Heathkit AD-1702 Active Crossover. enlarge.
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.
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, and 8 seconds in each of my other two.
Heathkit AD-1702 Active Crossover. enlarge.
All discrete high-pass stage.
Two NE5534N, one for 15 dB of gain in each low-pass channel.
Other assorted op-amps and comparators for overload indicator and summed and inverted LF outputs.
±18V DC supply rails.
BYPASS is a hard-wire bypass, no power required for signal.
Crossover Frequencies (switched with 1% resistors and 5% capacitors)
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.
-3dB at 15 Hz
Measured as 0 ±0.2 dB depending on sample.
Low-Frequency Gain Control
One 19" rack unit tall
Non-standard screw slots, only one per side.
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.
Measured Power Consumption
9.0 ~ 9.5 watts operating.
7.6 ~ 8.0 watts for the first few seconds before the demute relays click-on.
0 watts, power off.
Schematic Diagram top
Heathkit AD-1702 Schematic Diagram. much bigger.
Interior, Heathkit AD-1702 crossover. much bigger.
These tests are on a unit with its frequency scaled by 10x, having had its capacitors changed.
Measured with Rohde &Schwarz UPL.
Maximum Output Levels
MOL is at least 10 V RMS (limit of the Rohde &Schwarz UPL) at 0.008% THD.
The lower left channel of this sample had an 8mV DC offset.
In July 2014, at 11.0V RMS output from the bass section, adding gain from the 10V output of the UPL, I measured 0.4% THD.
The Bass Overload light is just a guide to overdriving your subwoofers, not the cross over itself. It lights at about 50% brightness at 750mV RMS output from either LF section, and gets brighter until it's 100% bright at 1.5V RMS out from either LF section.
Output Source Impedance
High: 568.7Ω / 569.5 Ω at 1 kHz (repaired beater Nr. 41743).
Low: 648.7Ω / 613.5 Ω at 50 Hz (repaired beater Nr. 41743).
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).
In March 2015, Nr. 41743 measured -0.123 dB left, -0.130 dB right.
This is a very quiet crossover. It will not degrade your amplifier with noise.
Nr. 41743 measures -106 dBV A.
The outputs are capacitor coupled.
Nr. 41743 measures +140 µV left, +10 µV right.
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.
Infrasonic frequency response to 0.1 Hz, low-pass section, infrasonic filter out.
Infrasonic frequency response to 1 Hz, low-pass section, infrasonic filter in.
Infrasonic filter response, Nr. 41743:
-0.5 dB at 26 Hz.
High-pass response to 110 kHz.
High-pass response, 18 dB/octave.
High-pass response, 18 dB/octave, Nr. 41743.
High-pass response, 6 dB/octave.
Low-pass response, 18 dB/octave.
Low-pass response, 18 dB/octave, Nr. 41743.
Low-pass response, 6 dB/octave.
LF and HF frequency responses
Only left channels shown.
40 Hz, 18 dB/octave.
60 Hz, 18 dB/octave.
80 Hz, 18 dB/octave.
100 Hz, 18 dB/octave.
125 Hz, 18 dB/octave.
150 Hz, 18 dB/octave.
Distortion is essentially nonexistent. This is excellent performance.
HF THD, 1 kHz, set to 40 Hz at 18 dB/octave.
HF THD, 1 kHz, set to 40 Hz at 18 dB/octave, Nr. 41743. (note extended scale down to 0.0001%)
HF THD at 30 mV, set to 40 Hz at 18 dB/octave, Nr. 41743. (note extended scale down to 0.0001%)
HF THD at 100 mV, set to 40 Hz at 18 dB/octave, Nr. 41743. (note extended scale down to 0.0001%)
HF THD at 300 mV, set to 40 Hz at 18 dB/octave, Nr. 41743. (note extended scale down to 0.0001%)
HF THD at 1 V, set to 40 Hz at 18 dB/octave, Nr. 41743. (note extended scale down to 0.0001%)
HF THD at 3 V, set to 40 Hz at 18 dB/octave, Nr. 41743. (note extended scale down to 0.0001%)
HF THD, 1V RMS, 40 Hz at 18 dB/octave.
HF THD, 200 mV RMS, set to 40 Hz at 18 dB/octave. (note extended scale down to 0.0002%)
Harmonic Distortion Content
THD is mostly noise below 1 volt, and mostly second-harmonic at and above 1 volt.
HF THD at 30 mV, set to 40 Hz at 18 dB/octave, Nr. 41743.
HF THD at 100 mV, set to 40 Hz at 18 dB/octave, Nr. 41743.
HF THD at 300 mV, set to 40 Hz at 18 dB/octave, Nr. 41743.
HF THD at 1 V, set to 40 Hz at 18 dB/octave, Nr. 41743.
HF THD at 3 V, set to 40 Hz at 18 dB/octave, Nr. 41743.
19+20 kHz Difference-Frequency Distortion
DFD is very low; this is excellent!
HF 19/20 kHz DFD at 100 mV, set to 150 Hz at 18 dB/octave, Nr. 41743.
HF 19/20 kHz DFD at 300 mV, set to 150 Hz at 18 dB/octave, Nr. 41743.
HF 19/20 kHz DFD at 1 V, set to 150 Hz at 18 dB/octave, Nr. 41743.
HF 19/20 kHz DFD at 3 V, set to 150 Hz at 18 dB/octave, Nr. 41743.
LF Section THD
LF THD, 1V RMS, set to 150 Hz at 18 dB/octave.
It's happy driving 600Ω loads. With a 600Ω output source impedance, the levels drop 6dB and the distortion climbs a little at a few volts, but otherwise, no surprises.
The LF section's frequency response drops 1.5 dB at 20 Hz and 4 dB at 10 Hz.
MOL at 0.1% THD is 5.0 V for LF at 50 Hz and 2.7 V for the HF section. (Of course the input voltages were twice this.)
HF THD at 1 kHz, set to 40 Hz at 18 dB/octave.
HF THD at 1 volt, set to 40 Hz at 18 dB/octave.
HF THD at 100 mV, set to 40 Hz at 18 dB/octave.
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:
Interior, Heathkit AD-1702 crossover. enlarge.
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.
Low-pass response, 6 dB/octave, crossover frequency-scaled 10x.
Low-pass response, 18 dB/octave, crossover frequency-scaled 10x.
High-pass response, 18 dB/octave, crossover frequency-scaled 10x.
High-pass response, 6 dB/octave, crossover frequency-scaled 10x.
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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