Hi, not sure if i am at the correct forum... been having some fun for a while with this preamp and now i am having questions about some details in its design.
Can someone explain the reason of the FETS in front of the final stage opamp ?
Ive played a lot with different opamps in it and ended up using Burson audio opamp V5ID (no change so far compared to ne5532 or lm4262).
This preamp generate a fair amount of hiss at medium volume no matter what, i would like to modify it to reduce it if it is ever possible. Ive considered tossing away completely the circuit and using it only as a passive volume and switch but that removes the tone control.
I croped the schematic to ease the process, it is in multiple pages and it wasnt easy.
Thanks for any details.
Can someone explain the reason of the FETS in front of the final stage opamp ?
Ive played a lot with different opamps in it and ended up using Burson audio opamp V5ID (no change so far compared to ne5532 or lm4262).
This preamp generate a fair amount of hiss at medium volume no matter what, i would like to modify it to reduce it if it is ever possible. Ive considered tossing away completely the circuit and using it only as a passive volume and switch but that removes the tone control.
I croped the schematic to ease the process, it is in multiple pages and it wasnt easy.
Thanks for any details.
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Probably to reduce the equivalent input noise current. Because of the fairly high impedances used everywhere, the equivalent input noise current has to be low to keep it from dominating the overall noise level. Nonetheless, it is still a fairly high noise design, because the high resistances everywhere lead to a highish thermal noise voltage.
Assuming you use the line output and a main amplifier:
If the output level it reaches is greater than needed to drive the main amplifier, you could simply put voltage dividers between the preamplifier and the main amplifier. (Another option is to reduce the impedance levels everywhere, but then you need to redesign the whole thing.)
Assuming you use the headphone output:
Does reducing R603 and R604 help?
Edit: you can find a datasheet for the headphone amplifier IC here: https://www.digchip.com/datasheets/parts/datasheet/413/LA4170-pdf.php It's in Japanese, but the schematics and graphs are pretty international.
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Replace the carbon resistors with metal film? That made quite a difference in my old phono stage.
That only helps for the noise floor when they carry DC current. R710, R717, R721, R725 and their colleagues in the other channel do.
i already replaced most if not all the resistors with RN50/55 metal film resistors. didnt noticed much difference.
To give an example of what I proposed in post #2:
Suppose you have 11 dB more gain than you actually need to play loud with the volume set high. You could then change R753 and R734 from 560 ohm to 1500 ohm, reduce R735 and R736 from 68 kohm to 560 ohm and increase C711 and C712 from 2.2 uF to 22 uF, 50 V. (I hope I have all the reference designators right, they are hard to read.) That would reduce the output level and output noise by about 11 dB.
The load on the op-amps is then about 2 kohm, which almost all op-amps can handle. The output impedance of the amplifier is slightly less than it is now, so treble loss due to cable capacitance will not be worsened. The time constant of R753 + R735 with C711 will be far above 10 ms, preventing bass loss. You would have to check whether the turn-on mute function still works properly; chances are it would.
Suppose you have 11 dB more gain than you actually need to play loud with the volume set high. You could then change R753 and R734 from 560 ohm to 1500 ohm, reduce R735 and R736 from 68 kohm to 560 ohm and increase C711 and C712 from 2.2 uF to 22 uF, 50 V. (I hope I have all the reference designators right, they are hard to read.) That would reduce the output level and output noise by about 11 dB.
The load on the op-amps is then about 2 kohm, which almost all op-amps can handle. The output impedance of the amplifier is slightly less than it is now, so treble loss due to cable capacitance will not be worsened. The time constant of R753 + R735 with C711 will be far above 10 ms, preventing bass loss. You would have to check whether the turn-on mute function still works properly; chances are it would.
there is, indeed, most probably, too much gain overall. one thing for sure is that the volume potentiometer gives its worst noise figure at 50% level....so i would need to adjust overall gain to give best performance out of this range. right now my usual listening level is like 30-40% of the volume pot i would say. So it would make sens to renduce overall gain. Also, the opamp i am using can drive 600 ohm load per specs.
Personnaly i would be more tempted to reduce the gain of the opamp circuit itself rather than attenuating the existing noise at its output?? i dont know, i am asking. thanks very much for the helpfull answers.
Personnaly i would be more tempted to reduce the gain of the opamp circuit itself rather than attenuating the existing noise at its output?? i dont know, i am asking. thanks very much for the helpfull answers.
i saw that the beta 50 has a magnitude of 10x for 330k/56k so 33k and 5.6k but it doesnt have the tone controle in that loop (connection at C715/R731) i guess this could help reduce the overall noise
thanks for all the replies.
thanks for all the replies.
Hi Pat,
I did warranty for Nikko back then. I also have a Beta 30, not a bad preamp at all.
If you look at that circuit, you'll see some pots earlier on. With JFets, there is next to zero gate current (unlike op amps and BJTs). They eliminated some capacitors by doing this, probably raising the input impedance also. I have never had to examine that circuit in detail.
Anyway, improving the power supply would be helpful. Much of what you did wasn't helpful - as you discovered.
I did warranty for Nikko back then. I also have a Beta 30, not a bad preamp at all.
If you look at that circuit, you'll see some pots earlier on. With JFets, there is next to zero gate current (unlike op amps and BJTs). They eliminated some capacitors by doing this, probably raising the input impedance also. I have never had to examine that circuit in detail.
Anyway, improving the power supply would be helpful. Much of what you did wasn't helpful - as you discovered.
i already did some changes to the power supply, as well as adding some shielding to the transformer-on/off switch area, there was always a presence of humm. the shield eleminated it all, but the back ground noise really drives me nuts. this is a 80's era preamp. i cannot believe there wouldnt be anything that could be done to reduce the hiss, especially with newer opamp or a better circuitry. maybe i am chasing myself or a am too adh/ocd with that....but the hiss is defenitively too present. i am not calling it a bad preamp but it is surely not to my taste.
Looking at the tone control circuit, I think the gain of the op-amp stage is about 5 at frequencies in the middle of the audio band. That is, at frequencies above the range that the bass control is supposed to control and below the frequency where the treble control kicks in, C503 and C506 can to a first approximation be treated as short-circuits, and C507 and C509 as open circuits. What you are left with then is R501, R503, R506 (or whatever the 12 kohm is called) and R729 and R731. R501 and R503 would attenuate the feedback signal four times (1 + 10 kohm/3.3 kohm), but the rest increases the attenuation a bit to about five or so. (I would normally calculate this more accurately, but I'm a bit too tired for that.)
Assuming you just want to reduce gain and don't want any tone control at all:
If you should want to reduce the gain to 1, so about 14 dB less, it would suffice to short R729 and adjust the frequency compensation to the lower closed-loop gain/smaller feedback attenuation. Increasing C709 and decreasing R723 by about the same factor as the gain reduction should do the trick, so 4.7 nF and 220 ohm instead of 1 nF and 1 kohm.
If you want to reduce it to some value greater than one, you could disconnect the tone control circuit from R731, replace R729 and R731 with resistors with much smaller values to reduce their thermal noise (but not so low that you overload the op-amp output), choose R729/R731 + 1 equal to the gain that you want, increase C715 such that C715 R731 >> 10 ms and again increase C709 and decrease R723 by about the same factor as the gain reduction.
Hi Pat,
Not defending it, and I don't use it in my systems. I have improved a number for people, and it pinch hits when needed.
It is a better preamp than most give it due. This is the measured response on one I did. Not too shabby. -10dB is 1 volt out. The peak at 60 Hz is the test setup.
Not defending it, and I don't use it in my systems. I have improved a number for people, and it pinch hits when needed.
It is a better preamp than most give it due. This is the measured response on one I did. Not too shabby. -10dB is 1 volt out. The peak at 60 Hz is the test setup.
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dear Anatech, i think its fair to ask what have you done to them. have you kept the original opamps ?
should i put them back in (did try several, 5532, tlo, bursons, lm4562,...).
i like to try things.
thanks.
should i put them back in (did try several, 5532, tlo, bursons, lm4562,...).
i like to try things.
thanks.
among the opamps in my parts bin....LM4562, OPA2134, NE5532, TLO's.....most of them are old or NOS. used them when i was a tech full time.
i found the original NJM2043D that where in the BETA so i may just slap them in to hear the difference compared to the Burson...(never actually made such test)
yes that it a dead Alpha220 in the background (exploded like nuclear war for no apparent reason, played at very low volume, just went pow, still under investigation as to what happened (still in the WTF mode))
i found the original NJM2043D that where in the BETA so i may just slap them in to hear the difference compared to the Burson...(never actually made such test)
yes that it a dead Alpha220 in the background (exploded like nuclear war for no apparent reason, played at very low volume, just went pow, still under investigation as to what happened (still in the WTF mode))
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Hi Pat,
I can't remember which op amps I used. It doesn't matter greatly as the NJM2043 isn't bad. Probably LM4562. Discrete op amps always have higher distortion than good ones. I did pay attention to a lot of different things, a holistic approach. Nothing radical and it is still got tone controls that work as they should.
I have some Alpha 220 and 230 amplifiers. The Alpha 440/450 aren't a good option, those do go bang all by their lonesome. Your Alpha 220 is actually very reliable. Keep the original DC servo Op Amp.
It had a good reason to bang, the design is sound. By this time it is possible a bias control got noisy and went open. Use a single turn control. To really tell you have to see what died. I can't say anything if it isn't on my bench. All I can say is that these are not failure prone. Also watch for oscillation given the age of the amp.
I can't remember which op amps I used. It doesn't matter greatly as the NJM2043 isn't bad. Probably LM4562. Discrete op amps always have higher distortion than good ones. I did pay attention to a lot of different things, a holistic approach. Nothing radical and it is still got tone controls that work as they should.
I have some Alpha 220 and 230 amplifiers. The Alpha 440/450 aren't a good option, those do go bang all by their lonesome. Your Alpha 220 is actually very reliable. Keep the original DC servo Op Amp.
It had a good reason to bang, the design is sound. By this time it is possible a bias control got noisy and went open. Use a single turn control. To really tell you have to see what died. I can't say anything if it isn't on my bench. All I can say is that these are not failure prone. Also watch for oscillation given the age of the amp.
fwiw about the 220, i am member on a FB page for Nikko vintage lovers, i am more active there. did ask about Alpha 220 going up rave like mine and i had like 10-15 peoples who told me exact same patern. Blew up at very modest volume.
Mine got serviced a while ago by someone else and he gave up at some point, i finished it. I had it in my basic set-up for at least 5 years in biamp mode, playing the upper band. Never ever got abuse. Just went fubar last week, it sparked and transistor hoods flew. All 4 outputs on the left channel are short, pre-drivers and pre-pre drivers as well. Two resistor toasted too.
Noted for the opamps. Still just would like to reduce hiss.
thanks.
Mine got serviced a while ago by someone else and he gave up at some point, i finished it. I had it in my basic set-up for at least 5 years in biamp mode, playing the upper band. Never ever got abuse. Just went fubar last week, it sparked and transistor hoods flew. All 4 outputs on the left channel are short, pre-drivers and pre-pre drivers as well. Two resistor toasted too.
Noted for the opamps. Still just would like to reduce hiss.
thanks.
Well, I have done warranty on them since new and serviced them ever since. Properly serviced they do not often fail unless abused. I am not defending Nikko at all here. I worked on all kinds of equipment.
What I do see are units overheated (in a rack system), and units improperly serviced. Those I get in. It sounds like your amp wasn't properly serviced or had fake semiconductors used. The design is solid aside from the bias pot in a position where losing contact would create excessive bias current. You wouldn't believe how many hacked units I see where the hacker is proud of their work and is convinced they did it properly.
This goes for all brands and types of equipment. "Everyone is an expert and this is easy work." Most do not have the equipment required to do the work properly, or assess it after they are done. "I can hear distortion". Nope, many exceed 1% THD.
What I do see are units overheated (in a rack system), and units improperly serviced. Those I get in. It sounds like your amp wasn't properly serviced or had fake semiconductors used. The design is solid aside from the bias pot in a position where losing contact would create excessive bias current. You wouldn't believe how many hacked units I see where the hacker is proud of their work and is convinced they did it properly.
This goes for all brands and types of equipment. "Everyone is an expert and this is easy work." Most do not have the equipment required to do the work properly, or assess it after they are done. "I can hear distortion". Nope, many exceed 1% THD.