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Direct Servo Subwoofer Amp A370PEQ

A370PEQ amp 

These amplifiers are showcases of our pursuit of perfection for those who seek more than just basic functions.

Features include:

The A370PEQ is rated at 370 watts of power into a 4 ohm load.

Harmonic distortion is less 0.01% at 200WRMS @4ohms (-3 dB from full output). This amp can surely qualify as an ultra low distortion amp.

All OP amps on the signal paths are OPA2134 and TL072 for better sound quality (SQ) and richer bass sound.

MOSFET transistors for power amp's input stage for a more smooth sound.

Class A/B operation for A series amplifiers and class-H operation for H series amplifiers. These are pure analog amplifiers (no digital sound). We use bigger transformer, bigger filter caps, and bigger one piece 3/16" thick back plate/heat sink construction for better heat transfer and rigid structure. Power rating is tested for 4 continuous minutes.

A defeatable extension filter with separate frequency and damping control. The extension filter can be bypass with 14 Hz and high damping setting. Frequency is adjustable between 14, 20 and 28 Hz. Damping factor is adjustable between high damping (Q=0.6, or bessel filter), medium damping (Q=1.07, or 1.5 dB boost Chebyshev), low damping (Q=1.33, or 3 dB boost Chebyshev). There are a total of 9 settings available. You can use this filter to boost the bottom end or do minor correction for room response. In case you don't know, our competitor has a 6 dB boost at 30 Hz which cannot be defeated. To defeat that, you have to unmount the circuit board and do some soldering, voiding the warranty if there is one. Incorrect rumble filter not only makes the bass sound boomy, it can even bottom out the woofer.

Damping factor: 160 @ 4ohms. (Note: damping factor is speaker wires dominated. Without speaker wires, the native damping factor is >800).

Signal to noise ratio: better than 107 dB (A-weighted).

Input impedance: 30k ohms.

Air tight by designed. No need for a back cover or separate chamber. It will give you an extra 0.15 cu ft enclosure net volume compared with those with back covers.

Auto ON/OFF on left channel input.

Accept both high level (from speaker) and low level (from preamp or processor) inputs.

Fixed high pass line level output for satellite speakers.

Continuous phase adjustment (0-180) and low pass filter adjustment (25-120 Hz).

Gold plated connectors.

Build-in thermal and short circuit protection circuitry.

The phase control is vastly improved so that it has better control in the non-HT setup.

Subwoofer crossover adjustable range is increased to 25-120 Hz for more flexibility.

HP filters for satellite is modified to -3 dB@100 Hz with Q=0.6.

This amp also carries a 2yr warranty against defects (does not cover abuse or installation errors). All other features such as protection circuits are same as regular amps.

Dimension: (W)10-3/8"x(H)12-7/16"x(D)4".

Cutout Dimension: (W) 9 1/8" x (H) 11 1/8".

Big power transformer with 650W max input power. Net weight is 15-18 lb.

Quality components

The op amps used here are audiophile grade OPA2134 and TL072. OPA2134's are used in the high pass section (the one for front speakers filtering). On the subwoofer signal path, the op amps are mixtures of OPA2134 and TL072. The reason for us to use TL072 is for its richer bass sound. At half of full output rating, the distortion is less than 0.01%. In addition, similar to the A370 servo amps, there is no electrolytic capacitors on the signal and feedback paths.

On the top is this defeat-able PEQ. The gain level is from 3db to -12db. The bandwidth controls how wide the notch/peak is. Frequency is from 20hz to 80 with each dot position represents 10hz increment. I don't recommend boost below 35hz. But boost above 40hz is ok as our subs
have more headroom there. The first plot is the gain control at 3 different positions: 3db, 0db, and -12db. The frequency setting is at 20hz. The bandwidth is at min.


In terms of sensitivity, the 12 o'clock position gives us about -5db attenuation while 3 o'clock position gives us -10db attenuation. In other words, the range between 0db and 3 o'clock is what we will use most.


Next is the width control. The comparison is between 7 o'clock, then 9, 12, and 3. I didn't plot 5 o'clock as it is same as 3 o'clock position. The gain is at -12db and frequency is set to 20hz.


Next is the frequency setting comparison at various dotted positions, from 20hz to 80hz. There is a bit of inaccuracy. But overall it is still very good.


In the middle are our regular controls. The one worth discussion is the phase/delay. It is same as the previous one except I improve the resolution and change the label. The circuit is a simple RC all pass filter. Similar circuit is used for adding delay time (such as those in Linkwitz's all active speakers). That is why I add delay to the label. While the circuit does provide delay, but it will top out at 180 degrees. Theoretically, given a fixed delay, the phase shift should be proportional to frequency. However, this RC circuit will not go beyond 180 degrees. Even though I can cascade more stages, I don't think it is useful. I put in 180 there to stress the fact that the circuit tops out at 180degress. So that 180 degrees is not constant to all frequency. Even if you put in 90degree (at 12 oclock position), that is not constant for all frequencies. To get accurate phase delay, one really needs to first figure out the xover frequency and the reciprocal of that gives use the period. 90 degrees is a quarter of period. Then find the position of that on this dial. Your adjustment range is from 7 o'clock to that position to get 0 to 90 degrees shift, you can more by turning more to the right, but it may saturate at 180 degrees even before you turn all the way to the right. Anyway, a plot is worth a thousand words, and here it is from 8 o'clock position to 3 o'clock position. I only plot to 3 o'clock as there is no change beyond 3 o'clock position.


The bottom row has 4 switches. The left 2 are new. One is to replace our 12-24db switch. So this model will replace our 12-24db model going forward. For those with HT receiver and use its bass management, they should get it to middle position 12db/EXT. EXT means the main xover is external. In this case, the xover control knob is mainly for fine tuning. For those don't have HT receiver, or don't use the bass management function on receivers would require the plate amp implement this xover function and that is what this 50hz/24 and 80hz/24 are for. 50hz/24 means the plate amp implement a LP filter with a fixed 12db at 50hz, plus the 12db adjustable knob to make up a 24db total slope. Similarly for 80hz/24. The following shows the FR of this 50hz and 80hz fixed filter.



In either case, one should always use the xover knob in the middle to fine tune the overlap between subwoofer and front speakers. It is more important to combine this control and phase control to achieve the best results. Personally I always use an SPL meter, a test CD (for warble tone), and a spreadsheet.


The rumble filter is for vinyl user or those who wants to play louder. It is a 3rd order HP at 20hz.
Lastly, the HP filter on the line out.


The price is $459.

PEQ3, XLR2 and XLR3 models are also available. See them listed on the Rythmik Audio Site.  

Those models are an additional $40. Please specify the model that you want in the comments section when ordering. 

Currently only the PEQ3 amps are available. 



Your Price: $459.00
Item Number: A370PEQ
Manufacturer: GR Research
Manufacturer Part No: A370PEQ


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