I have been trying to verify whether I can eliminate the preamp and hook the AUX output of different DVD players, Direct TV box etc. through a stereo volume pot, then drive directly into the power amp.
1) I have been looking on the web, sounds like the output spec of standard AUX output of DVD/CD and other devices is +4DBv which is about 1.228Vrms. The input spec of power amp is the same thing. So the voltage do match up.
2) What is the output impedance of the output devices mentioned above? What is the input impedance of power amps?
3) The final question is what value of volume pot I should use?
I am thinking of getting rid of the preamp and get a video mux to route different video outputs to the TV and different audio outputs to the power amp. Do you think this will sound better?
Thanks
Alan
1) I have been looking on the web, sounds like the output spec of standard AUX output of DVD/CD and other devices is +4DBv which is about 1.228Vrms. The input spec of power amp is the same thing. So the voltage do match up.
2) What is the output impedance of the output devices mentioned above? What is the input impedance of power amps?
3) The final question is what value of volume pot I should use?
I am thinking of getting rid of the preamp and get a video mux to route different video outputs to the TV and different audio outputs to the power amp. Do you think this will sound better?
Thanks
Alan
Search "passive preamplifier" with Google, since that's what you refer to and it's currently in vogue so there's plenty of overpriced product out there.
Hi, Are you Gingertube in Guitar Amp forum?
Do you have any recommendation on one or should I build one myself since it is totally passive. I don't see any reason to buy one if they want to charge a lot of money.
Nope, you lose your membership here with more than one username.
To be honest, I haven't used a passive preamp in any form other than just adding 20-50k stereo pots at DIY amplifier inputs. If you Google "passive volume control" or search the threads and literally hundreds of audio DIY sites, you'll know as much as anyone about how to make 3 connections to each pot. section. There are dozens of illustrations on the web and all amount to the same simple deal.
Commercial passives are often active anyway, by virtue of buffers used to connect to source switching and drive the amplifier properly with low impedance after the volume control and minimize noise. Then there are remote controls, power supply, indicators and other convenience features that make it a virtual preamplifier anyway, with no good reason to pretend otherwise.
For the cost of a stereo 50k log. pot and a cheap stereo lead cut in half, you can wire up a basic passive control in minutes. It's a cheap no-brainer to try the idea before buying anyway. A small, grounded metal box to avoid noise and RFI is also a good idea for any external signal control.
Thanks, That's what I am going to do, wire one up myself and test it. So 50K pot is a good value?
What is the usual output circuit of any DVD or other type of boxes? Is the output impedance usually just a low output impedance amp ( like an opamp) driving a resistor? If so, should I try 25K to lessen the capacitive effect of the coax? I have plenty of volume, attenuation is not my concern, high frequency roll off is my concern.
go with 50k ONLY if your Source components cannot drive a 10k pot and the cable connecting to it.
Otherwise choose a 10k dual track log law volume potentiometer.
Otherwise choose a 10k dual track log law volume potentiometer.
There is a very good reason to use a preamp. It has to do with the loading on the amplifier input stage. Read Self and Cordel to understand this better. I wondered why my passive preamp sounded lifeless, then I learned. Give me a simple op-amp buffer any day.
What is the reason of lifeless? If you have no frequency roll off, what is the reason.
Yes, it's so easy for me to put an opamp buffer, but does it hurt the sound? Any particular restriction on the opamp circuit? Do I just make it a simple unity gain voltage follower, Do I have to use a pull down resistor at the output to make the output of the opamp class A? Any circuit suggestion?
A cheap lash-up and an audition would be a great source of the truth for your system - go to it.
Concerns about buffers outside the black boxes called source and amplifier should be seen in the light of the number of buffers already in the total signal path. Correctly implemented, one more or less buffer in the path makes no odds in regard to hurting the signal. What does hurt is a loading of the source with something too low and then loading down the input when the volume setting - lets say 200R to ground at a modest listening level with a log. pot - is unexpectedly low across the input filter, and that impacts on the bandwidth of the amp, specifically bass when the input cap C1, which may be the entry point and determine the lower roll-off frequency, is in the order of 1uF. There sure can be frequency roll-off as in this simplified illustration and you may need to add a series resistance to avoid that loading, which brings its own problems.
So, if you have a high source impedance and a low resistance pot and listen at low levels, you'll find lacklustre (lack of bass punch and probably treble extension too) sound is the result.
Maybe easy to fix with a little thought, not often considered for all amplifier input filter designs.
Concerns about buffers outside the black boxes called source and amplifier should be seen in the light of the number of buffers already in the total signal path. Correctly implemented, one more or less buffer in the path makes no odds in regard to hurting the signal. What does hurt is a loading of the source with something too low and then loading down the input when the volume setting - lets say 200R to ground at a modest listening level with a log. pot - is unexpectedly low across the input filter, and that impacts on the bandwidth of the amp, specifically bass when the input cap C1, which may be the entry point and determine the lower roll-off frequency, is in the order of 1uF. There sure can be frequency roll-off as in this simplified illustration and you may need to add a series resistance to avoid that loading, which brings its own problems.
So, if you have a high source impedance and a low resistance pot and listen at low levels, you'll find lacklustre (lack of bass punch and probably treble extension too) sound is the result.
Maybe easy to fix with a little thought, not often considered for all amplifier input filter designs.
I see. This is a simple design of a buffer amp that has input impedance of 50K and low output impedance using a 10K pot. I pull 1mA from output to make it a Class A output. Do you think this is good enough to use in my existing setup? I can easily change to 200K resistor at the input to make the input impedance to 100K.
As I said I am very new in audiophile electronics, I have no idea what buffer can be used for a very expensive audiophile quality system. Is there any black magic circuit for a simple buffer like this?
Do you really think different opamp make a difference to the sound? I know I need to get good slew rate, unity gain stable.......But any black magic parts? I am not being sarcastic. At audiophile level, there must be certain amount of black magic that I am not aware of.
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As I said I am very new in audiophile electronics, I have no idea what buffer can be used for a very expensive audiophile quality system. Is there any black magic circuit for a simple buffer like this?
Do you really think different opamp make a difference to the sound? I know I need to get good slew rate, unity gain stable.......But any black magic parts? I am not being sarcastic. At audiophile level, there must be certain amount of black magic that I am not aware of.
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the buffer and the vol pot are back to front.
It's the vol pot that does not have the ability to drive the interconnect cable.
The added buffer, AFTER the vol pot, is what gives the OUTPUT of the vol pot the capability to drive the interconnect cable.
Why use an opamp with a gain of >100000 and then castrate it down to a gain of 1 to act as a buffer?
An EF (emitter follower) or SF (source follower) can do this gain of 1 duty with just ONE transistor.
There are very good ICs that are designed to only work as buffers.
Read up on the Pass B1 (uses two transistors for a CCS loaded SF).
It's the vol pot that does not have the ability to drive the interconnect cable.
The added buffer, AFTER the vol pot, is what gives the OUTPUT of the vol pot the capability to drive the interconnect cable.
Why use an opamp with a gain of >100000 and then castrate it down to a gain of 1 to act as a buffer?
An EF (emitter follower) or SF (source follower) can do this gain of 1 duty with just ONE transistor.
There are very good ICs that are designed to only work as buffers.
Read up on the Pass B1 (uses two transistors for a CCS loaded SF).
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The opamp has much lower distortion compare to even SF.
Is Pass B1 just SF with CCS loading to minimize distortion. But still the distortion is more than opamp as it is loaded by the cable.
So you just want to put the volume pot in front and buffer the output of the volume pot?
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I am not convinced one way or the other, I am just asking a question because I am so new in audiophile electronics that I don't know what to think.
Why opamp is not as good compare to just a SF?
Thanks, I got that already, looks like it's just a SF with CCS at the bottom.
Another stupid questions, why not using BJT? Why not opamp?
Hi,
A 100K dual linear pot with 10K wired to the wiper and
common will do the job very simply, ideally placed in
or very near the power amplifier.
rgds, sreten.
A 100K dual linear pot with 10K wired to the wiper and
common will do the job very simply, ideally placed in
or very near the power amplifier.
rgds, sreten.
Read the reports of designers of jFET input buffers.
All are convinced that jFETs as part of the buffer is almost ideal.
A buffer is ideal for driving a line when no gain is needed. That is what it is designed for.
An opamp with gains of 50000 to 250000 is not designed as a buffer. It just happens to be a package that can sometimes be converted to buffer duty.
All are convinced that jFETs as part of the buffer is almost ideal.
A buffer is ideal for driving a line when no gain is needed. That is what it is designed for.
An opamp with gains of 50000 to 250000 is not designed as a buffer. It just happens to be a package that can sometimes be converted to buffer duty.
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