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Interesting Soundcraft 1600 mod results
Interesting Soundcraft 1600 mod results
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Old 6th March 2020, 12:57 PM   #131
dreamth is offline dreamth  Romania
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Quote:
Originally Posted by dreamth View Post
OPA2228(or 2227) is ze best but mc33178 should be cheaper and good enough.

Pioneer CT-95 / T-1100S up close and personal - Tapeheads Tape, Audio and Music Forums
And when somebody shows you the direction in a deep forest, you don't ask why: Music and Audio Electronics
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Old 6th March 2020, 02:20 PM   #132
dreamth is offline dreamth  Romania
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Lowest noise BJT transistor?
may be of interest too...
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Old 8th March 2020, 12:55 AM   #133
sgrossklass is offline sgrossklass  Germany
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Originally Posted by dotneck335 View Post
It would seem that MM phono and tape head repro amps have a lot in common----both are very low level (a coupla mV), are inductive sources, and are amplifying signal sources with less-than-ideal signal-to-noise ratios. I am still looking for a single opamp for tape heads---considering NE5534, OPA 1641, and OPA1611. What are you finding?
OPA1611 has 1.7 pA/(Hz) worth of current noise (essentially as much as LM4562), that's a tad on the high side. Tape EQ doesn't have the very heavy downwards slope of RIAA EQ, it would penalize high current noise levels even more.

Adapting my phono preamp noise sim w/ A-weighting to the NAB preamp circuit (with head = 650R + 560 mH, input Z = 10p || 40k) gave this:
Interesting Soundcraft 1600 mod results-tapepre-noise-2-2-png
That's 20 dB on the vertical. Not minor differences!

Red: OPA6141
Green: LM833
Blue: NE5534A
Teal: LM4562

The classic NE5534A isn't such a bad deal right out of the box, and being partially decompensated, also is somewhat faster than e.g. the NJM2068 (a part otherwise very similar in noise). It can also be hacked in several ways, e.g. deactivating and replacing its input stage altogether, only the AD744 is even more flexible.

OPA1656 may also be promising, despite its relatively high 1/f corner. I'll have to try and work that in (currently 1/f noise is not being taken into account). It's got high GBW right out of the box. Tape preamp circuits tend to use highish gain by nature, so instability should rarely be an issue.

BTW, while I haven't gotten around to writing the article on everyman's audio opamp selection yet that's been on my to-do list (somehow it just keeps growing while I'm on vacation), the section on jellybean opamps has been revamped recently. It was rather a mess and should be much easier to follow now. I am still unsure about which content to move to dedicated pages - I'm at over 400K of HTML even with the calculator bits already taken out, making this my second largest page. I do actually care about user experience a lot, it's just that I'm working within the confines of a bunch of handwritten static HTML pages.

Oh, and if you are willing to go down tape-realted rabbit holes, I'm sure you've seen this as well. It seems like tape EQ may well be worth a look at depending on what kind of speeds you are using. In any case I would very much be looking at playback noise levels with the tape stopped vs. running - if tape noise is smothering everything else, you might be barking up the wrong tree trying to optimize tape head preamps.
Quote:
Originally Posted by dotneck335 View Post
Got a whole bag o' 100nFs. Learned about this practice from guru Rod Elliott. AND got a scope---my trusty ol' Tektronix 465 (ancient, I guess, but still a good piece of test gear)
Ah yes, I think you mentioned. Better than my old portable Hameg for sure. Probably about the same age.

Another part you may need to keep a stash of is very small caps, like 2-3 pF maybe. These can be handy to add in parallel to the feedback resistor, as otherwise input capacitance tends to appear in parallel to the ground leg resistor and cause ultrasonic peaking or even oscillation. It is generally seen with feedback resistor values of 10k and higher, though low-noise hot-rod parts may require this treatment as low as 1k.

Ideally the ratio of feedback capacitor and input capacitance should be the inverse of resistor ratio, but generally it works out to 5 pF max.
Quote:
Originally Posted by dotneck335 View Post
Well, maybe a 55MHz IC is too fast for this old mixer, but......the OPA1688 is sorta slow & noisy, and I have a hard time trusting ANYthing from New Japan Radio due to their very sneaky datasheets. They try to hide stuff. Also, I'm hoping for something with a single op amp footprint, as it's a TL071 that's in there now.
Ah well, that's a bit of a problem. I think it's different from the NE5534 pinout, too.

Some creativity may be required there, like paralleling two OPA1688s and combining their outputs much like the O2 headphone amp does. (I don't think anyone makes an adapter board like this, now do they?) And then you get 5.6 nV/√(Hz), hardly an input noise level worth complaining about.

I know what you mean by "sneaky" datasheets, but I would blame the datasheet trends of the '80s for this as much as anything, which is how old these parts generally are. At that time manufacturers were getting increasingly tight-lipped, I think both because of fear of their devices being cloned and to conserve space in their ever thicker databooks. Only in the digital age have comprehensive datasheets become more common again.
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Originally Posted by dotneck335 View Post
Yes, it is -30 db. I missed this before, but it's actually labeled as such on the panel next to the line/mic switch. I am thinking to just let it be at this point; before I was concerned about overloading, but now that doesn't seem to be a problem; the CFP mod should offer a dramatic improvement in the front end.
30 dB is still rather a lot. With all your mods, you should be able to run a fair bit higher internal levels, and not require that much attenuation.
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Last edited by sgrossklass; 8th March 2020 at 01:08 AM.
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Old 8th March 2020, 04:34 AM   #134
dreamth is offline dreamth  Romania
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Thanks for these new info!
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Old 8th March 2020, 06:17 AM   #135
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by sgrossklass View Post
The classic NE5534A isn't such a bad deal right out of the box......
That's what I've been thinking also. I found some NOS Philips NE5534N ICs that I am going to use in my tape preamp applications. Seems to be the best bang for the buck, and probably better than the tape itself.
Quote:
Originally Posted by sgrossklass View Post
...... the section on jellybean opamps has been revamped recently.
THAT is a VERY interesting article! Thanks for pointing it out. I do disagree, however, with your assessment that the NJM2043 has lower noise than the NE5534A---once again, the NJR spec sheet 'cheats' a little by quoting 'A' weighted noise specs in their tables; but if you look further down on the graphs, they do show the 'flat' noise, and it definitely appears to be worse than the 5534.

Quote:
Originally Posted by sgrossklass View Post
.... if you are willing to go down tape-related rabbit holes,.......... I would very much be looking at playback noise levels with the tape stopped vs. running - if tape noise is smothering everything else, you might be barking up the wrong tree trying to optimize tape head preamps.
My thought on that is that if Studer engineers deemed the NE5532 as the best for their tape audio electronics in the later years of production, I'm inclined to agree that an excellent I.C. well may make a viable difference. Their tape machines are certainly among the best ever made.
Quote:
Originally Posted by sgrossklass View Post
Another part you may need to keep a stash of is very small caps, like 2-3 pF maybe. These can be handy to add in parallel to the feedback resistor, as otherwise input capacitance tends to appear in parallel to the ground leg resistor and cause ultrasonic peaking or even oscillation. It is generally seen with feedback resistor values of 10k and higher, though low-noise hot-rod parts may require this treatment as low as 1k. Ideally the ratio of feedback capacitor and input capacitance should be the inverse of resistor ratio, but generally it works out to 5 pF max.
Hmmmm.... a good point, but I can't seem to find input capacitance specification on any IC datasheets. How do I determine that?

Quote:
Originally Posted by sgrossklass View Post
I think it's [TL071 pinout] different from the NE5534 pinout, too.
Nope, they're the same---standard single op amp pinnage.

Quote:
Originally Posted by sgrossklass View Post
30 dB is still rather a lot. With all your mods, you should be able to run a fair bit higher internal levels, and not require that much attenuation.
I'll look into that, and maybe lower the attenuation.
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Old 8th March 2020, 07:41 AM   #136
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by sgrossklass View Post
.... the section on jellybean opamps has been revamped recently. It was rather a mess and should be much easier to follow now.
A couple of questions on your very good article:
  • No mention of some very good, fairly low-cost opamps, such as the LM4562 and OPA1642----?????
  • I noted your warnings about the T.I. versions of the NE5532; those have been verified by others as well. Have you evaluated the ON Semi version of the NE5532? Unfortunately, it appears to only be available in SOIC-8, but that's not a total game-breaker. Is there a viable NJM (or other) version of the NE5532 that would be better than the T.I. part?
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Old 8th March 2020, 06:53 PM   #137
dotneck335 is offline dotneck335  United States
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Originally Posted by dotneck335 View Post
I do disagree, however, with your assessment that the NJM2043 has lower noise than the NE5534A---once again, the NJR spec sheet 'cheats' a little by quoting 'A' weighted noise specs in their tables; but if you look further down on the graphs, they do show the 'flat' noise, and it definitely appears to be worse than the 5534.
"Upon further review......."
I re-read the "Op Amp Measurement" by NwAvGuy, and I see that he says:
"NE5532 NOISE: The professional audio favorite, the NE5532, is about 3 dB worse at 7X gain than the less expensive and less current hungry NJM2068 above but still very quiet. I have heard some versions of the 5532 are a few dB better than others. This was a TI NE5532AP."
So it appears that, indeed, the NJM2068 does measure lower noise than the NE5532 or the LM4562. But the NE5534A should measure lower than the 4562 or the 5532. However, at 100 db down, noise really doesn't matter except maybe in mic preamp applications. Vinyl and tape are both way noisier than that, and at line level, noise of any of these opamps is inaudible.


Last edited by dotneck335; 8th March 2020 at 07:02 PM.
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Old 8th March 2020, 07:18 PM   #138
dreamth is offline dreamth  Romania
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Originally Posted by dotneck335 View Post
"Upon further review......."
I re-read the "Op Amp Measurement" by NwAvGuy,
Finally...
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