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-   -   BC550 BC560 Very low noise RIAA (http://www.diyaudio.com/forums/analogue-source/91497-bc550-bc560-very-low-noise-riaa.html)

lineup 3rd December 2006 07:24 AM

BC550 BC560 Very low noise RIAA
 
2 Attachment(s)
.

Hello, lineup here.

By paralleling low noise transistors, we can lower the noise further.
There is a formula for this. Number of parallel devices and dB lower noise.
But I do not remember.

This RIAA attached should give very good results
even though using only standard components.
4 transistors in parallel.
If course, with adjustments, you can parallel even more.
But the gain in low noise per added transistor
has largest value for the first 4 four.
(Each doubling of number of devices gives same noise improvement in dB)
-----------------

Discrete Op-Amp configuration.
It is using only BC550C BC560C transistors and +-15 Volt regulated.
The output stage works at ~ 9 mA push-pull Class A.

Some approximate data:

Gain at 1 kHz: +35dB, ( x 55 )
THD for 1 kHz, 10mV RMS input, 0.55V RMS output: 0.001%
RIAA curve, Using 1% film caps and 1% resistors: < +-0.20 dB

Max Output: > like 5-7 Volt RMS with distortion ~ 0.01%
Depending on the load.

A good and matching very low noise
regulated dual 15 volt supply is important!
For best results.

Enjoy!

( PS. If somebody would be kind design a nice fit PCB board,
this would make a nice low cost RIAA project for www.diyaudio.com members.
I know there are still plenty of turntables spinning, here and there.
I just don't have the tools to make a PCB myself. DS )

lineup

peranders 3rd December 2006 07:35 AM

It looks like the Elektor SUPA, decades ago. I made a board long time ago and called it QSX-001 Mark I and II. My design got 160-220 nV noise which is good for a medium level MC cartridge.

My very first QSM-001 http://home5.swipnet.se/~w-50674/hif.../qsxm2_old.jpg

Nordic 3rd December 2006 08:22 AM

Hey LU, I have a few of those, I bought for the class A buffer, but were told they sound too cold in that application, and my VSPS is lying open and semi dissasembled at my feet, as I am no longer happy with the sound, just too bass shy.

I looked at peranders's RIAA amps last night, but they seem terribly hardcore for someone more comfortable with things being only as complicated as they need to be.

Maybe I should spend a few minutes today to give a PCB a try...
We'll see what happens I guess. soo lazy and stoned allready and its not even 12 in the morning.

AndrewT 3rd December 2006 08:30 AM

Hi,
I read a long time ago that adding the other half of the LTP doubles the noise (I don't know if that was +3db or +6db).

On that basis, removing the other half should make your circuit quieter and save half of those 550s and 560s.

Now, if it were single sided and single ended would it be quieter still?

BTW,
BC550c has a fairly high Cob for a small signal device. Eight of them is going to be quite a load.

Nordic 3rd December 2006 09:17 AM

Shouldn't there be another R like R17 on the other rail too?

peranders 3rd December 2006 09:40 AM

The formula for noise reduction is 20*LOG(SQR(N)) where N is the amount of transistors in parallel.

More than eight pairs aren't very useful for this. Better to get low noise JFET's of BJT's.

Holger Barske 3rd December 2006 09:52 AM

2 Attachment(s)
Quote:

If somebody would be kind design a nice fit PCB board...
I once did an "as compact as possible" pcb for that one.
RIAA caps are Wima FKP2, output cap is 22 Wima - forgotten what type.

Nordic 3rd December 2006 10:48 AM

couldn't beat that layout if I tried

Holger Barske 3rd December 2006 12:37 PM

Thx Nordic.
Maybe there's some work to be done on it, some traces have very small clearance towards their neighbours. I am willing to do so if there's some interest.

Nordic 4th December 2006 04:59 AM

Wouldn't mind, especialy if you include board files.


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