Plate Amp Price Comparo? (1 Viewer)

[Mel Silva]

Just curious if anyone knows if these two plate amps are the same?
Adire AVA250
And
Parts Express 300-794
Anyone?
Just for info, this is for a Shiva 142.5L Vented Subwoofer I am building.
Thanks,
Mel

 

[Peter Jessee]

Almost. The AVA250 has been modified to have a slight (~1 dB) boost around 25 Hz and is -3 dB around 17 Hz, IIRC. The 300-794 now ships with no boost, and is -3 dB at 20 Hz with a Q=.7. The AVA250's curve works really well with sealed Shiva and Tempest designs, but the way they modify it makes further mods difficult. The 300-794 is very easy to modify to whatever response you need, and may be the better choice for your system.

What tuning frequency are you planning? There is an Excel spreadsheet that makes resistor selection quite easy. I'd be happy to run some calcs for you, or e-mail the spreadsheet.

Peter

 

[Mel Silva]

Uh, yea.. Tuning frequency, eh? Sure, I know that... I've got it right here behind this "exit" door. Yoink. (door slams shut and you hear tires screeching away from you).
Honestly, I have no idea what I should be tuning to. I am just copying the box plans from Adire Audio. Can you give a newbie a hint? :b
OK, the Alignment chart gives me numbers like:

• Anechoic F3 = 17.8 Hz
• Anechoic F8 = 14.5 Hz
• Anechoic > 105dB SPL = 16.3 Hz
• In Room F3 = 16.3 Hz
• In Room F8 = 13.7 Hz
• In Room > 105dB SPL = 13.7 Hz

The document also states: "In this case (of the 142.5L box), we chose to maximize base extension with 18.1 Hz tuning."
Is this what you mean? I'm only guessing that this is what you are referring to. Hang on, my ignorance is showing again.
Thanks for your help,
Mel

 

[Baldemar Garcia]

"The AVA250's curve works really well with sealed Shiva and Tempest designs, but the way they modify it makes further mods difficult."

Peter, what exactly do they do to it? Just curious, as I've always wondered.

Mel,
I just built this Shiva enclosure. The Adire AVA250 would work great with it 'as is', or you can save a little money and buy the PE 300-794 if you can modify it yourself for a tiny bit better performance (mated to this sub design). If you want, I can email you a pic of the performance graphs for all three options.

 

[Mel Silva]

Baldemar,
I'm not scared of modifying the electronics of the amp, I've done worse to Ham Radio equipment. But, I have no idea what the modifications are. I could probably do an internet search and come up with few dozen, but honestly I am an audio newbie and dont yet trust my own judgement to find the correct mod.

If I can LEARN SOMETHING by modifying the PE 300-794 then it is worth the extra effort, otherwise I'll just buy the AVA250 when I buy the Shiva.

If you would, send me that graph you mentioned, my email is in my signature line,

Thanks,
Mel

 

[Jack Gilvey]

The stock AVA250 is tailor-made for the Shiva EBS with its 18Hz Fc high-pass. I'm using it with one, and it's a great combo.
The mods for the 300-794 should be on PE's page for it. I believe you can achieve the same rumble filter with the PE amp.

 

[Mel Silva]

Thanks Jack. I never thought to look on PE's page for the mods.

Mel

 

[Manuel Delaflor]

Peter,
Im interested in the spreadsheet, please email it to me.

 

[Mel Silva]

Can anyone confirm my chart observations?

According to the 300-794 Spec Sheet EQ modification chart, to get 1dB of Bass Boost at 25Hz with a Corner (Fc) at 17Hz and a filter Q of 1, the doc says to change R26 from 56K to 47K and R27 from 120K to 180K.

Compare this to the AVA250: which states
Q of 0.96 (almost 1?)
Corner at 18Hz (almost 17Hz)
1dB bass boost at 25Hz

If there's more to it than this, it might be worth it to pay Adire the extra $50 for the AVA250, but for changing a couple of resistors...

Thanks from the newbie for all the help I have received so far.
Mel

 

[Peter Jessee]

Im interested in the spreadsheet, please email it to me.

Manuel, you can download a copy from
http://marv829.homestead.com/Home.html
Look for "Plate Amp Filter", which downloads a copy of Sallen_Filter.xls. There are instructions in the spreadsheet for using the macros. I'll e-mail you a copy I have tweaked a little. Big thanks to Marv829, whoever you are.
Mel, with the EBS design from the Adire website, the AVA250 tuning should work great. What you need to do with any ported sub is to cut the amp's gain below the port tuning frequency (18.1 Hz in this case), since the cone is basically uncontrolled below this frequency. High power below the tuning frequency will cause the driver to over-excursion and be damaged. The AVA250 is -3 dB at 18 Hz, and dropping fast.
Applying the resistor values you mentioned earlier to a 300-794 will, as you point out, give you virtually the same result.
Peter

 

[Allen Ross]

so which plat amp is good for a tempest ported big box enclosue, 15ft^3?

 

[Jack Gilvey]

The AVA250 is -3 dB at 18 Hz, and dropping fast.

Actually, that's the corner frequency (Fc). I think the f3 is 13Hz or something. In my opinion, the PE amp is a better deal since it's more flexible and cheaper. The AVA250 will work perfectly for the Shiva EBS, though, its filter is right where you want it.

 

[Mel Silva]

That settles it, since I have a free shipping coupon, the PE amp is on sale and I would like to buy the PE Speaker Spikes as well, I think we're done here. I plan on modifying the pre-amp board as I have indicated previously. I don't think I want to be "safe" and move the Fc to 21Hz, because my room calculations have exposed a possible 1st order Axial Mode at 40Hz, but this has not been tested or measured, so...

Thanks again Peter, Jack, and Baldemar.

Mel

 

[Manuel Delaflor]

Thanks Peter
I will play with it for a while...

 

[Ryan Schnacke]

A word of caution to those modifying the PE plate amps: Switching the unit off and unplugging it from the wall outlet does NOT drain the power supply for the amp board. It will retain charge for DAYS.

This is okay during normal operation because the power supply for the pre-amp board does drain of in a matter of seconds - you can watch the LED light dim out. So long as the unpowered pre-amp board is connected to the still-powered amp board then the input signal to the amp board will be grounded.

But following the directions for modding the filter, you would disconnect the bus from the pre-amp to the amp. At this point the input signal is free to float to whatever level it pleases. If it floats away from ground (positive or negative) then it can turn on the output transistors. If you happen to let the speaker wires touch then *POP* ... visible spark and small weld marks where the wires touched. You guessed it - this is the voice of experience.

No damage was visible on the components, but after the hit the plate amp would hum with no signal. And it sounded a bit sloppy or distorted. I was curious just how much charge was on the power supply so I investigated and found:

There's a 6800uF (6.8mF!!!) cap for the positive supply and another for the negative supply. During no-load operation supply voltage is +/-70V (140V total!). After being unplugged for 4 hours - around +/-62V. After another 24 hours - around +/-55V. That's a LOT of charge.

To prevent this from happening to you, there's a few options:
1) Discharge the amp power supply. You'd have to remove the amp board and use a hi-valued resistor (around 30k ohms or higher) and it'd take a while to drain in down (30 minutes or so)
2) Don't disconnect the wire bundle that connects the pre-amp board to the amp board. This will keep the input signal grounded.
3) Take extra care to isolate the output wires each other - and from anything else that can conduct electricity.

BTW, Parts Express is great! Though I was unable to identify the damaged component(s) their tech support was VERY helpful, sending countless emails with great tips and generously offering to send replacement parts if I identified the problem. Eventually I decided to just replace the unit and PE offered to exchange it for only the cost of shipping! They're among a small, select group of (mostly internet) businesses that are setting a new standard for customer service.