Re: Re: Re: Opamp line preamp questions
OK, do I understand right that you recommend a low attenuator value (1-2k) only for the 2 stage solution and that most high end solutions only use an attenuator (with high output impedance >10k) followed by a gain stage?
analog_sa said:
I certainly don't recommend running an LM4562 into 600ohms. 600ohms in any case is not a magical value (at least not anymore). 1-2k is often more practical but still sufficiently low.
Typical digital sources provide levels in excess of 2v. While this is not likely to overload any modern opamp with proper supply voltage, a gain of x2 still significantly reduces the available headroom. I have heard of dacs which output 5v. A first stage with gain may provide a minor advantage in noise but i doubt if it's worthwhile.
At least 90% of all high end preamps (not counting dubious CS3310 and derivative solutions) have an attenuator at input followed by a gain stage. A notable exception are some Pass preamps in which the attenuator as at output but their high supply rails give them additional immunity against overload.
Two stage commercial solutions are almost entirely absent. Maybe the advantages they provide do not sufficiently compensate the disadvantages - reduced transparency, DC coupling difficulties, reduced headroom, reduced S/N, increased distortion.
So, my point is: if you don't at least exploit the only obvious advatage - low attenuator value, why bother?
OK, do I understand right that you recommend a low attenuator value (1-2k) only for the 2 stage solution and that most high end solutions only use an attenuator (with high output impedance >10k) followed by a gain stage?
Levinson preamps would be exception too, where you have gain stage, followed by a balancing circuit, volume control and output buffer.
Similarly BA-labo: http://www.ba-labo.com/balabo/f_page/fe_win-bc1.htm The latter even provide short explanation with regards to chosen configuration
A traditional control amplifier has been employing the pre attenuation method that the input signals are made to decline with attenuaters at the first stage of an amplifier, after that the signals are amplified. This method, however, degrades the sound quality, as low level signals once attenuated are buried into the thermal noises and fluctuations. No matter how excellent amplifier after that the signal made to decline once, the original signal quality can't be restored again. For example, the contents on a paper which were made by size reduction copy once can't be 100% restored by means of a copying machine with a size expanding copy, as detailed information have been lost at the first size reduction and the reproduced copy is felt rough outlined and isn't vivid on the expanded copy. BC-1 uses "Post Attenuation Method" to avoid these degradation. The input signals are amplified at the input stage of BC-1 before attenuation, then made to decline at the later stage of the amplifier to the necessary level to the output. It becomes possible at the next generation high end control amplifier, BC-1, of the dynamic range as well hardly to make it deteriorate and keeping the high purity as to. The high voltage power supply in BC-1 enables "Post Attenuation Method" without loosing dynamic range and low level linearity.
Similarly BA-labo: http://www.ba-labo.com/balabo/f_page/fe_win-bc1.htm The latter even provide short explanation with regards to chosen configuration
A traditional control amplifier has been employing the pre attenuation method that the input signals are made to decline with attenuaters at the first stage of an amplifier, after that the signals are amplified. This method, however, degrades the sound quality, as low level signals once attenuated are buried into the thermal noises and fluctuations. No matter how excellent amplifier after that the signal made to decline once, the original signal quality can't be restored again. For example, the contents on a paper which were made by size reduction copy once can't be 100% restored by means of a copying machine with a size expanding copy, as detailed information have been lost at the first size reduction and the reproduced copy is felt rough outlined and isn't vivid on the expanded copy. BC-1 uses "Post Attenuation Method" to avoid these degradation. The input signals are amplified at the input stage of BC-1 before attenuation, then made to decline at the later stage of the amplifier to the necessary level to the output. It becomes possible at the next generation high end control amplifier, BC-1, of the dynamic range as well hardly to make it deteriorate and keeping the high purity as to. The high voltage power supply in BC-1 enables "Post Attenuation Method" without loosing dynamic range and low level linearity.
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Peter Daniel said:Levinson preamps would be exception too, where you have gain stage, followed by a balancing circuit, volume control and output buffer.
Similarly BA-labo: http://www.ba-labo.com/balabo/f_page/fe_win-bc1.htm The latter even provide short explanation with regards to chosen configuration
[I No matter how excellent amplifier after that the signal made to decline once, the original signal quality can't be restored again.
Peter's comment is spot on. I think the only reason to attenuate first is to avoid overload (clipping) of the subsequent stages. However, I expect most good DACs (eg Wolfson) have an absolute maximum analog output voltage around 2 Vrms and an OP AMP such as OPA2134 using +/- 15V rails has more than eadequate head-room even with a gain of 5 to amplify with low distortion such signals.
On the other hand, if an attenuator is used at the input of a pre-amp, the effect of the load on the driver would need to be considered carefully, particularly if the load were less than 10k.
Yes, I still use Levinson in one of my secondary system. It is nice preamp with great features and ergonomics, but no match for truly audiophile line stages, either passive or active.
I replaced 2604 with LM6174 as they sounded better in this application. The volume control is great and accurate, but again, maybe it's one of the reasons the preamp does not have ultimate resolution.
Even the output buffer, after more extensive comparisons, didn't sound that good. It certainly increased dynamics, soundstage and spatial effects, but added coloration and slight veiling.
I replaced 2604 with LM6174 as they sounded better in this application. The volume control is great and accurate, but again, maybe it's one of the reasons the preamp does not have ultimate resolution.
Even the output buffer, after more extensive comparisons, didn't sound that good. It certainly increased dynamics, soundstage and spatial effects, but added coloration and slight veiling.
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