Line stage modification

[electronic]

Hello there.

I want to change some things in one of my amplifiers.

The CD input features a non-inverting OP-AMP with a gain on 2.9***.

I want to change the IC with:
1. A discrete preamplifier
2. A better IC
***a. OPA2132
***b. LM4562

I`ve found some discrete preamplifiers like this, but the first issue is the minimum load.

A more simple option, is to change the IC.

Both options will have a dedicated PSU. The default PSU is unregulated.

The Muting circuit is imperatively required ?

A drawing of the schematic.
http://img293.imageshack.us/img293/705/p9047599dv9.jpg

 

[electronic]

Found an interesting preamplifier.
Look at page 9.

http://www.2shared.com/file/3877130/f34b7a8a/feb77article.html

 

[AndrewT]

Have a read of Nuuk's buffer site. Particularly Pedja's
Go back to the Pass site and find the B1 buffer.

Why do you need gain on the CD input? 0dBfs~7Vac. That is far too high for any domestic power amp.

 

[electronic]

Well, as you can see in the first post, the gain is 2.9, ~ 9dB.

It is not a unity gain buffer.

 

[electronic]

Some suggestions guys ?

 

[AndrewT]

you have a suggestion but you seem to be ignoring it.

 

[electronic]

I aint ignoring it. I just want to improve the line stage. If this is designed with 2.9 Gain, the upgraded stage will have.... 2.9 gain.

This in fact is a OP AMP preamp, and a buffer will not suit it.

 

[electronic]

I`ve studied the B1 Buffer.

Sounds pretty neat.

If I replace the pot with a fixed resistor, the input impedance will be stuck, but what about the gain .

 

[electronic]

Found the AX596 line stage.

Same gain = 10db

http://dc98.2shared.com/download/4182371/b48253d3/untitled.JPG?tsid=20081029-145947-be66662e

Some thoughts ?

 

[lineup]

it is a good discrete opamp
dual complementary input with resistor bias, 22 kOhm

will perform with very low distortion
needs a regulated +-15 VDC supply, as can be seen in your image

I would add 220 Ohm EMITTER resistors into all 4 input transistors
to balance and get ~0.3 mA into each input transistor

 

[electronic]

Thank you lineup.

What substitute should I use for 2SC1740/2SA933 ?

I have for the moment the MJE340/MJE350.
50V/0.15A vs 300V/0.5A.

Should I use this 2 transistors with a higher supply voltage ?
Also I would like a higher bias, if possible.

 

[lineup]

how much bias is the output?

Voltage across R360/value of R360, 68 Ohm = Current
Volt/68ohm=current in resistor
you should not need more than 2-5 mA Class A for best result
(to drive any power amp input)

the 4 input transistors works at ~0.3 mA per each
thi is a good value and should NOT be raised to more than ~0.5 mA
(the current in each is like 50 % of current in 22k resistor=
Volt/22k= current in resistor

Replacements:
2SC1740S:
BC550C, 2SC2240 ... TO-92 low noise, high gain NPN

2SA933S:
BC560C, 2SA970 ... TO-92 low noise, high gain PNP
--------------------

I would not go more than 0.5 mA for each input transistor
Example: use 15k (instead of 22k)
15Volt/15k = 1 mA
1 mA/2 = 0.5mA for each T

I would not use more than 5 mA for output stage transistors
0.340 Volt across 68 Ohm resistors

 

[lineup]

Predator864

These are the values I would used.
With R2, R14 at 2k2, the output stage will idle at 3.8 mA Class A.
This would be well enough for 99% of everything one Line preamp will need to do.

The gain is bit below x4.
-3 dB bandwidth, with my capacitors: 4 - 800.000 Hz
Distortion 1Vrms/10kohm output: < -100 dB for all harmonics.
(-100 dB = 0.001 %)

Not bad for 6 standard BC550C/560C TO92 transistors!
Enjoy