AMZ FX ANALOG ALCHEMY GEOFX MEF ROG DIY WIKI

Figured I would ask...

Started by Govmnt_Lacky, January 21, 2011, 10:00:56 AM

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Govmnt_Lacky

January 21, 2011, 10:00:56 AM
Simple yet noob-ish question:

Could I plug a standard set of headphones into my Dr. Boogie output and will it have enough to drive the headphones for a decent output?

Thanks and please be kind...  ;D
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Mark Hammer

#1
January 21, 2011, 10:23:43 AM
Not at all an unreasonable question.  The answer is: it depends. It depends on the headphones, and it depends on the circuit.  My guess is that the Dr, B will not be able to send enough current to your headphones to produce audible movement of the drivers.

That being said, one does not always need a power amp to drive headphones.  A humble NE5532 or LM833 is quite capable of driving headphones, and is regularly used for the headphone output on a great many mixers and similar devices.

I had an on-board preamp once, that was based around an NE5534 (half a 5532), and I could plug headphones into my guitar jack and play in private.  It wasn't punishng levels, mind you, but I could hear above normal conversation elswhere in the room.

Govmnt_Lacky

#2
January 21, 2011, 10:28:20 AM Last Edit: January 21, 2011, 10:47:00 AM by Govmnt_Lacky
Thanks Mark.

So basically, the Boogie WILL NOT do it all by itself. I pretty much thought that was the case but I wanted to consult before causing un-needed damage to my headphones and/or Boogie.

I did build the Headphone Amp from GGG however, I really do not care for the output. Sounds TOO low and grainy!!  :P
A Veteran is someone who, at one point in his or her life, wrote a blank check made payable to The United States of America
for an amount of 'up to and including my life.'

Mark Hammer

#3
January 21, 2011, 10:59:12 AM
What impedance are the headphones you are using with it?

Govmnt_Lacky

#4
January 21, 2011, 11:04:20 AM
Quote from: Mark Hammer on January 21, 2011, 10:59:12 AM
What impedance are the headphones you are using with it?

They are Koss UR-20s.

Freq Response: 30-20,000Hz
Impedence: 32 ohms
Sensitivity: 97dB SPL
Distortion: <0.3% Total
8ft. cord.
A Veteran is someone who, at one point in his or her life, wrote a blank check made payable to The United States of America
for an amount of 'up to and including my life.'

Govmnt_Lacky

#5
January 21, 2011, 12:01:38 PM
This is the Headphone Amp that I built:

http://www.generalguitargadgets.com/diagrams/mxr_headphone_amp_sc.gif?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

I realize that Impedence matching is what you are referring to Mark however, I still cannot grasp how to calculate the Impedence of this circuit. I have tried but I just cannot get it.

Can someone lend a hand so I can try to match the circuit better?

Thanks  ;D
A Veteran is someone who, at one point in his or her life, wrote a blank check made payable to The United States of America
for an amount of 'up to and including my life.'

PRR

#6
January 21, 2011, 11:53:56 PM Last Edit: January 21, 2011, 11:57:47 PM by PRR
> I still cannot grasp how to calculate the Impedence of this circuit. I have tried but I just cannot get it. Can someone lend a hand so I can try to match the circuit better?

Start with the BASICS. Lamps and wires. If you have a 10 ohm lamp and a 100 ohm wire, how much power you put in the wire gets to the lamp?

The Dr Boogie's output stage is a LOT like the tonestack and driver from a 5F6A, except only 9V supply and 1/10th the impedances. The 5F6A is adapted from tonestacks designed to drive 5V into 220K loads. We may assume that when built with 1/40th of supply voltage and 1/10th impedances it will happily drive 0.15V into 22K loads. And, duh, that's a fine level for amp inputs.

So pretend the Dr Boogie can output 0.15V from 22K. What if we load a 22K "wire" with 32 ohms? We get 32/(22,000+32) or 0.0014 of the unloaded voltage. 0.15V*0.0014= 0.000,2V. Output power is 0.000,000,001 Watts, 0.000,001 miliWatts, 120dB below headphone nominal 1mW sensitivity, or about 28dB below the threshold of hearing.

No harm done, just inaudible.

> This is the Headphone Amp that I built:

The small-signal output impedance is dang-near zero. Emitter-followers plus heavy overall NFB.

We need to know the large-signal output.

There are many little tricks to know because different designs run into different compromises. I'll just jabber to an answer.

Power supply is apparently 9V? The TL07x will lose 1V each side, 7V peak-peak or 2.5V RMS.

The emitter-followers are pulled-up/down with 3.9K resistors. The closer the output gets to a rail, the less base-current is available. The martini-napkin approximation is to divide the 3.9K by the hFE of the transistors. Say 3900/250= 16 ohm large-signal output impedance.

The load is two 100 ohm resistors and two 32 ohm ears. Total 132/2= 66 ohms. Out of 2.5V 16 ohm amp we get 2V at top of resistors, 0.5V on the earpieces.

0.5Vrms in 32 ohms is 0.008 Watts or 8mW. 9dB up from rated 1mW sensitivity, typically 100dB SPL max. Pretty loud. For 97dB SPL (1mW?) cans you get 108dB SPL. Plenty loud.

The MXR headphone amp SHOULD be very ample in low-Z (16 to 100 ohm) headphones. MXR are no fools.

Are you saying the MXR is _not_ total bliss?

If not loud enuff, check for wiring error.

The MXR seems to be intended for "line level" input. For faint guitar you want more gain. I would remove pot R2 and replace with a short (4.7K+4.7uFd from opamp to ground). Then remove R7 and replace with a 500K normal-audio-taper pot (wired as 2-terminal, like R2 was). That gives sensitivity similar to a classic Fender.

If still unhappy (going-deaf, or less-sensitive headphones), you may reduce R10 R11 to 33 or 10 ohms. Don't go lower: the amp has no short-protection except these resistors. And 10 ohms here gives 28mW, a really ample level.
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