Tube Preamp input/output wiring questions

[joshvito]

I am building a tube based preamp (ptp wiring), and have 2 questions about wiring the inputs and outputs.

I have planned for 3 pairs of inputs (rca) and 3 pairs of outputs (rca).
The 3 inputs are planned to go to a rotary switch. The wiring I have planned from the switch is where I have questions.

1. I would like to have an output that is just a pass through from the input selector switch to an output. I plan to run this to another preamp (ss). Is this as simple as just running a second pair of wires from the switch output to a pair of rca jacks? Or is this impossible without a manual switch?
2. I would also like to have 2 pairs of output from the preamp, in case I want to bi-amp, etc. Can I just wire the preamp according to schematic, and then just daisy chain the 2 pairs of output RCA's to the signal output of the circuit?

Thanks for any help.
p.s. If you need more information, I am planning on cloning this circuit .

 

[shelly_d]

That link does not identify the preamp tube on the schematic. Assuming ii is a 6L6GC, coming off the plate as you are, you have a pretty high output impedance. Paralleling that into 2 inputs, especially if those are modern amps with about 20K input impedance or so, may load the output of the preamp to much (draw too much current). I would recommend adding another stage to bring the output impedance down. If you want to use tubes, then do a cathode follower output. If you want something very easy to implement, use a unity gain Op Amp buffer. Either will have a low enough output impedance to allow you to parallel the outputs to 2 or even 3 amps.

Having said that, the best way to multiamp is to take only one output from the preamp and feed it into an electronic crossover which will split the frequency spectrum, low to one set of outputs and high to another. With a high enough input impedance on that device, you would not need a buffer.

Hope this helps and have fun with your project.

Shelly_D

Just went back and read the description. He says the design is based on 6V6 with a plate resistance and therefore output impedance of 1.8K ohms. Using a 1 to 10 engineering guideline, that means your combined input impedance of all devices on the output of the preamp cannot drop below 18K ohms. The load that 2 amps in parallel presents to this is the product of the two input impedances divided by the sum of the two input impedances. The only way to keep this design happy with what you plan on is if both of the two amps have an input impedance of 36K ohms or more.

 

[gadget73]

The trouble with paralleling the outputs from the selector switch is the load the sources see. You'll always have a 100K load across your sources from the volume pot in the preamp. If you output to another preamp with a 100K input impedance, the sources will see a 50K load. If they will drive that, no worries. Most SS gear won't care, and generally tube gear with a cathode follower built in won't either. Things that really care about such things will tell you what the minimum load to connect is.

If you want to be absolutely sure that it won't cause trouble you can use an op amp in a zero-gain buffer configuration to allow whatever load you want directly off the switch without having effect on your sources.



Looks like the 6L6 is just being used as a shunt regulator in the power supply. Quickly glancing over the rest I'm guessing the 6V6 is the audio tube.

 

[joshvito]

The trouble with paralleling the outputs from the selector switch is the load the sources see. You'll always have a 100K load across your sources from the volume pot in the preamp. If you output to another preamp with a 100K input impedance, the sources will see a 50K load. If they will drive that, no worries. Most SS gear won't care, and generally tube gear with a cathode follower built in won't either. Things that really care about such things will tell you what the minimum load to connect is.

If you want to be absolutely sure that it won't cause trouble you can use an op amp in a zero-gain buffer configuration to allow whatever load you want directly off the switch without having effect on your sources.



Looks like the 6L6 is just being used as a shunt regulator in the power supply. Quickly glancing over the rest I'm guessing the 6V6 is the audio tube.

Yes. I plan on using a 6V6 for the audio. Isn't the volume pot usually after the input selector switch?

More specifically, I was thinking about the "pass through" related to the first question would get connected to the input of the Aux RCA (20k ohms) of my Marantz PM8005.

.. and the tube preamp circuit would be output to the power amp direct input (15k Ohms) of the power amp section of the PM8005. This way I could hook up multiple inputs to the tube pre, but choose to switch which preamp is active via remote control.

When the PM8005 is in power amp direct mode, it's pre-amp section is turned off.

Thanks for your time and help. If you can't tell, I'm a bit new to this. But I'm treating it like a learning project/experience

EDIT: I just reread this, and realized you what you were referring to by "sources". I completely misread it earlier. Sounds like a buffer or a switch (maybe?) in between the source selector and volume pot would be a better design to achieve what I am after.

 

[joshvito]

That link does not identify the preamp tube on the schematic. Assuming ii is a 6L6GC, coming off the plate as you are, you have a pretty high output impedance. Paralleling that into 2 inputs, especially if those are modern amps with about 20K input impedance or so, may load the output of the preamp to much (draw too much current). I would recommend adding another stage to bring the output impedance down. If you want to use tubes, then do a cathode follower output. If you want something very easy to implement, use a unity gain Op Amp buffer. Either will have a low enough output impedance to allow you to parallel the outputs to 2 or even 3 amps.

Having said that, the best way to multiamp is to take only one output from the preamp and feed it into an electronic crossover which will split the frequency spectrum, low to one set of outputs and high to another. With a high enough input impedance on that device, you would not need a buffer.

Hope this helps and have fun with your project.

Shelly_D

Just went back and read the description. He says the design is based on 6V6 with a plate resistance and therefore output impedance of 1.8K ohms. Using a 1 to 10 engineering guideline, that means your combined input impedance of all devices on the output of the preamp cannot drop below 18K ohms. The load that 2 amps in parallel presents to this is the product of the two input impedances divided by the sum of the two input impedances. The only way to keep this design happy with what you plan on is if both of the two amps have an input impedance of 36K ohms or more.

Thanks. This is helpful, looks like I will keep one output instead of two from the preamp circuit. Especially, since this is my first custom build. I still want to try and incorporate a pass through if possible, so I can leave this tube preamp off and pass the signal out to another preamp.

 

[shelly_d]

Now that we know that the input impedance of your power amp is 15K, we can tell you that the load on the 6V6 is marginally too high (in my opinion). To keep the load on the output of a preamp down (low current drain) you want to maintain a 10 to 1 ratio between the input of the power amp and the output of your preamp. At 15K to 1.8K you have a ration of 8.3. I would consider that too low. I would recommend an output buffer, again either an opamp buffer, which can drive loads as bad as 600 ohms (typically) or a cathode follower circuit.

Shelly_D

 

[joshvito]

Now that we know that the input impedance of your power amp is 15K, we can tell you that the load on the 6V6 is marginally too high (in my opinion). To keep the load on the output of a preamp down (low current drain) you want to maintain a 10 to 1 ratio between the input of the power amp and the output of your preamp. At 15K to 1.8K you have a ration of 8.3. I would consider that too low. I would recommend an output buffer, again either an opamp buffer, which can drive loads as bad as 600 ohms (typically) or a cathode follower circuit.

Shelly_D

Can you tell me how you calculated the 1.8k Ohm output impedance? Sorry, I am just trying to learn as I go.

I only as, as the author of the circuit mentions an Output impedance of 1200 Ohm. Which puts the ratio at12+.

Thanks again, now I am off to read more about cathode followers.

 

[shelly_d]

I used the plate resistance of 1.8K for the 6V6 as stated in the articles chart. If that is not the correct value to use then my post would be in error.

6V6GT – Mu 9, Rp 1800, Gm 5, beam pentode, easy to find and well documented

I don't see how he arrived at his output impedance, he did not show the calculation.

Shelly_D

 

[joshvito]

I used the plate resistance of 1.8K for the 6V6 as stated in the articles chart. If that is not the correct value to use then my post would be in error.

6V6GT – Mu 9, Rp 1800, Gm 5, beam pentode, easy to find and well documented

I don't see how he arrived at his output impedance, he did not show the calculation.

Shelly_D

Yea, he didn't show his work on that write up (he does document it else where tho). But if I look up formulas for Grounded Cathode Amplifier Output Impedance, I find...
Zo = Rl || Rp (plate load resistor "in parallel to" internal plate resistance)
= 5k || 1.8k
= 1323.53 Ohm

That gives me a ratio of ~11.3.

Now I have 3 numbers, I am a bit less confused about calculating resistance, and I have more research to do (got to do something while I wait for parts

here is my references (mostly for my future ref):
- http://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers
- https://wtfamps.com/inputoutput-impedance/
- last page of (http://www.tubecad.com/articles_2003/Grounded_Cathode_Amplifier/Grounded_Cathode_Amplifier.pdf)

 

[gadget73]

Yes. I plan on using a 6V6 for the audio. Isn't the volume pot usually after the input selector switch?

It is, but if you look at it, you'll see that the output of the selector switch goes into the top of the 100k volume pot. Bottom of the volume pot goes to ground, so essentially your sources see a 100k resistor across them. If you feed that into a second device with a similar 100k control, you now have two 100k in parallel for a 50k load on the source. It may not matter any, its just something to be aware of in case you have some fiddly source that doesn't want to see under 100k or whatever. It also won't matter if the preamp is turned on or off, the volume control is always in the circuit.

 

[joshvito]

It is, but if you look at it, you'll see that the output of the selector switch goes into the top of the 100k volume pot. Bottom of the volume pot goes to ground, so essentially your sources see a 100k resistor across them. If you feed that into a second device with a similar 100k control, you now have two 100k in parallel for a 50k load on the source. It may not matter any, its just something to be aware of in case you have some fiddly source that doesn't want to see under 100k or whatever. It also won't matter if the preamp is turned on or off, the volume control is always in the circuit.

Thank you for the explanation. Makes sense.

I suppose, if I wanted to remove the resistance of the volume pot, I could put a switch in between to remove the pot from the circuit. Downside would be, more wire and the extra switch in the signal chain.