Perf-board 500 series headphone amp

[mitsos]

PRR, the 1M resistor is in the feedback loop of the distribution amp.... See attached.  BTW, can you explain what's with the +V going to the 100K resistors at the inputs? 

Rob, to me, the gain of this seems too hot for line signals. as your test has shown. There is 11x gain in the headamp, and I think 11x gain in the TL074. That's 121x voltage gain, a bit more than 40dB with the volume pot wide open.

Before you build the other channels, you might want to rewire it to skip the first stage of the TL074 and go direct to the buffers, to see if that gets you enough gain. If you find you need more gain than that, then you can add the first TL074 back in and either decrease its gain a bit or decrease gain in the headphone amps by either lowering the feedback resistors (and increasing the feedback caps proportionately) or by increasing the resistance from the -IN to ground.

You could also remove the THAT chip and use the first part of the TL074 as a balanced line receiver, OR conversely, rewire it as a buffer like the other 3, and build a 4x headamp.

 

[rob_gould]

Hi Mitsos.  Thanks for your input.

Rob, to me, the gain of this seems too hot for line signals. as your test has shown. There is 11x gain in the headamp, and I think 11x gain in the TL074. That's 121x voltage gain, a bit more than 40dB with the volume pot wide open.

Well, I've put a 430Ω build out resistor at the outputs of the TL074 buffer and a 39Ω build out resistor at the output of the headphone amp and that has tamed things quite nicely.  The reason that a lot of gain is actually quite useful is that I'm feeding this project from an (Igor) monitor controller and I rarely go above -40 or so on that (too loiud when driving speakers otherwise), so it is useful to be able to make up that gain on the headphone amp because otherwise I need to keep adjusting the monitor controller when going between speakers and headphones.  The only downside is that with a low level output from the monitor controller and a big gain factor in the headphone amp, there's a fair bit of hiss.

Before you build the other channels, you might want to rewire it to skip the first stage of the TL074 and go direct to the buffers, to see if that gets you enough gain. If you find you need more gain than that, then you can add the first TL074 back in and either decrease its gain a bit or decrease gain in the headphone amps by either lowering the feedback resistors (and increasing the feedback caps proportionately) or by increasing the resistance from the -IN to ground.

Interesting ideas and I might experiment with these.  One thing JR also mentioned that I wasn't sure about...  Which are the feedback resistors and feedback caps in each circuit?  And how do you calculate how to match the two when you change their values.  I have done some reading on this but it's not making a lot of sense to me.

You could also remove the THAT chip and use the first part of the TL074 as a balanced line receiver, OR conversely, rewire it as a buffer like the other 3, and build a 4x headamp.

Nice ideas.  I wasn't aware that the TL074 would do a balanced > unbalanced conversion.  With regard to 4 x headphone amp, there's not enough space on a 500 series front panel unfortunately

 

[mitsos]

The only downside is that with a low level output from the monitor controller and a big gain factor in the headphone amp, there's a fair bit of hiss.

That's a gain staging problem. I don't know the Igor monitor controller, but ideally you'd be splitting the signal with one side going to your monitors and the other going to headphones, with separate volume controls for each. From what it seems you are taking a line level, padding it, then adding more gain to bring it back to where it needs to be.  That added gain is where the hiss is coming from.

The feedback resistors are the 1M in the TL074 and the 22K in the headphone amp.  Those and the resistors going to ground (100K and 2K2 respectively) are what determine gain.  The feedback caps are the ones in parallel with the 1M and 22K.  Basically, if you halve your feedback R, you should double your feedback C to keep the same LPF happening.

Nice ideas.  I wasn't aware that the TL074 would do a balanced > unbalanced conversion.  With regard to 4 x headphone amp, there's not enough space on a 500 series front panel unfortunately

The TL074 is a quad opamp. Opamps can be wired many different ways, google balanced line receiver, or take a look at the GSSL schematic, the 5534's on the input are balanced line receivers. You can do the same with the TL074, if you rewire that first amp like the GSSL input.

good luck!

 

[PRR]

> I used this

Ah.

That's got gain of 11. Why? Perhaps just to show how to find gain in there.

What is YOUR problem? Let's figure it out.

You've usually got a hot output from the console.

Headphone voltage is often _lower_ than line level. Especially at ~~32 Ohms.

You have gain in the headphone amp. The PCBgrinder plan has gain of 11.

You do NOT want more gain.

You have added Gain=11 distribution amp.

Gain is 121 which is a LOT.

There's a log pot on the headphone amp but to kill excess gain of 121 you have to turn-down to "0.5" on a 0 to 10 scale.

The distribution amp should run unity-gain. Change "1Meg" to like 10K and remove the 100K it connects to (NOT the other 100K to the "+ In" pin).

For many-many purposes the headphone amp should run lower gain. Change R13 to like 6.8K, gain about 4. Run your console HP feed at "normal" knob setting. Put the headphone amp knob around "7". Is that loud enough for 90% of situations? Fiddle the value of R13 until it is.

Do you even need the distribution amp? Simple Y-type splitting often works great. This case is borderline.

For great transparency, note that the headphone amp input is 9K, and TL07x is super-happy only for loads over 2K. So four amps is OK on ONE TL07x, more suggests something fancier. The first step is to go 5532 (since you already have them in the HP amp), which will drive 600 Ohms quite fine. That gets you to 15 HP-amp loads on one opamp section.

 

[rob_gould]

Hi All,

Thanks for your replies.  Sorry it took a while to get back to this but finally I've had a chance to look at it again.

As a test, I simply bypassed the first stage of the TL074 as per Mitsos' suggestion, and with the build out resistors that are already in place that seems to have done the trick. 

I get enough volume when my monitor controller attenuating a lot (so maybe -40dB on the readout), but the HP amp volume is up high, and crucially no there's pretty much no noise (barely audible hiss when no music is playing). 

If I set the output of the monitor controller to not attenuate at all, I can set the volume control nice and low on the HP amp and still hear music clearly. 

So it seems I've found the 'sweet spot' where I can attenuate a lot on the monitor controller and make up the gain on the HP amp but with barely no hiss, or I can drive the whole circuit with unattenuated signal and still be able to turn down the volume enough on the HP amp for comfortable monitoring.

Is there any benefit now to adding in the first stage of the TL074 at unity gain PRR?  It strikes me that adding tthat he first stage back in now would add nothing to this except more noise...?

And Geoff004 is right.  This thing does sound good!

Now to build the other amps and sort out the metalwork!

Thanks all!