Why don't cabinet record players have phono ground?

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scrapjack+
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Why don't cabinet record players have phono ground?

Post by scrapjack+ » 15 May 2019 00:24

I've gotten to examine the internals of two 60's cabinet type record players. Neither one seemed to have a phono ground to the preamp like I'd expect on a modern turntable. Even Crosleys have a ground. The first cabinet was a Magnavox Astro Sonic with a Magnavox Micromatic turntable which had only 3 wires for the tone arm and no ground connecting the amp to the record changer. The second was a Packard Bell model 15 TU3, equipped with a Garrard 3000, which has two phono sockets for the tone arm, and no obvious phono ground. Perhaps on the Garrard the tone arm is grounded to the turntable chassis, as there is what looks like a safety ground on the Garrards 4 pin power connector, even though the amp doesn't have a 3 prong safety cord.

I know the grounds tend to be needed on modern equipment to equalize the electric potential of the two separate units. The only reason I can think of is that the cabinets didn't need them because the turntable is powered through a plug that is part of the circuits of the pre-amp. Is that hypothesis correct? Both turntable models are idler drives, if that bears any significance.

Lets say I want to put a modern (1980) record player in the 60's Astro Sonic cabinet which offers two wall current power leads for the turntable which could be wired as a standard wall plug. Do I need to create a connection on the pre-amp for the phono ground the new turntable has?

Next, in reverse. If I wanted to use a 60's cabinet record changer that has no built in phono ground as a stand alone turntable to be plugged into a modern pre-amp, Do I need to create a phono ground on the turntable?

Thank you for your help.

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Re: Why don't cabinet record players have phono ground?

Post by smee4 » 15 May 2019 02:09

I think, if you look closely at some of those old tables, the three wire signal connection has 2 signal wires (left and right channels) and a common shield/ground wire, and that common ground connects to the metal work of the table at one point for safety and to stop hum, often at a tag strip mounted underneath where the fine wires from the tonearm join to the cables that go to the amp.

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Re: Why don't cabinet record players have phono ground?

Post by scrapjack+ » 15 May 2019 03:44

So old cabinet tables did have phono grounds, they just hid them, like slight of hand, or rather slight of wire. That would explain the micromatic. Looking at the Garrard 3000 again it has two phono/rca connectors. Looks like the rca socket bracket is grounded to the chassis, so the "safety ground" with the 4 pin power connector is likely the phono ground.

So to connect a cabinet table to a modern amp with out adding wires, I need to do a continuity test if it's not a 3 wire unit to find which wire is the phono ground.

I suppose if the cabinet had a table with 3 wire tonearms, I just need to short the phono ground to one of the rca grounds. If the cabinet had a table with more than 3 wires, I must hope the old table is still present to figure out what they did.

Thank you for the explanation.

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Re: Why don't cabinet record players have phono ground?

Post by smee4 » 15 May 2019 04:18

Maybe part of the confusion is that there is only one ground, not a separate phono ground from the safety ground. Any metalwork you can touch needs to be grounded, and so the usual option taken was everything uses the metal of the turntable as a central ground point, and this is connected to the amplifier chassis by one wire only to avoid ground loops.

Also 3-wire vs 4 wire signal connection, is essentially the same thing for most common turntables back then. The two shields are the return path for the left and right signals, and in most designs, they return to the same point - hence they are really only one wire. So it's all really 3 wires, left, right, and ground.

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Re: Why don't cabinet record players have phono ground?

Post by Damien Taylor » 16 May 2019 03:03

Common on standalone players historically and also a common modification for the SL-1200 is to incorporate the chassis ground into the signal ground.

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Re: Why don't cabinet record players have phono ground?

Post by scrapjack+ » 16 May 2019 03:43

Damien Taylor wrote:
16 May 2019 03:03
Common on standalone players historically and also a common modification for the SL-1200 is to incorporate the chassis ground into the signal ground.
Interesting, when I rewired my Technics SL-Q2, I just gave it an EMI filtered C14 3 prong socket for a safety ground, and a insulated panel mount banana jack for the signal ground as the old cable was way too short, and something of a pain. the original signal ground is zip tied and connected to the banana jack. It works fine despite the naysayers, I don't get any hum.
smee4 wrote:
15 May 2019 04:18
Maybe part of the confusion is that there is only one ground, not a separate phono ground from the safety ground. Any metalwork you can touch needs to be grounded, and so the usual option taken was everything uses the metal of the turntable as a central ground point, and this is connected to the amplifier chassis by one wire only to avoid ground loops.

Also 3-wire vs 4 wire signal connection, is essentially the same thing for most common turntables back then. The two shields are the return path for the left and right signals, and in most designs, they return to the same point - hence they are really only one wire. So it's all really 3 wires, left, right, and ground.
Having been shocked by supposedly functional electronics that had grounding issues, I always thought any metal enclosure for electronics should have a safety ground even when the amps/volts isn't fatal. I have only seen one component from a component stereo that has ever had a 3 prong cord.

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Re: Why don't cabinet record players have phono ground?

Post by smee4 » 16 May 2019 06:19

scrapjack+ wrote:
16 May 2019 03:43

smee4 wrote:
15 May 2019 04:18
Maybe part of the confusion is that there is only one ground, not a separate phono ground from the safety ground. Any metalwork you can touch needs to be grounded, and so the usual option taken was everything uses the metal of the turntable as a central ground point, and this is connected to the amplifier chassis by one wire only to avoid ground loops.

Also 3-wire vs 4 wire signal connection, is essentially the same thing for most common turntables back then. The two shields are the return path for the left and right signals, and in most designs, they return to the same point - hence they are really only one wire. So it's all really 3 wires, left, right, and ground.
Having been shocked by supposedly functional electronics that had grounding issues, I always thought any metal enclosure for electronics should have a safety ground even when the amps/volts isn't fatal. I have only seen one component from a component stereo that has ever had a 3 prong cord.
Yes, they should always be connected to ground for safety. I wasn't saying anything to the contrary.

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Re: Why don't cabinet record players have phono ground?

Post by scrapjack+ » 16 May 2019 07:04

smee4 wrote:
16 May 2019 06:19
scrapjack+ wrote:
16 May 2019 03:43

smee4 wrote:
15 May 2019 04:18
Maybe part of the confusion is that there is only one ground, not a separate phono ground from the safety ground. Any metalwork you can touch needs to be grounded, and so the usual option taken was everything uses the metal of the turntable as a central ground point, and this is connected to the amplifier chassis by one wire only to avoid ground loops.

Also 3-wire vs 4 wire signal connection, is essentially the same thing for most common turntables back then. The two shields are the return path for the left and right signals, and in most designs, they return to the same point - hence they are really only one wire. So it's all really 3 wires, left, right, and ground.
Having been shocked by supposedly functional electronics that had grounding issues, I always thought any metal enclosure for electronics should have a safety ground even when the amps/volts isn't fatal. I have only seen one component from a component stereo that has ever had a 3 prong cord.
Yes, they should always be connected to ground for safety. I wasn't saying anything to the contrary.
I Understood that, and wasn't trying to argue. I was saying that I agree with you, but the companies that make the stuff don't. I think the companies just want to save a few cents on power cords.

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Re: Why don't cabinet record players have phono ground?

Post by markcass » 16 May 2019 09:53

Many modern products with metal casings are 'double-insulated' and should not be earthed/grounded. They have to be designed as such, not simply made the same way but without the third core to the power cable - it's definitely a very bad idea to remove the ground from a unit that is meant to have one!

Such double-insulated items have a 'rectangle within a rectangle' symbol on the back-panel label.

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Re: Why don't cabinet record players have phono ground?

Post by smee4 » 16 May 2019 09:59

markcass wrote:
16 May 2019 09:53
Many modern products with metal casings are 'double-insulated' and should not be earthed/grounded. They have to be designed as such, not simply made the same way but without the third core to the power cable - it's definitely a very bad idea to remove the ground from a unit that is meant to have one!

Such double-insulated items have a 'rectangle within a rectangle' symbol on the back-panel label.
Absolutely right. Of course these "cabinet record players" are way before that :)

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Re: Why don't cabinet record players have phono ground?

Post by Coffee Phil » 16 May 2019 18:22

OK, here is my treatise on phono grounding.

First off cabinet record players of the '60 typically used ceramic cartridges which have a much higher output voltage than magnetic cartridges. Often the arm wiring had three wires. One for each channel and the third for the signal return (ground). The return was also connected to the arm and the chassis of the player and also served to connect the chassis of the player to the chassis of the amplifier. That setup works well for units with isolated power supplies. (double insulated in today's language) Some cabinet record players had non-isolated power supplies as transformers are costly and heavy (shipping costs). On such machines tricks were used, such as connecting the chassis of the player, which can be touched, to the chassis of the amplifier with RC networks to balance low hum with safety. Magnetic cartridges are pretty much out of the question in such machines.

Now moving to high fidelity component systems using UNBALANCED (RCA connectors) interconnects there are a couple of ways to deal with "grounding". In the first and most common arrangement each channel of the cartridge is connected to the phono stage via a coaxial cable with the returns (grounds) of each channel connected to the phono stage chassis via the coax shields. The signal returns are isolated from each other, the arm, and the player chassis. The player chassis is connected to the phono stage chassis via a "ground" lead.
An alternate method which also works has one of the signal returns (usually left) connected to the arm and table chassis providing the connection to the phono stage chassis. A separate ground wire should not be used with this arrangement.

Any decent audio system with unbalanced cable interconnects should have isolated power supplies (double insulated). Connecting more than one component to "earth ground" (three wire plug) is inviting a ground loop. (read hum) If there is a tuner with an external antenna, the antenna will provide an earth ground if the antenna installer has good sense.

Phil


scrapjack+ wrote:
15 May 2019 00:24
I've gotten to examine the internals of two 60's cabinet type record players. Neither one seemed to have a phono ground to the preamp like I'd expect on a modern turntable. Even Crosleys have a ground. The first cabinet was a Magnavox Astro Sonic with a Magnavox Micromatic turntable which had only 3 wires for the tone arm and no ground connecting the amp to the record changer. The second was a Packard Bell model 15 TU3, equipped with a Garrard 3000, which has two phono sockets for the tone arm, and no obvious phono ground. Perhaps on the Garrard the tone arm is grounded to the turntable chassis, as there is what looks like a safety ground on the Garrards 4 pin power connector, even though the amp doesn't have a 3 prong safety cord.

I know the grounds tend to be needed on modern equipment to equalize the electric potential of the two separate units. The only reason I can think of is that the cabinets didn't need them because the turntable is powered through a plug that is part of the circuits of the pre-amp. Is that hypothesis correct? Both turntable models are idler drives, if that bears any significance.

Lets say I want to put a modern (1980) record player in the 60's Astro Sonic cabinet which offers two wall current power leads for the turntable which could be wired as a standard wall plug. Do I need to create a connection on the pre-amp for the phono ground the new turntable has?

Next, in reverse. If I wanted to use a 60's cabinet record changer that has no built in phono ground as a stand alone turntable to be plugged into a modern pre-amp, Do I need to create a phono ground on the turntable?

Thank you for your help.

smee4
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Re: Why don't cabinet record players have phono ground?

Post by smee4 » 16 May 2019 23:54

Coffee Phil wrote:
16 May 2019 18:22
Some cabinet record players had non-isolated power supplies as transformers are costly and heavy (shipping costs). On such machines tricks were used, such as connecting the chassis of the player, which can be touched, to the chassis of the amplifier with RC networks to balance low hum with safety. Magnetic cartridges are pretty much out of the question in such machines.
Thankfully, such transformerless machines were largely unable to meet safety regulations here, probably because of the higher mains voltage (240Vac), our rules pretty much demanded isolation via a transformer. It also meant we seldom encounter weird heater voltages on tubes as they didn't string together many tube heaters in series to a total equal to or near the mains voltage. All our tubes tended to be 6.3 or 12.6 volt (occasionally a 5V regulator maybe) running from transformer secondaries in parallel.

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Re: Why don't cabinet record players have phono ground?

Post by Coffee Phil » 17 May 2019 18:42

Hi smee4,

Transformerless, electronics with series tube heaters were budget items, however the AA5 (All American Five) AM radio was developed to the point where they were safe as long as the OEM knobs remained and the cabinet was intact. Some were actually decent performing little radios.

There were plenty little record players which used the same approach and most were relatively safe as long as the case was intact.

In the '60s my girlfriend's roommate and a tabletop record player with a dying ceramic cartridge and I offered to fix it. I took the fool notion to "upgrade" it to a mid price MM cartridge. I bought such a cartridge and a little phono stage and made the switch. Huge hum! Grounding the chassis of the changer to the amp would have created a death trap. To bail myself out I did the grounding and added a 1:1 isolation transformer to power the electronics including the tube heaters. I did bring it off but the transformer probably cost and weighed more than the MM cartridge and phono stage together.

Lesson learned: Before doing a mod give some thought to the possible complications.

Phil
smee4 wrote:
16 May 2019 23:54
Coffee Phil wrote:
16 May 2019 18:22
Some cabinet record players had non-isolated power supplies as transformers are costly and heavy (shipping costs). On such machines tricks were used, such as connecting the chassis of the player, which can be touched, to the chassis of the amplifier with RC networks to balance low hum with safety. Magnetic cartridges are pretty much out of the question in such machines.
Thankfully, such transformerless machines were largely unable to meet safety regulations here, probably because of the higher mains voltage (240Vac), our rules pretty much demanded isolation via a transformer. It also meant we seldom encounter weird heater voltages on tubes as they didn't string together many tube heaters in series to a total equal to or near the mains voltage. All our tubes tended to be 6.3 or 12.6 volt (occasionally a 5V regulator maybe) running from transformer secondaries in parallel.

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Re: Why don't cabinet record players have phono ground?

Post by scrapjack+ » 19 May 2019 20:50

Damien Taylor wrote:
16 May 2019 03:03
Common on standalone players historically and also a common modification for the SL-1200 is to incorporate the chassis ground into the signal ground.
What are the pros and cons of such a mod?
Doesn't this create a safety ground on the table if the amp has a 3 prong cord?

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Re: Why don't cabinet record players have phono ground?

Post by smee4 » 19 May 2019 23:18

scrapjack+ wrote:
19 May 2019 20:50
Damien Taylor wrote:
16 May 2019 03:03
Common on standalone players historically and also a common modification for the SL-1200 is to incorporate the chassis ground into the signal ground.
What are the pros and cons of such a mod?
Doesn't this create a safety ground on the table if the amp has a 3 prong cord?
It does ground the chassis and make it safer. Maybe not as safe as a dedicated ground in a 3 prong power cord though.

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