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Old 9th November 2013, 09:54 PM   #1
AC439 is offline AC439  United States
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Smile XLR input coupling cap replacement ?

I've been heavily modding a Crate Taxi 15Watts acoustic amp and come to the final stages:-

I'd like to change out the non-polar electrolytic coupling caps at the XLR input. They are 10uF and I can't find MKT or MKP that big. I know the XLR is a 600 ohms low impedance circuit and therefore the coupling caps has to be bigger to maintain a reasonable low frequency roll off. Should I parallel a few MKT caps to make up the value ? What is the smallest value you will go for XLR input cap ?

I did search and found not a lot of related info about XLR inputs. Thanks in advance.

-AC
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Old 10th November 2013, 12:00 AM   #2
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PP (Polypropylene) caps are easily available in 10uF, and larger.
Just search Ebay - "polypropylene 10uF".

Dan.
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Old 10th November 2013, 02:13 AM   #3
AC439 is offline AC439  United States
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Thanks. I didn't realized they are avail in such a big value. I just went to parts-express and they do have 10uf PP type. The only thing I don't like about is their shipping + handling charge. I will keep looking.
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Old 15th November 2013, 03:05 AM   #4
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Just asking, what would happen if you were to use lets say a 1 uf cap? Would the low end roll off be that exaggerated?
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Old 15th November 2013, 09:26 AM   #5
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Depends on the impedance of the preceding and following stages....try it, it might be fine.
Sometimes it can be useful to roll out the low bass.

Dan.
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Old 15th November 2013, 09:52 AM   #6
AndrewT is offline AndrewT  Scotland
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Most 600ohm specified gear is able to drive a load of 600r.
But few Receivers use this 600r as their input impedance.
W.Jung shows that better rejection of interference is available by using very high input impedance at the receiver end of the balanced impedance connection.
From memory he talks of 1M5

Expect a few kohms to many kohms as a typical Zin of balanced receivers.
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Old 15th November 2013, 11:34 AM   #7
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Quote:
Originally Posted by AndrewT View Post
Most 600ohm specified gear is able to drive a load of 600r.
But few Receivers use this 600r as their input impedance.
Because that would be BAD - matching impedances provides maximum POWER transfer, and isn't something you want. You're looking for maximum voltage transfer (matching impedances halves the voltage!) and high quality - matching impedances would load the microphone excessively, drastically reducing performance and quality.
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Old 15th November 2013, 11:58 AM   #8
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Quote:
Originally Posted by AC439 View Post
I'd like to change out the non-polar electrolytic coupling caps at the XLR input. They are 10uF and I can't find MKT or MKP that big. .......... I know the XLR is a 600 ohms low impedance circuit....-AC
amp - 01.jpg

amp - 02.jpg
Data says 20k ohm.
What are you trying to do ?.

Dan.
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Old 15th November 2013, 04:14 PM   #9
AndrewT is offline AndrewT  Scotland
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Nigel,
many 600ohm balanced impedance transmitters have a low output impedance of well below 100r and some are down around 5r.

That leaves the voltage arriving at a 600r input impedance virtually at the maximum level. Certainly not aimed at maximum power transfer.
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Old 15th November 2013, 09:14 PM   #10
AC439 is offline AC439  United States
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[QUOTE=Max Headroom;3704605Data says 20k ohm.
What are you trying to do ?.

Dan.[/QUOTE]

It is interesting to watch the directions the responses are getting into......

Well, I do have the schematic of the amp. What I'd to do is to not having any electrolytic cap in the signal path, in which the last electrolytic caps I want to change out are those at the XLR input of the amp. I will also build a 48v phantom power to inject voltage at the XLR input for my condenser mic to eliminate a preamp that I now put in between the condenser mic and the Taxi amp. Pin 2,3 of XLR feed to the + and - inputs of the TL074 op-amp via a pair of 10uF non-polarized electrolytic caps.

If the input impedance is 20k and the output impedance of the condenser mic is standard 600R (XLR), then I think a lower value input cap should be sufficient. But I read many XLR amplifier schematics and they are all using bigger input caps such as 10uF (some in the range of 20uF) therefore I think there is a reason (my guess is probably to preserve low freq roll off). Other coupling caps in the Taxi amp are of 1uF only.

I may just have to do some trial and error and see how low the value I can get by.....
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