Neumann SE66

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Gold
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Post by Gold »

JR. wrote: The Bootstrap (actually a unity gain buffer) has no voltage gain so cannot improve the slew rate capability. It does provide higher output current to support driving the transformer load.
I thought when an opamp runs out of output current it slews. No?
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JR.
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Post by JR. »

Gold wrote:
JR. wrote: The Bootstrap (actually a unity gain buffer) has no voltage gain so cannot improve the slew rate capability. It does provide higher output current to support driving the transformer load.
I thought when an opamp runs out of output current it slews. No?
If driving a large capacitive load, output current limiting may look like slew limiting but that is a separate different mechanism.

Not running out of output current, but deep inside the op amp circuitry the internal current available to charge/discharge the compensation cap... This relationship with the compensation cap (internal integrator stage) is a trade off of gain for stability (the integrator stage rolls off the voltage gain at -6dB octave so gain is <unity by time internal delay turns the negative feedback into positive feedback). There are a number of interrelated mechanisms to define an op amp's max slew rate.

A booster stage can increase the slew rate by simply adding fast voltage gain in series with the output, so the former N volts per uSec, becomes N x X volts per uSec, of course the added gain stage needs to be fast.

[edit- adding a dual active output doubles the effective slew rate when the signal gets summed in a following receiver. /edit]

If you really want to learn about how op amps work, I discussed it some in my old 1980 article about console design, but I gave you the short hand version above.

JR

PS: sorry about the veer...
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Gold
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Post by Gold »

JR. wrote: If driving a large capacitive load, output current limiting may look like slew limiting but that is a separate different mechanism.
Thanks for the explanation. I think slewing and current limiting often sound similar.

I've keep Lancasters OpAmp Cookbook out for browsing for at least ten years. Some of it may eventually sink in.
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Post by JR. »

Gold wrote:
JR. wrote: If driving a large capacitive load, output current limiting may look like slew limiting but that is a separate different mechanism.
Thanks for the explanation. I think slewing and current limiting often sound similar.
Not in my experience.... Slew (limiting) is generally a HF phenomenon and could sound like IMD (LF hash under loud HF content, like cymbals) . Current limiting can occur at any frequency. Loudspeakers with un-kind crossovers can present nasty impedance loads to power amps and cause current limiting at different frequencies.

All amps do not treat current limiting the same. For some it may look/sound like clipping. If the amp has a clip limiter, that detects for current limiting too, it will just reduce the gain until it stops current limiting.

I've keep Lancasters OpAmp Cookbook out for browsing for at least ten years. Some of it may eventually sink in.
I never read that book, but the genre is usually just a collection of generic op amp circuits, like recipes to accomplish simple circuit tasks,(but maybe that book goes deeper into the underlying technology).

JR

PS: having a bad day on the WWW debating with people about what stuff sounds like,,, not usually productive. :lol:
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Gold
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Post by Gold »

never read that book, but the genre is usually just a collection of generic op amp circuits,

PS: having a bad day on the WWW debating with people about what stuff sounds like,,, not usually productive. :lol:
The first few chapters of the Lancaster book is theory. Then it goes in to typical audio applications and does both theory and cookbook circuits.

I don't have a lot of experience listening for distortion and assigning a mechanism. Your description of what the mechanisms sound like to you is helpful to me.
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mediatechnology
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Post by mediatechnology »

In addition to putting voltage gain in series with the output to increase slew rate or creating a bridge output to double it it's also possible to bootstrap the supply rails in feed-forward to accomplish SR improvement.
Like the Funaska posted earlier: http://www.proaudiodesignforum.com/imag ... Supply.pdf
They get 300 V/µs out of a 4558.

When you mentioned the SE66 used bootstrapping I thought they might have modulated the supply rails to increase 709 slew rate.

Self, Danyuk and others (maybe Gilbert was the first?) also use bootstrapping to reduce common mode distortion. http://electronicdesign.com/analog/supp ... p-circuits

Paul - The voltage level at C3 on the SE-66 inverse RIAA board input after the EE loop is what I was looking for. http://www.proaudiodesignforum.com/imag ... 66_800.jpg
That would give me an idea what level the µA709 runs at.
I really think an LME49710 - or for that matter a 5534 with an adapter would be a huge improvement in the inverse RIAA.
The SE66's ultrasonic "Neumann pole" is after the µA709 inverse RIAA rather than before the LF356 in the SAB74.

Paul - You mentioned that before the SAB74 was modified to use LF356s that it used a "Motorola" part. Was that by chance an MC1458S? Motorola made a high slew rate MC1458 the "MC1458S" which were 10V/µs SR parts. We used those at Micmix briefly before the TL082 became available. When the SAB74 was introduced it was during the brief period of time the "S" parts were made. Dollars to donuts they used those.

The SE66 board already operates at 30V according to the print so it's voltage wouldn't need to be lowered to accept an LME49710.
The WV66 board shows to operate at 35V and I'm not sure the HR reduction there would be as much as an issue as it would be in the SE66's inverse RIAA stage.

John - The choice of LME49710 is driven due to the fact that its available in a metal can.
In that spot a lot of different op amps could be used except for the can requirement.
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Post by Gold »

mediatechnology wrote: Paul - The voltage level at C3 on the SE-66 inverse RIAA board input after the EE loop is what I was looking for.
That's system Input so the reference level is +4dBu. The EE's are almost never installed in the VG66. The EE's were usually located in the consoles. I can't remember whether the transformer secondary is strapped for Low or High. I think High which looks like 1:1.
Paul - You mentioned that before the SAB74 was modified to use LF356s that it used a "Motorola" part. Was that by chance an MC1458S? Motorola made a high slew rate MC1458 the "MC1458S" which were 10V/µs SR parts. We used those at Micmix briefly before the TL082 became available. When the SAB74 was introduced it was during the brief period of time the "S" parts were made. Dollars to donuts they used those.
Yep, I think those were the ones. I'll dig an older drawing out and check. The .01 set of docs had the number.
John - The choice of LME49710 is driven due to the fact that its available in a metal can.
In that spot a lot of different op amps could be used except for the can requirement.
Right. I wouldn't want to use an adapter. Remember there is a very expensive and delicate transducer at the other end of the signal path. Reliability is much more important to me than a slight sound quality improvement. I've made it close to 20 years with this setup. I've never lost a shoot out that I could trace to the sound of the electronics. Maybe when I restore the Shure lathe I'll try it.
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Post by Gold »

I mis remembered what my friend said. I found the email where he described what he did.

" As far as the LM709 goes, I replaced it with an LF356, the JFET input op-amp that Neumann used in everything at the end. SAL 74s and 84s are loaded with them, and I must have 200-300 of them. I kept T4, the output transistor, however I did remove C13, R21 and C15. These op-amps are internally compensated so no need for those components. No problem with the single supply either; the output floated at almost exactly half the supply voltage. I did the same thing in the WV66IC except there I used an NE5534 for IC1 and an LF356 for IC2. I got a noise level in the phono preamp of close to -80dBu."

Would a bipolar input opamp like an LME49710 be good in those positions or would a FET input opamp be better?
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Post by mediatechnology »

Would a bipolar input opamp like an LME49710 be good in those positions or would a FET input opamp be better?
I think either would be a big improvement over the µA709.
The inverse RIAA is very demanding of any op amp.

In the WE66 front-end his choice of a 5534 was probably best.
Neither the LF356 or the LME49710/49720 are optimum.
The LF356 due to higher voltage noise, the LME497X0 due to current noise.

I agree that based on the cutterhead being downstream one needs to make those mods carefully.
Oscillation would be very harmful.

Thanks for all the info Paul. The SE66 schematics are very hard to find online.
Vintage Windings doesn't seem to have it.
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Post by Gold »

mediatechnology wrote:
Neither the LF356 or the LME49710/49720 are optimum.
The LF356 due to higher voltage noise, the LME497X0 due to current noise.

The only other op amp available in a TO-99 at Mouser that I've seen used in audio is an OPA627. At the bargain basement price of $50.10 ea. It looks better than the LF356 spec wise.
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