Who can explain this preamp circuit to me?

[joe]

As an interim solution until my Pass pre is ready I am using a old English/Taiwanese budget pre. It is not too bad for the money but I thought a little bit of cap replacement can't hurt after 10+ years.

The attached schematic is basically the same as in my pre. I don't have a good photo of mine so this can serve as a reference here. In my pre is a tone control bypass switch (connects C525 with a volatge divider to ground with the output) and there is no C513.

I have changed the PSU caps and the output coupling cap so far with Panasonic FC and Rubycon ZL. My next canditates are C525 (47u in the (tone control?) feedback loop?) and C517 (47u, kind of bootstrap?) but I don't understand what they are doing so I don't dare to change the values of them. I have plenty of 470uF Rubycon ZL lying around I would like to use. They are a perfect fit mechanically as well.

Next step I would love to do is to reduce the gain as it is to high in my setup. I don't want to use voltage dividers on the input or output for this purpose. But as I don't understand the circuit I need some help to identify the resistors to change for gain setting (maybe R519?) and how much they can be altered without running into stability problems.

Any other ideas how to improve the performance?

Thx for your help
Joe

[darkfenriz]

Yes, you can increase R519

[joe]

Just found the schematics here:
original schematics

May be my questions more specific now:
- What is the function of C324 and C314, can I increase them to 470uF?
- Which resitors determine the closed loop gain (without tone control)?
The gain should be 6.25 V/V.
- Is it possible to reduce the open loop gain?

Thx

[currentflow]

C324 lies in the negative feedback path between the tone controls and the amplifier's inverting input (Q305 emitter) and is used in conjunction with R322 to roll-off the bass response below a certain frequency. It also serves to block any DC component from reaching the tone controls and the tone defeat switch to prevent clicks when operating. The bass roll-off appears to be set at around 1.3Hz.

C314 is a bootstrap capacitor designed to primarily increase the effective input impedance of the output stage around Q306.

Increasing the values of C314 and C324 as you suggested may have a slight effect on bass response, but I doubt it would be very noticeable. If you wanted to experiment, you could try 100uF in these positions.

Resistor R322 will control the closed-loop gain, but changing its value will most likely have an impact on the tone control performance. Increasing its value will decrease the gain. You will probably have to modify R326 / R329 so that the new gain level is maintained when the tone defeat switch is in the defeat position. This modification wouldn't be ideal IMO if you value the tone control performance.

Regards,
Steve

[lineup]

Originally posted by joe
Just found the schematics here:
original schematics

May be my questions more specific now:
- What is the function of C324 and C314, can I increase them to 470uF?
- Which resitors determine the closed loop gain (without tone control)?
The gain should be 6.25 V/V.
- Is it possible to reduce the open loop gain?

Thx

You have also one download at one members website:
NAD-106-100S-pre-sm.pdf
http://www.audio-circuit.dk/Schematics/List%20files.php


These amplifiers may look old, using Capacitors at input and output
and bootstrap caps, too.
But the designers are very clever at NAD. Have always been
And these preamps are often tuned/trimmed for very good performance.
This is why they have managed to sell in many 1.000
and still can beat some amplifiers using OP-amps for pre-amplifier.

Now upgrade with new capacitors using same values
is a very good idea, to get a fair comparison
what these little transistor amplifiers can do