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Heathkit AD-1702
Active Crossover
(1979-early 1980s)
© 2011-2012 KenRockwell.com. All rights reserved.

Intro   Specs   Schematic   Measurements   Recommendations

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Heathkit AD-1702

Heathkit AD-1702 Active Crossover. enlarge.

 

Heathkit AD-1702

Heathkit AD-1702 Active Crossover (about $350 used). enlarge. This free website's biggest source of support is when you use these links, especially this link directly to Heathkit crossovers at eBay (see How to Win at eBay), when you get anything, regardless of the country in which you live. Thanks! Ken.

April 2013, May 2012   Audio Reviews   All Reviews

See also:

Stereo Subwoofers

Bass Management

 

Introduction         top

Intro   Specs   Schematic   Measurements   Recommendations

The Heathkit AD-1702 is an active low-level crossover ideal for adding subwoofers to any stereo playback system. It's fed from your preamp, and has outputs to feed to the power amplifiers for both your high and low speakers.

The Heathkit AD-1702 crossover is unusually handy because it provides separate adjustment of frequency and slope for each of the high pass and low pass sections. For example, with a normal sealed-box speaker, set the high-pass crossover to 6 dB/octave at the speaker's -3 dB rolloff point, which makes the combined acoustic response of the regular speaker 18 dB/octave. Set the subwoofer's crossover to 18 dB/octave at the same frequency, and you've got perfect acoustic response for the complete system, taking the speakers' own responses into account.

It's a stereo crossover, ideally intended for stereo subwoofers. Even though our ears can't localize a mono woofer crossed-in at 60 Hz or below at 18 dB/octave, we can localize woofers crossed over at higher frequencies or with shallower slopes. Most importantly, music and movies on CD, DVDs and better has stereo bass. You can slum it with a mono woofer, but if you want serious high fidelity for music, unless you only listen to LPs and 78s, you need stereo subwoofers. CDs and movie DVDs have always had stereo bass. See more at stereo subwoofers.

I built my AD-1702 specifically when I upgraded to stereo subwoofers in 1986.

You feed the low outputs either to your own amp and passive subwoofers (I used JBL B460 18" subwoofers fed from a Hafler DH-200), or feed the crossover's outputs to the RCA inputs of today's popular self-powered subwoofers.

It has a hard-wired bypass, a level control for the low output, separate mute controls for each of the high and low outputs. The level control knob is on a splined, not set-screwed, shaft, so if you want to index the knob to zero for a certain gain, it only goes in steps.

The only thing missing is a polarity switch for the low outputs.

The "Bass Overload" LED lights whenever either low output exceeds about 1 RMS. The crossover still has 20 dB of headroom at 1 V RMS; this light is letting you know that most power amplifiers are going to be at about maximum output at 1 V RMS input.

Yes, 5.1 has only one mono low-frequency effects (LFE) channel, however the other 5 channels are full-range, and contain all sorts of multichannel low-frequency effects if you'd only listen to them. 5.1 is 5 point 1, not (5 x 0.9) point 1. See also Bass Management.

If you settle for a mono subwoofer, the AD-1702 also has a summed mono woofer output. It thirdly has a Right and (minus) Left pair of outputs as a sub-optimal attempt to feed any stereo amp as a high-power mono subwoofer amp. Ideally, one ought to redesign it so that pair of outputs is R+L and -(R+L).

It's in a steel sheet metal case, covers held by sheet metal screws. The board hangs upside down: removing top cover reveals the bottom of the board. Remove the bottom cover to see the top of the board and the components

Relays provide about an 8 second turn-on delay. The relay shorts the outputs to ground after the 560 ohm build out resistor. I measure 20 seconds delay in my factory-built unit.

Heathkit AD-1702

Heathkit AD-1702 Active Crossover. enlarge.

 

Specifications         top

Intro   Specs   Schematic   Measurements   Recommendations

Circuitry

Discrete, with two NE5534N.

 

Crossover Frequencies (switched with 1% resistors)

40 Hz

60 Hz

80 Hz

100 Hz

125 Hz

160 Hz

High-pass and low-pass filters individually adjustable for frequency.

 

Crossover Slopes (switched)

6 dB/octave and/or 18 dB/octave, Butterworth.

High-pass and low-pass slopes are individually selectable.

 

Infrasonic Filter

-3dB at 15 Hz

12 dB/octave.

 

High-Frequency Gain

Unity.

Actual measured: -0.2 dB.

 

Low-Frequency Gain Control

±15 dB

 

Size

One 19" rack unit tall

Non-standard screw slots, only one per side.

 

Power Supply

Power consumption rated 0.1 A at 120 V RMS, 0.25A fuse.

One unswitched outlet.

+-18V regulated DC supplies, LM317 and LM 337 on heat sinks.

 

Measued Power Consumption

  9.25 ~ 9.5 watts operating.

  7.75 ~ 8.0 watts for the first few seconds before the demute relays click-on.

  0 watts, power off.

 

Schematic Diagram         top

Intro   Specs   Schematic   Measurements   Recommendations

Heathkit AD-1702 Schematic

Heathkit AD-1702 Schematic Diagram. much bigger.

 

Heathkit AD-1702 crossover, inside

Interior, Heathkit AD-1702 crossover. much bigger.

 

Measurements         top

Intro   Specs   Schematic   Measurements   Recommendations

These tests are on a unit with its frequency scaled by 10x, having had its capacitors changed.

Measured with Rohde &Schwarz UPL.

 

Output Levels

MOL is at least 10 V RMS (limit of the Rohde &Schwarz UPL) at 0.008% THD.

Lower left channel of this sample had an 8mV DC offset.

 

Gain

High-Frequency section gain measured at -0.11 dB (fixed). (Another sample measured +0.2 dB)

Low-Frequency section gain measured at -16 to +14.5 dB (variable).

 

Noise

This is a very quiet crossover. It will not degrade your amplifier with noise.

Noise, A-weighted, HF, set to 100Hz
Left
Right
18 dB/octave
-108.4 dBV
-108.6 dBV
6 dB/octave
-108.4 dBV
-108.6 dBV
"OFF"
-113.7 dBV
-113.7 dBV

 

Noise, A-weighted, LF, set to 100Hz
Left
Right
18 dB/octave
-102.4 dBV
-101.9 dBV
6 dB/octave
-102.5 dBV
-101.9 dBV
"OFF"
-102.5 dBV
-102.0 dBV
18 dB/octave (max level)
-100.5 dBV
-97.1 dBV
18 dB/octave (minimum level)
-104.2 dBV
-103.8 dBV

 

DC Offset

Left
Right
HF
+60 µV
-64 µV
LF
+2.3 mV
< ±10 µV

 

Frequency Responses

The traces are color coded for the Left Channel and for the Right Channel. When they don't lie on top of each other, it's due to channel imbalance.

Heathkit AD-1702 crossover

Infrasonic frequency response to 0.1 Hz, low-pass section, infrasonic filter out.

 

Heathkit AD-1702 crossover

Infrasonic frequency response to 1 Hz, low-pass section, infrasonic filter in.

 

Heathkit AD-1702 crossover

High-pass response to 110 kHz.

 

Heathkit AD-1702 crossover

High-pass response, 18 dB/octave.

 

Heathkit AD-1702 crossover

High-pass response, 6 dB/octave.

 

Heathkit AD-1702 crossover

Low-pass response, 18 dB/octave.

 

Heathkit AD-1702 crossover

Low-pass response, 6 dB/octave.

 

LF and HF frequency responses

Only left channels shown.

Heathkit AD-1702 crossover

40 Hz, 18 dB/octave.

 

Heathkit AD-1702 crossover

60 Hz, 18 dB/octave.

 

Heathkit AD-1702 crossover

80 Hz, 18 dB/octave.

 

Heathkit AD-1702 crossover

100 Hz, 18 dB/octave.

 

Heathkit AD-1702 crossover

125 Hz, 18 dB/octave.

 

Heathkit AD-1702 crossover

150 Hz, 18 dB/octave.

 

THD

Heathkit AD-1702 crossover

HF THD, 1 kHz, set to 40 Hz at 18 dB/octave.

 

Heathkit AD-1702 crossover

HF THD, 1V RMS, 40 Hz at 18 dB/octave.

 

Heathkit AD-1702 crossover

HF THD, 200 mV RMS, set to 40 Hz at 18 dB/octave. (note extended scale down to 0.0002%)

 

Heathkit AD-1702 crossover

LF THD, 1V RMS, set to 150 Hz at 18 dB/octave.

 

Frequency Scaling

One huge gotcha of buying these used today is that some people have changed parts to change the crossover frequencies. If someone did, this makes the crossover useless until you change the parts back to the correct values, or at least to the values correct for your use.

The best way to test is by plotting frequency response with an audio analyzer, but barring that, look at the capacitors on the lower left, flanking each of the frequency selector switches:

Heathkit AD-1702 crossover, inside

Interior, Heathkit AD-1702 crossover. enlarge.

 

Heathkit AD-1702 crossover, inside

Closer, scaled Heathkit AD-1702 crossover. enlarge.

The original capacitors are green Mylar, somewhat smaller than the outlines for them on the board's silk-screened printing.

Replacement capacitors can be any size, shape and color.

In this sample, the white rectangular capacitors on the left, near the HF frequency selector, are only 1/10 the correct values, increasing the crossover points for the HF section by a factor of 10.

The capacitors near the LF frequency selector switch in this sample are the correct stock capacitors.

Therefore, the LF section of this sample is normal, while the HF section passes only above your selection of 400 Hz, 600 Hz, 800 Hz, 1 kHz, 1.25 kHz or 1.6 kHz!

After the original owner has changed this, normally no one knows until the next unhappy buyer — and only if that buyer has an audio analyzer to figure out what's going on. The bypass switch is often disabled if the crossover was scaled for use with tweeters.

Amazingly, I've seen listings on eBay with sufficiently detailed interior shots to let me see that a crossover had been scaled, saving me the trouble of buying it. (my technician charges too much to reswap capacitors.)

To scale or rescale a unit, you'll need at least the skills of a licensed technician, and the original manual with the values and locations of the capacitors.

 

10x frequency-scaled sample

Measurements of a simple with both the HF and LF sections scaled by 10x. In other words, the "100 Hz" position was really a 1 kHz crossover.

Heathkit AD-1702 crossover

Low-pass response, 6 dB/octave, crossover frequency-scaled 10x.

 

Heathkit AD-1702 crossover

Low-pass response, 18 dB/octave, crossover frequency-scaled 10x.

 

Heathkit AD-1702 crossover

High-pass response, 18 dB/octave, crossover frequency-scaled 10x.

 

Heathkit AD-1702 crossover

High-pass response, 6 dB/octave, crossover frequency-scaled 10x.

 

Noise, A-weighted, High-pass, 1 kHz (scaled)
Left
Right
"OFF"
-115.7 dBV
-108.5 dBV
6 dB/octave
-107.3 dBV
-105.5 dBV
18 dB/octave
-105.5 dBV
-104.7 dBV

 

Noise, A-weighted, low-pass, 1kHz (scaled)
Left
Right
"OFF"
-101.4 dBV
-101.1 dBV
6 dB/octave
-101.1 dBV
-101.1 dBV
18 dB/octave
-99.2 dBV
-99.1 dBV
18 dB/octave (max level)
-88.7 dBV
-87.3 dBV
18 dB/octave (minimum level)
-103.3 dBV
-102.3 dBV

 

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Recommendations         top

Intro   Specs   Schematic   Measurements   Recommendations

It's great for use in any bi-amped system. I used one for years to use a pair or 8-cubic-foot 18" B460 (JBL 2245H) subwoofers with a pair of Quad ESL-63s. This crossover let me cross them over at 40 Hz, which lets the Quads sing and the JBLs do nothing other than move air.

The bridged outputs are done incorrectly: they aren't an out-of-polarity mono sum; instead, they are the right output and the inverted left output.

These were all sold as do-it-yourself kits, so there is always the possibility that you might get one today assembled by an idiot.

WARNING: Of three I've seen for sale, all three were frequency-scaled, meaning that the prior owner had severely altered the crossover frequencies by changing internal capacitor values, but had not noted this while selling these.

One tip-off is to look for green Mylar capacitors near the frequency and slope switches in any online photos, which were the original capacitors. Different capacitors suggest different frequencies, especially if you see any rectangular or cylindrical polypropylene capacitors, which were not standard.

I have no idea why people felt the need to scale these by 10x, especially since that's a relatively useless frequency shift, but was the one outlined as an example in the manual. If you buy one that's been shifted, simply changing capacitors can change it back.

If you've found this report helpful, this free website's biggest source of support is when you use these links, especially this link directly to Heathkit crossovers at eBay (see How to Win at eBay), when you get anything, regardless of the country in which you live. Thanks! Ken.

 

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Mr. & Mrs. Ken Rockwell, Ryan and Katie.

 

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