Transiet response in low pass filter

[pvh1987]

Hi, this is my first post - I hope this is the right place to ask this question:


I'm a newbie in analog electronics. I'm building a RIAA-preamplifier and I'm stuck implemting the 75 us low pass filter. It's a passive filter implemented in between to JFET amplifiers. Well, it looks like it works just fine, the only problem is, that if i have a square wave on the input, it looks not exactly like a square wave at 1 kHz... at 10 kHz it's triangular

That tells me that the transient response of my circuit is very, very bad. What is the source of this problem and/or how can I improve it?

 

[crutschow]

You don't want to improve its response. You filter is working exactly as designed. It is a 2.12KHz low pass filter and is rolling off the high frequency edges of the 10kHz square-wave, giving an output that looks more or less triangular.

The purpose of the filter is to exactly compensate for the 75µs high-pass increase in the recorded signal (the purpose of this increase in high frequency signal and subsequent roll-off during playback is to minimizes the effect of high frequency record noise). Thus a recorded square-wave would be reproduced correctly since the low-pass filter roll-off matches this high-pass recorded gain increase, giving an overall flat-response at the output.

 

[pvh1987]

Aha... Thank you very much!

 

[crutschow]

I assume you are also implementing the 318 µs and 3180 µs roll-offs that are part of the RIAA equalization.

 

[dknguyen]

"Oh no! My circuit does exactly what it's supposed to!"

 

[pvh1987]

Yeah, I'm going to implement the 318/3180 roll-off as well. This is my first serious analog project, so I might have more questions soon - I feel so stupid about the last question... well, nobody told me it should be like that - the few books I have is either very basic or very advanced - searching the internet for explanations, formulas and so on is sometimes even more confusing.

 

[audioguru]

Old vinyl records?
Before compact cassette tapes?
Before CDs?
Before MP3 players?
Before the North America was discovered?
Why?