Nikko Preamps, etc.


newaag
 


Hi Radu
Glad to hear your Beta II worked out, you got a great deal. These units do have well thought out phono sections, all discrete, using the best selected parts available (back in the day). Nikko was serious about this market, at least for awhile...The later components were still good, but not made to the same level as the earlier stuff. All in my opinion, of course.
I actually had a couple 80's Nikko receivers here, which had really nice black face cosmetics (digital tuned). They were a lower profile than most receivers back then, and worked great. I gave one away to my friends son, who uses it daily. These came from a retired EE / electronics hobbyist who passed, and left this huge cache of mostly old parts, etc for his family to dispose of. They were charging me $10 / wheelbarrow to take it away...
But my wife was not happy...there was a lot of stuff in there I still don't know what to to with, old 50s, 60s parts.
Bob   

On Sun, Mar 8, 2020 at 07:16 PM, Radu Bogdan Dicher wrote:
To your points on the Beta preamps - now that I recapped the Beta II I got, and had some time with it in my system - they are truly great units (from the vantage point of the Beta II I have here). 
 
I don't feel I'm giving much up from my AI M2D (which can finally get on the bench for some well deserved TLC). With a Sony TA-3200F (which I also recapped, adjusted, and measured via AP box), it sounds like anything first grade I've ever heard. Both the Sony and the Nikko are really great units, btw. They sing into a pair of Thiel CS2s, playing very competently either digital from my homebrew DAC or vinyls from my Thorens turntable. 
 
A couple of points on the Beta II:
  • gain is indeed on the high end of what's out there, but the dedicated -20dB attenuation button makes up for it all and is very convenient. As anything additional on the signal path I'm weary of it, but I don't feel it harms the signal in any way. 
  • the stepped attenuator volume control is also a very nice touch. All mechanical controls and contacts etc. were in dire need of DeoxIT. 
I almost feel no rush to get my AI up to snuff and back on the main rack... That says a lot. 
 


Radu Bogdan Dicher
 

Bob,
Not sure if you've seen this (as maybe I and III don't have this), but the differential input pair (FETs) for the phono stage has a pair of variable resistors which seem to be for  adjusting the pair's balance (R419/R420) on the Beta II. However, no mention of this adjustment in the service manual. Thoughts?

My interpretation of this is that the adjustment only needs to be redone if the two input FETs are replaced and the pair loses factory balancing. In that case, the 6.5V voltage on the two sides needs to be brought back to same value. This is my assumption, and could use feedback.

If you need the service manual to evaluate this, I have it and can send it over.

Thanks!


On Tue, Mar 10, 2020, 7:42 AM newaag via Groups.Io <newaag=yahoo.com@groups.io> wrote:

Hi Radu
Glad to hear your Beta II worked out, you got a great deal. These units do have well thought out phono sections, all discrete, using the best selected parts available (back in the day). Nikko was serious about this market, at least for awhile...The later components were still good, but not made to the same level as the earlier stuff. All in my opinion, of course.
I actually had a couple 80's Nikko receivers here, which had really nice black face cosmetics (digital tuned). They were a lower profile than most receivers back then, and worked great. I gave one away to my friends son, who uses it daily. These came from a retired EE / electronics hobbyist who passed, and left this huge cache of mostly old parts, etc for his family to dispose of. They were charging me $10 / wheelbarrow to take it away...
But my wife was not happy...there was a lot of stuff in there I still don't know what to to with, old 50s, 60s parts.
Bob   
On Sun, Mar 8, 2020 at 07:16 PM, Radu Bogdan Dicher wrote:
To your points on the Beta preamps - now that I recapped the Beta II I got, and had some time with it in my system - they are truly great units (from the vantage point of the Beta II I have here). 
 
I don't feel I'm giving much up from my AI M2D (which can finally get on the bench for some well deserved TLC). With a Sony TA-3200F (which I also recapped, adjusted, and measured via AP box), it sounds like anything first grade I've ever heard. Both the Sony and the Nikko are really great units, btw. They sing into a pair of Thiel CS2s, playing very competently either digital from my homebrew DAC or vinyls from my Thorens turntable. 
 
A couple of points on the Beta II:
  • gain is indeed on the high end of what's out there, but the dedicated -20dB attenuation button makes up for it all and is very convenient. As anything additional on the signal path I'm weary of it, but I don't feel it harms the signal in any way. 
  • the stepped attenuator volume control is also a very nice touch. All mechanical controls and contacts etc. were in dire need of DeoxIT. 
I almost feel no rush to get my AI up to snuff and back on the main rack... That says a lot. 
 


Herb Ward
 

Radu,

That pot is for the phono stage DC balance as measured at the output ...junction of R437, R435

HRW

On Wed, Mar 11, 2020 at 11:16 AM Radu Bogdan Dicher <vondicher@...> wrote:
Bob,
Not sure if you've seen this (as maybe I and III don't have this), but the differential input pair (FETs) for the phono stage has a pair of variable resistors which seem to be for  adjusting the pair's balance (R419/R420) on the Beta II. However, no mention of this adjustment in the service manual. Thoughts?

My interpretation of this is that the adjustment only needs to be redone if the two input FETs are replaced and the pair loses factory balancing. In that case, the 6.5V voltage on the two sides needs to be brought back to same value. This is my assumption, and could use feedback.

If you need the service manual to evaluate this, I have it and can send it over.

Thanks!

On Tue, Mar 10, 2020, 7:42 AM newaag via Groups.Io <newaag=yahoo.com@groups.io> wrote:

Hi Radu
Glad to hear your Beta II worked out, you got a great deal. These units do have well thought out phono sections, all discrete, using the best selected parts available (back in the day). Nikko was serious about this market, at least for awhile...The later components were still good, but not made to the same level as the earlier stuff. All in my opinion, of course.
I actually had a couple 80's Nikko receivers here, which had really nice black face cosmetics (digital tuned). They were a lower profile than most receivers back then, and worked great. I gave one away to my friends son, who uses it daily. These came from a retired EE / electronics hobbyist who passed, and left this huge cache of mostly old parts, etc for his family to dispose of. They were charging me $10 / wheelbarrow to take it away...
But my wife was not happy...there was a lot of stuff in there I still don't know what to to with, old 50s, 60s parts.
Bob   
On Sun, Mar 8, 2020 at 07:16 PM, Radu Bogdan Dicher wrote:
To your points on the Beta preamps - now that I recapped the Beta II I got, and had some time with it in my system - they are truly great units (from the vantage point of the Beta II I have here). 
 
I don't feel I'm giving much up from my AI M2D (which can finally get on the bench for some well deserved TLC). With a Sony TA-3200F (which I also recapped, adjusted, and measured via AP box), it sounds like anything first grade I've ever heard. Both the Sony and the Nikko are really great units, btw. They sing into a pair of Thiel CS2s, playing very competently either digital from my homebrew DAC or vinyls from my Thorens turntable. 
 
A couple of points on the Beta II:
  • gain is indeed on the high end of what's out there, but the dedicated -20dB attenuation button makes up for it all and is very convenient. As anything additional on the signal path I'm weary of it, but I don't feel it harms the signal in any way. 
  • the stepped attenuator volume control is also a very nice touch. All mechanical controls and contacts etc. were in dire need of DeoxIT. 
I almost feel no rush to get my AI up to snuff and back on the main rack... That says a lot. 
 


newaag
 

Yes, Herb is right, that is a common amplifier adjustment as well for 0 DC at the output.
Those pots are for adjusting current from the current source, to balance the long tailed pair, but also correct for other circuit imbalance from downstream stages. The long tailed pair circuit itself should already be pretty balanced, as it uses matched-pair-in-one-die JFETS (2SK68A) and cascode transistors (2SC1775).

This is a really premium preamp phono gain stage design, as it uses both a current source on the tail (versus a resistor) and cascode circuit on top. It may look like a current mirror on first glance, frequently used there, but it's cascode, some feel the best architecture you can have there for audio stages used w/ feedback.
Marsh detailed the cascode circuit advantages in Audio Amateur way back in the old days, with plenty of
example circuits and measurements.    
More background here
https://www.electronics-notes.com/articles/analogue_circuits/transistor/long-tailed-pair-circuit.php
https://en.wikipedia.org/wiki/Cascode

from the wiki on cascode
"The major advantage of this circuit arrangement ... the upper transistor permits the lower FET to operate with minimal negative (Miller) feedback, improving its bandwidth."
Bob


Radu Bogdan Dicher
 

Thank you both.

Interestingly enough, it mirrors part of the modification I've made to the Triode Electronics board I have in my Dynaco MKIII monoblocks - namely, in my case, a cascode MOSFET CCS far more linearly loading the long tail pair pre-driver to the PP finals. It's been so long since I put this together it completely slipped my mind (in all honesty, I need to spend more time reading the schematic to get all this - thank you Bob for expanding on the details of the circuit). I enclose the schematic  for my monoblocks for reference. 

If anyone can point to (a source of) Marsh' article in AA, I'd love to see it. I'll try to locate it too. 

I guess then the pot is to be adjusted to null the DC balance on the PP as seen at the output of the stage - thank you Herb for clarity. I'll measure what DC I see on there first, to calibrate the effort.

Thank you,
Radu. 

On Wed, Mar 11, 2020 at 2:49 PM newaag via Groups.Io <newaag=yahoo.com@groups.io> wrote:
Yes, Herb is right, that is a common amplifier adjustment as well for 0 DC at the output.
Those pots are for adjusting current from the current source, to balance the long tailed pair, but also correct for other circuit imbalance from downstream stages. The long tailed pair circuit itself should already be pretty balanced, as it uses matched-pair-in-one-die JFETS (2SK68A) and cascode transistors (2SC1775).

This is a really premium preamp phono gain stage design, as it uses both a current source on the tail (versus a resistor) and cascode circuit on top. It may look like a current mirror on first glance, frequently used there, but it's cascode, some feel the best architecture you can have there for audio stages used w/ feedback.
Marsh detailed the cascode circuit advantages in Audio Amateur way back in the old days, with plenty of
example circuits and measurements.    
More background here
https://www.electronics-notes.com/articles/analogue_circuits/transistor/long-tailed-pair-circuit.php
https://en.wikipedia.org/wiki/Cascode

from the wiki on cascode
"The major advantage of this circuit arrangement ... the upper transistor permits the lower FET to operate with minimal negative (Miller) feedback, improving its bandwidth."
Bob


Radu Bogdan Dicher
 

(This is about to get even further off-topic. Hope that's OK. bI am also not starting a new topic as this is just as off-t. as the original)

More interestingly, maybe, though, is that the DC offset seems to be haunting me now. I have a Sony TA-3200F I've recapped, then adjusted and measured via AP box. Measured very well (whoever's so inclined, can read the measurements here:  https://audiokarma.org/forums/index.php?threads/sony-ta-3200f-restoration-saga.906201/). 

However, after a few hours of flawless operation, POP!, both channels stop outputting signal. A burned smell accompanies all this (no smoke, though), traceable to the speaker protection board, which is designed to trigger upon either low frequency output (maybe targeting motorboating?), or DC. The latter triggers mine, as I measure 0.85V and 1.38V at output L/R, respectively. 

I've been suspicious of the pre/final transistors, but finals being soldered, I came to reconsider deconstructing the amp too much before thinking this through. Thank you very much in advance for thinking on this with me.
Radu. 

On Wed, Mar 11, 2020 at 9:02 PM Radu Bogdan Dicher via Groups.Io <vondicher=gmail.com@groups.io> wrote:
Thank you both.

Interestingly enough, it mirrors part of the modification I've made to the Triode Electronics board I have in my Dynaco MKIII monoblocks - namely, in my case, a cascode MOSFET CCS far more linearly loading the long tail pair pre-driver to the PP finals. It's been so long since I put this together it completely slipped my mind (in all honesty, I need to spend more time reading the schematic to get all this - thank you Bob for expanding on the details of the circuit). I enclose the schematic  for my monoblocks for reference. 

If anyone can point to (a source of) Marsh' article in AA, I'd love to see it. I'll try to locate it too. 

I guess then the pot is to be adjusted to null the DC balance on the PP as seen at the output of the stage - thank you Herb for clarity. I'll measure what DC I see on there first, to calibrate the effort.

Thank you,
Radu. 

On Wed, Mar 11, 2020 at 2:49 PM newaag via Groups.Io <newaag=yahoo.com@groups.io> wrote:
Yes, Herb is right, that is a common amplifier adjustment as well for 0 DC at the output.
Those pots are for adjusting current from the current source, to balance the long tailed pair, but also correct for other circuit imbalance from downstream stages. The long tailed pair circuit itself should already be pretty balanced, as it uses matched-pair-in-one-die JFETS (2SK68A) and cascode transistors (2SC1775).

This is a really premium preamp phono gain stage design, as it uses both a current source on the tail (versus a resistor) and cascode circuit on top. It may look like a current mirror on first glance, frequently used there, but it's cascode, some feel the best architecture you can have there for audio stages used w/ feedback.
Marsh detailed the cascode circuit advantages in Audio Amateur way back in the old days, with plenty of
example circuits and measurements.    
More background here
https://www.electronics-notes.com/articles/analogue_circuits/transistor/long-tailed-pair-circuit.php
https://en.wikipedia.org/wiki/Cascode

from the wiki on cascode
"The major advantage of this circuit arrangement ... the upper transistor permits the lower FET to operate with minimal negative (Miller) feedback, improving its bandwidth."
Bob