Where can I find mod info for the 250W plate amps? (1 Viewer)

[Scott Parker]

Hey guys,
I thought a read a post on here a couple days ago that had 3 different ways of modifying the plate amps. An audiophile version, LF version, and HT. Has anyone come across that site, or is there some more good ones? Is there a way to change the 2nd order crossover to a 4th order crossover?

 

[Ryan Schnacke]

You could add another op-amp circuit - a 4th order Sallen-Key filter requires two op-amps and as it comes there's only one op-amp dedicated to this purpose.

I suppose if you're wanting a 4th order rumble filter but you don't care about the low-pass filter you could "borrow" that op-amp. But it'd be a little challenging to "borrow" the rumble filter op-amp for the low-pass filter since this is a pot-adjustable filter and you'd need 4 pots to move simultaneously. Besides, a rumble filter is a good thing to have.

What are you wanting to do? I'm pretty familiar with this circuit.

 

[Scott Parker]

Thanks Ryan,
Actually I did find the page but I'm still not sure exactly what its telling you to do. It's under http://www.vacuumstate.com/sub/index.htm
and then go to the mod for the audiophile version. What got me wondering was that at Stryke audio the guy said he could give me a 18db/octave crossover just by changing one thing. So I was wondering if I could do other simple things to improve it.

 

[Ryan Schnacke]

Hmm. I'm looking at the schematic on the ApexJr site, which has proven to be almost exactly the circuit used in the PE 300-794. The mods listed on the vacuumstate web site make no sense.

Modify R9 and R14? Well, those are each in the input structures, and one is in the left input while the other is in the right input, but they don't correspond to the same device. And only one of the two has an original value of 15k.

Modify R23 to flatten the 30Hz peak? No, that resistor is in the variable gain section, not the rumble filter/bass boost section.

It goes on and on. These mods were obviously defined for some amp other than the 300-794. You might be able to get 18dB/octave somewhere, but unless you're noticing a problem with the existing crossover I wouldn't worry about it. In fact, most guys are using a 24dB/octave filter built into their receiver's subwoofer output so they just turn the plate amp's "crossover frequency" control up as high as possible to get it out of the way.

 

[Ronnie Ferrell]

Here and here are the best places for mods to the parts express plate amps. I have not done any of these mods so don't come crying to me if you try them and they screw up your amp. I am thinking of doing the 4th order crossover mod to around 55hz to 60 hz.
Ronnie

 

[Ryan Schnacke]

John's mod "borrows" the rumble filter's op amp. So you'll only be left with a little bit of 1st order RC rumble filter with a very low Fc. Definitely not ideal for vented subs, especially if they're tuned to 20Hz or higher. But this could work really well for sealed enclosures and infinite baffles.

These kind of mods only make sense if you're actually using the plate amp to crossover the signals between the sub and the speakers. If that's the case, I'd go ahead and mod the high-pass on the line outs to get something more like Q=0.6 - 0.7 and Fc=80Hz. If your sub is near or between your main speakers then localization shouldn't be a problem and you can keep the 2nd order low-pass.

If you're using the receiver's bass management then you've already got a 4th order at (probably) 80Hz and you're cascading the plate-amp's low-pass filter on top of that!

 

[Ronnie Ferrell]

If you're using the receiver's bass management then you've already got a 4th order at (probably) 80Hz and you're cascading the plate-amp's low-pass filter on top of that!

Right now I do not even have a HT receiver, but x-mas is around the corner! The receiver I am looking at only has a 2nd order at 100Hz. with not many bass management options. That is ok, because I am having to skimp of some of the bells and whistles. 2-channel audio is a main priority of mine and it limits the receivers in my price range. :frowning: I am going with a receiver that concentrates on great sound over all the bells and whistles... You got to make do with what meets your requirements AND that you can still afford...
Ronnie

 

[Ryan Schnacke]

Yes, this is true for us all ... at least to some degree. 2-channel audio is also very important to me. My use is probably 90% music / 10% movies. And I always listen in stereo - no pro-logic 2 or dsp modes. I've been pretty pleased (ha) so far with a budget Onkyo - the 595. But I certainly don't have the golden ear. I hope you find something that meets your needs and pleases your wallet.