I think simple diode clamps should suffice. I'll scan the 1632 data sheet again see if TI recommend anything special otherwise good old 1N4148s.
I started of with 9V and went down to 1V5.
I can't say I've tried all possible devices but IME,
- 'LN' devices of that time, eg BC109, 214, 184, 2n5087
- medium power devices including the Zetex(s)
- da supa low rbb' ones like Hitachi & Toshiba
The THD profile of this circuit is interesting and I did a full analysis after getting some 'unbelievably good' results.
You could of course insert diodes, but ask Russ White whether it has ever occurred that a 1632 was blow, I would be surprised.
I never ever had any problem with floating differential Cart inputs.
Hans
I did think about this last night. The plausible failure mode is owner walking across carpet to turntable and accidentally touching the cartridge leads before the (grounded) headshell. This is unlikely but possible (and I have a wooden floor).
If building for personal use I cannot see that being a case, but as THAT say in their literature in the ESD sections 'This is unlikely, but certainly not impossible'
Aside, maybe I should tell people to wear an ESD wrist strap when changing records. I wonder how many would actually try that..Nearly April 1...
If building for personal use I cannot see that being a case, but as THAT say in their literature in the ESD sections 'This is unlikely, but certainly not impossible'
Aside, maybe I should tell people to wear an ESD wrist strap when changing records. I wonder how many would actually try that..Nearly April 1...
Yup, should have waited a few days ......🙂
Perhaps.....
More perhaps charge accumulation on an ungrounded cantilever making a giant leap for cartridgekind........plus RCA connector plug in can be centre first, plus as many ways as Murphy's law will accommodate +1 😉
Carts don't necessarily have grounded cantilevers..........
Just be careful out there, best to earth yer listening seat and always wear a wriststrap 😱
LD
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Real men don't use RCA connectors 😛 (although the RCAs on my SME are actually balanced with seperate ground wire).
I can't see a cantilever storing enough charge to actually punch through a junction after getting through a 250Ohm resistor. Lowering onto a highly charged record maybe..
Measurements required. I think I have a aload of antistat stuff around somewhere. I used to wear antistatic sandals in the office, which were a great idea until the lab space flowed out into areas where there was std carpet not antistat stuff.
I can't see a cantilever storing enough charge to actually punch through a junction after getting through a 250Ohm resistor. Lowering onto a highly charged record maybe..
Measurements required. I think I have a aload of antistat stuff around somewhere. I used to wear antistatic sandals in the office, which were a great idea until the lab space flowed out into areas where there was std carpet not antistat stuff.
The circuit when in operation keeps both virtual inputs firmly at zero volt.
Any charge coming from whatever direction will be immediately absorbed by those low ohmic virtual inputs, unless it is a megajoule lightning strike entering your system, but normally you can simply touch the Cart wires with anything alarming to happen.
That's the benefit of having a high CMRR.
When inserting an RCA plug into an isolated RCA input, nothing will happen either because no current will be flowing, and if a static charge has to be discharged it will be absorbed directly into the virtual Gnd.
In the power off situation, there is an undefined situation, but that is also just as true for the hot input of a SE preamp where disasters never seem to happen.
There is nothing against inserting two diodes over the virtual inputs, although to my opinion it is an overkill.
And that must be the reason why this circuit board has no provision for input protection.
Hans
Any charge coming from whatever direction will be immediately absorbed by those low ohmic virtual inputs, unless it is a megajoule lightning strike entering your system, but normally you can simply touch the Cart wires with anything alarming to happen.
That's the benefit of having a high CMRR.
When inserting an RCA plug into an isolated RCA input, nothing will happen either because no current will be flowing, and if a static charge has to be discharged it will be absorbed directly into the virtual Gnd.
In the power off situation, there is an undefined situation, but that is also just as true for the hot input of a SE preamp where disasters never seem to happen.
There is nothing against inserting two diodes over the virtual inputs, although to my opinion it is an overkill.
And that must be the reason why this circuit board has no provision for input protection.
Hans
Well, IMO, whilst it surely is jolly fun, it's unnecessary to require floating cart inputs nor a balanced input stage, with transimpedance loading. Any noise benefit is surely at risk exactly because one is then relying on CMRR, rather than intrinsic low input impedance of the interface, as to rejection of many and various external noise sources. I think it misses a big part of the whole point, but hey............
And Bill, yes, of course real men don't use RCA's 😉
Nevertheless, since there's momentum behind a balanced PCB version, and it's fun, one might as well do a proper job I figure. So omit protection if it pleases anyone, I can add my own 😉 No whining later about changing 1632's though.......
LD
In a SE preamp, one end of the coil is grounded and high speed op-amps are seldom used..........🙄
LD
I was referring to your RCA plug insertion, power off and this time in a SE preamp, centre first, so no ground at the other end of the coil, but never a disaster happening.
And with power off, what's the difference between a high speed amp to a lower speed version ?
But be my quest and insert all the diodes you may wish, if properly done it has no negative effect and costs are only a few cents, if that is what makes you feel better.😀
Hans
The disaster scenario that worries Lucky is most probably during normal operation the discharge of the statically charged vinyl through the needle, to cantilever, to the preamp input.
I have 3 cartridges here. (Shure M97xE, AT110E, Stanton MKV H2)
I studied the patents describing these cartridges.
There is no electrical connection direct or indirect btn cantilever and coils shown on the figures or mentioned in the text.
By "indirect" I mean the internal construction is such that there is no way for the static to discharge through an arc from the cantilever arm to any of the coils.
I anticipate that the only route for static to discharge from the cantilever to the preamplifier input is through a contact with the exterior metallic cartridge body thanks to the grounding lug.
I had to check it. I used a Keithley 175 (*).
1. Shure M97xE (cartridge body ground lug disconnected from the R- coil pin):
Open circuit btn cantilever arm and any of the coil pins.
Open circuit btn cantilever arm and ground lug.
Short btn the cantilever guide tube and ground lug.
165KOhm btn the brush and the ground lug.
So for this cartridge, vinyl static discharge is either through the brush to the metal body and to the ground lug
or
when brush is raised, from the cantilever arm through micro arcing (**) to the cantilever guide tube, to metal body and to the ground lug.
2. Audio Technica AT110E (cartridge body ground lug disconnected from the R- coil pins):
Open circuit btn cantilever arm and any of the coil pins.
Open circuit btn cantilever arm and ground lug.
Open circuit btn connection pipe or mounting pipe and ground lug
There is no brush on this cartridge.
So for this cartridge, vinyl static discharge can only follow the route from the cantilever arm through micro arcing to metal body and to the ground lug.
3. Stanton MKV H2 (cartridge body ground lug disconnected from the R- coil pins):
Open circuit btn cantilever arm and any of the coil pins.
Short btn cantilever arm and cantilever guide tube.
Short btn the cantilever guide tube and ground lug.
So for this cartridge, vinyl static discharge is from the cantilever arm to the cantilever guide tube, then to metal body and to the ground lug.
With these cartridges, discharge can pose a threat to the preamp input circuitry only if the ground lug is left connected to the coil pin (R-) and the braid of the interconnect is either improperly wired or is of high impedance. Otherwise, there is no problem (single ended or balanced input)
(*) I used an unused razor blade as the probe for the cantilever (sharp tip needed to cut through the aluminum oxide to ensure contact)
(**) micro arcing is the standard way vinyl static discharges on the cantilever (diamond tip is an electrical isolator)
George
The momentum for me is entirely because there is a board that might work exceeding well vs veroboarding something that might not. The delightful overkill of a balanced input is just something that comes along for the ride. Scott in his retirement is of course going the otherway and Munzing things as far as he can.
I'll have a rummage and see if I have the parts to glom up a single ended one as well. I know have a pair of LT1028 somewhere left over from an MC headamp project many moons back.
Pop quiz for Scott. I note the 1992 linear data sheet for LT1028 shows a BF862 being used for a photodiode detector. When was this little beauty first discovered for low noise applications?
I'll have a rummage and see if I have the parts to glom up a single ended one as well. I know have a pair of LT1028 somewhere left over from an MC headamp project many moons back.
Pop quiz for Scott. I note the 1992 linear data sheet for LT1028 shows a BF862 being used for a photodiode detector. When was this little beauty first discovered for low noise applications?
That could be it, I don't remember but the idea there was to make a bootstrap circuit for the PD capacitance that does not add noise. I just said gee, let's order a bunch and put them on the QuanTech, the first anticipation was that they would have the lousy 1/f like on the DS and be useless for audio.
Mutzing is good, a JFET diff pair into a load is -96dB THD at 26dB of gain and 1V p-p why do you need any more?
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These days, that is my design philosophy, and partly why I became fascinated with realising the potential of MM/MI carts in the first place.
Yes, I can get that.
Protection, in some cases.
Contrast with 1642, for example.........
LD
Trying to get some more insight in serious static discharge problems, I fed one side of the Cart with a 1usec pulse from a 200pF capacitor charged to 20KV with a 2K resistor in line to simulate the human body.
Simulation below shows the output of the first and the second 1632 when the preamp is simultaneously fed with a 1 KHz Sinus wave.
All the diodes can be 1N4148 and the two input resistors have to be able to withstand a 10A/1usec pulse.
Quite a lot of diodes are needed, but the 1632 stays alive under these extreme conditions. So if you want to be prepared for all mishaps, this is what you will need.
Hans
Simulation below shows the output of the first and the second 1632 when the preamp is simultaneously fed with a 1 KHz Sinus wave.
All the diodes can be 1N4148 and the two input resistors have to be able to withstand a 10A/1usec pulse.
Quite a lot of diodes are needed, but the 1632 stays alive under these extreme conditions. So if you want to be prepared for all mishaps, this is what you will need.
Hans
earthed cantilevers
Tony Emerson & Don Barlow, who designed turntables & cartridges for LEAK, used to insist on earthed cantilevers.
If fact they also insisted on earthed platters on turntables. Many turntables used a Teflon thrust pad on the spindle cos that helped rumble. But earthing via the usual sintered bronze 'shaft bearing' was inconsistent. The LEAK Truspeeds all used a SS thrust pad and 'conductive' mats.
SHURE always had crude tie wires on their cantilevers and the 'antistatic brush' was also supposed to make an 'earth' connection between record and cartridge even when raised like a lightning conductor.
B&O used a more elegant tie wire. Not sure about Ortofons.
ADC cantilevers were never earthed.
IIRC, all MC cantilevers were earthed due to their construction.
The advantage? Less ticks & pops on the earthed cantilevers ... & less crud on your stylus. With unearthed cantilevers, you are getting ESD into your phono inputs but without permanent damage.
I'm sorry I can't replicate the evidence this Millenium as I no longer have a vinyl player and access to these cartridges.
A cure for this is of course wet playing.
Tony Emerson & Don Barlow, who designed turntables & cartridges for LEAK, used to insist on earthed cantilevers.
If fact they also insisted on earthed platters on turntables. Many turntables used a Teflon thrust pad on the spindle cos that helped rumble. But earthing via the usual sintered bronze 'shaft bearing' was inconsistent. The LEAK Truspeeds all used a SS thrust pad and 'conductive' mats.
SHURE always had crude tie wires on their cantilevers and the 'antistatic brush' was also supposed to make an 'earth' connection between record and cartridge even when raised like a lightning conductor.
B&O used a more elegant tie wire. Not sure about Ortofons.
ADC cantilevers were never earthed.
IIRC, all MC cantilevers were earthed due to their construction.
The advantage? Less ticks & pops on the earthed cantilevers ... & less crud on your stylus. With unearthed cantilevers, you are getting ESD into your phono inputs but without permanent damage.
I'm sorry I can't replicate the evidence this Millenium as I no longer have a vinyl player and access to these cartridges.
A cure for this is of course wet playing.
Ha, ha, that seems like a wild guess.
But in fact the OPA1632 is protected with 4 diodes clamping the Vin+ and Vin- to 0.6 volt above and below the supply voltages, just like the OPA1642. 🙂
Hans