LM1875 balanced input circuit with volume control

[matej123]

Hi, in this picture i made this schema of an input for lm1875 amp and i wonder if its all good or if i can make anything better.

The CN3 is 50K potenciometer.

And i will connect it directly to amp input, no other filters.

Thanks for help guys 😉

 

[m10chalk]

R4 should be 100k not 100R. The back to back electrolytics for DC blocking would be better to be higher values if you want to ensure they have no effect (distortion) in the audio band. Why 4558 opamp? 5532 would be obvious choice in this context and be fine with those component values. R10 - why 22k rather than something like 100k given you are using 50k pot?

 

[JMFahey]

Looks fine except for R4 error as posted above.
10uF in series with 10k means 1.6Hz cutoff, not sure why would anybody ask for even larger caps.

I bet 4558 was in the Cad package library as a generic Op Amp, many can be used instead 😉

Remember to use an Audio/Log Volume pot for smooth adjustments.

As a side note: this schematic is way more readable than an earlier one 🙂

 

[rayma]

Remember 0.1uF to ground at each op amp pin 4 and 8.

Instead of back to back 22uF, use this, much better.

https://www.digikey.com/en/products/detail/nichicon/UES1V100MEM1TD/4317421

 

[PRR]

R1 R2 C1 C2 filter cut-off is 16 Megahertz. Why so high? In most places the big offenders are down close to a Mhz.

 

[rayma]

And use more like 100pF for C7 and C8.

 

[matej123]

R4 should be 100k not 100R. The back to back electrolytics for DC blocking would be better to be higher values if you want to ensure they have no effect (distortion) in the audio band. Why 4558 opamp? 5532 would be obvious choice in this context and be fine with those component values. R10 - why 22k rather than something like 100k given you are using 50k pot?

Thank you for advice
So what is better value for r10? And yes im using 50k pot.

 

[matej123]

R1 R2 C1 C2 filter cut-off is 16 Megahertz. Why so high? In most places the big offenders are down close to a Mhz.

So what values are better? Im new in op amps and i dont really know what im doing 🙁

 

[matej123]

And use more like 100pF for C7 and C8.

Why? Can you explain it please?

 

[matej123]

Looks fine except for R4 error as posted above.
10uF in series with 10k means 1.6Hz cutoff, not sure why would anybody ask for even larger caps.

I bet 4558 was in the Cad package library as a generic Op Amp, many can be used instead 😉

Remember to use an Audio/Log Volume pot for smooth adjustments.

As a side note: this schematic is way more readable than an earlier one 🙂

So is the cutoff good or it will be better to use different values?

 

[m10chalk]

Looks fine except for R4 error as posted above.
10uF in series with 10k means 1.6Hz cutoff, not sure why would anybody ask for even larger caps.

I bet 4558 was in the Cad package library as a generic Op Amp, many can be used instead 😉

Remember to use an Audio/Log Volume pot for smooth adjustments.

As a side note: this schematic is way more readable than an earlier one 🙂

For the value of DC blocking I follow the analysis that Doug Self sets out in his small signal book (Chapter 2). Even at that cutoff there are distortion artefacts of electrolytics that extent into the audio band

 

[rayma]

And use more like 100pF for C7 and C8.

Why? Can you explain it please?

The frequency is set by 1 / ( 2 x Pi x R x C )

On the order of 100kHz is about the right cutoff frequency for audio, and 1 / ( 2 x Pi x 10k x 100pF ) = 159kHz

 

[matej123]

The frequency is set by 1 / ( 2 x Pi x R x C )

On the order of 100kHz is about the right cutoff frequency for audio, and 1 / ( 2 x Pi x 10k x 100pF ) = 159kHz

Okey thanks😉

 

[rayma]

For the value of DC blocking I follow the analysis that Doug Self sets out in his small signal book (Chapter 2). Even at that cutoff there are distortion artefacts of electrolytics that extent into the audio band

See post #4.