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Old 8th June 2015, 08:32 PM   #1
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Default A little compilation Erwin's Amp

Hi everybody. I'm sorry for my poor English but it's not my native language.

I'ts my first post so sorry if written wrong.

It's my compilation of some designs and test. I would like to ask what do you think about it, because next week I'm will be routing PCB and then ordering it.
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Old 9th June 2015, 12:42 AM   #2
Bigun is offline Bigun  Canada
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i dont know much about these circuits, it looks ok to me though. Does the + input of the op amp need some dc path to ground, through a resistor for example? I dont know how good is the PSRR of your design, do you have a good power supply in mind for this project?
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Old 9th June 2015, 01:16 AM   #3
agdr is offline agdr  United States
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Quote:
Originally Posted by erwin008 View Post
I would like to ask what do you think about it, because next week I'm will be routing PCB and then ordering it.
Welcome to the forum!

Someone who is more current on discrete circuits than me would have to answer this one, but something doesn't look quite right with your AC negative feedback path, R6. Instead of connnecting that to the output of the op-amp you will probably get more milage by connecting it to the output of the transistors (the output of the overall amp from T2 & T12), so the negative feedback linearizes the transistors' cross-over region.

Also it looks like your gain from the feedback path might be set for 24K/1K +1 = 25, which is way high for a headphone amp. Somethiing like 2x - 4x is probably more in the ballpark (what is your intended audio source?). And then given 4x (or 5x) maximum voltage gain, consider replacing those un-compensated OPA552s with compensated opa551's to reduce your chances of oscillation. The 552's are most useful for gains over 5 or so.

The DC path I know more about. So another suggestion is consider replacing the DC servo (the bottom op-amp) with a DC precision op-amp (one that has very small input offset voltage) such as the OPA140, OPA827, or OPA627. Those three are also FET-input op-amps so they will further help the DC servo situation by not creating big voltage drops across the input resistors (R20 & R21) from the op-amp input bias currents.

Otherwise the lowest DC output offset voltage you could correct down to is the input offset voltage of the OPA551/2, which is 1-3mv. The OPA140 would give you around 50uV = 0.05mV.

I would agree with Bigun too, you need a DC path to ground on that op-amp input, otherwise C7 & C5 will probably charge up to somewhere around the postive or negative power supply rail. Some resistance from that input to ground around the parallel value of R16, R17, and R6.

Last edited by agdr; 9th June 2015 at 01:33 AM.
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Old 9th June 2015, 01:48 AM   #4
T in AZ is offline T in AZ  United States
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Seems a lot of power for a head amp.
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Old 12th June 2015, 07:37 AM   #5
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Nah, not with those 2k2 emitter resistors. 550 ohm output impedance, anyone?

Looks like an adaption of a higher-power design gone slightly wrong. Nobody needs an OPA552 for +/-12 V rails, there are far better/cheaper parts available then. Impedance balance on inverting vs. noninverting input is pretty bad, too.

And as mentioned, no DC path to ground on the input (100k would do), and gain a bit on the high side. Speaking of which, headphones come in such different kinds of sensitivities that it is hard to accomodate all of them with a single gain setting.
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Old 12th June 2015, 02:51 PM   #6
agdr is offline agdr  United States
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Quote:
Originally Posted by sgrossklass View Post
Nah, not with those 2k2 emitter resistors. 550 ohm output impedance, anyone?
Hey sgrossklass, just curious - where are you seeing those 2.2K's? The emitter R's on his T2, T4, T10, and T14 are all 2.2R.

Yeah the 551/2 wouldn't be a first choice. The 551/2 is one of the few high(er) voltage parts out there though. Since the rest of the output stage is discrete maybe this one could be bumped up +/-24Vdc to +/-28Vdc. Not the the op likely needs that, but just something useful that could be done with it as sits. The servo could probably be run from +/-12Vdc if there was a resistive voltage divider on the integrator input.

Last edited by agdr; 12th June 2015 at 02:56 PM.
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Old 14th June 2015, 11:45 AM   #7
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Oops, my bad - I probably should have looked at the schematic in somewhat bigger size. What I thought read 2.2k is in fact 2.2R.
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