GGG Headphone Amp - Done - sortof...

[mydementia]

Hi guys.
I just finished putting together the MXR Headphone amp (http://www.generalguitargadgets.com/index.php?option=content&task=view&id=134&Itemid=166) and it works - sortof...

Sounds like it works but the output is REALLY low.  I think the pot I found is suspect (it's a linear 500k mini pot that I found in a bin at the local electronics surplus store - hard to find reverse log mini without ordering online...).  With the volume pegged, the output clips unusably - a very small rotation off of max (zero resistance) brings the tone back in reason but the overall volume is really low.  Here's a snapshot of my board layout in case anything jumps out at you:


I also bought a 'Multimedia Y-Cable Adapter' (Radio Shack PN 42-2496).  It's a 1/8"M+1/8"M to 1/8"F (all stereo) combiner cable that lets me listen to my MP3 player while I play guitar - cool, eh?  My girlfriend likes this DIY project best so far...now that she knows what it does!

Let me know what you think about the volume problem.
Thanks in advance.
Mike

[mydementia]

UPDATE:
I swapped out the suspect pot with an 500k pot I had laying around (from a guitar conversion) and now I get more volume.  I'm still not happy about having so little volume control (it's a whisper for 80% of the rotation, then wham - better roll off the guitar volume or it clips like a beast).  I've tried three types of headphones and all are similar in output level.

So...
For those of you who've built this amp:
1) What can I do to get pleasent clipping? (usable distortion) ...or no clipping with more volume?
2) What will the reverse log pot give me ?  Where do I get one?  (I didn't see a 500k RL pot at SmallBear as was recommended in a previous thread).
3) I found an LM741 op-amp at Radio Shack today and tried swapping it for the TL061 - pretty much the exact same tone profile.  I assume the LM741 is the same as the TL061 aside from the 'low power consumption' moniker - any thoughts?  Is there a better low consumption op-amp that will perform better?  I just tried using an LM386 (jockying around pins 5 and 6 to get the right i/o) and got cruddy sounding blurbs - probably not a good idea.  Just looking for ideas.
Thanks.
Mike

[mydementia]

Bump...
I'm contemplating putting a stratoblaster booster (from GGG) behind this circuit (with a 47k trim pot for gain adjustment) - would it help?  ...or is there a resistor I should try changing first?
Any help will be greatly appreciated - I want to get this one boxed up!
Mike

[JimRayden]

The rev log will make it sound better through the pot's range while the linear pot piles the response up to one side of the rotation. I don't know the schemo but you should be able to use a normal log pot, which means having a "max" setting  on the CCW position.

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Jimbo

[Nasse]

Hard to tell without all the info, low volume might be because some mistake, cold solder joint or bad component. Or maybe it is the phones, some just  need more power. It should be easy to test if a preamp helps, or some hotter input signal. For distortion I would look for some separate distortion box.

More clean headroom... use two batteries in series or convert for bipolar supply +/- 15 volts...

I never have built this, but almost similar circuit and when listening music trough it I always liked it more when removed those 100 ohm resistors, I know it is not "right" but bass sounds litle bit tighter...

Just few quick thoughts, don´t take too serious

[mydementia]

Interesting thoughts guys.

Nasse - do you think I could use the charge pump (below) to bump the power up to 17(ish)V?
ref: http://www.generalguitargadgets.com/diagrams/charge_pump2_lo.gif

I built one of these charge pumps when I thought I was going to build the PT80 delay box... I know the MAX1044 will increase the power output, but what about battery consumption?  I'm trying to make an efficient headphone amp here...
Mike

[Nasse]

Never used those charge pumps, dunno but if it has enough current and does not whine...

There was once interesting theory in Elektor magazine in one guitar headphone amp project, the author suggested bipolar supply with two batteries and claimed they last longer together than two one after one. And I thought this would be worth thinkin in some opamp pedal circuits. But if it was so people at this forum would have mentioned it before

EDIT but really dunno if it is wise to bother converting this for two batteries, just guess it should work with one