Neumann SE66

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Gold
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Neumann SE66

Post by Gold » Sun Jan 15, 2017 5:16 pm

When Neumann started using an LM709 in the VG66 they were always bootstrapped. I'll scan and post the SE66 Input and RIAA encode. The RIAA decode is in the monitor/phono module(s). Some had functions combined in one module and some had functions split between them. There are also different versions of each flavor. A surprise in every rack!

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mediatechnology
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Neumann SE66

Post by mediatechnology » Mon Jan 16, 2017 9:31 am

When Neumann started using an LM709 in the VG66 they were always bootstrapped. I'll scan and post the SE66 Input and RIAA encode.
Wow. Very interesting! I would love to see those scans. I've been looking for a VG66 schematic.
I think they might have done that to extend its slew rate.

I recall a JAES paper where they extended the SR of a 4558:

EIICHI FUNASAKA AND HIKARU KONDOU, "Feedforward Floating Power Supply (High-Response-Speed Equalizer Circuit)," J. Audio Eng. Soc., Vol. 30, No. 5, 1982 May.

https://www.proaudiodesignforum.com/ima ... Supply.pdf

Image
Wide Bandwidth RIAA Phono Preamp With Feedforward Power Supply

One of the examples is a voltage-boosted phono preamp.

FWIW The Jim Williams series on op amp booster stages is one of the most popular pdf downloads from this site. https://www.proaudiodesignforum.com/for ... ?f=12&t=13
It's consistently in the top ten.
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Gold
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Neumann SE66

Post by Gold » Mon Jan 16, 2017 1:37 pm

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JR.
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Neumann SE66

Post by JR. » Mon Jan 16, 2017 2:25 pm

thanx.

From a quick glance*** interesting how the discrete transistor buffer is floating and cap coupled in/out (perhaps because of single rail PS), running open loop. No voltage gain in that buffer, just unity gain common collector followers. Seems like one could connect the NF loop to the output side to reduce distortion. Maybe not if stability was already marginal. I never used 709s but recall them being very expensive back in the 60's and very easy to blow up (two reasons I was never allowed near them as a junior tech). Lots of typical old school circuitry like LPF wrapped around a simple emitter follower, using a bipolar transistor backwards as a DC controlled switch/shunt, etc.

JR

*** caveat lector not a detailed review so read my comments with a grain of salt.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Gold
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Neumann SE66

Post by Gold » Mon Jan 16, 2017 4:31 pm

JR. wrote:thanx. I never used 709s but recall them being very expensive back in the 60's
A VMS 66/VG 66/SX 68 (lathe,drive amps and cutter head) was $44,000.00 in 1970...

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mediatechnology
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Post by mediatechnology » Mon Jan 16, 2017 8:07 pm

What I find interesting is that the so-called 50 kHz "Neumann pole" is actually second-order 32.5 kHz in the SE-66 (T4 sheet 2.)
People get far too wrapped around the axle about that ultrasonic pole...

I took the liberty of assembling the Neumann SE-66 schematic on one page:
Image
Neumann SE-66 Inverse RIAA Record Equalizer


With regard to the "pickup amp" it looks like to me that it ran on 35V single-supply.
Not sure why they AC-coupled the input to the current booster stage but IIRC the µA709 had no short-circuit protection.

Paul - Was there ever an upgrade to change out the µA709 that was made available?
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Neumann SE66

Post by Gold » Mon Jan 16, 2017 9:54 pm

mediatechnology wrote:What I find interesting is that the so-called 50 kHz "Neumann pole" is actually second-order 32.5 kHz in the SE-66 (T4 sheet 2.)
People get far too wrapped around the axle about that ultrasonic pole.
People usually are talking about the later SAL/VG74 rack when they talk about that.
It looks like to me that it ran on 35V single-supply. Not sure why they AC-coupled the input to the buffer stage but IIRC the µA709 had no short-circuit protection.
I don't think anything anywhere is DC coupled. Those Germans were very conservative in that way. I at least double the value of all the coupling caps when I restore the electronics. The line level audio runs on +35VDC. There are three +35VDC supply modules per channel. There are separate supplies for Left and Right. The cutter head Drive (Power) Amps bridge two +35VDC supplies to make a 70VDC rail for those. There is also a +24VDC supply for lamps and Datacell opto switches. They are pass transistor in a shunt configuration.
Paul - Was there ever an upgrade to change out the µA709 that was made available?
The µA709 was the upgrade to the discrete circuitry. After that they must have focused their attention on the SAL74 design. That is almost entirely IC based. The original IC they used was a Motorola. I forget the model. They quickly switched over to using LF356 for almost everything.

A friend and super lathe tech guru put a LME49710 metal can in to try it. He said it worked. He's not the type of guy to comment on the glorious sound. He said he left the boot strapping in. I've stuck with the 709. I figure since it's bootstrapped it won't slew limit.

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Post by mediatechnology » Tue Jan 17, 2017 7:04 am

What I think is silly about implementing the Neumann pole on playback is the misguided assumption that the 50 kHz second-order filter the SAB-74 is representative of every record made with RIAA EQ.

I'm not seeing where there is supply bootstrapping but may have missed it.
I do see the AC-coupled current-booster.

What is the typical level at C3, the Inverse RIAA input?

IMHO an LME49710 would be a big improvement though I would lower the supply rail in the pickup amp to 30V. (it looks to be 30V in the record amp.)
The LME49710, 20 etc. really do have a 36V maximum. They are very touchy in that area.
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JR.
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Post by JR. » Tue Jan 17, 2017 10:09 am

Gold wrote:
A friend and super lathe tech guru put a LME49710 metal can in to try it. He said it worked. He's not the type of guy to comment on the glorious sound. He said he left the boot strapping in. I've stuck with the 709. I figure since it's bootstrapped it won't slew limit.
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.

There is probably a long list of op amps better than the 709, just about every op amp ever designed since then. That said even a so-so op amp can be better than many discrete designs. (The ua741 was a big deal when it was introduced because of the internal compensation and good performance for back then.)

Should be relatively easy to test on the bench objectively whether the electronic path is "rate of change" sensitive. I like two-tone IMD (19k/20kHz) for characterizing RIAA stages because of the LPF in the RIAA EQ that makes simple THD tests look better than the reality.

JR
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Neumann SE66

Post by Gold » Tue Jan 17, 2017 11:47 am

mediatechnology wrote: What is the typical level at C3, the Inverse RIAA input?
Do you mean the SE66 or the WV66?
IMHO an LME49710 would be a big improvement though I would lower the supply rail in the pickup amp to 30V. (it looks to be 30V in the record amp.)
The LME49710, 20 etc. really do have a 36V maximum. They are very touchy in that area.
I'm not sure reducing headroom is a good tradeoff for improved noise and distortion.

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