Remove riaa stage from cheap phono pre

[DIP8]

Hello guys!
Do not give me crazy please and sorry for my very poor english.
I want to try to remove riaa stage from one of the cheaper phono amp (tc 750 or pyle pp444 etc..)
I would to only use the gain stage and use pc to rip the vinyls using a riaa software.
I read a brief guide here to tweak opamps and discrete phono stage
Stereo Lab - Flat response phono preamplifier
but i can't understand well, how to do in these circuits (i'll buy one of these 3 to do an experiment)

What are the capacitors to be eliminated in these circuits?

1)

2)

3)

An externally hosted image should be here but it was not working when we last tested it.

Thank you very very much if someone can help me

 

[rayma]

I would to only use the gain stage and use pc to rip the vinyls using a riaa software.

You'll need the schematics to do a redesign. The article is reasonably correct.

 

[DIP8]

Thank you Rayma for your reply. There are schrmatics of first board and tc 750. Thank you so muchif you can help me.

 

[scottjoplin]

Hi, the equalising network is in the feedback loop of the op amp. Basically the idea is to remove the RC network and adjust the gain as required

 

[DIP8]

Hi Scottjoplin
thank for reply

In the tc750 that have discrete transistors, I have to remove c4,c5?

 

[scottjoplin]

Yes, and you'll need to lower the resistance in the feedback loop or you'll have excessive gain

 

[DIP8]

Thankyou.
In that circuit, what are the feedback loop resistances?
Sorry... i'm a total newbie..

 

[scottjoplin]

R5,6 and 15, in conjunction with R3 and 4. In the article you linked to it suggests shorting the larger capacitor which in your circuit also shorts out R6, this would be a good way to start as it just leaves R5 and 15 in circuit

 

[DIP8]

I really appreciated scottjoplin
Thankou very very much

 

[scottjoplin]

You're welcome. This will help you understand Negative Feedback and Gain

 

[DIP8]

VEry interesting for a newbie like me

The base formula is β = R1 / (R1+R2) = 0.0909 = 1 / 11

If I use an opamp'd phono preamp instad of the transistor one, is the same stuff?
I'm always lowering resistances in feedback loop ?

 

[scottjoplin]

Yes, lowering resistance of feedback resistor increases the amount of feedback and so lowers the gain. If you just remove the capacitors in your circuits you are left only with the resistors which on their own will set the feedback and the gain will be too high

 

[DIP8]

Scott...

 

[DRONE7]

A very interesting discussion, I would like to try this..

For the attached circuit, if I follow correctly, I should disconnect C4 and C6 and short C5 and C7 then adjust gain by reducing the value of R14. Would that be right ?

http://www.pcb-audio.com/pcb-audio_com/riaa-super-low-noise-preamp-circuit.jpg

 

[scottjoplin]

Yes, but leave R14 and change R17 if required

 

[DRONE7]

Thanks for that Scott

 

[scottjoplin]

You're welcome

 

[DIP8]

Hi Scottjoplin
thank for reply

In the tc750 that have discrete transistors, I have to remove c4,c5?

I've tried today.. but nothing to do if I remove c4 and short c5..no sound..

If I leave on board c5 and remove c4, there is an heavy equalization
if i remove c5 (without short it) there is a very boomy bass equalization

 

[scottjoplin]

I've tried today.. but nothing to do if I remove c4 and short c5..no sound..

Then you need to increase the gain by reducing the feedback

 

[DIP8]

can i try with a potentiometer? and then fixed the value, put a resistance?