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Old 2nd January 2015, 09:14 PM   #1
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Default Suggestion for buiding a selector box to drive power amp.

I am considering in building a switch box to multiplex 4 different video/audio sources (DVD players and VCR) to drive the power amp. Here is my schematic. I want to get comments on whether this is good enough for higher end audiophile.

1) I don't know what opamp is very low distortion, please give me some suggestion. Notice I use 4.99K to pull the output of the opamp down to -15V to ensure Class A operation.

2)Attached is the schematic. Let me know if opamp circuit is just not good enough and has to be other designs.

3) Do I need any frequency compensation. Or is straight pass through with gain of 2 is good enough?

Thanks
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Old 2nd January 2015, 09:22 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by Alan0354 View Post
1) I don't know what opamp is very low distortion, please give me some suggestion. Notice I use 4.99K to pull the output
of the opamp down to -15V to ensure Class A operation.
2)Attached is the schematic. Let me know if opamp circuit is just not good enough and has to be other designs.
3) Do I need any frequency compensation. Or is straight pass through with gain of 2 is good enough?
Your sources should all have output coupling capacitors, so this ought to be ok. There still could be some pops when switching the selector.
Perhaps a hf filter at the input? Some of the best op amps need a well designed pcb layout because of their bandwidth. How will you be building this?
There are a number of unity gain stable audio quality op amps that would work well here. With a bipolar op amp, R5 should equal R6 parallel R7,
and the same for the other channel.

Last edited by rayma; 2nd January 2015 at 09:28 PM.
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Old 2nd January 2015, 09:40 PM   #3
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Quote:
Originally Posted by rayma View Post
Your sources should all have output coupling capacitors, so this ought to be ok. There still could be some pops when switching the selector.
Perhaps a hf filter at the input? Some of the best op amps need a well designed pcb layout because of their bandwidth. How will you be building this?
There are a number of unity gain stable audio quality op amps that would work well here. With a bipolar op amp, R5 should equal R6 parallel R7,
and the same for the other channel.
I am going to use "dead bug" hand wiring. That is flatten the IC pins and solder onto a ground plane pcb. All the resistors and caps are point to point. I did a lot of RF bread boarding using wide band opamps of over 200MHz with this technique with very good result.

RCA connectors are isolated from the chassis, run from connector to the pcb and ground only onto the pcb. The only concern I have is interference from the 3MHz to 4MHz composite video signal interfering into the audio channel. That's the reason I use very low resistance value in everything.

The best low noise opamp I've found is MC33178 with GBW of 3MHz, so loop gain is very high for gain of 2 here. Particular I pull down to ensure the output of the opamp is operating in pure class A to avoid the gm doubling problem. The signal swing should be about 2V only, I don't think slew rate limit is a factor in this case.

Let me know what you think.

Thanks and Happy New Year.

Last edited by Alan0354; 2nd January 2015 at 09:45 PM.
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Old 2nd January 2015, 09:51 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by Alan0354 View Post
I am going to use "dead bug" hand wiring. That is flatten the IC pins and solder onto a ground plane pcb. All the resistors and caps are point to point. I did a lot of RF bread boarding using this technique with very good result. RCA connectors are isolated from the chassis, run from connector to the pcb and ground only onto the pcb. The only concern I have is interference from the 3JHz to 4MHz composite video signal interfering into the audio channel. That's the reason I use very low resistance value in everything. The best low noise opamp I've found is MC33178 with GBW of 3MHz, so it's plenty high for gain of 2 here. The signal swing should be about 2V only, I don't think slew rate limit is a factor in this case.
Ok, then you'll have to stay with lower bandwidth op amps. Still, add the decoupling caps right at the pins and to the ground plane.
Use a moderate hf filter on the video input, say with a corner of 30kHz. If there are switching pops, you can add an input capacitor.
Do try to equalize the resistances seen by the bipolar op amp inputs, as well as keeping them low in value.
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Old 2nd January 2015, 09:56 PM   #5
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Any better or lower distortion opamps that is popular for hifi?

I think the composite video is up to about 4MHz(3.57MHz or something). My other thinking is to buffer every single audio input before even driving to the selector switch so the line is very low impedance to avoid crosstalk. But that will be 8 opamps instead of 2.

Last edited by Alan0354; 2nd January 2015 at 09:59 PM.
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Old 2nd January 2015, 10:29 PM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by Alan0354 View Post
Any better or lower distortion opamps that is popular for hifi? I think the composite video is up to about 4MHz(3.57MHz or something). My other thinking is to buffer every single audio input before even driving to the selector switch so the line is very low impedance to avoid crosstalk. But that will be 8 opamps instead of 2.
Don't worry about crosstalk, just turn off the other sources, or use a series 1k in the inputs, grounding it when deselected.
Ti has a lot of good audio op amps, even free samples are available. The LM4562, LME49723, and LME49990 are some examples.
These may test your breadboarding skills, though.
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Old 3rd January 2015, 12:06 AM   #7
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Quote:
Originally Posted by rayma View Post
Don't worry about crosstalk, just turn off the other sources, or use a series 1k in the inputs, grounding it when deselected.
Ti has a lot of good audio op amps, even free samples are available. The LM4562, LME49723, and LME49990 are some examples.
These may test your breadboarding skills, though.
I took a quick look, only LM4562 comes in PDIP. But it's the highest GBWP of 50MHz. Why do you need such high frequency? This is going into video opamp range.

I am not afraid of the high frequency, I breadboard lots of Comlinear and Elantec amps back in the days that ran into hundreds of MHz with rise time of less than 1.5nS. So this is not exactly a challenge. Yes, the 0.01u ceramic is going to by right on pin 4 and pin 8. Pin 2 and 6 are absolutely point to point. Or else it's a non starter. Ground plane is a must.

Thanks
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Old 3rd January 2015, 12:10 AM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by Alan0354 View Post
I took a quick look, only LM4562 comes in PDIP. But it's the highest GBWP of 50MHz. Why do you need such high frequency?
Yes, the 0.01u ceramic is going to by right on pin 4 and pin 8. Pin 2 and 6 are absolutely point to point. Or else it's a non starter. Ground plane is a must.
This is the trend now in high performance audio op amps, high gbw product. Make sure the 0.01uF cap is of good quality, even some X7R types are marginal.
You can place a sample order with TI for up to 3 pieces, and delivery is fast, too.
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Old 3rd January 2015, 12:20 AM   #9
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Quote:
Originally Posted by rayma View Post
This is the trend now in high performance audio op amps, high gbw product. Make sure the 0.01uF cap is of good quality, even some X7R types are marginal.
You can place a sample order with TI for up to 3 pieces, and delivery is fast, too.
Ha!! I use 0.01 spacing ceramic bypass caps all this time for opamps of much higher frequency, never ones have any oscillation or even ringing problem. I got good square wave. I never stop and think about what cap!!! Just the usual TTL bypass caps. Most of my designs are for pulse circuits for mass spectrometer that need rise time of 1 to 1.5nS. Ringing is deadly for our application as it likely be counted as extra ion pulses!!!

I wasted my time registering and all. Then it said my allowance was "0" piece!!! I couldn't get samples.

Last edited by Alan0354; 3rd January 2015 at 12:22 AM.
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Old 3rd January 2015, 12:25 AM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by Alan0354 View Post
Ha!! I use 0.01 spacing ceramic bypass caps all this time for opamps of much higher frequency, never ones have any oscillation or even ringing problem. I got good square wave. Most of my designs are for pulse circuits for mass spectrometer that need rise time of 1 to 1.5nS. Ringing is deadly for our application as it likely be counted as extra ion pulses!!!I wasted my time registering and all. Then it said my allowance was "0" piece!!! I couldn't get samples.
What bypass caps do you use? It sounds like they're very effective.
Try improving you fibbing. If your current job wouldn't be credible as requiring these parts, you can be a consultant.
It's worth registering for all the support they offer.
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