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benproiii 1st January 2012 12:56 AM

My amp is clipping with new speakers
I just built the Zaph|Audio SR71 Kit from Madisound and I hooked them to my DX HRII amplifier running off +/- 35V and it is much quieter then my old speakers (Polk Audio M10's) and when I turn the volume up pretty loud the speakers are all distorted. I know its the amp because my AKM4396 DAC and Pass B1 Preamp are not clipping the signal (checked full vol. sign wave with scope). Would it be OK the increase the supply voltage to 49.7V without any modifications. the transformer I'm using has a 25V tap (using now) and a 35V tap. Thanks.

Stefanoo 1st January 2012 02:20 AM

nope, don't do that unless you have an overkill heatsink.
Also, if there is distortion, there might be something not properly soldered or something defective.
you need to make some measurements and check dustortion vs input level.

gootee 1st January 2012 05:34 AM

Maybe the speakers are just much less efficient than whatever you were using previously. When running 150 W/ch into my 4-Ohm Magnepan MG-12 speakers (supposedly rated 86dB/2.83v/m @500Hz), my amp would start to clip before I could get the speakers as loud as I sometimes wanted them to be. I bought an amp that can push 400 W/ch @ 4 Ohms (Adcom GFA-585) and I've never seen it even get close to clipping, and can now get the speakers to play so loudly that they begin to self-distort. But that would just be as a test because they would be "too loud" for me at that point, even if they weren't distorting. So now I am happy with the system's SPL capability.

In other words, you might simply need either more-efficient speakers or more available power.

jaycee 1st January 2012 06:10 AM

Check your DAC is happy to drive a 12K impedance. As you have a 'scope, this is pretty easy, just check the signal at the base of Q1.

godfrey 1st January 2012 06:54 AM

Changing from the 25V to the 35V taps will allow about double the power, but some other changes will be needed:

a) The zener diodes that regulate the supply voltage for the front end of the amplifier will have to be changed from 16V to (IMO) 22V.
b) Check that all the capacitors are rated to withstand the higher voltages.
c) As Stefano mentioned, you may need bigger heatsinks depending what you have and how hot they get.
d) I'd strongly recommend adding a second pair of output transistors (and associated resistors).

AndrewT 1st January 2012 10:57 AM

Changing the PSU voltage requires a complete re-assessment of the amplifier working conditions. It is in effect a re-design.

tvrgeek 1st January 2012 12:42 PM

That is a superb speaker design, so I suspect something else terribly wrong. They are not that inefficient. I hate to go there, but have you triple checked everything like having a woofer out of phase, or peculiar impedance? Are you expecting a monitor size speaker to fill a stadium, boost the bass or anything not for what it was designed?

There are some speakers that just seem to want big power to "open up", as a totally non scientific term. But I have used enough 10W amps and 200W amps winding up with most of my amps are about 60W in my serious room, and 30 in my office.

benproiii 1st January 2012 03:35 PM

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They only distort on the highest volume of my pass B1 preamp which has no gain, I will only have it on the highest volume on quietly recorded songs. the distortion is not that bad just some edgy vocals. I was not going to listen to them that loud but I am worried about the ability to handle very dynamic passages.

I plugged them into my Onkyo receiver and they don't distort even on extremely high SPL. The amp in it only has 1 output pair per channel and runs off 65V+/- would that exceed the SOA of the output transistors? Why can't I run the HRII amp off 50V+/- this amp runs the amp off 65!

5th element 1st January 2012 07:30 PM

If you take 0dB digital to equal the maximum volume possible (usually about 2Vrms on the DAC output), then you could play with sine-wave sweeps @ this level and scope the amplifier output to see at what volume you can safely set the pre-amplifier to such that the amplifier does not clip. If the amplifier is clipping because you're running into limits of the rail voltages then you need not have the loudspeakers connected to do this. However if the amplifier is clipping due to the protection kicking in, then you will need to carry this out with the loudspeakers connected. If done at high levels (which this is) do it with caution as you might exeed the woofers physical capabilities if the sine sweep sends 10hz etc through them.

As 0dB is the absolute maximum that can be produced by the DAC, if you therefore set the max volume such that the amplifier does not clip when driven at this level, then you will know that regardless of how dynamic the music you will never cause the system to clip.

benproiii 1st January 2012 07:57 PM

It did the test with my scope and the output of the preamp at 0db is a perfect sine wave and the output of the amp with a 8ohm load will start to clip at about 15V p-p:confused: which is the volume control at 9 o'clock. I thought that the amp would swing more volts than that before clipping at +/-35V supply

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