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.
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.
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.
In normal use there will be very little across the 200 ohm but you have to look at all possibilities.
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.
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.
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.
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.
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.
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.
On the other side of those 3 paralleled capacitors is a cathode follower, cathode 125V and a 20k to a j-fet to ground.If that fet is completely conduction (probably is, sitting there doing nothing) the current is 6mA and so is the max amplitude of the output via the 200Ω.Since P=RxI^2=200x0.006x0.006 Watt , well let's put 2W to be on the safe side.
I don't like Audio Research and there audiophile nonsens.
Mona
I don't like Audio Research and there audiophile nonsens.
Mona
Ketje,
I'll grant you your dislike of Arc gear, it would take extreme force to part me from mine, if you'll grant me my dislike of Swiss gear. I once worked for a company that was almost run into the ground by the Swiss. If our Canadian president hadn't come along when he did I'd have been asking, "Would you like fries with that sir?".
Cheers, Steve
I'll grant you your dislike of Arc gear, it would take extreme force to part me from mine, if you'll grant me my dislike of Swiss gear. I once worked for a company that was almost run into the ground by the Swiss. If our Canadian president hadn't come along when he did I'd have been asking, "Would you like fries with that sir?".
Cheers, Steve
Axeman,
I can understand your desire to replace some parts, I've been tempted at times too. The output caps are probably film types and would be the least of your problems in a 20 year old piece of gear. The power supply electrolytic caps will likely need attention first. If you really can't resist the temptation to replace the output caps I can vouch for Clarity Cap MR and CMR. They're big and pricey and if memory serves made in Blighty. Poetic huh? Just don't judge the performance of any new part until you've put about 100 hours on them.
Steve
Cheers, Steve
I can understand your desire to replace some parts, I've been tempted at times too. The output caps are probably film types and would be the least of your problems in a 20 year old piece of gear. The power supply electrolytic caps will likely need attention first. If you really can't resist the temptation to replace the output caps I can vouch for Clarity Cap MR and CMR. They're big and pricey and if memory serves made in Blighty. Poetic huh? Just don't judge the performance of any new part until you've put about 100 hours on them.
Steve
Cheers, Steve
Thank you, Steve, I was thinking of doing the Power SUpply caps. To be honest, although they're not around anymore, I've been reading a lot about GNSC (Great Northern Sound Company) and the Upgrade Company, and how they have modded gear in the past. I'm not a novice, I have installed better power caps and transformers etc in a number of different amps (Copland, PS Audio, Lehmann), with great success.
So any further advice you can give, I'd very much appreciate it. Cheers!
So any further advice you can give, I'd very much appreciate it. Cheers!
Two types of capacitors found in audio gear are likely to 'age':vintageaxeman said:
1. electrolytics, especially in the power supply - after 20 years of heavy use or maybe 50 years of occasional use they may need replacing
2. paper dielectric caps found in very old equipment and some 'high-end' equipment - after about 40 years they absorb moisture and become leaky
No other caps need changing unless they are found to be faulty. Plastic film caps will last more or less for ever.
From my days in the Sand Box rather than my days in the hollow state world, I know that the worst and most objectionable capacitor "warts" happen when the absolute voltage (signal + DC idle) across the cap reverses. With the cathodes of the 6DJ8 sitting at +125V that is NEVER.
Just use a decent 4uF/200V Polypropylene or Poly Phenylene Sulphide (PPS) Cap and don't bother about the parallel caps. (or 2 off 2uF caps in parallel if you cant find a 4u7 or 3u9)
Your $50 Audiophool Caps will not make any difference here. They ONLY makes a difference when there is no DC bias across the cap, that is, when the voltage across the cap reverses with the signal.
The 200 Ohm series resistors are essential for stability when driving long cables (high capacitance) to your power amp. A good "rule of thumb" thing to keep in mind is that when considering parasitic oscillations then anode loaded circuits are susceptible to inductive loads and cathode loaded circuits are susceptible to capacitive loads.
The series 200 Ohm resistors are 2W because at switch on and switch off you get very high peak currents, in this case it is the switch off which is a problem, 125V gets dumped across that 200 Ohms when the mute relay operates.
ASSUMING (check your assumptions at the door)
That the mute relay is NORMALLY (unpowered) ON. Do the math, the instantaneous peak power at the moment of relay switching is 78 Watts. Hence a 2W resistor AND it should be a carbon film rather than a metal film since carbon film have superior peak power handling, they hang in there when the metal films say F this and goes short or open.
Cheers,
Ian
P.S.
For the single cap (no parallel caps) even something like a Digikey standard stock part like this:
https://www.digikey.com/product-det...onic-components/ECW-F2395JA/P13751-ND/1635194
should be good.
Just use a decent 4uF/200V Polypropylene or Poly Phenylene Sulphide (PPS) Cap and don't bother about the parallel caps. (or 2 off 2uF caps in parallel if you cant find a 4u7 or 3u9)
Your $50 Audiophool Caps will not make any difference here. They ONLY makes a difference when there is no DC bias across the cap, that is, when the voltage across the cap reverses with the signal.
The 200 Ohm series resistors are essential for stability when driving long cables (high capacitance) to your power amp. A good "rule of thumb" thing to keep in mind is that when considering parasitic oscillations then anode loaded circuits are susceptible to inductive loads and cathode loaded circuits are susceptible to capacitive loads.
The series 200 Ohm resistors are 2W because at switch on and switch off you get very high peak currents, in this case it is the switch off which is a problem, 125V gets dumped across that 200 Ohms when the mute relay operates.
ASSUMING (check your assumptions at the door)
That the mute relay is NORMALLY (unpowered) ON. Do the math, the instantaneous peak power at the moment of relay switching is 78 Watts. Hence a 2W resistor AND it should be a carbon film rather than a metal film since carbon film have superior peak power handling, they hang in there when the metal films say F this and goes short or open.
Cheers,
Ian
P.S.
For the single cap (no parallel caps) even something like a Digikey standard stock part like this:
https://www.digikey.com/product-det...onic-components/ECW-F2395JA/P13751-ND/1635194
should be good.
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5uF at 125V stores 156mJ of energy. Looking at it another way, 5uF and 200R have a time constant of 1ms. I don't think that is going to harm a film resistor. Also, bear in mind that the short is being applied at a point which is already nominally at zero voltage so it is only as the voltages collapse that any current will flow; 125V is not dumped across the resistor. No need for CC.
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.
well one of the major reasons why the 200 ohm is over rated is so that there is dissipation headroom. Because ideally, you want the resistance of the resistor pair in the channel to be the same. Otherwise, the phase angle of the output will change with temperature of the resistors. If you replace these resistors, they should be replaced with 2-5W 100ppm type. I use Mills/vishay MRA series or vishay z-foil types on these. The caps in parallel probably isn't that big of a deal with your application. I've only seen a real use for it for instances where it needs to maintain a phase coherency, if this was like a mic pre output, you would add or subtract caps in parallel to remove comb filtering effects when using it along with different mic preamps in the studio.
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