Reslo SR1/L Ribbon Mic Refurbishment

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rodabod

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May 12, 2005
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2,896
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Hi,

I thought I would post pics of these two Reslo studio ribbon microphones which I am "refurbishing" - ie. ribboning and repairing.

The mics are the rare SR1/L models. Similar to the RBT models, these models use a similar construction except they have much longer ribbons, better wiring method (lower noise) and I think they have better transformers. They supposedly have a frequency response of 30Hz to 20KHz - deviation in dB not given though.

Anyway, I will hopefully post pictures of the different stages of repair, in case it interests anyone.

Currently, the microphones have had bad water damage. I have stripped them and removed most of the corrosion. I have also glued the magnets back in place and fixed a snapped ribbon frame.

Roddy

reslosr16gg.jpg
 
Ok,

I thought I'd re-ribbon one of them tonight which I have now done. I accidentally snapped my first attempt by ripping it with a screw driver when tightening up the clamp screws. I swore a lot.

Here is a photo:

reslosr1ribbonelement0li.jpg
 
I personally use artist's aluminium leaf. I get it from an art supplies store.

There are a few threads here about ribbon foil and quite a lot of debate.

Some people use 2 or 3 micron foil. My leaf is around 1 micron. STC used to us 1 micron foil.

In my Reslos at least, I think it sounds better. Since it is lighter, you should achieve better transient response. It also increases the output impedance of the mic (30-50 Ohms) to a higher level since thinner foil has higher resistance. It's a bit tricky to work with though.

I've just got the other one done too. No breakages this time! Minimal vulgar language!
 
Well, as far as I know, the transformer reflects impedance across from what is connected to the primary to the secondary.

So, if you have a 1:10 transformer, you will get a 10 squared impedance ratio.

So, if you connected a 1 Ohm ribbon to the primary, it would be seen as 100 Ohms (1 X 100) at the secondary.

Then using the same theory, if you were to connect a 2 Ohm ribbon at the primary, this would be seen as 200 Ohms (2 X 100) at the secondary.

Can anyone else confirm that this is correct?
 
Ok, I see what you are saying here.

Impedance is a result of resistance, plus resistance caused by capacitive and inductive elements at different frequencies.

But, the ribbon is simply a piece of aluminium foil. Does its AC impedance not roughly equal its DC resistance since it has very little capacitance or induction?

The ribbon does have mechanical resistance at different frequencies (like a loudspeaker) , but I don't think this is taken into account when looking at transformers.
 
But, the ribbon is simply a piece of aluminium foil. Does its AC impedance not roughly equal its DC resistance since it has very little capacitance or induction?

ever stuck a meter on an 8 ohm speaker or the 600 ohm winding on an audio transformer ?

Or maybe the best example a piece of 75 ohm co ax cable. In this case the core & screen are open circuit so there`s no way you`ll measure 75 ohms !!
 
[quote author="Rob Flinn"]
ever stuck a meter on an 8 ohm speaker or the 600 ohm winding on an audio transformer ?
[/quote]

Yes, I see what you are saying here - speakers and transformers have complex impedances which vary (sometimes widely) from their resistance value. This is due to complex components being involved which have different reactions at different frequencies.

But, my point is, does the resistance (impedance) of a strip of aluminium foil vary greatly at different AC frequencies. I'm thinking it will stay fairly constant.

I'm not familiar with 75 Ohm cable - all I know is that it is used for TV and high frequency signals to prevent electrical reflections. Is there anything interesting about it? I thought it was just rated at 75 Ohms per metre length.
 
Just had a look at 75 Ohm co-ax out of interest. It is not rated per length at all. Just an assumption I must have made.
 
Just had a look at 75 Ohm co-ax out of interest. It is not rated per length at all. Just an assumption I must have made.

That`s because its impedance is only there when an a.c signal is being sent & to a degree the length is unrelated to its Z.

A d.c meter reading will measure open circuit whatever the length of the 75 ohm cable, because it is `an open circuit`

An impedance is made up from reactive & resistive properties. This is kind of difficult to explain without me typing in a whole chapter but comes under the banner of "complex Waveforms" & "a.c theory". I`m no expert in either of these, & it may be easier to understand if someone else can explain them to you.

In your mic you have both inductive & capacitive elements to consider & not just the resistance of the foil. The way the inductive & capacitive elements effect your impedance will change depending on the signal frequency. This means that the Z of the mic will change with frequency.

It may be that in the case of you mic most of the z is due to the resistive element. I don`t want to speculate there, but I think I would be suprised if that were the case.
 
[quote author="Rob Flinn"]
An impedance is made up from reactive & resistive properties. This is kind of difficult to explain without me typing in a whole chapter but comes under the banner of "complex Waveforms" & "a.c theory". I`m no expert in either of these, & it may be easier to understand if someone else can explain them to you.

In your mic you have both inductive & capacitive elements to consider & not just the resistance of the foil.[/quote]

Ok, I undertsand impedance - I studied AC circuits for a year.

But, I can't see where there would be any significant reactive components as a result of just having a strip of aluminium suspended between two magnets.

If the impedance of the ribbon varied with frequency then would this not have a significant effect on frequency response (since at higher impedances, there would be a larger voltage drop accross the ribbon)?
 
But, I can't see where there would be any significant reactive components as a result of just having a strip of aluminium suspended between two magnets.

The ribbon suspended between the magnets is definately a reactive element, since it is like a half turn coil & is a basic generator. Two pieces of wire an inch long twisted together have a capacitance of a few pf`s & in that mic you have that scenario also. Since the d.c resistance of the foil is very low these reactances may well be significant.

I think we both know what principles are involved. Maybe we need someone with a bit more knowledge to step in & clarify the situation.

I must say I`ve been looking for one of these mics on the cheap for a while, without success, since as you say they are supposed to be good.
 
Ribbon is basically a half turn coil, so its inductance and capacitance are small. On LF the mechanical impedance becomes of a great influence and ribbon impedance increases, while it is constant and resistive on HF. It is widespread taking the ribbon impedance as its DCR.
 
The DC resistance of a ribbon is typically between 0.1 Ohms and 2 Ohms.

It depends on the the thickness of the ribbon, the length and the width.

The DC resistance gets reflected onto the secondary which is why you need to have a suitable turns ratio on the transformer.
 

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