Phono preamp fix

Hello,
I gutted Non working NAD 701 that I had for its phono preamp.
It works pretty well with very good power supply, but I am trying to make it better and don’t know how to. Does any one know what parts to replace or take out of this circuit to make it better?
I don’t want to buy 100-200 dollar phono preamp just to find out its not better than mine.
Turntable is Thorens TD150 with mm Pickering cart and project debut 3 with stock Ortofon omg 5 e.
As a newb i need “buy this part and put it here or replace this part for that part”
Instructions.
Many thanks
 

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You might try the most famous NAD preamp, the NAD 3020 mm preamp...You might see some similarities :)
 

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Last edited:
Carl T Hess,
Thank you, I replaced the resistors in signal path with better ones.

I was hoping if there is a way to make the transistors have higher idle current , change or take out parts ( electrolytes to non polarized caps in signal path withe different value ) some change in frequency response coming in and outgoing... I don’t know that much about it, but this preamp is supposed to be good as is as mentioned and similar to 3020.
Thank you both
 
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If you retain an electrolytic at the output then I would measure the DC offset of the preamp and make sure the cap is fitted according to the measured offset.

The diagram shows what appears to be +0.2V printed on the paper, which means the cap as shown is fitted incorrectly... but don't worry, such discrepancies in service manuals are common.

The same applies to the feedback return cap. Fit it according to the measured voltage across it.

You won't gain anything swapping transistors and you could actually make things worse if you get mismatched parts.
 
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If you mean the standing current that flows in the two transistors at the output then that is pretty much fixed by the circuit design rather than individual parts... that is the beauty of feedback, DC as well as AC. It makes the design repeatable and independent of the parts used.

The two diodes and their combined volt drop help set the bias current but I wouldn't really recommend you go altering things on a whim :) You won't improve it audibly.
 

PRR

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...to adjust higher bias etc ....

It's a pretty rich design. I've run whole preamps on less current than Q203 alone sucks. It all interacts: changing current in one part will throw-off the design unless some other part is changed. Holistic.

It violates my design rules. But my preamps are not acclaimed.

I'm quite sure NAD's gurus spent a LOT more time tweaking this than I ever could.

I'd leave it alone.
 

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Odd design, Q205 is the output bjt, relative high Zout. Lowered by feedback at low freq but might proof sensetive at high freq, #13 is an issue indeed (simulate phase resp).
Q207 is a current source topsy turvy with bootstrap known from generic circuits.
You can make a 180Ω resistor out of six 100Ω resistors: one 100Ω resistor parallel to four 100Ω resistors in series (yields 80Ω), and this network in series with another 100Ω resistor.
 
hi all, looking at the schematic more I realized I omited an input cap of 4,7mf electrolytic and a ground resistor of 330k right next to this cap... I installed them back into the circuit and it sounds much better.
it was anemic before... thats why I was looking to fix it...

thank you all!