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Old 20th April 2017, 06:45 PM   #1
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Default Capable Electronics Advice required please

I have a balanced valve line preamp which I am working on.
Just before the output sockets (XLR) there are 3 output capacitors in parallel (4uF, with 0.68uF and 0.01uF as bypass caps), followed by a 200ohm 2 watt metal film resistor in series, leading to each balanced leg of each XLR socket.
So there are 4 sets of these in total.
I was considering replacing these 2w resistors with some extremely high quality Charcroft ones, but now I know that they are 2 watt ones, I am concerned because my replacements are only 0.4w.
Surely the output from a preamp won't be more than 0.4watt, will it?
Desperately seeking advice on this, please.

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Old 20th April 2017, 06:59 PM   #2
Mooly is offline Mooly  United Kingdom
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I think the worst case scenario here is when the mute relay is active and dumping the output to ground. You should perhaps look at what level of signal voltage the amp can develop across the 200 ohm when the relay is active.

In normal use there will be very little across the 200 ohm but you have to look at all possibilities.
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Old 20th April 2017, 08:05 PM   #3
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Thanks, Mooly. That's really helpful advice.

I did wonder whether the original design used 2 watt resistors in this location so that under working condition, the resistors would be extremely stable compared to a smaller one which might forever be heating up and cooling down under the stress of the signal.

The signal from a preamp tends to be nominally between 1.77 and 2v but I understand it can peak at up to 7v on full output.
Does this info help you give me any further advice about whether or not a 0.4watt resistor could cope?

The last thing I want to do is replace these 4 resistors and find the amp blows straight away.:-)
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Old 20th April 2017, 08:08 PM   #4
DF96 is offline DF96  England
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Unless they are carbon composition or faulty there is no need to replace them. They are simply there to aid stability of the cathode follower, in case a capacitive load is applied.
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Old 20th April 2017, 08:11 PM   #5
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Hi DF96. Thank you for your comment.
Do you mean they're actually unnecessary and can be replaced with a wire link?
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Old 20th April 2017, 08:42 PM   #6
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And..DF96,please....
When you say a 'capacitive load', how much capacity are we thinking of? (I have two 8 meter balanced XLR cables going direct to an active crossover, before going to 3 power amps....)
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Old 20th April 2017, 09:06 PM   #7
DF96 is offline DF96  England
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No, don't remove them. They do a useful job. I said don't replace them, unless they are faulty or poor quality (e.g. carbon composition). Actually, in that position with no DC current through them even a CC resistor will do little harm. Why do you want to replace them?

What sort of capacitive load might create instability depends on the details of the stray capacitances and inductances in the cathode follower circuit, including inside the valve. A longish cable might be enough to cause trouble, but difficult to say. This is why the resistors are there. They prevent potential trouble, without having to work out in detail what might create that trouble.
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Old 20th April 2017, 09:11 PM   #8
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I'm trying to replace some of the caps and resistors which are directly in the signal path. I hace some better quality polyproplene caps to replace the output caps, for example.
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Old 20th April 2017, 09:18 PM   #9
DF96 is offline DF96  England
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I suspected that might be the reason. In most circuits almost all components are 'in the signal path', yet almost none of them need to be anything other than normal competently-made commercial quality components.

As I often say, in order to improve a circuit you first need to understand it even better than the original designer. In this particular case, how can you improve a circuit by replacing resistors whose circuit function you were unaware of until a few hours ago?

What is wrong with the output caps, by the way - apart from the unnecessary 'bypassing' which may introduce HF phase shifts.
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Old 20th April 2017, 09:24 PM   #10
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I am sure you're right, and I apologise for my inexperience! That's why I am looking for advice like you're kindly giving! Was initially replacing the output caps as they are around 20 years old....
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