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Question: Controlling input Impedance Load.
Old 4 weeks ago
  #1
Question: Controlling input Impedance Load.

Hello dear Space friends.
I am trying to get my head around Impedance and how to "control it".

I am currently building an EQ that I want to use in Eurorack format.
It is stated that the Eurorack standard for modules is Input: 100K and Output: 1K

My EQ that I am building want to see as low load as possible. So a standard 600ohms is my main goal.

So, if the scenario will be that the output from Eurorack modules (around 1K) will go into my EQ that wants 600ohms, Am I then correct that using a a "step down transformer would do the job?
For example I have a Carnhill VTB9046 in front of me, so perhaps I could use it wired to be from 2.4K to 600Ohms? It´s not 1K I know, but maybe a good start?

Now, I could ofc just try it, but that leaves me to my next question for another thread, I currently do not understand the wiring principles of transformers...

But most importantly want to understand the options I have for using a low impedance unit in Eurorack.

Thank you for your time.
//Ted
Old 4 weeks ago
  #2
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jaddie's Avatar
 
🎧 10 years
A load impedance is placed on an output.

A source impedance feeds an input.

Why does your EQ need to see a low source impedance? And why did you pick 600 ohms? Is it a passive EQ?

Transformers reflect their secondary load back to their primary. So if you have a 1:1 transformer and load the secondary with 600 ohms, the primary will load the device feeding it with 600 ohms too. If you raise that secondary load to 1500 ohms, the primary will act like a 1500 ohm load too.

The only reason transformers specify input and output impedances is for optimization. The work best at those impedances, but still reflect the load back to the primary.

The point of the Eurorack standards is that a relatively low 1K source impedance for outputs keeps stray noise pickup low, and can drive many inputs. The point of having a 100K input is that you can feed many in puts from a single 1K output without overloading it. I would suggest you stay with the standards.

Rather than use a transformer, why not just a unity gain buffer amp? That way your EQ circuit can see a very low source impedance, but the actual input can still be at the 100K standard.
Old 4 weeks ago
  #3
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nosebleedaudio's Avatar
 
🎧 15 years
The one spec sheet I read stated:
Input Imp; 100K with 47K being a low min.
Output Imp; 1K but LOWER is better for some modules..

Having a Lower output Imp can not be a bad thing...
The only reason I can see for the 1K is for accidental shorts on the Output of a module...A Bad cable ect..
Old 4 weeks ago | Show parent
  #4
Quote:
Originally Posted by jaddie ➡️
Why does your EQ need to see a low source impedance? And why did you pick 600 ohms? Is it a passive EQ?
.
Hi Jaddie.
Thank you for your very informative reply.
Yes, its a passive inductor EQ design, so the impedance will affect the inductance.

Quote:
Originally Posted by jaddie ➡️
If you raise that secondary load to 1500 ohms, the primary will act like a 1500 ohm load too.
aah. Ok. So in other words, if you use a transformer as a "Step down" or "Step up" configuration. and even though it changes some 13dB, it still does not affect the impedance?

Quote:
Originally Posted by jaddie ➡️
Rather than use a transformer, why not just a unity gain buffer amp? That way your EQ circuit can see a very low source impedance, but the actual input can still be at the 100K standard.
Excellent! I just read up on this, and this is the defenitely the best solution for me. So how can one determine the output Load impedance of this? does it differ between Op amp types?

Thank you again. Excactly the information I was hoping for.
//Ted
Old 4 weeks ago | Show parent
  #5
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jaddie's Avatar
 
🎧 10 years
Quote:
Originally Posted by Ted Krotkiewski ➡️
Hi Jaddie.
Thank you for your very informative reply.
Yes, its a passive inductor EQ design, so the impedance will affect the inductance.
Ok, that explains it. I'm not sure why, in 2021, anyone would do that, but it's your choice.
Quote:
Originally Posted by Ted Krotkiewski ➡️

aah. Ok. So in other words, if you use a transformer as a "Step down" or "Step up" configuration. and even though it changes some 13dB, it still does not affect the impedance?
With transformers, everything is a ratio. Voltages, impedances, everything. Yes, the secondary load with be reflected to the primary relative to the ratio.
Quote:
Originally Posted by Ted Krotkiewski ➡️
Excellent! I just read up on this, and this is the defenitely the best solution for me. So how can one determine the output Load impedance of this? does it differ between Op amp types?
You can consider most opamps output impedance to be extremely low, but with limited current capability. So you use a build-out resistor of 1K to establish a reasonable source impedance as well as a load the opamp can work into if there's short. The source (output) impedance is essentially the value of the build-out resistor. 1K in this case.

But, caution! If your passive EQ is designed to work from at 600 ohm source, and actually presents a 600 ohm load, you need the build-out resistor to be 600 ohms. You will loose 6dB in level that way. On the output, the circuit may be designed to require a 600 ohm load. That should be a resistor bridged with a high impedance buffer amp to supply the signal to the outside world through a 1K resistor. If you want to maintain unit gain, the output amp will need to be +6dB plus whatever insertion loss there is in the EQ.

I think you got to 600 ohms by copying a vintage circuit. Nobody actually uses 600 ohm audio circuits anymore, not since facilities went to a voltage distribution method. 600 ohms is a power distribution method, and went out long ago. If you must stay with the old circuit, you'll need unity gain buffer amps at the input and output.
Old 4 weeks ago | Show parent
  #6
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Quote:
Originally Posted by jaddie ➡️
Nobody actually uses 600 ohm audio circuits anymore, not since facilities went to a voltage distribution method. 600 ohms is a power distribution method, and went out long ago. If you must stay with the old circuit, you'll need unity gain buffer amps at the input and output.
????

So nobody uses Pultecs or LA2a's or makes modern clones of gear that uses 600 ohm inputs and outputs?
Old 4 weeks ago | Show parent
  #7
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jaddie's Avatar
 
🎧 10 years
Quote:
Originally Posted by JLast ➡️
????

So nobody uses Pultecs or LA2a's or makes modern clones of gear that uses 600 ohm inputs and outputs?
Sure, but we don't design 600 ohm distribution systems, and haven't for decades. Which means to operate "as originally intended", both devices need something a bit special to fit into a modern insert. They need a true 600 ohm source (or whatever impedance the input is strapped for) and a 600 ohm output load. The won't get either just sticking them into an insert or anywhere in a modern studio system, as sources will be 100 ohms or lower, and loads will be way above bridging. Without these specific impedances, there will be unintended response variations and undamped transients.

As far as I can tell, the above applies to the modern clones too.
Old 4 weeks ago | Show parent
  #8
Quote:
Originally Posted by jaddie ➡️
Ok, that explains it. I'm not sure why, in 2021, anyone would do that, but it's your choice.
Thank you so much Jaddie for putting answering my questions. I learn a lot!

I would say, in 2021 it is more relevant then ever?
However, on a "technical engineering" point of view, I wouldn´t know. But in my book based on a aesthetically, sound point of view, There is just something with a good designed inductor EQ that is hard to beat. And, these old designs, that adds a footprint to the sound, put in a modern form factors is what still sells in the analog domain.

Quote:
Originally Posted by jaddie ➡️
I think you got to 600 ohms by copying a vintage circuit...
...If you must stay with the old circuit, you'll need unity gain buffer amps at the input and output.
Yes, its a good old Helios Type69 design. (Only copying the EQ section from Ruffrecords)

At the moment when prototyping, I run Line level audio directly into the EQ from my Audio interface, which has 600ohms Load on output.
600ohm seems to be a standard Load impedance for line outputs for most audio interfaces from the little research I´ve done.
and then I use a simple Gain buffer amp on EQ output to restore the 15dB insertion loss from the EQ circuit.

Thanks again!
//Ted

Last edited by Ted Krotkiewski; 4 weeks ago at 06:23 AM..
Old 3 weeks ago | Show parent
  #9
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ruffrecords's Avatar
 
🎧 10 years
Quote:
Originally Posted by Ted Krotkiewski ➡️
Thank you so much Jaddie for putting answering my questions. I learn a lot!

I would say, in 2021 it is more relevant then ever?
However, on a "technical engineering" point of view, I wouldn´t know. But in my book based on a aesthetically, sound point of view, There is just something with a good designed inductor EQ that is hard to beat. And, these old designs, that adds a footprint to the sound, put in a modern form factors is what still sells in the analog domain.


Yes, its a good old Helios Type69 design. (Only copying the EQ section from Ruffrecords)

At the moment when prototyping, I run Line level audio directly into the EQ from my Audio interface, which has 600ohms Load on output.
600ohm seems to be a standard Load impedance for line outputs for most audio interfaces from the little research I´ve done.
and then I use a simple Gain buffer amp on EQ output to restore the 15dB insertion loss from the EQ circuit.

Thanks again!
//Ted
If you are using my Helios 69 design then it is not intended for 600 ohms. It expects a low input impedance (1K is fine but anything lower is also OK) and it expects a high impedance load (100K would probably be OK but higher would not hurt). Are you including a gain make up amplifier?

Cheers

Ian
Old 3 weeks ago | Show parent
  #10
Quote:
Originally Posted by ruffrecords ➡️
If you are using my Helios 69 design then it is not intended for 600 ohms. It expects a low input impedance (1K is fine but anything lower is also OK) and it expects a high impedance load (100K would probably be OK but higher would not hurt). Are you including a gain make up amplifier?

Cheers

Ian
Thank you Ian.
oh! I thought 600 ohms was classified as a low impedance.

So, if I would aim for a very low impedance feeding into the EQ.
What are your thoughts on achieving a "step down" in impedance from a high impedance source?
As mentioned above, by adding a Unity Gain Buffer seems to give us an impedance of around 200ohms (Based on googling) Is that considered "Low impedance"?

I just found an explanation by DIYREC about making a variable input impedance control. Please see attached pic.
https://www.youtube.com/watch?v=r1yPMDHiRuc
Thoughts on using that on input?

Thank you for your time!
//Ted Krotkiewski
Attached Thumbnails
Question: Controlling input Impedance Load.-screenshot-2021-04-15-10.00.43.png  
Old 3 weeks ago
  #11
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nosebleedaudio's Avatar
 
🎧 15 years
I think your'e confused on Input imp & Output/source imp.
The Output/Source Imp can be & should be much lower than the Input its feeding.
What Feeds the Eq should be much lower than the Input Imp.
Another thing happening is frequency, the lower the frequency the output Imp goes up a little to a lot depending on the "Source" circuit, the cap size for one thing..

That's why you always want the Source/Output to have at least 5 times Lower than the input, 10 times is better and even higher.
For example: a 600 Ohm source/output into a common 10K input Imp is 16 times higher..
Old 3 weeks ago | Show parent
  #12
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ruffrecords's Avatar
 
🎧 10 years
Quote:
Originally Posted by Ted Krotkiewski ➡️
Thank you Ian.
oh! I thought 600 ohms was classified as a low impedance.
Sorry, I did not explain myself properly. A 600 ohm EQ would have a 600 ohm input impedance and a 600 ohm output impedance. My Helios 69 design is not like that which is why I said it was not a 600 ohm EQ.

My design has a relatively high input impedance and works best from a relatively low source impedance. 600 ohms IS a low source impedance.
Quote:
So, if I would aim for a very low impedance feeding into the EQ.
It will work with any source impedance in the range from zero to about 2.5K. There is no need for it to be 'very low'
Quote:
What are your thoughts on achieving a "step down" in impedance from a high impedance source?
As mentioned above, by adding a Unity Gain Buffer seems to give us an impedance of around 200ohms (Based on googling) Is that considered "Low impedance"?
Whether you need a buffer depends on the actual source impedance of whatever you are driving the EQ with. What exactly will you be driving the EQ with?
Quote:
I just found an explanation by DIYREC about making a variable input impedance control. Please see attached pic.
https://www.youtube.com/watch?v=r1yPMDHiRuc
Thoughts on using that on input?

Thank you for your time!
//Ted Krotkiewski
I am not sure what that circuit thinks it is ahcieving but it definitely not what you need Ted.

As others have mentioned, the basic principle is to feed from a relatively low source impedance into a relatively high load impedance.

So for the EQ input, it typically looks like a 22K load so anything driving it should have an output impedance of anything up to a couple of Kohms. Does not matter if it is low of very low.

The output impedance of the EQ is typically around 5K so loading it with anything above 50K should be fine. Really high, like 500K, is still fine.

Hope that clears things up a bit.

Cheers

Ian
Old 3 weeks ago | Show parent
  #13
Quote:
Originally Posted by nosebleedaudio ➡️
...That's why you always want the Source/Output to have at least 5 times Lower than the input, 10 times is better and even higher.
For example: a 600 Ohm source/output into a common 10K input Imp is 16 times higher..
Indeed I have been very confused about it.
This is made me totally understand how the principle works and at least gave me a ballpark to throw things at.
Thank you so much for taking your time explaining this to me.
Old 3 weeks ago | Show parent
  #14
Quote:
Originally Posted by ruffrecords ➡️

....The output impedance of the EQ is typically around 5K so loading it with anything above 50K should be fine. Really high, like 500K, is still fine.

Hope that clears things up a bit.

Cheers

Ian
Yayyy Ian to the rescue .. AGAIN..
You cleared up A LOT of things for me.
Thank you!

Regarding Transformers, I know I need to read up on it properly.

But, just to throw me in the ballpark figure as you and @ nosebleedaudio did with explaining impedance. What would be a good transformer fit to put before and after the EQ?
The EQ input will be feeded with a Eurorack nominal level of +13dBU and about 1K impedance.

I thought a 1:1 600:600 ohms should work as an output xformer efter the EQ (with a op amp buffer making up the insertion loss), but since the Output/source impedance of the EQ is around 5K as you stated, I get confused again. EDIT I just realized, that the Op Amp output gain buffer after the EQ sets the Source impedance into a low impedance output.

I am still going to ask the original question below, even though I realized the above:
Should I choose a transformer based on the impedance it is given from the EQ (5K), or the impedance it will feed? Or both?

FYI, At the moment I have at home a VTB9046 input xformer and a VTB2281 600:600 output transformer. Can they be put into use?

I am learning so much! Thank you.

Last edited by Ted Krotkiewski; 3 weeks ago at 04:46 PM..
Old 2 weeks ago
  #15
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ruffrecords's Avatar
 
🎧 10 years
As I have said many times before, transformers do what they say on the tin - they transform input voltages, currents and impedances into output voltages, currents and impedances. A transform does NOT have an intrinsic impedance of its own. However, if you want to design a transformer to operate over the entire audio range, you soon discover that, for a given design, there is a limited range of impedances it will work best with. And these are the values that manufacturers assign to their transformers, like 600:600 or 10K:10K. The transformer knows absolutely nothing about this, it just obeys the laws of physics.

OK, so for the Helios EQ, this is designed as a modern day line level EQ so for a transformer balanced input you would normally use a 10K:10K input type transformer. The VTB9046 transformer you have is the modern version of the Neve standard line input transformer from the 70s. Neve usually wired this up 10K:600 so it dropped the input signal level by 12dB which brought it close to the internal operating level of the Neve mixers back then. You don't need to do that because the EQ will drop the level by 15dB anyway. If you want to use this transformer, probably the best way is to wire it 2K4:2K4 so there is no insertion loss. The EQ will be a light load for it and most sources you feed in the transformer will be of a low enough impedance not to compromise the bass response of the transformer.

After the EQ, before you even think of an output transformer, you need a gain male up amplifier with a high input impedance so as not to distort the EQ curves. There are some simple TL072 based designs you can use shown under the DIY page of my website:

https://www.customtubeconsoles.com/diy

You should not be planning on adding a transformer directly after the EQ, it is not designed to drive one.

So, after you gain make up amp you can than add a transformer if you want.

Remember way back when I said my Helios EQ design is not a 600 ohm design; by changing all the component values you could make it into a 600 ohm design. You could then add a 600:600 transformer at the front and the output would already be a 600 ohm balanced source so you would not need a transformer. My latest mastering EQ project works that way.

To summarise, the best input transformer would be a 10K:10K type.
After the EQ you need a gain make up amplifier
After the gain make up amplifier you can add an output transformer but which one you need depends on the actual amplifier you use.

Cheers

ian
Old 2 weeks ago | Show parent
  #16
Quote:
Originally Posted by ruffrecords ➡️
As I have said many times before, transformers do what they say on the tin - they transform input voltages, currents and impedances into output voltages, currents and impedances. A transform does NOT have an intrinsic impedance of its own. However, if you want to design a transformer to operate over the entire audio range, you soon discover that, for a given design, there is a limited range of impedances it will work best with. And these are the values that manufacturers assign to their transformers, like 600:600 or 10K:10K. The transformer knows absolutely nothing about this, it just obeys the laws of physics.

OK, so for the Helios EQ, this is designed as a modern day line level EQ so for a transformer balanced input you would normally use a 10K:10K input type transformer. The VTB9046 transformer you have is the modern version of the Neve standard line input transformer from the 70s. Neve usually wired this up 10K:600 so it dropped the input signal level by 12dB which brought it close to the internal operating level of the Neve mixers back then. You don't need to do that because the EQ will drop the level by 15dB anyway. If you want to use this transformer, probably the best way is to wire it 2K4:2K4 so there is no insertion loss. The EQ will be a light load for it and most sources you feed in the transformer will be of a low enough impedance not to compromise the bass response of the transformer.

After the EQ, before you even think of an output transformer, you need a gain male up amplifier with a high input impedance so as not to distort the EQ curves. There are some simple TL072 based designs you can use shown under the DIY page of my website:

https://www.customtubeconsoles.com/diy

You should not be planning on adding a transformer directly after the EQ, it is not designed to drive one.

So, after you gain make up amp you can than add a transformer if you want.

Remember way back when I said my Helios EQ design is not a 600 ohm design; by changing all the component values you could make it into a 600 ohm design. You could then add a 600:600 transformer at the front and the output would already be a 600 ohm balanced source so you would not need a transformer. My latest mastering EQ project works that way.

To summarise, the best input transformer would be a 10K:10K type.
After the EQ you need a gain make up amplifier
After the gain make up amplifier you can add an output transformer but which one you need depends on the actual amplifier you use.

Cheers

ian
Very well explained Ian. This makes me understand the philosophy much better!
I cant thank you enough.

Re: adding Gain makup amp efter the EQ.
Yes, Ofc. As stated above, I do have that in my design.
At the moment I am running a TL072 just for prototyping, but plan on using something like a OPA134.
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