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Old 8th October 2006, 08:08 PM   #1
phn is offline phn  Sweden
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Default Coupling cap questions

Let's say we have this preamp. At the output we have this low-pass type network, a cap and a grounded resistor. (From what I can tell, this isn't always present in ss gear.) The cap, as I understand it, is there to block DC.

In a phono preamp, having a low-pass filter is not a bad thing. The only sub 20Hz information the cart will pick up is rumble. But the problem with the output coupling cap and grounded resistor is that you usually need relatively high values. If I'm to use a coupling cap, I want to use something like a silver mica.

I'm trying to understand why you "have" to have this kind of network. Ditching the resistor eliminates the roll-off. And placing the cap after the resistor can still produce the, in phono stages, welcome roll-off while keeping the cap value in the picas. So what is going on here?

Could somebody briefly explain the reason/background for this network? As well as explaining why ditching the resistor (using only a DC block cap) or placing the cap after the resistor would be a bad idea.

While we're at it, what is the main cause or source for DC? Is any technology (tubes, ss) more susceptible? I know DC is a reality in SE tube amps.
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Old 8th October 2006, 08:33 PM   #2
lineup is offline lineup  Sweden
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hi

Anything you add to an amplifier output has got an impedance.
This is like a resistance seen and it is frequency dependent.
A loudspeaker can have lower impedance at 200 hertz and high impedance at 50 hertz.
The nominal impedance for a load is most often given for 1 kHz.

This is the resistance for AC signals,
and can be different than for only DC current.

8 ohm loudspeaker has got ~7-8 ohm impedance.

A power amplifier input, attached to preamplifier output
has got an input impedance, often like 22k - 47k.

Headphones can have 16-32 or 300 or even 600 ohm impedance.

So an output cap has got some impedance to work into,
depending on what is attached to the output.


Sure there are many more topics in the past, that try to explain this.
If we can search and find them
I tell a little bit about formula to use to get different rolloff frequency in this post:
http://www.diyaudio.com/forums/showt...40#post1024640

I am sure somebody will fill in with more and better information here.

Regards
lineup
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Old 8th October 2006, 09:24 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
the in line cap followed by the grounding resistor is a high pass filter.
The DC and very low frequencies are blocked.
The input impedance of the next stage must be paralleled with the grounding resistor to find the "load" that the cap forms a filter with.
There may also be a further DC blocking cap at the input of the next unit (receiver).

Let's suppose you have a 1uF blocking cap and a 1M0 resistor on the output of the source unit.
Now the receiver has a DC coupled input with a 22k ohm impedance setting resistor to ground.

The 1uF sees the 22k//1M0 which is equivalent to 21.526k, the 1M0 has very little effect.
The -3db frequency of the high pass filter is 1/2 Pi R C=1/(2*3.14159*22526*1*10^-6)=7.1Hz The effect of this filter is audible up to about 70Hz.
You then have a choice to raise the RC time constant. Either increase the capacitor or increase the input impedance. That's why you often find 50K used at the front end of receivers.

If the receiver also has a DC blocking capacitor then the two caps are in series and the effect of both need to be checked.
Lets make the arithmetic easy and say the receiver also has a 1uF cap. The effective capacitance is 0.5uF and the -3db frequency is now up to 14.2Hz and it's effect now extends up to 140Hz. Not much low bass left in the signal now.
2.2uF for both source and receiver moves you in the right direction, but I would fit a good quality 10uF polypropylene at the output of the pre-amp and hope that the power-amp has at least a sensible cap to match the chosen input impedance. I aim for F(-3) of between 1Hz and 2Hz (RC=80 to 90 mS).
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Old 25th October 2006, 10:32 PM   #4
phn is offline phn  Sweden
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Thanks guys. I had to rethink before posting, and then time flow by.

Maybe I'm looking for a simple answer when there is none. It still bothers me. I mean, say I put in all this effort making a direct-coupled tube whatever doing everything to avoid caps and then, wham, toss it out by adding an output cap.

Maybe chips are different, having low capacitance. This active crossover, for example, has no output caps.

And HIGH pass. Doh!
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Old 25th October 2006, 10:57 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
but it has inherent DC offset on the outputs and these offsets will vary with temperature, whether caused by ambient conditions, heat generated within the equipment or by internal heat generated within the ICs.
The high pass filters have DC blocking for offsets coming in from previous stages, but the low pass filters aggregate all the offsets as the signal passes towards the final output.

BTW. that crossover had better have socketed IC's because the standard tl07Xs are pretty poor at driving reactive loads, even small capacitances on the outputs can upset them.
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Old 30th October 2006, 07:38 PM   #6
phn is offline phn  Sweden
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I will have to regroup on this issue. But I have a related question.

I got some 600:600Ohm matching transformers for my PlayStation as CD player. I had already modified the output on the PSX, now consisting of a coupling cap and grounded resistor. The idea was to replace the caps and resistors with a transformers.

I made one crucial mistake. I never tested the transformers before installing them, being sure I had got everything right. It didn't go as planned. The sound was very low with lots of hum. Since I never tested the transformers, if I got the leads right and things like that, I didn't know what was wrong. It could be anything. So I had to remove them. I later hooked RCA connectors to the transformer leads and connected them between the PSX and amp. Works perfect. From that I can conclude that I had connected them right in the first place.

My problem now is that I don't know what went wrong. And I don't want to do it all over again without making sure it will work.

So my question is, shouldn't it be possible to replace the output cap and resistor with a transformer?
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Old 30th October 2006, 11:08 PM   #7
Pano is offline Pano  United States
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Quote:
Originally posted by phn
So my question is, shouldn't it be possible to replace the output cap and resistor with a transformer?
Depends on the transformer, the circuit and how you hook it up.

You probably have DC between the point where you connected the transformer and ground, or wherever you connected the other side of the transformer primary. Not good for the transformer or the circuit that proceeds it.

That DC blocking cap keeps the DC where it belongs, out of the following stages - like the amp or the transformer primary.

Got a schematic of the PS1 output?
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Old 31st October 2006, 04:34 PM   #8
phn is offline phn  Sweden
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In short, the original out put (caps and muting resistors) is replaced with a DC block cap and grounded resistor.

http://dogbreath.de/PS1/output/output.html
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Old 19th November 2006, 06:06 PM   #9
phn is offline phn  Sweden
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Quote:
Originally posted by panomaniac

You probably have DC between the point where you connected the transformer and ground, or wherever you connected the other side of the transformer primary. Not good for the transformer or the circuit that proceeds it.

That DC blocking cap keeps the DC where it belongs, out of the following stages - like the amp or the transformer primary.
I'm back.

I really know nothing about the transformers other than they are "standard" (whatever that means) pro line/balancing transformers made by Haufe.

Anyway, the transformer output works perfect when I use coupling caps. But when I remove the caps, the sound gets very low with hiss as loud as the music.

This is apparently caused by DC. Is there a way to solve this? Or am I forced to use coupling caps?
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Old 19th November 2006, 06:54 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
are you using transformers for a fully balanced interconnect?
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