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Seeking Soundcraftsmen DX4000 preamp op amp upgrade experience

Schmidlapper

Schmidlapper

Super Member
DX4000 uses 4 ea RC4136 op amps, one in the line amp, two in the headphone amp section, and one for an inverted output. I wish to upgrade the first three, not interested in the inverted output. I will use the browndog RC4136 adaptor board built for this purpose. I would be interested in getting input from anyone who has successfully upgraded one of these pre amps. My concern is that doing this modification could introduce hf oscillation. I am not educated enough to know for certain if changing would work safely or not, or how to test and correct for it. Even when people caution about changing op amps willy-nilly, they will often caveat it with unless it’s from the 741 family, as the RC4136 is. The OPA2134 seems to be a good mildish upgrade from what I have researched. Thanks for any input.

Bill
 
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Wanted to close this out by saying I am easily enchanted by the lure of improvements to be had by various mods that I read about. I have always tempered this with research sufficient to feel comfortable I’m not degrading anything in the process. Op amp swapping in theory can bring gains, but it also if done incorrectly could bring degraded performance. Just saying a chip is compatible is woefully insufficient to ensure a swap will improve performance. The likelihood of dropping an op amp into an old circuit and having it perform optimally is like the lottery. I don’t ever win at the lottery. I had hoped someone may have successfully tested an op amp swap, tested being the key word. Until I get a better oscope so I can perform the testing myself I will leave as designed. The DX4000 has a great soundstage already, so maybe this would be chasing rainbows anyhow.
 
Not any longer, it took about 15-20 minutes then it sounded pretty darn good. Didn’t want to go thru restoring it.
 
I’ll assume you meant recap it. If a person has the schematic it can be pretty straight forward. Buying the correct cap lead spacing and physical fit is often the biggest challenge. I’ve never seen inside a Carver unit myself, have a buddy who has a lot of it. These Soundcraftsmen units are usually very nice to work inside off, reminds me of military equipment in some respects just without the excellent wire and harness handling.
 
Caps yes, there were lots of them from what I recall when I opened it up. There were a whole bunch of updates for it also. Just couldn’t get into doing it.
 
Appended. As RC4136 chips are still produced by TI, I purchased four from Mouser and replaced them all in the preamp. My motivation being that these chips were manufactured and installed prior to 1982 and because IC chip manufacturing process have likely improved sufficient enough that the chip performance parameters are now at the upper end of their design specifications, and likely exceed them for noise. Being these are the as designed for IC it certainly won’t hurt, and for the $5 it cost me they like the electrolytic caps are now refreshed too.
 
Hello, I replaced the 4136 for the preamp section with a TL075, the only drop-in replacement that exists and I did not measure but it does sound substantially better. I did not replace the EQ nor the phono section ones because I don't use those. The headphone outputs are all 4 sections of each 4136 bridged in parallel as seen below, but the 4556 is the one I would use for that function. A home made adapter or retracing is needed.

https://www.analog.com/en/technical-articles/paralleling-amplifiers-increases-output-drive.html#
 
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rickedd46 said:
Hello, I replaced the 4136 for the preamp section with a TL075, the only drop-in replacement that exists and I did not measure but it does sound substantially better. I did not replace the EQ nor the phono section ones because I don't use those. The headphone outputs are all 4 sections of each 4136 bridged in parallel as seen below, but the 4556 is the one I would use for that function. A home made adapter or retracing is needed.

https://www.analog.com/en/technical-articles/paralleling-amplifiers-increases-output-drive.html#
Click to expand...

Yeah I could have done it, but I don’t have test equipment capable of verifying I didn’t introduce any HF instability, nor can I find where anyone else has actually tested for it. It is not something one can listen for. HF instability when swapping chips is quite likely if circuit was not designed for it or it isn’t mitigated, so I’ve chosen to just install newly manufactured RC4136 chips and be happy. Thanks for responding.

Bill
 
Did you ever change the opamps? I just designed and ordered adapter boards to mate TSSOP14 with DIP14 for both the 4136 pinout and conventional. Hope to receive them this month yet. I'm going with the excellent OPA1604 and will do some before and after tests. I should have some extras if anyone is interested. Here's a pic.
TSSOP14_p4_n11-to-PDIP14_p11_n7.jpg
 
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I am aware of those boards. I can print 300 of them for what they charge for me to do one preamp.
 
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PabloTincho said:
:lurk::lurk::needpics:
Click to expand...
Your popcorn is going to get cold.
He ordered the boards and put up a pic.
I'm thinking there will be a finished product with testing and results Pretty soon.
If they pan out, I have 3 sp4002s that may get his, but 2 for sure.
Can't wait.
 
PabloTincho said:
:lurk::lurk:

:needpics:

Does the OPA1604 have the same power requirement as the original op amp ?
Click to expand...
It only requires 2.6mA quiescent current per amp compared to the 4136's 6mA. The 1604 also has a output current capability at 50mA or more depending on temp, the 4136 doesn't specify, but likely isn't that high. 50mA+ is quite a bit from an opamp. Many are in the 20mA range.
 
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Racingh11 said:
It only requires 2.6mA quiescant current per amp compared to the 4136's 6mA. The 1604 also has a output current capability at 50mA or more depending on temp, the 4136 doesn't specify, but likely isn't that high. 50mA+ is quite a bit from an opamp. Many are in the 20mA range.
Click to expand...
So there is not oscillation or any side effects in replacing the original op with the 1604 ?
Were you able to measure distortion with the new opamp ?
 
I did just find a spec for the 4136, it is 45mA. Better than I thought.

The opamp is unity gain stable, 35Mhz bandwidth compared to 3Mhz on the 4136, and the feedback circuits in the preamp determine the actual gain and frequency cutoff. I'm waiting for a new hot-plate soldering station to assemble these. Should arrive today. The boards are here already and look good. I also don't have any 1604 right now. I have 1664 and something else that's close.
 
The main benefit to this upgrade would be lower noise and greater channel separation. I don't believe the 4136 restricted signals in the frequency and voltage range used in. The closest may have been the slew rate being 1V/uS compared to the 1604 20V/uS. But 1V/uS can support about 90khz at rated amp sensitivity, and about 16khz at 10V, which would be very high transient spikes. It may open up some sonics, but who knows what can be actually heard. I will try to see if I can measure a S/N difference and distortion. But I 'm not sure if I have any DX4000 around. I would have to use a DX4200 or something which makes it a little harder to swap chips quickly for comparison.
 
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