Dear Sajti,
Thanks, I am taking advice for a location of the buffer, ahead or behind the volume control, your opinion?, some one suggest front and others suggest put it behind the volume.
😕
Yes it may not be necessary. lots people object the idea of adding buffer, all have reasons.🙁😀
you did not read my answer.
A Buffer is required where the Source cannot adequately drive the cable and any capacitance that follows.
The Buffer, if it is required, goes on the OUTPUT of the source.
This is because the Source cannot drive the cable, it needs the Buffer to drive the cable.
if the output from the volume control comes out of the box, you need a buffer to drive the output cable. be sure to add a 330 to 680 ohm series resistor after the op amp to prevent oscillation.
you can scope the response to the amp by inserting a 20khz tone ( iphone app) into an input measuring the output at the end of the cable at the amp. Worst response will be at the 50% of V in point
you can scope the response to the amp by inserting a 20khz tone ( iphone app) into an input measuring the output at the end of the cable at the amp. Worst response will be at the 50% of V in point
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I feel like to put a buffer ahead of the volume pot (20k) to avoid high impedance from the previous device if it happens. CD, DVD or DAC might already has a output buffer and no need of this additional buffer. The volume actual used may be at 5-10 k, if the following pre-amp is a op with high input impedance (FET input), and we use a Rin 100k, with a series R and a parallel C of 100pf to reduce noise and limit the OP impedance, this 100k Rin meet the rule of ratio of 10 to 1, even 20 to 1, so there is no need to put a buffer after the volume control. This is what I learned in this thread, thanks everyone, if I am not right, pls correct me. I think add buffer or not is really circumstantial.
It's the old passive preamp argument. I don't buffer anything and am quite happy, but none of my cables are long. Self brings up an interesting point in his signal book. We now have very good opamps capable of driving low impedances. You can avoid the noise penalty of 20-100 kohm volume pots by driving a much lower value with a suitable opamp. You can make a good case for a 1 kohm volume pot or step attenuator, resulting in lower noise than can be had with the traditional arrangement.
Don't, before the Receiver is the wrong place for a Buffer.
This is quite likely the correct conclusion. If true then your Source probably does not need a Buffer.
The logic in this last half is completely mixed up.
Analyse your Vol Pot output impedance.
Can it adequately source current to feed the cable and capacitance that follow?
Probably not if it's approaching 5m.
If the cable is very short, say less than 1m then a 20k vol pot with a maximum output impedance of ~5k will not suffer treble roll-off.
But between 1m and 5m you may have an audio loss when the capacitance is high.
insert buffer on a pre-amp
Andrew,
So what should I do, to avoid this audio loss? if a buffer is not likely be put in front of the 20k pot, simply put a buffer after the volume control; the 20 k ohm pot ?😕
Andrew,
So what should I do, to avoid this audio loss? if a buffer is not likely be put in front of the 20k pot, simply put a buffer after the volume control; the 20 k ohm pot ?😕
Have a look at PRRs post #7, it will give you an idea of the high frequency loss due to cable capacitance. If you decide you need a buffer put it after the volume pot, its purpose is to drive the cable
the volume control is only few cm away from the pre-amp input, no need to drive any long wire, so I gave up the idea of driving any long cable. Since the buffer should be installed at the source end to drive the cable btw source and pre-amp. So it is the job of previous instrument (CD, DVD or other AUX, tape etc.). The benefit of put a buffer behind the volume control is also diminished, because you guys told me in many times, the 20k ohm, 10k ohm is OK if the Rin of the pre-amp to be 57k ohm or 100 k ohm, to fit the 10: 1 ratio, even 5 : 1 ratio for modern OP, so there is no need for a buffer there. Am I correct this time? I apologize to being so slow.
New question will be the buffer function. We all know its mission. If I put a two stage amplification, say stage one 6X, stage two 1.5 X to make it 9 times in total, will the stage two forms a buffer in function? If so, no extra buffer for output of the pre-amp is needed. Next box is the active crossover. I see circuits put a buffer before the active crossover (low pass +high pass). I think it is the wrong place to put the buffer, if cable capacitor rises problem. Or if I am wrong again, simplyto say, the buffer after the cable can cure the problem not before the cable.
Yes
If it has a very low output impedance (which it probably has) it will effectively act as a buffer
The buffer here serves two connected purposes, the designer doesn't know what is driving the circuit and so the high input impedance allows a wide variety of drive capabilities, it also provides a low output impedance for the following filter network so it's performance is independent of the source and constant
Sometimes low output impedance is needed not only for driving capacitive (cables load).
It is other important question, the power amplifiers input stage.
For example, If Your amplifier is BJT amplifying input stage without any emitters degeneration resistors, the input impedance could be quite low. For example if beta for input transistor is 100, and collector current is about 1mA, then input impedance is only 2.5kOhm. The problem is not quite low input impedance, but that this impedance is not linear. If current is varying by the signal, then input impedance is varying non linearly, it could drop and rise depending on the signal half-wave. And it could produce distortion directly at the non inverting input terminal - that will not be compensated by Negative Feedback, if You driving it from high impedance source. 5 kOhms (20 k Volume control at the middle) would produce not acceptable distortion in this case IMHO.
By contrast, for example Gainclone type amplifiers has emitter followers before highly degenerated amplifying diff.pair stage like "LM3875" and will not produce distortion at non inverting input even if source impedance is 5kOhms or a bit higher.
So if You know that Your amplifier is BJT non inverting, without input buffers, then I would recommend to have one linear buffer j-Fet, or other High impedance type After Volume control.
It is other important question, the power amplifiers input stage.
For example, If Your amplifier is BJT amplifying input stage without any emitters degeneration resistors, the input impedance could be quite low. For example if beta for input transistor is 100, and collector current is about 1mA, then input impedance is only 2.5kOhm. The problem is not quite low input impedance, but that this impedance is not linear. If current is varying by the signal, then input impedance is varying non linearly, it could drop and rise depending on the signal half-wave. And it could produce distortion directly at the non inverting input terminal - that will not be compensated by Negative Feedback, if You driving it from high impedance source. 5 kOhms (20 k Volume control at the middle) would produce not acceptable distortion in this case IMHO.
By contrast, for example Gainclone type amplifiers has emitter followers before highly degenerated amplifying diff.pair stage like "LM3875" and will not produce distortion at non inverting input even if source impedance is 5kOhms or a bit higher.
So if You know that Your amplifier is BJT non inverting, without input buffers, then I would recommend to have one linear buffer j-Fet, or other High impedance type After Volume control.
Perhaps a little calculation of the current noise density of this preamp will help. IMO a 10k Pot will be "drivable" for the most modern audio sources while avoiding noise problems for the amp.
yes, for active crossover deal, I have following power amp that is FET input, and another amp is bjt input. So, at the out put stage of the crossover， I will give a gain adjustment and a low gain, 2 OP to put out 100 ohm impedance, good enough to drive the bjt amp. No gain 1 buffer, but close.
100ohm output impedance should be able to drive long cables.
Do you know what the maximum output current is?
You can use that to check how much capacitance you can drive at low RF frequencies.
You don't want current clipping when an RF transient passes through.
Do you know what the maximum output current is?
You can use that to check how much capacitance you can drive at low RF frequencies.
You don't want current clipping when an RF transient passes through.
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