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Old 31st October 2008, 09:12 AM   #1
Tolu is offline Tolu  Germany
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Default Opamp line preamp questions

Some questions arise during reading through many threads and websites.

1. Why have many designs a two stage setup? input > 1st stage > attenuator > 2nd stage > output
Is it superior to a simple input > attenuator > output stage design? What is the reason for such a two stage setup?

2. Most designs with two stages that use dual opamps like LM4562 or OPA2134 have a L/R (1st opamp) + L/R (2nd opamp) setup. What effect would it have to use one opamp per channel with two stages? E.g. left channel opamp (1st stage/2nd stage) + right channel opamp (1st stage/2nd stage)

3. How important is a good power supply? Most modern opamps have a very good PSRR of about 120dB! Many designs use simple 7815/7915 voltage regulators. Would there be an audible difference when using e.g. 317/337?
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Old 31st October 2008, 10:15 AM   #2
lineup is offline lineup  Sweden
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There can be different reasons
for use input buffering + output buffering surrounding potentiometer.

Can be one or several of:

1. One resistor/pot gives an extra load, for the source.
With one input buffer opamp, the source does not see this extra load.
Only see the input resistor/impedance of opamp.
If the potentiometer is low in value (large load) the source output may begin to distort or even start clipping.

2. A potentiometer can have a special curve. Often logarithmic for audio.
If the wiper to ground is parallelled with the input resistance/impedance of the load coming after pot
then this curve is upset.
Usually we like to have the load impedance 10 times higher to the pot value
in order to avoid this.
An output buffer can provide such a high impedance put in parallel with wiper/ground.

3. There is often a filter at power amplifiers input.
This is one small value cap C going to ground to restrict too high freq to enter power amp.
This filter is one RC. Where R = the output impedance of the potentiometer.
Now when you turn pot, this impedance changes
and so the upper cutoff of this RC filter will change.

Now we may want to not let potentiometer level eefect the bandwidth of the power amplifier
One output buffer can provide such an constant output impedance.
The power amplifier input filter capacitor will not see potentiometer. Only the output buffer opamp.
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Old 31st October 2008, 10:23 AM   #3
lineup is offline lineup  Sweden
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Whether to use input buffer before Volume control
and any output buffer after Volume control
depends on what is the source output parameters
and the parameters of load coming after Volume control.
Also the resistance value of the Potentiometer is one important factor.

Sometimes we can do well without any. And this is a lucky case Because the sound signal does not have to pass another stage.

Other times we can benefit from input and/or output buffering.
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Old 31st October 2008, 10:41 AM   #4
AndrewT is offline AndrewT  Scotland
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I prefer to see a buffer/gain stage AFTER each stage of the source/attenuator etc.

i.e. CD player with buffer, Tuner with buffer, Attenuator with buffer.

If these output buffers are fitted then no stage needs a buffer immediately before it.
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Old 31st October 2008, 01:21 PM   #5
Tolu is offline Tolu  Germany
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Thanks for anwering question 1 so far! There are two questions left.
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Old 31st October 2008, 02:21 PM   #6
AndrewT is offline AndrewT  Scotland
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the performance of a 317/337 is poorer, in most respects, than a 78xx/79xx.
The upgraded version of the 337 with the cap across the lower resistor gives the 317/337 a slight edge.
The biggest area where these IC regulators fall down is the attenuation of High Frequency noise (glitches & pulses).
The region that opamps are weakest in PSRR is the same high frequencies.
There are dozens if not hundreds of copies of genuine 317/337 on the market. I wonder how many of mine get anywhere near to matching the published specifications of the genuine article?

Together they allow a significant HF through to the audio output and can give rise to various distortions in or near the audio band even though the initial noise is way above the audio band.
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Old 31st October 2008, 03:24 PM   #7
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Default Re: Opamp line preamp questions

Quote:
Originally posted by Tolu
Most modern opamps have a very good PSRR of about 120dB!

Really? At what frequency? Problems with psrr exist even in the audible range but get really serious at higher frequencies. Which means rectifier switching noise, mains RF noise, locally induced RF in PS lines etc don't get attenuated much. And according to many who bother to listen this is quite audible.

Different regulator brands also seem to matter. As a more repeatable solution i have chosen to use only discrete (mostly Jung type) regulators in all analogue stages. The subjective improvement compared to 3-terminals is very substantial.

Btw, why do you say there are many 2-stage preamp designs? I am only aware of very few. Any links?
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Old 31st October 2008, 03:34 PM   #8
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>why do you say there are many 2-stage preamp designs? I am only aware of very few. Any links<

such config can be seen in all MBL preamps
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Old 31st October 2008, 03:59 PM   #9
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Default Re: Re: Opamp line preamp questions

Quote:
Originally posted by analog_sa
Really? At what frequency? Problems with psrr exist even in the audible range but get really serious at higher frequencies. Which means rectifier switching noise, mains RF noise, locally induced RF in PS lines etc don't get attenuated much. And according to many who bother to listen this is quite audible.

Different regulator brands also seem to matter. As a more repeatable solution i have chosen to use only discrete (mostly Jung type) regulators in all analogue stages. The subjective improvement compared to 3-terminals is very substantial.
Agreed. However, a simple capacitance multiplier before the 3 pins reg will already greatly improve the high frequency rejection (with careful selection of parts). And this can often be easily retrofitted into existing designs.
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Old 31st October 2008, 04:45 PM   #10
Tolu is offline Tolu  Germany
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Default Re: Re: Opamp line preamp questions

Quote:
Originally posted by analog_sa



Really? At what frequency? Problems with psrr exist even in the audible range but get really serious at higher frequencies. Which means rectifier switching noise, mains RF noise, locally induced RF in PS lines etc don't get attenuated much. And according to many who bother to listen this is quite audible.

Different regulator brands also seem to matter. As a more repeatable solution i have chosen to use only discrete (mostly Jung type) regulators in all analogue stages. The subjective improvement compared to 3-terminals is very substantial.

Btw, why do you say there are many 2-stage preamp designs? I am only aware of very few. Any links?

Can you post the PS circuit? What is a recommended PS for such an opamp pre?

Links to two-stage-setups:

ESP P88
Silicon Chip Pre
and
Unisonus
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