A thread for my sex amp
Oct 15, 2010 at 1:02 AM Thread Starter Post #1 of 25

nikongod

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Hello Head-fi 
 
A little while ago I bought a busted SEX amp off of the FS forums. It had been up there for a while, I have wanted to hear this amp for a while now (nobody has SEX in the greater NYC area it seems), and the price was right so I said why not.
 
I payed for sex, and it came shortly thereafter tightly packed in a nice little box.
 
The problems with the amp were:
The base is broken in half at 2 corners
One channel was stone dead
 
I got it all unpacked and immediately set to work. I visually checked for obviously broken stuff but found nothing and initially suspected that the big Russian teflon caps that were installed had broken free. They are floating around unsecured on their leads, so that seemed to be the most likley suspect. I hooked up my full test rig ready to trace signal through the circuit to what I thought would be the failure point (a busted solder joint at one of the ends of the cap) Signal generator, Oscilloscope, the whole 9 yards. I like using my signal generator & oscilloscope but have STILL not had to use them to diagnose a problem which is pretty much why I bought them. 
 
I flipped the amp on its back because thats offers the best access to the bits I wanted to work on, and fired it all up. Thankfully I began by measuring voltages and comparing them between channels. The one side lit up at reasonable voltages (there is ~450V hanging out in some places! EEK!), the other side showed 0V everywhere in the audio section and (whoa a lot) at some places in the power supply. I turned the amp off, and let it sit for the caps to drain. I came back later and measured voltages to make sure that there werent any high voltages stored in some dum cap like I have found on other amps. After this I measured resistances around the power supply and the audio section and most things made good sense. There was however a resistance that did not measure up. The ~650 ohm resistor between the voltage doubler and the second PS cap on one side read infinite, on the other ~650ohms. Yea, that sucks. 
 
I got new parts and put them in. The voltages between the 2 channels are now equal to within a few percent. The amp sounds quite nice.
 
A picture is worth a thousand words....
 

 
A slightly different view:
 

 
ooh snap! 
 
You cant tell from these photos, but the break is nearly impossible to see with the naked eye. To make matters worse that resistor is tucked between 2 tag strips underneath a big power supply cap. I am curious why BottleHead didnt instruct builders to loop the leads around from the outsides, offering some (at least a little) bit of give should the strips move. I *ASSUME* that when the base broke (I didnt ask when this happened, perhaps in transit?) everything went to hell. A hard shock like dropping the amp (even in a nice box) could certainly cause all of this. 
 
Mneh, pseudo-scientific failure analysis is no fun. Here is what I did to repair it.
 

 
I wanted to try some KOA Speer carbon film resistors. They seem well regarded and I had never used them before. They are not particularly expensive (like $0.22 each) so why not?
 
You can see how I looped the resistor leads around the outside of the terminal strips here. I did the same thing on the side that was working. We cant have mismatched resistors in the power supply screwing up the sound, can we now... ooh no.
 
Here is the amp playing merrily away:
 

 
The blue tape is there because I read it makes the audio gear sound better. Its on all of my gear. Just kidding. Thats one of the busted corners.
 
As mentioned above the amp sounds quite nice with a few songs on it. Im going to take a bunch more time to get to know it more intimately, but plan to replace most of the resistors with these pretty green KOA resistors. I'l update the thread as I do that.
 
As always, stay safe, and use any info you get in this thread at your own risk.
 
Oct 15, 2010 at 7:54 AM Post #4 of 25

MrTom

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Fantastic!!
 
Regarding the blue tape, I wrap it around all my cables and it dramatically reduces RF induction. I also like to cut small rectangles out of it and place it over most of my opamp IC's. Some people say that 2 layers of the tape is better but I can't hear the difference after one layer.
 
Oct 15, 2010 at 7:56 AM Post #5 of 25

00940

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Be careful with tape though... if you put red tape rather than blue, you'll experience a big reduction in performances.
 
Nice amp though, it's always good to see good gear back in shape.
 
Oct 15, 2010 at 8:05 AM Post #6 of 25

Nebby

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You're wrong, it's much more important to ensure that the tape is 3M branded, otherwise you lose all your bass!
 
Nikongod: So you're the person that bought that amp! Thanks for saving my wallet some pain
smily_headphones1.gif

Quote:
Be careful with tape though... if you put red tape rather than blue, you'll experience a big reduction in performances.
 
Nice amp though, it's always good to see good gear back in shape.

 
Oct 15, 2010 at 8:17 AM Post #7 of 25

Avro_Arrow

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I actually had that happen!
I used some blue tape I got off fleabay and all my bass disappeared.
It turned out that the tape was counterfeit. If you looked closely you
could see where they had erased the original name and written 3M
in with crayon.
I only buy from reputable sources now.
 
Quote:
You're wrong, it's much more important to ensure that the tape is 3M branded, otherwise you lose all your bass!
 

 



 
Oct 16, 2010 at 2:25 PM Post #11 of 25

fatcat28037

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Excellent post Nikongod, highly informative and entertaining. I would however like to suggest one change that I believe would be an improvement. Replace the blue painters tape with wood grain Duct Tape. The end result would have a more professional appearance..
 

 
Oct 16, 2010 at 9:58 PM Post #12 of 25

NoPants

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I'm glad things are working out for this amplifier, I definitely enjoyed the channel I heard when I first tested it out of the box however many eons ago that I bought it. I wish I could have gotten around to fixing it but without the right tools (not to mention experience) what can you do? Are you you planning to add anything extra to the amplifier that isn't exotic tape?
 
The base did break in transit btw, just so I keep my name clear from amplifier abuse
 
Oct 16, 2010 at 11:03 PM Post #13 of 25

studeb

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Nice job on the amp. i too thought of buying it.
About the tape, i think you need to apply it vertically though. Two horizontal strips just are not enough, you will not get enough shielding.
This is little known fact, and it works with the fake tape too. I saved a boatload of cash on it, now i have bass out the wazoo too.
 

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