DC offset from pre-amp affects power amp how?

[62vauxhall]

My plan to replace a female friend's Yamaha receiver with a Hafler pre and Adcom power amp went sideways. Her new boyfriend that she just moved into her place thinks anything other than streamed music is archaic and anything needed for analog playback is a waste of space. I can see this new relationship of hers lasting weeks maybe months at best. I really feel like kicking his f***king teeth in when he gave a condescending smile and shake of the head after I expressed being partial to vintage audio.

But I digress. With said amp and pre-amp, I noticed the power amp was warmer on one side after playing a CD. I thought that odd because I re-set the bias to factory and it's DC offset was reasonable. That prompted me to check offset at the pre-amp's outputs which is something I've never done before. One side was near zero but the other had 6 volts. I read elsewhere of the exact same problem with the same pre-amp and what the fix is. The parts are on order. But is my assumption correct that this circumstance would cause an abnormally warm temperature to the corresponding channel of the power amp?

Was also wondering if a sustained input of 6 volts for 30 or so minutes could have harmed the power amp?

 

[laatsch55]

6 volts DC is a BUNCH!! Joe can give you the technical version, but I know it's VERY bad.....

 

[Gepetto]

My plan to replace a female friend's Yamaha receiver with a Hafler pre and Adcom power amp went sideways. Her new boyfriend that she just moved into her place thinks anything other than streamed music is archaic and anything needed for analog playback is a waste of space. I can see this new relationship of hers lasting weeks maybe months at best. I really feel like kicking his f***king teeth in when he gave a condescending smile and shake of the head after I expressed being partial to vintage audio.

But I digress. With said amp and pre-amp, I noticed the power amp was warmer on one side after playing a CD. I thought that odd because I re-set the bias to factory and it's DC offset was reasonable. That prompted me to check offset at the pre-amp's outputs which is something I've never done before. One side was near zero but the other had 6 volts. I read elsewhere of the exact same problem with the same pre-amp and what the fix is. The parts are on order. But is my assumption correct that this circumstance would cause an abnormally warm temperature to the corresponding channel of the power amp?

Was also wondering if a sustained input of 6 volts for 30 or so minutes could have harmed the power amp?

Why was the amp channel warm? Because it had DC on it's output reflecting the preamp input which was going into the 8 ohm loudspeaker load producing heat (a steady DC current with the AC signal riding on top of it). Doubtful any harm was done to the amp but get that preamp fixed.

 

[WOPL Sniffer]

Iffn ya got some spare heat, send it to Idaho..... The Nutella is too thick to spread.

 

[62vauxhall]

Iffn ya got some spare heat, send it to Idaho..... The Nutella is too thick to spread.

Yeh, I don't like torn bread or broken toast either.

Thanks Joe, it does seem to go hand in hand. And Lee, I knew 6 volts was bad wrong.

What seemed a bit odd. at least to my thinking, was during the time the CD was playing, the power amp was not outrageously warm but after the disc ended and they both idled a while (the 30 minutes or so) that one side of the power amp warmed up considerably. Would DC exiting the pre-amp unadulterated by amplified signal cause that?

The fix I read of was replacing J112 FET transistors in the muting circuit. I should get them next week.

Far too late to do anything about it now but I'm a bit annoyed with the guy I got the pre-amp from. I did not need it but wanted to home a pair of Wharfedale Dovedale III's I had no room for. There were many problems with those speakers and so me restoring them got spread out for over two years. I was offered the trade and it was difficult to let them go because of the time/work/money I invested but I figured a pre-amp took up a lot less space than the speakers did. He told me a local tech (who I happened to know of) went through the pre-amp, replaced some capacitors and gave it clean bill of health. I took the guy at his word. It was just now when I noticed the offset problem that I had a look inside and not one thing on the board was changed out. I'm fairly sure he just dropped the tech's name and traded me a bill of goods.

 

[Northwinds]

At least the pre is fixable, as for the streaming dude... you can't fix stupid

 

[Billboard]

Good luck with the preamp. Streaming does not have to exclude vintage. I love my turntable and have invested a ton of money for a basket case PL-570 turntable that one day will sit in my main system.
I stream from my iPad through the aux port into my system and it sounds great.

Make sure you leave a 1/8 jack to RCA cord connected to the new setup and maybe both will be happy with the system.

 

[62vauxhall]

Not that simple. Although my friend and this guy been a couple sort of for a few months, she would rather he had his own place. But he got "reno-victed" and could not find anyplace to rent. I have to sympathize because he's not alone in that situation. There are a lot of older, affordable rental apartments getting bulldozed to make way for high density expensive condos. Rental vacancy rates around here are something like 0.5% and it's not unusual for a one bedroom anything to be $1500 per month if you can find one. Bidding wars on rental units are pretty common. The boyfriend is only supposed to be there until he finds his own place, at least that's what my friend says. But I've got a feeling this guy thinks otherwise. She makes good money and has owned her condo a while so it's a nice one and I'm pretty sure it's more high end than where he used to live.

Anyway....this guy's opinion of technology older than yesterday is that it's obsolete, archaic and therefore undesirable. I would expect that from someone 15 to 25 but not someone his age. He equates physical media (records, CD's tapes) with oil lamps and he lumps books in there too. He is quite smug about having an e-reader that can accommodate several thousand books and thinks anyone who devotes space to printed books is foolish. I doubt he ever read anything, probably thinks it takes too much time. He's apparently got some shit hot gear of some kind that gets around a billion TV channels so I bet he'd rather watch the movie than read the book. Probably could not stand using just one sense at a time - gotta get as many as you can stimulated at once.

So for the time being, the new/old audio stuff I would fix my friend up with is on hold. Which is actually a good thing since the pre-amp needs attention. I've known my friend for maybe 15 years and this boyfriend has been around for all of 5 months. She rolls her eyes at his statements that up to date technology is the only way to go. I wonder if he has some kind of blue tooth thing to play the music on his phone through? And I also wonder, what kind of music would he listen to?

In any case, I can see her punting him out sooner than later because he is irritating. She's not someone that tolerates being annoyed and I'm sure there are lots of things he considers himself the expert on so it's just a matter of time.

 

[laatsch55]

Nice summation Gary, gotta love a woman like that, does not suffer fools gladly....

 

[62vauxhall]

I ordered the J112 transistors last week and picked them up today but what I got were J211's. Just noticed that now after I got home. I had a look at the respective data sheets, but I've never needed to do anything before involving a J-FET transistor and only have the vaguest comprehension of a BJT, I have no idea if this was an automatic substitute from the supplier or if the vendor got the number wrong.

All I know so far is the pinouts are the same but the J211's gate current is 10 vs a J112's 50 and their function is give as chopper transistor for J112 and RF amplifier for J211.

Can anyone here with knowledge of J-FET's say if I can use a J211 in place of a J112? For what it's worth, they are for the muting circuit in a Hafler DH110 preamp - parts position numbers Q15 & Q115.

I suspect the answer is no and that the order got screwed up.

 

[WOPL Sniffer]

Maybe he meant to replace it with a J111 since ( I believe the J111 and J112 are interchangeable).

 

[62vauxhall]

Maybe he meant to replace it with a J111 since ( I believe the J111 and J112 are interchangeable).

Thanks. I may be able to get back to that shop tomorrow and if they cannot get J112's I'll ask about getting J111's.

 

[grapplesaw]

Thanks. I may be able to get back to that shop tomorrow and if they cannot get J112's I'll ask about getting J111's.

Here is the spec sheet for both. There is some differences to note.

http://www.onsemi.com/pub_link/Collateral/J111-D.PDF

 

[62vauxhall]

She misread my e-mail and is ordering me some J112's.

 

[62vauxhall]

I picked up the J112 FET transistors yesterday and when I got home, took off the DH110's top and bottom covers, looked at the component location diagram in the manual to find Q15 and Q115 then took a close look at the transistors on the board and what I found there were K163's. I had a look at that data sheet and compared it to that for a J112 and the pinouts are different. For J112 - (1) drain, (2) source, (3) gate but K163 is - (1) drain, (2) gate, (3) source.

I looked at the manual again which clearly shows, in two places, that the transistors in question are J112. The K163's legs are not crossed so it looks to me that in this instance (mute circuit) legs 2 and 3 are interchangeable. Regardless if this particular preamp was factory assembled or user built, it appears that the boards were populated at the factory and shipped complete.

Would my assessment be correct? Due to my "research" I ran out of time last night to re & re so time allowing, will do so tonight.

 

[laatsch55]

When I'm confused by a component locator I refer to the schematic. There seem to be less mistakes on a schematic than a component locator, and then of course the component locators can be from the component side or foil side, I've seen em be very confusing as to which side they are referring to.

 

[62vauxhall]

Thanks Lee. The only online manual I've been able to find is the assembly/user manual. It does contain schematics but not a complete one. I believe the main board was shipped complete or nearly so and not populated by the end user therefore no schematic. The manuals on HFE and Electrotanya are the same.

I am going to draw the conclusion that for whatever reason the factory substituted K163's for J112's, they did not create a different board. And therefore, installing J112's will work despite the difference in pin outs.

I shall find out later tonight.

EDIT: But since you mention it, I will review the parts list again for any K163's. Maybe positions Q15/Q115 are mislabeled on the component locator.

SECOND EDIT: The J112's are in and all seems good. DC offset at the preamp's outputs is 0mv.

The board in this thing looks what Don Imlay uses for the DC protect boards so I took care when removing the transistors. With this type of board, what is the recommended method to remove components? It appears that solder is present on both sides of the holes and one must wick it away from both sides. Is that a correct assessment?

I ask because I will be receiving some new capacitors soon. Since I was falsely told that new capacitors were installed when I acquired this thing, I thought I might as well actually do it. But I am concerned about the board's construction.

When I had my problem with the 700B's DC protect board and changed relays, I realized too late that removing a component "the old fashioned way" can (and did) cause damage. Can someone enlighten me as to their most goof proof method of removing things without hurting the board in the process?

 

[laatsch55]

Well??

 

[62vauxhall]

Well....the sucker didn't suck and the wicker didn't wick all the solder out of those holes so the old fashioned way is what it was - but gently. There was a lot of solder in them thar holes. All new electrolytic capacitors are now in and it's working fine. During my brief dabblings, I've encountered a few non-polarized ones but not to the extent of what this pre-amp has. Most are bi-polar

 

[Gepetto]

Well....the sucker didn't suck and the wicker didn't wick all the solder out of those holes so the old fashioned way is what it was - but gently. There was a lot of solder in them thar holes. All new electrolytic capacitors are now in and it's working fine. During my brief dabblings, I've encountered a few non-polarized ones but not to the extent of what this pre-amp has. Most are bi-polar

The trick is to sacrifice the component, cut it out leaving enough lead left to heat with soldering iron and then use either tweezers or the capillary action of the solder on the iron tip to pull out the remaining portion of the lead. Then after all the leads are plucked, the solder suck or solder wick the hole. I usually apply more solder to wet the via and then suck it out or wick it out.

One lead at a time is much easier than dealing with multiple leads simultaneously.