Impedance load on phono cartridge feeding two preamps?

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As part of the reorganization of my home stereo system I wish to use my turntable/cartridge combination as a source for 2 separate receivers, each with its own integral phono preamp. I would only use one receiver powered up at a time and would leave all components connected all the time.

My uncertainty has to do with the impedance load on the cartridge. I am not sure if both receiver preamps would load the cartridge if one is not powered up while I use the other. My thought was to use a simple short "Y" cord to connect the turntable to the two receivers, but I don't want the impedance load on the cartridge to drop too much and possibly affect the frequency response. I am not sure of the grounding scheme of the phono preamps in each unit, neither of which has an earth ground mains cable (USA 120V).

I am happy with the sound of my turntable/cartridge combination through each receiver, but I have also considered the possiblity of running it through a separate modular phono preamplifier and using the output of that preamp connected by a "Y" cord to the auxilliary input of each receiver. I would prefer to not do that due to space and budget considerations.

I measured the phono input resistance of each receiver at the RCA input jacks and obtained values of 47Kohms on one and about 65Kohms on the other. Based on these numbers, if the cartridge sees the impedance of both receivers in parallel it would see a value of about 27Kohms. My cartridges are both vintage Shure: a V15 Type IV (output impedance 1,380 ohms) and an M97HE (output impedance 1,550 ohms). The turntable output cables are of a very low capacitance type.

My questions:

Would the cartridge see the impedance load of both receivers if connected in a "Y" configuration if only one receiver was powered up?

Would the frequency response of the cartridge suffer as a result?

I look forward to some knowledgeable opinions. Thank you.
 
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I am pretty sure you are right about the parallel load. Just put a switch in where one is connected other not and just the opposite when flipped the other way. Can put it in a external box , If you do want to modify your equipment.

Enjoy the ride
Tom
 
A generally bad idea. The cart will see half the 47k load and double the capacitance. Plus some non-linear and unknown impedance from the unpowered unit.

Can you not use the tape out of one amp in order to feed the other? Obviously both will need to be powered.

Having a separate phono stage and a splitter after it is much better, although feeding a signal into an unpowered input may also be problematic.
 
Its takes a special switch to reliable handle such low level signals as from a cartridge, there isn't enough current for self-cleaning of the contacts so ideally you'd want gold-plated contacts, so that corrosion can't build up.

Standard switches are designed for power, which would quickly arc through gold-plating, and use other, harder, materials that oxidize over time. A small signal relay might be a more appropriate switching device, they are typically used for routing signals in test-equipment and engineered specifically for the task.

Loading a MM cartridge doubly will alter its frequency response, reduce signal level, and increase noise, definitely not recommended. MM cartridges are highly inductive and sensitive to load resistance and load capacitance.

A better approach is use a separate phono preamp and split its output to the two receivers aux inputs. The preamp needs low-impedance output with series resistors to each receiver to prevent a powered-down unit from distorting the signal to the other one.
 
Unless.........
the phono preamplifiers are a tube type like ecc83 or even 6dj8 and in both cases the input of the amplifier is up to 1 Megohm without the 47k input resistors and replace it with 100k for each input, two of them in parallel see the same cart!
Why not, but the input switch is out of the question.
 
This is another solution, a distribution amplifier designed specifically for moving-magnet phono signals. Its 47 kohm input resistance is realized with a combination of series and shunt feedback. R4 is to be adjusted such that there is about 1 V DC voltage across R1. Q1 can be a J308, J309 or J310. The 7812 can supply four channels without cooling.
 

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Both receivers are solid state designs. In my opinion the combined parallel impedance of the auxilliary inputs of the two receivers should not load the output of a modular solid state phono preamp enough to cause distortion, assuming the output of the preamp is at a typically low level impedance and that it has even modest current capability. This would probably be the best option for audio quality considerations, but again, I hoped to avoid it for both space and budget considerations.

I like the idea of a gold-plated switch meant for low level signals, but is a permanent "Y" connection a guarantee of poor audio quality?

Thanks for the opinions so far, and please feel free to offer more.
 
I have abandoned the idea of a permanent "Y" connection from one turntable to two receivers. The reason is a discovery I just now made when I again measured the phono input resistance of one of my receivers, a 1970s vintage Sherwood.

This receiver has a multi-position mechanical rotary source switch with spring leaf connectors. When the switch is in the "phono" position the input resistance is what I stated, about 65Kohms, but when the switch is in any other position the resistance at the phono inputs changes to what is essentially a dead short! To use it in a "Y" hookup I would have to ensure that the input source switch was always in the phono position when using the other receiver, and even then I would not be guaranteed good-sounding audio.

The plan, then, will be to use an external modular phono preamp feeding a line-level switch box to isolate the output to one receiver's auxilliary input at a time. I can rearrange my audio equipment to make space for the additional components in my listening room. It will be a squeeze, but I can make it work.

I guess my experience reaffirms the old DIY adage, "Measure twice, enter into potentially destructive audio connections once or not at all."

Thanks to all who responded. Your comments were helpful.
 
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