Servo controlled Op Amp board on Ebay

[keilau]

The benefit of servo circuit on power amp is well document. Are there benefit to have servo on preamp board? I found 2 of these kits on Ebay. Any opinion or first hand experience?


OPA2604 Servo Preamplifier Kit AMP board kit for DIY


Top Preamplifier NE5532 Dual OP Amp And servo circuit kit for DIY

This is a cheap line amp board that may have a lot of use as output buffer. It is neither too cheap or too expensive. I have found some gems among the offers from these sellers and, obviously, burnt many times too.

 

[phase]

Are those 5v regulators I see?
Would think that a bit more would be needed to get decent sound from most popular op amps.
Neat board though, after adding an opa627, and Russian polystyrenes, may be good deal, and allow the use od a direct coupled amp.

 

[CharlieLaub]

How the heck do you pull off a servo using a 5532??? Oh, wait, this is from China.

Usually one would choose a FET input op-amp for a DC servo...

Also, I don't see an OP2604 anywhere in sight. Not sure how that got worked into the title of the first link!

 

[restorer-john]

The regulators are 7815/7915 +/-15v.

 

[CharlieLaub]

This is a cheap line amp board that may have a lot of use as output buffer.

If you can make a PCB, a good buffer is simple a 5532 as a follower (output directly connected to input). It has very little voltage offset in this configuration and can drive pretty low impedance loads. Just make sure to put a 100nF cap directly across the power supply pins of the 5532.

My advice is to pass on the boards you linked to.

 

[ammel68]

Just make sure to put a 100nF cap directly across the power supply pins of the 5532.

Why not use a .1uF ceramic cap from each supply pin to ground?

 

[keilau]

Are those 5v regulators I see?
Would think that a bit more would be needed to get decent sound from most popular op amps.
Neat board though, after adding an opa627, and Russian polystyrenes, may be good deal, and allow the use od a direct coupled amp.

The regulators are 15 volts. Agree with you that it would need to add a dual OPA627 to sound well.

 

[dragonweed]

Why not use a .1uF ceramic cap from each supply pin to ground?

Because ceremic caps (specially the larger size ones) all have mechanical resonance modes at random frequencies, also they function as piezo "microphones", and certainly you do not want AM modulated supply voltages on your opamps.

 

[ostripper]

Total junk ... as is the rest of it.

OS

 

[ammel68]

Because ceremic caps (specially the larger size ones) all have mechanical resonance modes at random frequencies, also they function as piezo "microphones", and certainly you do not want AM modulated supply voltages on your opamps.

So if .1uF ceramic caps aren't suitable, what type of capacitor is?

 

[Ian Finch]

The plan is to filter the supply tails at the opamp by shorting the noise between the rails, not contaminating ground as you would in a single rail microprocessor or logic chip supply. You save a wasted cap that way too. Normally, MKT film caps are an alternative used in audio applications. These are considerably larger but MKP is just impractical. See "bypassing" here: Audio Designs With Opamps

 

[dragonweed]

So if .1uF ceramic caps aren't suitable, what type of capacitor is?

Small value (0.1-0.47uF) polyprop or polycarbonate is the best, but even MKT is better than ceramic. And yes, it is better to connect them between + and - supplies,(pins 4 and 8) than supplies to ground.