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Worth replacing 4458 op amp in dbx 163X?
Old 20th November 2009
  #1
Worth replacing 4458 op amp in dbx 163X?

Two are used in the over-easy circuit, are they worth replacing? What are suitable drop in replacements?
Old 20th November 2009
  #2
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JohnRoberts's Avatar
 

Quote:
Originally Posted by upinflames View Post
Two are used in the over-easy circuit, are they worth replacing? What are suitable drop in replacements?
If they are used in the side chain and not the primary audio path I wouldn't worry about it.

JR
Old 20th November 2009
  #3
http://www.dbxpro.com/product_downlo...0Schematic.pdf

That's the schematic, I'm not really sure what they are used for. I replaced the VCA with a THAT 2180, dual input opamp for a National LME49860NAm and the output 5534 for a LME49710NA. I also rebuilt to power supply with with larger caps and a polyprop bypass, replaced all the electrolytics and gave the audio caps a polyprop bypass as well.
Old 20th November 2009
  #4
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what it is in that circuit is the VCA servo voltage buffering for itself and stereo strapping output, if you need to replace due to lack of compressor action, them then OPA2604 is a premium quality of that type, but the 4558 will still work becuase its operating as a dc voltage buffer.
Old 20th November 2009
  #5
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Hi
Only OA1 and OA2, IC1 and C5 (3u3) have a direct bearing on the sound when not compressing although bypasses and replaced caps on the power rails may alter things slightly.
All the rest is 'varying DC' so will not alter the sound.
Matt S
Old 20th November 2009
  #6
It is side chain control. If you replace them with precision very low noise opamps, the modulated noise that is reduced on the DC VCA control port lines will allow THD to drop a bit.

I have lowered THD in other higher end dbx products by doing this and replacing the 10 nv noise 4558's with precision LT and AD devices.

Jim Williams
Audio Upgrades
Old 20th November 2009
  #7
Thanks Jim. There's a thread over on Tape Op in the DIY section that kind of details what all I'm doing. It's based off of old post you made

Can't wait to get that Aphex 651 you upgraded back!
Old 20th November 2009
  #8
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OK, the part is not a dc servo for the primary audio path. If it were it would be worth replacing because parts used in servos do matter.. but this isn't so that is not the concern.

It is as I speculated in the VCA side chain so somewhat less critical than primary path, especially for older VCAs. But you are not using the older VCA.

Regarding noise for adding N to the THD+N, from your schematic there is a passive pad in the CV (80/50), and an RC filter so the opamp's noise will be attenuated more than 6dB but still present.

AFAIK the general design advice is to keep noise at the control ports down in low single digit nV/rt Hz. Noise on these control ports shows up as a multiplication with the audio signal, so perhaps measurable on the new parts with high level THD+N testing, while not very audible (IMO).

But yes, do use modern precision opamps in the side chain at least for the VCA control port driver, and look at updating the circuit design for the newer VCAs. Note: eliminating the resistors in series with the CV buffer will require re-scaling side chain math (to make up for pad you remove) and could cause some unintended consequences if you ever use the CV slave input for stereo linking and two opamps try to drive each other.

I would check the THAT corp website, they may have some applications note about swapping new VCAs for old, while they probably don't get down to individual models of old DBX designs.

There is an AES paper written by Gary Hebert (THAT Corp) talking about the new VCAs , but perhaps more math than you are interested in. For the record they do drive the VCAs control ports directly from a precision opamp (AD797 with RC to ground for low Z at HF) to showcase them in the paper.

I have a sense that there is also a distortion mechanism associated with too high source impedance driving the control ports but can't find any discussion about that in his paper. Maybe I'm imagining it?

The new VCAs are notably cleaner... not sure the lovers of classic coloration will approve.

JR
Old 20th November 2009
  #9
Thanks for the advice John. Since I removed R9 and R10 based on Jim suggestion for using the THAT 2180, do you suggest I put them back in since I intend to use them primarily in stereo? Or if I replace the other op-amps should I be safe? I re-read your post a few times and tried to understand it the best I could, but it's a little over my head.

If the AD797 a drop in replacement for the 4458? I believe the opa2134 is, but mouser and digikey do not have them in stock.
Old 20th November 2009
  #10
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Quote:
Originally Posted by upinflames View Post
Thanks for the advice John. Since I removed R9 and R10 based on Jim suggestion for using the THAT 2180, do you suggest I put them back in since I intend to use them primarily in stereo? Or if I replace the other op-amps should I be safe? I re-read your post a few times and tried to understand it the best I could, but it's a little over my head.
R9 and r10 are symmetry trim adjustments... The symmetry trim in newer parts is different, >>100k for r10, and R9 is a smaller value and inside the VCA so not needed.

The modern parts are quite good without trimming, and unless you have bench equipment you can't adjust the trim anyhow, so just leave them out.
Quote:
If the AD797 a drop in replacement for the 4458? I believe the opa2134 is, but mouser and digikey do not have them in stock.
I do not make specific opamp recommendations, there are others here more than happy to steer you to the latest opamp du jour. I only mentioned the 797 because that is what THAT used in their AES paper. I suspect there are cheaper parts that are significantly better than 4558.

My comments about stereo/slave mode were regarding different resistors and I am reluctant to suggest you get that deeply into a modification without doing more homework and a better understanding of the circuit.

I am a little unclear about how the "strapping" jack is used. If the "save" switch is actually a "slave" switch that makes some sense. When switched the VCA is controlled by remote CV while rms detector outputs from both combine. If "save: switch not toggled, the two opamp outputs 1/2 OA4 from each, may be shorted together. Presumably the 100 ohm buildouts inside the feedback loop are to help prevent opamp problems if shorted. Replacing that 4558 with a higher current opamp may have other consequences.

For mono operation I would suggest one thing, for strapped operation together, another.. I would have designed this part of the circuit differently, but what else is new, and that doesn't do you any good..

So for now, I would lose r9 and r10 as Jim suggests, and keep all other Rs (despite my earlier suggestion) . When upgrading OA4 perhaps avoid changing to an opamp with too much drive capability, if when strapped together neither unit is in slave mode and the opamps are fighting each other.

Have fun but do no harm.

JR
Old 20th November 2009
  #11
I think the "save" switch must be the "slave" because a "save" switch doesn't exist on the 163X.

Unless it's still noisy after all the other upgrades and modifications I make to it I'll consider replacing them. If anyone has brand and model suggestions for a drop in replacement, let me know. I've beefed up the power supply caps and added bypass caps so it should be able to handle a modern replacement.
Old 22nd November 2009
  #12
The AD8599 is a .9 nv noise dual opamp.

Jim Williams
Audio Upgrades
Old 22nd November 2009
  #13
Thanks Jim, maybe I'll get some the next time I order parts. Do I need to order an adapter, those are only available 8-SOIC from digikey.

Do you recommend removing the HF suppression caps from all the op-amps, or keeping them? You suggest removing the HF roll-off, and I assumed that is what you are talking about, or am I wrong?
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