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The $20 Passive iPod Preamplifier

Feb 2, 2011 at 12:06 PM Thread Starter Post #1 of 9
danobeavis

danobeavis

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Hooking up my iPod to the car stereo through the headphone jack, or the line out has never been much fun. The impedance mismatch through the headphone jack, and the underpowered voltage output leaves a lot to be desired. I've built several active preamp designs that all work great, but being active means another power connection in the car or batteries to change on a regular basis.
 
Here's s dirt-simple passive preamp design using two small audio transformers. Bottom line is that it gives a great boost, and almost completely eliminates the mudiness inherent in the headphone jack->line-in connection.
 

 
Full article, parts list and schematic: http://www.beavishifi.com/projects/Passive_IPOD_Preamplifier
 
beerchug.gif

 
Sep 29, 2011 at 5:50 PM Post #3 of 9
 
Thanks for this.  I originally found it on beavishifi.com, and built it, but since there are no comments there I am commenting here.
 
My local RadioShack had all the parts in stock for less than $13, and it was really simple to put together, though I am an amateur.  
 
The result?  It works, for sure.  Definitely increases the level of the signal going into my stereo, and sounds good.  
 
One question though:  On your webpage you have the specs for the transformers, and it says their output frequencies range from 300-10,000 Hz.  I did an A/B comparison, switching back and forth between my Walkman with the passive preamp, and the same track playing on my CD player.  There seems to be a definite increase in bass when I switch to the CD.  Is this because the transformers are only boosting frequencies above 300 Hz?  Or is there some other potential cause that you can think of?
 
Oct 4, 2011 at 10:44 AM Post #4 of 9
Quote:
filius said:
 
Thanks for this.  I originally found it on beavishifi.com, and built it, but since there are no comments there I am commenting here.
 
My local RadioShack had all the parts in stock for less than $13, and it was really simple to put together, though I am an amateur.  
 
The result?  It works, for sure.  Definitely increases the level of the signal going into my stereo, and sounds good.  
 
One question though:  On your webpage you have the specs for the transformers, and it says their output frequencies range from 300-10,000 Hz.  I did an A/B comparison, switching back and forth between my Walkman with the passive preamp, and the same track playing on my CD player.  There seems to be a definite increase in bass when I switch to the CD.  Is this because the transformers are only boosting frequencies above 300 Hz?  Or is there some other potential cause that you can think of?
Click to expand...

 
  • A transformer doesn't amplify sound.
  • While it's certainly possible that the decrease in bass is caused by the transformers, it's also equally likely that the CD player in your car simply sounds different than your walkman.
 
Oct 4, 2011 at 4:53 PM Post #5 of 9
 
Quote:
nullstring said:
  • A transformer doesn't amplify sound.
Click to expand...


I suppose technically its just AC voltage at this point but transformers can be set up to provide voltage amplification. 
 
The lack of bass could come from any number of things after that. I'd be curious how the poster above volume matched. 
 
Oct 5, 2011 at 12:53 PM Post #6 of 9


Quote:
filius said:
One question though:  On your webpage you have the specs for the transformers, and it says their output frequencies range from 300-10,000 Hz.  I did an A/B comparison, switching back and forth between my Walkman with the passive preamp, and the same track playing on my CD player.  There seems to be a definite increase in bass when I switch to the CD.  Is this because the transformers are only boosting frequencies above 300 Hz?  Or is there some other potential cause that you can think of?
Click to expand...


There are a couple of possible explanations for an apparent attenuation of bass, first to mind is the apples to oranges as another poster commented on. A better comparison would be to attenuate the output level of the passive transformer circuit (a resistor from output to ground). Throw it on the scope and find the correct resistance value to have the passive preamp output sound at the same level. Tack a switch onto the resistor and do the A/B comparisons that way.
 
The other likely issue is indeed the frequency response of the cheap radio shack transformers. Xicon makes a line of small signal audio xformers but I haven't taken the time to look up the frequency response specs.
 
Oct 5, 2011 at 2:36 PM Post #7 of 9
If I remember, or can be bothered, I will measure the FR of these transformers (actually wired as autoformers here, but thats splitting hairs) as used in this circuit tonight.
 
When used as true transformers at low power, they have bandwidth FAR wider than what the package says. 
Click for old thread, bandwith measured as true transformer:
http://www.head-fi.org/t/553094/continued-sidetrack-discussion-from-tiniest-portable-amp-i-can-build-nikongod-microtransformer-based-impedance-step-down-box
 
Oct 5, 2011 at 2:44 PM Post #8 of 9
A different application of a passive preamp to be sure.  Typically, these are used between components with relatively high output voltage and a power amp/amps, serving as an uncolored volume control.  In these big-rig applications the quality of the pots/attenuators and other "passive" devices is critical.  Nice science project nonetheless.
 
Oct 5, 2011 at 10:15 PM Post #9 of 9


Quote:
nikongod said:
If I remember, or can be bothered, I will measure the FR of these transformers (actually wired as autoformers here, but thats splitting hairs) as used in this circuit tonight.
 
When used as true transformers at low power, they have bandwidth FAR wider than what the package says. 
Click for old thread, bandwith measured as true transformer:
http://www.head-fi.org/t/553094/continued-sidetrack-discussion-from-tiniest-portable-amp-i-can-build-nikongod-microtransformer-based-impedance-step-down-box
Click to expand...


Fueled by boredom I set to checking this. 
 
Direclty from my function generator, with a 50ohm output impedance into the transformer as described in the article by the OP loaded with a 10Kohm resistor, I found pretty much exactly the same as in the linked thread. Please note, that in this test I used a 2vp-p input, and a 4vp-p output voltage as opposed to whatever it was above. 
 
-1db bandwidth is 55hz on the "needs moar basses" end to OMG way a lot on the "not quite enough treble" end. I didnt actually note the bandwidth on the high end, but it went past 100Khz. Interestingly, there was a bit of a ringing in the 80-100Khz range that was not present when wired as a transformer with any load in the above linked thread. -3db was in the neighborhood of 20hz, but extremely distorted, as noted above. 
 
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