Minimal Headphones Amp

[john12ax7]

Try OPA1678/79 instead.

Why not OPA1641/42? More expensive but superior performance.

 

[thor.zmt]

Why not OPA1641/42? More expensive but superior performance.

Actually in listening (blind, preference) the tests results showed a preference for OPA1652/54.

This in turn is the same die as OPA1678/79 but with increased tolerances for DC and some tests omitted, so to speak the "B-grade" OPA1652/54.

And for mass producing low cost audio gear "more expensive" is the wrong direction.

Thor

 

[ccaudle]

This thread inspired me to look into the current boost stage for op-amp like @thor.zmt described in post 83.
One thing that was not obvious to me right away was what appears to be a bootstrap capacitor C19 from the output to the junction of the R43 and R52 resistors that provide the current path to negative supply for the Vbe spreader.
The output of the transistor stage should be following the output of the op-amp pretty closely, so what advantage does adding C19 have over just making C20 somewhat larger?

 

[ccaudle]

By the way, who told Thor he could go on vacation and stop checking his thread?

 

[JimJhn]

Not to hijack a thread or insert something unrelated, but...(and this IS about headphone amps I promise!)...

I just finished Mark Howard's excellent book "Listen Up", which chronicles the Canadian recording engineer, producer and musician's musical journey so far.

He has allot of really cool thoughts about recording music, and especially about how to get away from soul-killing sterile recording environments.

I really empathize with this, regardless of the differences in what areas of music interest me.

Having spent most of my daily life in commercial studios for more years than I care to enumerate (and I am not an engineer or technical at all), I've gotten to the point where the LAST place I want to make music in is a commercial studio. (EXCEPT for REALLY well-maintained places, and only if that's totally STELLAR, but I digress!).

One interesting thing he mentioned is that he buys old consumer stereo-systems to use as headphone amps instead of dedicated studio headphone-systems or the ubiquitous Crown DC-60s with distro or whatever, and it got me to thinking about that as I read this thread...

 

[kags]

One interesting thing he mentioned is that he buys old consumer stereo-systems to use as headphone amps instead of dedicated studio headphone-systems or the ubiquitous Crown DC-60s with distro or whatever, and it got me to thinking about that as I read this thread...

There are some producers, like Joe Baressi, who change monitors for each album they work on in the hopes that the new “lens” will help create a different outcome. You may or may not like this approach but it is certainly valid.

 

[ccaudle]

@thor.zmt I think you may have been on holiday when I asked previously; could you take a look at my question in post #143 and explain the purpose of the bootstrap cap from the output back to the Vbe multiplier stage?

 

[thor.zmt]

One thing that was not obvious to me right away was what appears to be a bootstrap capacitor C19 from the output to the junction of the R43 and R52 resistors that provide the current path to negative supply for the Vbe spreader.

The bootstrap capacitor acts as impedance multiplier. This means the load on the op-amp output is mostly the headphone impedance multiplied by output stage Beta.

With BD139/140 or BCP53/56 (same die) in the high beta group we have typically ~ 200 Beta. So a 16 Ohm headphone will appear as 32k Ohm load to the Op-Amp output.

This appears in parallel with the "tail" resistance to -V which is 2.4kOhm for ~5mA. If we split the resistor into 1.8k + 620R we load the output stage with an extra 620 Ohm load, but with the bootstrap and assuming ~ 1 Ohm open loop Z Out and a 16 Ohm headphone, the 2.4k become around 29k, so the load on the Op-Amp output becomes ~ 15kOhm, instead of around 2.2kOhm.

It also allows the negative swing to go all the way to the rail, while the positive side will always be limited to 1VBE plus other losses.

One could use a CCS instead of the bootstraped resistor, that has its own limitations and drawbacks.

Thor

 

[cyrano]

Thanks, Thor. Learned a lot from this one post!

 

[ccaudle]

The bootstrap capacitor acts as impedance multiplier.

Thanks for confirming. You do not usually seem hesitant about having an op-amp drive relatively low impedances, so I just wanted to confirm there was not something more subtle going on.

 

[thor.zmt]

Thanks for confirming. You do not usually seem hesitant about having an op-amp drive relatively low impedances, so I just wanted to confirm there was not something more subtle going on.

It depends on the Op-Amp, I usually use modern parts that do very into low impedances. In our case here it is simply to maximise the open loop linearity ahead of the power follower.

I found measurable THD reduction And increased maximum output with that cap in place vs absent.

Thor