Hello,
I'm referring to the phono preamp section of this integrated amplifer. We see a dual FET in differential mode, followed by half a 4558 dual opamp. We all know that nowadays there are so many better opamps than that one, but would the amplifier benefit sound- or noisewise from substituting it by, e.g, a NE5332 or LM833 - or an even better, faster, low-noisier one? Or is this section's characteristics mostly dominated by it's FET input what would make substituting the IC more or less useless?
Best regards!
I'm referring to the phono preamp section of this integrated amplifer. We see a dual FET in differential mode, followed by half a 4558 dual opamp. We all know that nowadays there are so many better opamps than that one, but would the amplifier benefit sound- or noisewise from substituting it by, e.g, a NE5332 or LM833 - or an even better, faster, low-noisier one? Or is this section's characteristics mostly dominated by it's FET input what would make substituting the IC more or less useless?
Best regards!
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id probably shove an NE5532 in there. Wouldnt bother with faster/lower noise without seeing the rest of the circuit to know if it's even worth it. it probably isnt.
I agree a NE5532 would be a good drop in replacement here. The GBP of the 4558 is only 3 MHz which is kind of marginal for a phono preamp, and I'm guessing may be inadequate if the MC option is used.
Mike
Mike
depends on the gain. 4558 are hissy at high gain. With Rf/Ri of 40:1 I knocked a lot of hiss off the mm RIAA stage of my RA-88a by changing from 4558 to 33078. But I had to install power bypass & feedback resistor bypass caps to stop the 33078 from oscillating.
I don't think it matters low gain. Lots of dynamic mike mixers & graphic equalizers use 4558, but don't hiss much.
I don't think it matters low gain. Lots of dynamic mike mixers & graphic equalizers use 4558, but don't hiss much.
A project like this is not without its perils.
1. Note the supplies: +19.5 V / -20 V. Not much choice besides a 5532 here.
2. I won't be one bit surprised if local rail bypassing needs beefing up. (Possibly best try a 100n between the rails first... depending on grounding scheme, heavier bypassing may introduce ground pollution, even though these rails ought to be pretty quiet.)
3. External LTP compensation should have returned its gain to unity well below the fT of a faster opamp, but these kinds of circuits can still be iffy and are generally best used with opamps using straightforward compensation schemes... 2-pole or nested feedback may not be ideal. If in doubt, install a socket.
4. 4558s rated for voltages this high are not even made any more, so desolder carefully.
5. An extra 5 mA of current draw should not be upsetting the supplies too much.
The feedback network looks pretty low in impedance, certainly not an ideal match with the current driving abilities of a 4558. I wouldn't consider its GBW a problem if you have an external LTP up front though, as that is no doubt going to add a fair bit in the audible range.
1. Note the supplies: +19.5 V / -20 V. Not much choice besides a 5532 here.
2. I won't be one bit surprised if local rail bypassing needs beefing up. (Possibly best try a 100n between the rails first... depending on grounding scheme, heavier bypassing may introduce ground pollution, even though these rails ought to be pretty quiet.)
3. External LTP compensation should have returned its gain to unity well below the fT of a faster opamp, but these kinds of circuits can still be iffy and are generally best used with opamps using straightforward compensation schemes... 2-pole or nested feedback may not be ideal. If in doubt, install a socket.
4. 4558s rated for voltages this high are not even made any more, so desolder carefully.
5. An extra 5 mA of current draw should not be upsetting the supplies too much.
The feedback network looks pretty low in impedance, certainly not an ideal match with the current driving abilities of a 4558. I wouldn't consider its GBW a problem if you have an external LTP up front though, as that is no doubt going to add a fair bit in the audible range.
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The JFETs totally fix the 4558's hissiness.
They also give substantially more gain and GBW.
The slew-rate is limited by needle trackability and it is nearly impossible to get in trouble at any useful gain.
I vote "leave it". But like my dog, you may not listen.
Chiming in to say about the same:
1) without simulating/calculating it, those Fets most probably supply 20 to 26dB gain before even reaching the Op Amp ... which means its own noise contribution is 20/26dB less important/offensive.
2) replacing/improving the Op Amp does *nothing* to improve input FETs noise which is the dominant source here.
3) RIAA EQ strongly reduces high frequency gain, where we *might* have needed improvement ... but we don´t exactly stress the Op Amp there, so ....
And strongly boosts Bass .... where no conventional Op Amp has any problem.
4) don´t own a dog, so can´t comment on that issue 😉
That said, this is just thinking aloud at 02:43 AM , please feel free to do whatever makes you happy 🙂
5) just as a side note, in the 70's and 80's designed and sold tons of PA mixers , my front end of choice for transformerless balanced Mic input was a discrete differential pair, followed by a .... 4558 ..... go figure.
Never could use Fets in a commercial product, WAY too much inherent spread was a PITA, so after much experimenting and measurement, the solid winner was an input differential pair , bipolar, made out of PNP transistors, usually BC559
VERY low noise, I used them with quite low idle current, around 100uA each.
Then in the 90´s we got hit by no Tariff, open Customs doors and ultra cheap U$D and our Electronics Industry was hit by a Tsunami.
So no more PA equipment. 🙁
EDIT: if you want to reduce overall distortion, have a strong hard look at cheesy mute clamps Tr3/Tr4 ... grab a couple sharp nippers and amputate them from the board ... what were they thinking?
1) without simulating/calculating it, those Fets most probably supply 20 to 26dB gain before even reaching the Op Amp ... which means its own noise contribution is 20/26dB less important/offensive.
2) replacing/improving the Op Amp does *nothing* to improve input FETs noise which is the dominant source here.
3) RIAA EQ strongly reduces high frequency gain, where we *might* have needed improvement ... but we don´t exactly stress the Op Amp there, so ....
And strongly boosts Bass .... where no conventional Op Amp has any problem.
4) don´t own a dog, so can´t comment on that issue 😉
That said, this is just thinking aloud at 02:43 AM , please feel free to do whatever makes you happy 🙂
5) just as a side note, in the 70's and 80's designed and sold tons of PA mixers , my front end of choice for transformerless balanced Mic input was a discrete differential pair, followed by a .... 4558 ..... go figure.
Never could use Fets in a commercial product, WAY too much inherent spread was a PITA, so after much experimenting and measurement, the solid winner was an input differential pair , bipolar, made out of PNP transistors, usually BC559
VERY low noise, I used them with quite low idle current, around 100uA each.
Then in the 90´s we got hit by no Tariff, open Customs doors and ultra cheap U$D and our Electronics Industry was hit by a Tsunami.
So no more PA equipment. 🙁
EDIT: if you want to reduce overall distortion, have a strong hard look at cheesy mute clamps Tr3/Tr4 ... grab a couple sharp nippers and amputate them from the board ... what were they thinking?
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Oh yes, PRR, I will! 'Cause you said exactly what drove me to my initial question:
As the unit still isn't mine yet, but on it's way to me, I'll give it very strong listening sessions first before doing anything to it.
Thanks a lot to all of you for your thoughts and suggestions! You've been very helpful!
Best regards!
[Edit] Btw, those mute clamps. Yesterday I've had it's little brother on my workbench, a DUAL CV1260. The audio signal was way too calm, and the input indicator LED's didn't work. It took me some while to find the reason: An unimposing 'lytic, corresponding to C108 in the 1460's schematics, was shorted, shorting out the LED's supply voltage and permanently activating the mute function as well.[/Edit]
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My unit has arrived this morning!
It's in pristine condition and works perfectly. But it definitely needs these muting transistors, as otherwise it produces loud farting and popping noise when switching on and off. Perhaps I'll need to add an dedicated speaker protection board?
Best regards!
It's in pristine condition and works perfectly. But it definitely needs these muting transistors, as otherwise it produces loud farting and popping noise when switching on and off. Perhaps I'll need to add an dedicated speaker protection board?
Best regards!
Attachments
A delayed turn-on might help suppress turn-on pops. All in all I hate speaker relays as a common source of failures.
Perhaps Nippon Columbia, the original designer, maybe also manufacturer, of these units also hated speaker relays for the same reason and provided those clamping transistors?
Brst regards!
Brst regards!
You could use this instead -
I've been using that setup for years. It works well and reliably...cheaper than a high quality relay too.
Mike
I have to admit that today I was tempted. I had the unit on my workbench in order to decrease the CD input's sensitivity (yes, my actual unit differs from the schematic in this detail, and it has a tricky tape-to-tape recording and monitoring gut that isn't documented). So I quickly desoldered the 4588, soldered a gold plated DIL8 socket instead and plugged in a TI NE5332AP. Well, the first IC oscillated like hell, and as the 2nd one didn't provide any improvement over the original NJM4558DX, neither noisewise nor soundwise, the original IC went back into the new socket and will remain there.
Best regards!
Best regards!
You could make the muting transistors switch reed relays instead of the signal and have the relays mute the signal where the transistors do now. Just a thought.
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Why? It works perfectly in it's original estate, i.e. with these transistors! Others here advised me to try without them. Didn't you read the whole thread?
Best regards!
Best regards!
You can try a RC4559 which can often work as a drop-in replacement for the 4558!
http://www.ti.com/product/RC4559
(Product code: 1459652 at Farnell).
http://www.ti.com/product/RC4559
(Product code: 1459652 at Farnell).