#160a

[moe-so-journal]

[atomiumamps]

The DBX160A has pads on its PCB to install an optional 1:1 output transformer (made by Jensen). But you can use those pads to install any transformer you want. In this case, it’s a Neve-style 200:600 output transformer, wired in reverse as a -4dB step-down. The 160A has a bridged-mode output stage that can hit +26dBu before clipping. That’s wasted headroom when your converters clip at +22dBu, so this transformer makes the match ideal, while also juicing the low end with some odd harmonics -- especially cool for kick and bass guitar.

I added a 1.5k loading resistor across the secondary, since the added damping greatly reduces ringing with this particular transformer, and most line inputs these days have >10k input impedance.

The 160A’s unbalanced output is driven by a separate servoed/buffered opamp stage. That’s still available here as a +26dBu transformerless output, in case the extra level is needed. And when the unit is in bypass, the transformer is out of circuit; unity-gain buffered input signal is passed directly to all outputs.

[atomiumamps]

Two DBX 160A compressor/limiters. These are super common everywhere, but there’s a ton of confusion about the different versions of the so-called “1U 160” or “160 rack” -- more about this down below. Both of the above units are the later 160A, but obviously they don’t look anything alike inside. Both had problems, so part of the job was typical triage/repair, but the other part was to harmonize them so that they could be used as a stereo pair.

The older one (top) needed a full re-cap and one of its regulators was blown. It was hellish to get apart. The newer one (bottom) just needed a couple of new switches, but I bumped the supply caps while I was in there.

Both units got newer, lower-distortion THAT2180Bs, instead of the 2150 series parts they had before (older one had a THAT2159, newer one had a DBX-branded VCA that was probably a THAT2155). The 2180 is pre-trimmed, so some resistors needed to be clipped out for it to work properly.

Both units also got new opamps:

OPA1642 for the input buffer and transimpedance stages around the VCA, where the low bias/offset current of a FET amp helps, but you also need very low voltage noise. In older 160X/XT versions, this was an LF353 (FET opamp); in the 160A, it’s an NE5532 (bipolar). The 1642 gives you the good points of both, plus it’s far less sensitive to source impedance than most FET opamps.

LM4562 for the balanced output driver. The 4562 directly drives 600-ohm loads with vanishingly low distortion and noise, marginally superior to the stock NE5532. The 5532 is totally fine, though.

OPA2134 for the unbalanced output amp, which drives a push-pull pair of output transistors and runs from the unregulated 24V supply; benefits from high PSRR, high headroom, and graceful overload behavior, all of which the 2134 is good at. The other half of this dual is used as a DC servo, which the 2134 also works well for. It’s not quite as nice as the 1642 on audio specs, but has better DC specs, higher common-mode input voltage range, and clips just as gracefully.

I do not have the official factory calibration procedure for these units. If anyone does have those documents somewhere, please do get in touch. Update: I found the 160XT calibration procedure here, and there are some notes below about how to adapt it to the 160A.

Following are some notes (and myth-busting) about the different versions of this circuit.

There are three (and a half?) versions of the 1U rack-mount DBX160:

1. 160X - balanced or unbalanced input, unbalanced-only output. Terminal block I/O. Has PCB pads for an optional output transformer, the Jensen JT-123-DBX. No units came with the transformer stock. The transformer was the only way to get a balanced output from this unit. It has an LF353 dual as input buffer and I/V converter, and an LF351 single as the unbalanced output amp. There are 4558s in the sidechain, which is RMS-detecting with fixed attack and release times.

2. 160XT - balanced or unbalanced input, balanced or unbalanced output. This one has XLR & 1/4″ I/O. Same transformer option PCB pads, no units came with a transformer stock (everyone seems to think “XT” stands for “transformer” -- it does not). The unbalanced output is the same as the 160X; the balanced output is driven by two NE5534 singles (one for each signal polarity). The transformer option pads are on the balanced output and the transformer only adds galvanic isolation to that output; it’s already electronically balanced. The 160XT also introduced the non-linear attack/release characteristic, where the time constant gets faster the more the threshold is exceeded. This sounds pretty different from the 160X (better, IMO), and all subsequent versions have this. Check THAT Corp. Design Note #114 for details. Still LF353 around the VCA and 4558s in the sidechain.

3. 160A (top unit above) - balanced or unbalanced input, balanced or unbalanced output. Mostly the same as the 160XT, except that the LF353 around the VCA is an NE5532, and the balanced output is also an NE5532 dual instead of two 5534 singles. Coupling caps are much larger values, so the 160A has less phase shift in the bottom octave than the earlier models. The unbalanced output is an LF353 dual instead of an LF351 single, with the 2nd amp used as a DC servo to reduce offset on the DC-coupled outputs. With the servo, there is an added offset trimmer to zero the DC offset of the outputs (this is the unlabeled trimmer). Same transformer-option PCB pads. Has a strange internal jumper for “0dB” or “-26dB”. Sidechain is now TL074 quad opamps instead of 4558 duals. The stereo strap input is also changed (simplified) from the 160XT, so even though the functionality is the same, the TRS stereo strapping cable must be unplugged for two units to function independently. Likewise, there is no separate trimmer for gain calibration in slave mode, only for master mode. Hellish to disassemble if it requires repair. Front panel PCB is a single piece mounted to double-stick foam (really), with only a single ribbon cable connection to the mainboard.

3a. 160AD (bottom unit above) - update of the 160A, with much smaller PCB and dramatically revised layout. Same opamps as 160A, though VCAs are DBX-branded rather than THAT-branded. Some capacitors eliminated or tweaked in value. PCB is overall more fragile than previous 160A, but much easier to get in and out of the chassis. Offset trimmer is now labeled. Can be identified from the front because it has six socket-head hex screws in the front panel instead of four like the earlier version. Two front-panel PCBs, mounted on screws/braze-ons, each with its own ribbon cable. Much better layout than previous version. Removes 0/-26dB jumper. Test points are 0-ohm resistors instead of wire jumpers.

Using the 160XT calibration procedure for the 160A Procedure is largely the same, except that the component references for the trimmers are not accurate. Just go by the printed labels for what each trimmer does. Set the offset trim first -- with zero signal applied, adjust it so pins 2 & 3 of the XLR output are as close to 0V as possible -- I shoot for less than +/-1mV. After completing the whole calibration procedure, re-check the offset.

If you have replaced the VCA with a trimless 2180-series part, you can obviously skip the VCA symmetry adjustment.

The only other main difference is that the 160A lacks a separate adjustment for slave-mode gain calibration. The gain calibration trimmer will have no effect when the unit is in slave mode. Skip that part of the gain adjustment and just do the part for master mode.

Keep in mind that the metering is referenced to 0dBu for the duration of the procedure, but then set to +4dBu in the “final calibration” at the end. This is the factory setting, though of course you can set it wherever you like. Setting the 0dB reference to +4dBu is technically correct in terms of nominal pro line levels, but may be somewhat confusing because the threshold knob is referenced to 0dBu. Thus, once the meter is referenced to +4dBu at the end of the procedure, an input signal that hits -4dB on the meter will trigger the threshold with the knob at 0dBu.

[vironika]

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[hurrahmusic]

new bbs #dbx #160a