Adjusting gain on X-amp?

[lennon_68]

I picked up some X-amps off the classifieds and one of them has a much higher gain than the other three (like 10dB louder with the same signal). I opened them up to see if I could see anything that looked like a way to adjust it but nothing jumped out at me. Any ideas?

Also all of them - with the cheater plugs in use to reduce the hiss - have between 25 and 40v of AC on the chassis, should I be worried about that? I removed the cheater plugs for now as that doesn't seem right:shiftyeyes:

[dvenardos]

The gain was too high for the first batch off the line and they dialed it down for all subsequent runs. I forget the layout of the board. Can you post some pics (not that I will be able to help you anyway:ufo:)?

[lennon_68]

The gain was too high for the first batch off the line and they dialed it down for all subsequent runs. I forget the layout of the board. Can you post some pics (not that I will be able to help you anyway:ufo:)?

Thanks for the input... interesting :no clue: I'll get some pics tonight and post them up.

[madpoet]

I've got the opposite problem... one of mine went to zero gain ;) Not sure what happened, I hadn't used them in a few weeks and one is just plain dead.

[sbisheff]

I've got the opposite problem... one of mine went to zero gain ;) Not sure what happened, I hadn't used them in a few weeks and one is just plain dead.

You got one of the "special" ones. They don't make any hissing noise this way. Much better SQ :assshake:

[madpoet]

Actually neither one particularly hissed... which makes it all the worse.

[dvenardos]

One of mine was DOA and the other you could hear the hiss from 5ft on my Minis. I packed them up and shipped them back. That was it for me and MLS; to brag about how great the amp was and then deliver something that was mediocre at best. I had the first run, so hopefully they got better. :crazy:

Actually neither one particularly hissed... which makes it all the worse.

[lennon_68]

lol, ya I suppose I'm lucky they work at all :whew:

I figured for $40 each (shipped) they are worth trying out. If nothing else I can probably use them as sub amps - I don't know how much to trust it (I plan to test one on my IB sub) but Kyle told me they will push 150W into a 4ohm, -3dB at 10hz (I would be surprised if this were true though). Worth checking out...

I have 5 threads running on this and got some input on the others but not sure if it's going in the right direction or not though...

Curt C at techtalk (although he admittedly isn't familiar with class D) suggested I check the resistor values around the op amp in the input stage. I think that means I should be comparing the resistor values in this area to the ones on the amps not displaying the problem:



Here's the rest of the innards:

[lennon_68]

The gain was too high for the first batch off the line and they dialed it down for all subsequent runs. I forget the layout of the board. Can you post some pics (not that I will be able to help you anyway:ufo:)?

It seems this is exactly what's going on. I compared the circuit for the input and found that the amp that's louder than the others is significantly different. Note in the picture above the two caps... those have been replaced with jumper wire. Also some of the other components are different as well.

I emailed Kyle, we'll see if they can hook me up... I'm not holding my breath though :o

[dvenardos]

Any word? Sometimes you can adjust the gain by changing the values of resistors on the board.

[lennon_68]

So... looks like I'm on my own as far as advice from AV123...

I popped open all of the amps and found that the circuit board that I was looking at - right where the RCA jack is - is different on the "offending" amp. This pretty much confirms that particular amp is from the first run that had too much gain. I emailed AV123 asking how much a replacement board would be but got nothing and will get nothing.

I setup a quick measurement rig in my kitchen and played a 500hz tone through the offending amp and one of the "good" amps and found that the loud one plays 12dB more than the other.

I had been considering buying an inline attenuator to fix the issue but when I looked around nothing was the right price so I'm going to tackle building my own.

It looks like I can build a better quality one on my own and may be able to just build it right into the inside of the x-amp so should I sell these I won't have to worry about including an inline attenuator. I found the information on how to build one here:

http://www.gearslutz.com/board/geekslutz-forum/31023-diy-line-pad.html

with better pictures/etc here:

http://www.uneeda-audio.com/pads/

It's pretty much all there but I'm not sure about how to determine what impedance I should be planning for? Also I'm not sure what Watt rating resistors I should be getting? Someone in the forum post (first link) said 1W but it sounded like that wasn't correct. The second link says 250mW or less will be fine but how low can I go?

[dvenardos]

Too bad dweekie isn't on the forums anymore, he could probably tell you what to do. I think it is Dane that is an EE (he posted about modding the parts express amp for the ULW), maybe shoot him a PM.

[HAL]

Guy's,

I looked at this awhile ago since I have a few X-Amps.

If you download the spec sheet for the Philips TDA8924 Class D amp chip, the schematic for the X-Amp power amp section is pretty much what is shown. In Bridge Tied Load (BTL) mode, as the X-Amp is designed, the gain is 34dB. Putting an attenuator between the preamp section (the small PC Board) and the power amp section could be used to attenuate the input levels.

The front end opamp is not a low noise device if it is a TL072 dual JFET opamp. It is there to bring up the input impedance of the amp. The TDA8924 has fairly low input impedance, so needs an input buffer. Also if I read the board correctly, there is about 12dB of gain in the preamp.

An idea would be to design an input board that has two low noise opamps, that takes a single ended input and gives balanced output with about 6dB of attenuation.

The power supplies are +/-28VDC from the switching supply and the preamp board has +/-15VDC for the opamp.

Just some thoughts.

[dvenardos]

An idea would be to design an input board that has two low noise opamps, that takes a single ended input and gives balanced output with about 6dB of attenuation.

That sounds like a fun project. :yes:

[vfreeman]

I have 3 x-amps.

All of which have a different output level.

Perhaps we could measure our amps and do a trade if possible to get a matching pair. That would be too easy.... worth a look though.