Universal Audio 550-A Passive Filter

[MNJay]

Hello everyone. This is my first post so I apologize in advance for any n00b errors.

I recently found a Universal Audio 550-A filter and I have some questions about how to use it. Currently, I have line level from my mixer into some XLR pigtails wired to the back. Then I come out from the filter to the same mixer and use the preamp gain to drive it. So far I really love how it sounds but I also feel confident this is not the best way to interface with the filter. I definitely want to explore different preamps for make-up gain but my main question is - what do I need to know about the 600 ohm operation?

According to this UA catalog from 1967 this is a 600 ohm impedance device. I also saw this Reverb listing (I can't believe I missed out on this sale!!) which has a "termination resistor" wired to the in/out. Should I do something like this?

This is my first passive piece of equipment like this so I really have no idea what I'm doing ha!

 

[ruffrecords]

The UA-550 is a 600 ohm device. This means it expects to see a source impedance of 600 ohms and it also expects its output to be loaded with 600 ohms. So if you drive it from a modern low output impedance source you will need to add a build out resistor in series with the input to raise the source impedance to 600 ohms. Also, if you feed it into a modern 10K bridging input then you will need to add a terminating resistor across the output to make the load 600 ohms.

Cheers

Ian

 

[MNJay]

Thank you so much for your reply! Do you know if that’s what’s been done in the pics provided in the reverb listing? It looks like a resistor was attached to the in/out terminals in the back…could it be that simple?

 

[JMan]

Do you know if that’s what’s been done in the pics provided in the reverb listing? It looks like a resistor was attached to the in/out terminals in the back…could it be that simple?

Yes, a terminating resistor is as simple as it appears. Note that it does NOT go between input and output terminals, but rather across the hot and cold of the output. That is indeed what is shown in the reverb listing -- a 620 ohm (if my eyes aren't deceiving me; I struggle when reading the color codes!) resistor across the output.

 

[MNJay]

Yes, a terminating resistor is as simple as it appears. Note that it does NOT go between input and output terminals, but rather across the hot and cold of the output. That is indeed what is shown in the reverb listing -- a 620 ohm (if my eyes aren't deceiving me; I struggle when reading the color codes!) resistor across the output.

Thank you so much. I really appreciate the assistance with such a novice inquiry. Thank you for correcting me about the hot and cold, I’m not sure how I mixed that up.

Is it the case that I should put a resistor across the hot and cold of the output and another across hot and cold of the input if I wish to drive the filter with a modern line level?

I’m quite interested in experimenting with other passive devices from the same era as this filter so this will all come in handy to me.

 

[JMan]

I would say "no" to the resistor across the input.

EDIT: I had typed out more originally, but I have elected to delete it as I don't think I was contributing accurate, well-founded information so much as I was just giving speculation, and I prefer not to muddy the waters lest anyone mistake me for someone who knows something.

 

[scott2000]

It looks like a resistor was attached to the in/out terminals in the back

It's only across the outputs of the pair afaik...

The UA-550 is a 600 ohm device. This means it expects to see a source impedance of 600 ohms and it also expects its output to be loaded with 600 ohms.

Guess it kinda matters with the filter response which makes sense..
http://alteclansingunofficial.nlenet.net/publications/techletters/TL_192.pdf

 

[MNJay]

It's only across the outputs of the pair afaik...


Guess it kinda matters with the filter response which makes sense..
http://alteclansingunofficial.nlenet.net/publications/techletters/TL_192.pdf

This was a great read, thank you. It does seem that the filter will respond as it's meant to only with matched input and output impedance.

Could there be any harm in attaching a resistor to the input as well?

 

[dmp]

Could there be any harm in attaching a resistor to the input as well?

It could only degrade performance I think, depending on what is driving it. You say you are driving it with your mixer. Some mixers will have no problem driving a 600 ohm load, some may have problems.

if you drive it from a modern low output impedance source you will need to add a build out resistor in series

Note, Ruffrecords is saying a resistor in series with the input. A resistor across the +/- is a resistor in parallel. I would suggest not worrying about it as it will probably work fine just plugged in.

If you really want to pursue this, set the filters to 'out', and measure the frequency response of the unit. It should be flat with the settings at 'out'. Then engage the settings and measure the response.

 

[thomasdf]

Can you share picture and schematics of the unit ? That would be awesome and it appears to be pretty rare.

 

[dmp]

The pics from the reverb listing are pretty good (one attached here). It looks like a passive LC high and low pass. You can read some of the caps and even read the inductor. Not much in common with the API

 

[MNJay]

It could only degrade performance I think, depending on what is driving it. You say you are driving it with your mixer. Some mixers will have no problem driving a 600 ohm load, some may have problems.



Note, Ruffrecords is saying a resistor in series with the input. A resistor across the +/- is a resistor in parallel. I would suggest not worrying about it as it will probably work fine just plugged in.

If you really want to pursue this, set the filters to 'out', and measure the frequency response of the unit. It should be flat with the settings at 'out'. Then engage the settings and measure the response.

Thanks! I have found that simply plugging it in sounds quite good but bc this is from the 1950s (i think? I have serial no. 4) I just want to make sure I'm not missing any steps here.

And yeah, the pics from the reverb listing are good but if anyone wants different views lmk and I can post them.

Thanks everyone for all these helpful replies, I'm really impressed with how quickly people have joined in to help me out!

 

[ruffrecords]

Is it the case that I should put a resistor across the hot and cold of the output and another across hot and cold of the input if I wish to drive the filter with a modern line level?

You need to put a terminating resistor between the hot and the cold of the output (typically 620 ohms is close enough). You might also need to add a pair of resistors in series with each of the hot and cold terminal of the input to make the source impedance look like 600 ohms. The value you need depends on the source impedance of whatever is driving the input. If the driving source has an output impedance of 100 ohms then you need to add an extra 500 ohms in series so you could at a 240 ohm resistor in series with the hot input and another 240 ohms in series with the cold input.

Cheers

ian

 

[thomasdf]

And yeah, the pics from the reverb listing are good but if anyone wants different views lmk and I can post them.

Thanks ! I went through the reverb pics and they are quite interesting indeed, but I'd like to get a clear picture of the capacitors value, and a confirmation of the value of the small inductor, and value of the big one Yes I am that lazy ! haha
The unit looks unbalanced despite the connectors? Or am I missing something ?

I am quite interested in passive filters lately, would love to build a pair of clones of this one.

Thanks a lot , and cheers

Thomas

 

[BertBert37]

Classical filters of the reactive type (LC - consisting of capacitors and inductors only) act by presenting a mismatch to the source for the stopband. There are 2 different possibilities: Short circuit and open circuit. Although there is no difference in a perfectly linear system - in practice most amplifier outputs prefer the open circuit type unless the input impedance is given by a resistor anyway. From this point of view, it is strange why the designers have used a pi-type layout.

 

[MNJay]

Thanks ! I went through the reverb pics and they are quite interesting indeed, but I'd like to get a clear picture of the capacitors value, and a confirmation of the value of the small inductor, and value of the big one Yes I am that lazy ! haha
The unit looks unbalanced despite the connectors? Or am I missing something ?

I am quite interested in passive filters lately, would love to build a pair of clones of this one.

Thanks a lot , and cheers

Thomas

Are these helpful? I couldn't make out any values on the inductors except a few figures on the big one.

 

[MNJay]

Also, I'm sorry to keep asking such baby questions but does anyone have a suggestion about how I can set up a resistor across the output that is removable? From what I've been reading and from a few pics I have found it appears that when people are strapping the resistor entails soldering the resistor directly to the connector.

I'm asking because I can imagine scenarios where I might use this filter in conjunction with other 600 ohm devices and I think in that case the resistor on the output would be inappropriate.

 

[JMan]

Also, I'm sorry to keep asking such baby questions but does anyone have a suggestion about how I can set up a resistor across the output that is removable? From what I've been reading and from a few pics I have found it appears that when people are strapping the resistor entails soldering the resistor directly to the connector.

I'm asking because I can imagine scenarios where I might use this filter in conjunction with other 600 ohm devices and I think in that case the resistor on the output would be inappropriate.

If you look at the Reverb listing that you included in your first post, you'll see how they've done it on the terminal strip. Crimp, solder, or otherwise attach the leads of the resistor to fork connectors (cheap to buy from your local hardware store), insulate any exposed part of the leads to avoid shorting to chassis, and screw in place on the terminal strip. Easy!

 

[MNJay]

If you look at the Reverb listing that you included in your first post, you'll see how they've done it on the terminal strip. Crimp, solder, or otherwise attach the leads of the resistor to fork connectors (cheap to buy from your local hardware store), insulate any exposed part of the leads to avoid shorting to chassis, and screw in place on the terminal strip. Easy!

Ah thank you. I thought that's what I was seeing there but I could not quite make it out.

I was stressing myself out bc the closeup image looks like it is attached with fork connectors but then in the same listing there are different images which show different resistors that appear soldered. This made me paranoid I wasn't getting it!

 

[JMan]

Are these helpful? I couldn't make out any values on the inductors except a few figures on the big one.

Any chance you could take some more photos? These are great, and with detail of the wiring, switches, and connectors, I'm fairly certain we could reverse engineer this piece (the inductor tap values should, I believe, just be a matter of math once everything else is clear). There's also one capacitor that can't be read - third image, far left cap. Can you get a value off of that?