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Old 16th Jan 2021, 10:48 am   #1
Levente
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Default Vintage tube mixer input impedance

hello All... i hope all of you are safe nd sound and wonder if you could help me with some impedance question. ( I am pretty sure that you do ! )

Recently I did purchase a very nice looking portable tube mixer. She has 3 mic inputs labelled at 50 Ohms and a 600 Ohm output. It seems that it is transformer balanced too looking at those big cans at the rear. Guessing the year of early 60s or even 50s built for a radio station in the US ( found great information about this station and the person who owned it, he was a military tech person in the war) guessing that he himself built it.

My question is these input impedances labelled at 50 Ohms. Can I use this with a now days mic such as SM57? Looking at the specs, the SM57 rated at 150 ohms ( which in reality according to the spec sheet is 310Ohms).

Do I always need to match the input impedance with these old tube gear? Old mic's from that era were low 30-50 impedances ( or very high 10k?).

Please advise on this if you can as I would love to see how this mixer performs.

Many Thanks and my best wishes to you all...

PS: if you could guess what circuit this mixer might be based of, would also help... i did some research on Gates and RCA designs, the closest I get is maybe the Mix It with extended inputs to 3? http://www.preservationsound.com/wp-...Schematic1.jpg
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Old 16th Jan 2021, 11:32 am   #2
mark_in_manc
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Default Re: Vintage tube mixer input impedance

I suspect someone more knowledgeable than me will be along in a bit, but here is my 2p!

If you draw any current from your mic (that is - if the input impedance to your mixer is not very high) - then the source impedance of the mic starts to matter. You can think of a mic as a magic source of signal voltage proportional to the sound pressure incident on it, in series with a small impedance network whose components tell you something about what kind of mic it is. So for a moving coil mic, that source impedance will have some inductance (think coil!) and the impedance of an inductor is of the form j(2.pi.f)L. This forms a potential divider with your mixer input impedance, and as frequency goes up, more and more of your signal gets dropped across the source impedance and less and less across the mixer - so it sounds muffled.

So we make the mixer impedance huge - but then the signal current is so small, than tiny currents induced in the mic cabling by external noise become comparable and the signal gets noisy!

So we end up with a differential input amplifier at the front of our mixer which amplifies the difference between two conductors, where the noise is hopefully the same on both conductors. Your inputs on cannon plugs suggest you might be working on those lines - but this thing is older than I know much about and others here will know more. If your circuit is right and the input is a transformer primary, I don't know how they ever got that big enough compared to the mic source impedance.

But in general - I would say matching impedance at the input is a bad idea - you want voltage transfer, not power transfer, so Z_in wants to be much bigger than Z_source of the mic.
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Old 16th Jan 2021, 11:40 am   #3
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Default Re: Vintage tube mixer input impedance

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Originally Posted by mark_in_manc View Post
I suspect someone more knowledgeable than me will be along in a bit, but here is my 2p!

If you draw any current from your mic (that is - if the input impedance to your mixer is not very high) - then the source impedance of the mic starts to matter. You can think of a mic as a magic source of signal voltage proportional to the sound pressure incident on it, in series with a small impedance network whose components tell you something about what kind of mic it is. So for a moving coil mic, that source impedance will have some inductance (think coil!) and the impedance of an inductor is of the form j(2.pi.f)L. This forms a potential divider with your mixer input impedance, and as frequency goes up, more and more of your signal gets dropped across the source impedance and less and less across the mixer - so it sounds muffled.

So we make the mixer impedance huge - but then the signal current is so small, than tiny currents induced in the mic cabling by external noise become comparable and the signal gets noisy!

So we end up with a differential input amplifier at the front of our mixer which amplifies the difference between two conductors, where the noise is hopefully the same on both conductors. Your inputs on cannon plugs suggest you might be working on those lines - but this thing is older than I know much about and others here will know more. If your circuit is right and the input is a transformer primary, I don't know how they ever got that big enough compared to the mic source impedance.

But in general - I would say matching impedance at the input is a bad idea - you want voltage transfer, not power transfer, so Z_in wants to be much bigger than Z_source of the mic.

Thanks Mark, this explanation was very educational and thanks for that, i did not know the science behind this till now the other question is i am asking to myself, whoever marked these inputs as 50 Ohms, meant to say that the actual input is 50 Ohms or the mic to be plugged into these sockets should be 50 Ohms..
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Old 16th Jan 2021, 12:34 pm   #4
Ted Kendall
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Default Re: Vintage tube mixer input impedance

The usual practice at the time was to optimise for transfer of power, so the impedance of the input would be that of the microphone. Modern inputs present something like five times source impedance to the mic - about 1K for a 200 ohm mic.

You can directly measure the input impedance with a tone source and a variable resistance - connect the resistance in series with the tone source, adjust until the reading on the VU meter drops by 6dB and then measure the value. this will give you the input impedance directly.
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Old 16th Jan 2021, 12:46 pm   #5
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Default Re: Vintage tube mixer input impedance

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Originally Posted by Ted Kendall View Post
The usual practice at the time was to optimise for transfer of power, so the impedance of the input would be that of the microphone. Modern inputs present something like five times source impedance to the mic - about 1K for a 200 ohm mic.

You can directly measure the input impedance with a tone source and a variable resistance - connect the resistance in series with the tone source, adjust until the reading on the VU meter drops by 6dB and then measure the value. this will give you the input impedance directly.
Thank you. I am not sure that I am able to do these measurements... i do not have a tone generator sadly... I guess my only choice is to use this with vintage microphones of outputs of 30-50 Ohms ( which I dont mind at all) or ..altering/checking the input transformer taps and changing it to 200-250 Ohms input if it is possible at all...i did not check that... would you suggest doing so?

Thanks !
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Old 16th Jan 2021, 2:16 pm   #6
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Default Re: Vintage tube mixer input impedance

Quote:
Originally Posted by mark_in_manc View Post

But in general - I would say matching impedance at the input is a bad idea - you want voltage transfer, not power transfer, so Z_in wants to be much bigger than Z_source of the mic.
Is the answer I would give.
Looking at the piccies, it would seem that the input is via audio transformers. These would have been chosen with good characteristics for audio use, so the general impedance presented to the mic. would probably be a transformation of the value of the grid leak resistor in the amplifier. Don't forget that if you only know the turns ratio, the impedance ratio is squared going up or square-rooted going down.

Incidentally, one boon of using transformers rather than just a diff. amplifier is the elimination of earth loops where line inputs are concerned - also a transformer having an inter-winding screen excellently filters out RF.
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Old 16th Jan 2021, 3:02 pm   #7
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Default Re: Vintage tube mixer input impedance

I see that your mixer originated in West Virginia (see https://www.radiolineup.com/stations/WBUC-AM ) so will have 'heard' plenty of Country Music!

This thread has already given you a thorough grounding in microphone transformer ratios. My practical advice would be to give it a try. An inappropriate impedance match won't do any damage, and with the limited output of a dynamic mike won't be a cause of distortion.

The most likely limitation is that the input transformers won't have the ideal amount of winding inductance for a modern 150- 200 ohm mike. This would cause the input impedance to drop at low frequencies (say, below 100Hz) and hence give more low-frequency roll-off than you'd get with the 30-50 ohm mikes it was designed for.

Unless you're looking for critical broadcast-quality results, it will be fine.

Martin
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Old 16th Jan 2021, 3:46 pm   #8
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Default Re: Vintage tube mixer input impedance

WIth audio it's best (or even always) feed from a low source to a high input, it is only when you get long (more than a few hundred yards) run it becomes slightly important.
 
Old 16th Jan 2021, 4:53 pm   #9
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Default Re: Vintage tube mixer input impedance

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Originally Posted by Hartley118 View Post
I see that your mixer originated in West Virginia (see https://www.radiolineup.com/stations/WBUC-AM ) so will have 'heard' plenty of Country Music!

This thread has already given you a thorough grounding in microphone transformer ratios. My practical advice would be to give it a try. An inappropriate impedance match won't do any damage, and with the limited output of a dynamic mike won't be a cause of distortion.

The most likely limitation is that the input transformers won't have the ideal amount of winding inductance for a modern 150- 200 ohm mike. This would cause the input impedance to drop at low frequencies (say, below 100Hz) and hence give more low-frequency roll-off than you'd get with the 30-50 ohm mikes it was designed for.

Unless you're looking for critical broadcast-quality results, it will be fine.

Martin
Thank you.. I have several mic's to try...dynamic's also some condensers and one ribbon mic which is at 30 ohms if I am not mistaken. Will definitely give it a go and try.

Thank you all for the responses and advice I am really thankful to you all. For fun fact I found an original document issued by the owner of this radio station in West Virginia with regards their setup, and in this document, it is referred to a three mic input... could be this mixer or could be not..only Mr Newman knows from above now...his work will have a wonderful place in my studio...in use...

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Old 16th Jan 2021, 5:58 pm   #10
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Default Re: Vintage tube mixer input impedance

That letter give a nice insight into the rather homely simplicity of all those many American local radio stations back in the 1960s - and I don't think it's changed much today.

I think that your guess is right. Your mixer would probably have mixed those 3 mikes to pre-set the levels for perhaps the presenter, a newsreader/weather forecaster and from time to time an interviewee or two.

The output of the premixer would have been wired to one input of the main presenting console - probably nothing very spectacular, just something like the vintage 'Sparta' model in this video: https://www.youtube.com/watch?v=PWRiDaKApn4 . Such US stations were pioneers of 'self operation' where the presenter/DJ operates the desk as well as presenting the show.

Like most such US stations, and totally unlike most European practice, the letter describes that the transmitter and studio were in the same location. So, although the studio mixers are very uncomplicated, they have to be resistant to high RF fields from the transmitter. In this case, I see they had the luxury of the antenna tower being all of 380 feet from the studio! Often the tower is right on top of the studio building, leading to all sorts of potential breakthrough problems. It wouldn't be surprising to find a bespoke 1460 kHz rejection filter somewhere in the audio connections.
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Old 16th Jan 2021, 6:30 pm   #11
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Default Re: Vintage tube mixer input impedance

Quote:
Originally Posted by Hartley118 View Post
That letter give a nice insight into the rather homely simplicity of all those many American local radio stations back in the 1960s - and I don't think it's changed much today.

I think that your guess is right. Your mixer would probably have mixed those 3 mikes to pre-set the levels for perhaps the presenter, a newsreader/weather forecaster and from time to time an interviewee or two.

The output of the premixer would have been wired to one input of the main presenting console - probably nothing very spectacular, just something like the vintage 'Sparta' model in this video: https://www.youtube.com/watch?v=PWRiDaKApn4 . Such US stations were pioneers of 'self operation' where the presenter/DJ operates the desk as well as presenting the show.

Like most such US stations, and totally unlike most European practice, the letter describes that the transmitter and studio were in the same location. So, although the studio mixers are very uncomplicated, they have to be resistant to high RF fields from the transmitter. In this case, I see they had the luxury of the antenna tower being all of 380 feet from the studio! Often the tower is right on top of the studio building, leading to all sorts of potential breakthrough problems. It wouldn't be surprising to find a bespoke 1460 kHz rejection filter somewhere in the audio connections.
Ha this is fantastic I do not have this mixer in front of me yet as it is en route to my current location ( Madeira) where I will install this in my future small studio. Will check this thoroughly out.. also very keen to see what input and output transformer she has... I only can make out the 6x4 rectifier tube and possibly some 12ax7s and 12au7s in there... not sure...

Great to read and see how people did this in the old days and still outperforms many of the new tech "all for your eyes" gear... i will definitely stay analog. aaah... i wish to be young in the 60s.. these current times suks... ( pardon me)
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Old 18th Jan 2021, 6:11 pm   #12
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Default Re: Vintage tube mixer input impedance

Hey, it’s pretty good these days when considering the amazing quality of cheap musical instruments - or the wide availability of dental treatment and analgesia
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Old 18th Jan 2021, 6:55 pm   #13
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Hey, it’s pretty good these days when considering the amazing quality of cheap musical instruments - or the wide availability of dental treatment and analgesia
ha... not for me although tomorro will be my 4th root canal session at 6pm... ouch...
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Old 18th Jan 2021, 7:01 pm   #14
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Default Re: Vintage tube mixer input impedance

wow I don't have anything constructive to add I just wanted to say that is such a cool little mixer!

You should totally record a mono record!
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Old 18th Jan 2021, 7:03 pm   #15
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wow I don't have anything constructive to add I just wanted to say that is such a cool little mixer!

You should totally record a mono record!

That is my plan.. on drums.. one or two RCA mic's to this mixer into a mono tape...
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Old 10th Feb 2021, 3:24 pm   #16
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Default Re: Vintage tube mixer input impedance

Hey Guys

The mixer has arrived and I though would share some interesting details I found with it. I did not try or plugged this in in any way just yet.

I have noticed that that someone might have been worked on this since this was built at the first place as I found two diodes at the PSU which I don't know why is there? I have a 6x4 rectifier already + two diodes? I haven't seen this before ( not that means anything with my little knowledge )

Also, there is something .... please see the photos... when I saw it i was like..."what the hell is that?!" honestly i dont know...some knd of old capacitor? and old burnt out resistor? its like a little fat bee...

Please help me out here...
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Old 10th Feb 2021, 3:44 pm   #17
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Default Re: Vintage tube mixer input impedance

So far as I can make out from the photo, the two diodes are connected as a full wave rectifier, basically across the valve rectifiers socket.

Lawrence.
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Old 10th Feb 2021, 3:48 pm   #18
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So far as I can make out from the photo, the two diodes are connected as a full wave rectifier, basically across the valve rectifiers socket.

Lawrence.
Thanks Lawrence... and what does this do on the top of the 6x4? as that is also a full wave one...? jus trying to understand why are these there...
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Old 10th Feb 2021, 4:02 pm   #19
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Thanks Lawrence... and what does this do on the top of the 6x4? as that is also a full wave one...? jus trying to understand why are these there...
It's hard to say without better photo's showing what's what for sure.

Lawrence.
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Old 10th Feb 2021, 4:12 pm   #20
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Thanks Lawrence... and what does this do on the top of the 6x4? as that is also a full wave one...? jus trying to understand why are these there...
It's hard to say without better photo's showing what's what for sure.

Lawrence.
Of course... i understand..it is very difficult to take good pic's from there can not even go there with my phone to take a sharp..this mixer is so little and it is fully packed so far I made out the power transformer and what is written on it is 4710 2 STC 47.

So odd to see the phone jack output is right next to the PSU... wonder how much AC hum this will pick up... thanks for your help anyways
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