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65.12.113.3
UPS dropped off my Mouser order today, so I tore the top off the NHT MA-1 subwoofer amp. One of the things I'd ordered was a Texas Instruments TLE2072 op-amp to replace the TL072 the amp came with.
The TI site lists the 2072 as a direct replacement for the 072, and touts it as "the highest precision upgrade" over the TL072. For example, the TL072 has a typical slew rate of 13 V/us vs. 45 V/us for the TLE2072.
The op-amp is socketed, so replacement was simple. I listened to the stock amp for a few songs, shut it off, swapped op's, and listened again. Hmm. The bass wasn't as tight as before. More plodding.
To tell you the truth, I wasn't expecting to be able to hear much, if any difference. When Stereophile reviewed this amp in 1994, during the bench-testing phase they reported "amplifier's low-pass crossover functions, implemented using 4558 dual op-amp chips..", so I suspected THAT would be the op-amp to change for an audible improvement.
I upgraded the TL072CP to the TLE2072CP simply because it was easy, cheap ($2.25) and TI called the TLE2072CP "the highest precision upgrade" over the 072.
Could this simply be a burn-in issue with the new "improved" op-amp?
Thanks for any feedback!
Follow Ups:
You might check this site out. A lot of us utilize opamps for headphone amp use, both in SS and tube-hybrid designs.
...to taste. Watch out for oscillation though if untested.
Jfet's are great, and they even make some tasty cmos types -- but not for direct swaping.
Basic rule of Thumb IMO : If it's got OpAmps don't buy it. Well designed circuits manage to avoid these gizmos.
Wally Jung be damned :-)
I'm tellin'!
I might agree with you about avoiding Op-amps if this were a full-range amp. There might be well designed subwoofer amps out there, ones with adjustable frequency controls, but I haven't really looked.
I bought the NHT SuperZeros, along with the NHT sub and amp back in 1995. It makes a great rock system. As much as I love the magic of my Quad ESL's, they don't do rock so good.
I was in the mood to tweak, and this was easy. I'll be doing a full elecrolytic cap replacement on the amp as soon as my Digi-key order gets here. That's a bit more of a job.
I might as well replace that 4558 op-amp while I'm in there. I think that one will have a more direct effect on the sound. Of course I was wrong this time, so who knows?!
Definitely replace the 4558(s). I would not be able to sleep if I had one of those in the house even. A lot has changed in 35 years!
A few times the op amp swaps I made were audible and sometimes not. I'm very careful with bypassing/decoupling and always go with what the application notes for the op amp recommend and more.
Some people hear the words op amp and they freak out, waving a silver cross in front of themselves. They usually are not very knowledgable about anything circuit wise.
A lot has changed in 35 years.
Wow. An evil poltergeist, that 4558, eh?I have an OPA2604 sitting here, but it's slated to go into a Yamaha T-80 tuner I picked up, which also uses the 4558. That's a more critical application, for obvious reasons.
What I'd really like is a safe (non-oscillating) replacement for the 4558 which is known to have great bass. In this application, the sonics of anything above 100 Hz to 120 Hz will be less important, since it's a subwoofer amp.
I need to learn more about op-amps. I've done a lot of reading on this site and there's a lot of mixed information. At least it seems so to me. "Bypass this" "use ceramic caps", "NEVER use ceramic caps", etc.
Thanks for the input, nightdoggy. Just like your feedback on my JVC 1050 post, this is much appreciated!
WTF? I first wrote "unimportant", then decided "less important" would be the correct verbage, but forgot to delete my "un".
I love catching these things AFTER I post. Sheesh!
The rule is to use non-inductive caps on each supply rail and no sockets. This is so they will act as close to "ideal" capacitors at the very high frequencies, to provide stability and prevent oscillation. Remember that the op amps we want to use have open loop bandwiths in the MHz range. If you use a 1 uFd dipped tantalum paralleled with a 0.1 uFd ceramic disc or monolithic ceramic, you will not have any oscillation or supply noise issues. Remember to keep the cap leads short. The fact that both of these cap types are not appropriate for audio signal use is irrelevent. They are ideal in this dc application.
You can also add a .001 or .01 cap between V- and V+, (pins 4,8) but it is not essential.
There should also be a cap bypass across each feedback resistor. This is to reduce the op amp gain at very high frequencies. It will also address your slew rate concerns.
If you want to start rolling off the gain at about 100 kHz, use about a 33 pF cap between each output (pins 1,7) to each inverting input (pins 2,6). I generally use a 5 pF or 10 pF silver mica, depending upon the op amp and how much bandwidth I want. I like to minimize phase shift at the highest audible frequencies.
For subwoofer use I would go with an op amp with a bipolar input. Try a National LM4562 or 6172 or a Linear Technology LT1358CN8 or LT1361CN8 for example. Review the applications notes also.
I did find the 1050 service manual. It is pretty thick as service manuals go and it has excellent diagrams, board layouts, schematics, etc. etc. I'll never use it again at this point.
Happy tweaking!
Doggy
"the highest precision upgrade"
Usually means DC parameters to the manufacturer.
Tell us if it gets better with time.
I searched "slew rate" on here because I wasn't finding much info elsewhere, except for a definition of slew rate or tech graphs way over my electrical head.
I've included a link to a posting of yours from way back when. You mentioned that 20 V/us shouldn't be exceeded. Do you think this might be my issue, since I've trippled slew rate (up to 45 V/us) and bandwidth?
Just curious and trying to learn something.
Thanks!
The layout of most commercial pieces will not withstand the dropping in of a speed-demon opamp.
I've dropped in the OPA2604 for the LF353/TL072 and had better sound from top to bottom.
You could also try the AD823, a very neutral sound.
If you could get the schematic from the manufacturere I could suggest some real changes.
I'm going to contact NHT to see if they'll send me a schematic. I also looked up Ken Kantor's whereabouts, as I understand he's the designer of this amp. He might have a few suggestions if he's so willing.
I don't intend to get too crazy with upgrades on this thing. It's only an 80 watt amp, and I think something with more power than that might be useful driving a sub. I've seen various plate amps @ 200W with variable frequency controls going for $99 on sale. Those might be a better place to start any extensive tweaking.
I'm going to replace the 4558 op-amp with an OPA2604 while I'm in there changing caps. I suspect there's op-amps with better bass than the 2604, but I'm looking for the "drop in" reliability you spoke of.
Thanks for the feedback!
"The TLE207x series of JFET-input operational amplifiers more than double the bandwidth and triple the slew rate of the TL07x and TL08x families of BiFET operational amplifiers."
And a feature which was important to me, "Direct Upgrades to TL05x, TL07x, and TL08x BiFET Operational Amplifiers"
I'm not normally a spec chaser, but this seemed like a good upgrade to me, especially since TI says it's a direct replacement. I'm no expert, and when my fellow tweaks suggest "well, it should work, but it MAY oscillate" I get a bit uncomfortable.
I assumed that if nothing else, it would improve the performance (if not the sound) of the amp. And it very well may have, but at the expense of bottom end control...at least so far. And in a subwoofer amp, giving up the bottom doesn't make any sense.
I'll post again once I've used the amp for a while.
Thanks.
you are worrying about the slew rate performance of an op-amp in a sub-woofer than rolls off at (say) 200Hz ??
A complete mis-hit, IMO.
Could I explain why? Honestly, no, I couldn't. Not in terms of "the increased slew rate will cause the amp to perform in a more liner parameter at 80 CPS because blah blah blah".
I'm no expert, and don't pretend to be. I'm a music lover, kit builder, and parts upgrader. If the experience of many posters is that upgrading a 4558 op-amp to a 2604 results in better sound, with rarely a problem, I consider giving it a try.
In this case, the Texas Instruments web site says the TLE2072 is a direct replacement for the TL072 which was in my amp, and it's a better performer. Who am I to argue with TI? For $2.25 it was worth a shot.
I hope I don't sound rude in my reply. That's certainly not my intent. It seemed easy and cheap, and it was. It also may have been the complete mis-hit you describe. But I figured the "mis-hit" would be in the form of overkill, and no sonic change, rather than a somewhat degraded performance.
If there was no audible change from the increased slew rate and bandwidth of the new op-amp I never would have posted. It was the "better part = worse sound" that made me ask the question.
I'm sure that op amp changeout is much more audible then say the differences between two audiophile polypropylene caps. The TL2072 is orders of magnitude "better" than the ancient (30 year old) TL072.
For a couple of bucks, why not? I might be hesitant to go with a $25 op amp, but I'd still be tempted.
Also get rid of that 4558 as fast as you can.
It's bothering me and It's not even mine!
LOL
doggy
I've got a couple of AD827's I took out of the JVC 1050 a few months ago which I replaced with OPA 2604's.
Think the 827 would work in place of the 4558 or should I wait and order another 2604?
I'm sure it will be fine. I don't know what purpose your 4558s serve. I have Luxman tuners that use them to drive signal strength LEDs.
And you lef them in?!! Oh my gash! LOL!
Below is a section from my original post on the function of the 4558 in my subwoofer amp. Looks (and sounds!) to me as if it's in the signal path.
When Stereophile reviewed this amp in 1994, during the bench-testing phase they reported "amplifier's low-pass crossover functions, implemented using 4558 dual op-amp chips..",
To tell you the truth, it's the other chip (the TL072) which I'd like to know the function of, now that I've played with it.
Doggy, thanks for all your input!
And stomped up and down on them and then dropped the remnants down into the sewer in front of my house!
Actually I stole a bunch of OP270EZ (ceramic dips-nice!) op amps from work and used those.
The OP270 is a great op amp for dc applications like that and less than ideal for ac applications.
It is a hundred times better than the RC4558.
Yes, a hundred times exactly!
Gotta go, my cat just ran by me with another mouse in her chops.
doggy
Did the mouse have a 4558 in his chops? That's some well trained cat you have there doggy!
Kinda gross but I guess she loves me!
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