amp on).

ilyaa

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Re: music man hd130 opamp
« Reply #2 on: August 05, 2015, 02:27:34 PM »
cool - ill track one down -

i tried an LM358 and it biased up fine but there was still a tiny oscillation - coming out at like 1 V p-p on the output but still there....

just to clarify, are you suggesting a 100pF across BOTH R48 and R50 (so two caps total), or somehow between them?

J M Fahey

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Re: music man hd130 opamp
« Reply #3 on: August 05, 2015, 03:22:24 PM »
Sorry, I noticed after posting that I might have not been clear enough: one 100pF cap across each resistor mentioned.

Noticed they are the resistors going from output to NFB application point, thus reducing high frequency gain (above the audio band anyway).

As of the LM358, that's one of my secret weapons: despised by many, obsolete, inexpensive (that's the main Sin I believe) , it can easily do some things impossible for others, mainly being able to amplify DC, down to ground/0 , both at input and output, fed from a single supply, which can be go as low as +3v (a single button cell) , it's *designed*  for 5V single supplies.

I use them all over the place in sensors (thermal and overcurrent) , as meters, etc.
Once it was a Saturday night and a desperate friend knocked at about 1130 PM with a dead 100W mixer, where a TL072 was dead and I had run out of Dual Op Amps (Murphy's Law) ... until I remembered the stick of LM358 .

In a pinch slapped one in: it worked (which was enough ;) ) , only it had a somewhat duller sound .... but quite usable still, and saved the night.

I asked him to come on Monday afternoon , (I'd go parts shopping in the morning) and repair it properly ......................................... have you seen him?  :o .................... because I have not  :lmao:

When we met at other moments he just told me: works fine, "someday" I'll take it to your shop.

Not recommending you do that, but in a pinch ..... you already know.

Once I mentioned this at DIY Audio .... they almost burnt me at the stake  :lmao:


ilyaa

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Re: music man hd130 opamp
« Reply #4 on: August 05, 2015, 05:50:34 PM »
damn i think it works well in there but i cant seem to get rid of the oscillation -

seems like it makes more sense to get a 1458 and see if that fixes the issue, rather than messing around with shunting caps here and there (already fried a couple LM358s touching my alligator clamps where they arent supposed to go!) -

ill let you know if that works!

thanks JM!

phatt

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Re: music man hd130 opamp
« Reply #5 on: August 06, 2015, 03:59:00 AM »
Now I'm not the sharpest tool in this shed,,, but I think the clue is the LM358 has very limited band width and hence if you use a higher performance chip it will need compensation.

Am I right? ;)
Phil.

g1

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Re: music man hd130 opamp
« Reply #6 on: August 06, 2015, 03:57:58 PM »
  Some amps are picky about only using 1458 in certain positions, many are not.
I think this is one of the fussy ones.  (along with the input IC on some of the smaller Marshall amps)

ilyaa

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Re: music man hd130 opamp
« Reply #7 on: August 07, 2015, 02:35:56 AM »
true -

put in a 1458 and good as new.

this is one situation where the original part does have the 'mojo,' even though its technically inferior, i guess!

Enzo

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Re: music man hd130 opamp
« Reply #8 on: August 13, 2015, 09:00:57 PM »
Don't think of it as inferior, it is just the part the circuit was designed for.  Newer parts that are faster, cleaner, wider in bandwidth, and so on are not better here.  If your mom's sedan needed a new carburetor, a four barrel Holley would not likely be better there, regardless of how much better it may be elsewhere.