Hi ! looking at the schematic of some great measuring solid state line preamps i see that some have buffers just after the inputs before the attenuator
Nevertheless other preamps that shows great performance do not have buffered inputs and the signal goes directly to the volume pot
I wonder if a low ohm value attenuator could make inputs buffering avoidable
And in this case how low a pot can go ? 5kohm ? 2kohm ?
Is the rule of the ratio 10 between the input impedance of the preamp and the output impedance of the source always valid?
Thank you all for any kind and valuable advice
Good day
Nevertheless other preamps that shows great performance do not have buffered inputs and the signal goes directly to the volume pot
I wonder if a low ohm value attenuator could make inputs buffering avoidable
And in this case how low a pot can go ? 5kohm ? 2kohm ?
Is the rule of the ratio 10 between the input impedance of the preamp and the output impedance of the source always valid?
Thank you all for any kind and valuable advice
Good day
'I wonder if a low ohm value attenuator could make inputs buffering avoidable.'
No, not a good idea. Its other way around. You should design preamp that it does not load the signal source too much. That way any signal source with any output impedance will provide best signal transfer.
If you want to use low ohm attenuator, prudent way is to put buffer infront of it.
Splitting the stage in two with volume pot in between has potentially some other benefits, especially if very high gain is required.
No, not a good idea. Its other way around. You should design preamp that it does not load the signal source too much. That way any signal source with any output impedance will provide best signal transfer.
If you want to use low ohm attenuator, prudent way is to put buffer infront of it.
Splitting the stage in two with volume pot in between has potentially some other benefits, especially if very high gain is required.
Hi thank you very much for your kind and very valuable advice.
When i think of a low value pot personally i would not think below the 10K value that i have seen used in many preamps without input buffers
(I am only on solid state because i do not like the bass response provided by tubes)
And moreover may main focus is on line stage If i ever had the need of phono i for sure would think to a separate phono stage placed very close to the turntable outputs This would allow much more flexibility like using even tubes etc.
I am not in phono because for me a good reproduction of an LPs is a huge challenge from different points of view ... i am sure of this
Going back to the pot ohmic value and eliminating dacs with tube output stages i guess that dacs with around 200 ohm of Zout should be the norm ?
10k/200= 50 times Even 5kohm would be 25 times But 10k should be fine
I conclude noticing that there are way more preamps that do not buffer the inputs of those that actually use input buffers
This to me means that when the attenuation stage is well designed the need for buffering inputs becomes less stringent ?
When i think of a low value pot personally i would not think below the 10K value that i have seen used in many preamps without input buffers
(I am only on solid state because i do not like the bass response provided by tubes)
And moreover may main focus is on line stage If i ever had the need of phono i for sure would think to a separate phono stage placed very close to the turntable outputs This would allow much more flexibility like using even tubes etc.
I am not in phono because for me a good reproduction of an LPs is a huge challenge from different points of view ... i am sure of this
Going back to the pot ohmic value and eliminating dacs with tube output stages i guess that dacs with around 200 ohm of Zout should be the norm ?
10k/200= 50 times Even 5kohm would be 25 times But 10k should be fine
I conclude noticing that there are way more preamps that do not buffer the inputs of those that actually use input buffers
This to me means that when the attenuation stage is well designed the need for buffering inputs becomes less stringent ?
Ideal output impedance of signal source is zero.
Ideal input impedance of preamp or amp is infinitely high.
Neither of these is achievable, but we should strive to approach it.
Whatever you calculate, is meaningless. Weather ratio is 50 or 25, its all compromise. It should be as high as possible.
If you use input pot too big, its a source of noise. If you choose too small, some signal sources will sound terrible. Choose wisely.
Ideal input impedance of preamp or amp is infinitely high.
Neither of these is achievable, but we should strive to approach it.
Whatever you calculate, is meaningless. Weather ratio is 50 or 25, its all compromise. It should be as high as possible.
If you use input pot too big, its a source of noise. If you choose too small, some signal sources will sound terrible. Choose wisely.
Hi thanks again Message received strong and clear I am always look for an easy solution but it is not the case
The discussion has gone for long time speaking of passive preamps and their optimal impedance
I guess that a lot depends also on the receiving end i.e. the power amp impedance That would impact the choice of a suitable attenuator
Exactly ! that was what i was thinking Statistically one common value is around 10k for the attenuator in passive preamps to be used with solid state power amps.
A tube power amp having a high Zin is more kind to passive preamps I do not use tubes
A tube power amp having a high Zin is more kind to passive preamps I do not use tubes
It depends on the signal path design of the preamp and the noise floor objective of your design.
Typically a source will have an output impedance of 50-150 ohms SE. Loading that with less than 5K input impedance may impact on the -3 db point of the source low frequency response.
The reason for an active stage after the input is to add voltage gain so that there around 9db gain or 3 x times. The benefit of this is to ensure there is enough signal level to drive the power amp. Also to ensure that at nominal signal levels the wiper position on the volume pot presents a low source impedance to the unify gain output buffer. This is important for low noise performance of a bjt Opamp buffer. Because pots are log taper this is quite achievable. The buffer offers a high impedance input to ensure accuracy of the volume pot over the range of its attenuation.
10K ohms is a good value.
Always add voltage gain first and attenuate last to maximise the SN of a preamp.
Typically a source will have an output impedance of 50-150 ohms SE. Loading that with less than 5K input impedance may impact on the -3 db point of the source low frequency response.
The reason for an active stage after the input is to add voltage gain so that there around 9db gain or 3 x times. The benefit of this is to ensure there is enough signal level to drive the power amp. Also to ensure that at nominal signal levels the wiper position on the volume pot presents a low source impedance to the unify gain output buffer. This is important for low noise performance of a bjt Opamp buffer. Because pots are log taper this is quite achievable. The buffer offers a high impedance input to ensure accuracy of the volume pot over the range of its attenuation.
10K ohms is a good value.
Always add voltage gain first and attenuate last to maximise the SN of a preamp.
Hi thank you so much for your valuable advice I would like to observe that inputs buffering is more an exception than a norm at least on commercial units But also on DIY projects Maybe i am missing something ?
Almost all the preamps i have seen skipped on buffering the inputs For sure a buffer stage adds complexity
But aside buffering and attenuation i understand now better that some voltage gain is almost unvoidable to be sure to be able to drive any power amp around
I mean deciding for just a passive attenuator followed by a buffer circuit can be a viable option only with high sensitivity power amps but hardly a solution for all situations
Unfortunately because it is so much easier to design a buffer with extremely low THD than a line preamp with some voltage gain with very low THD
Gain corrupts
Almost all the preamps i have seen skipped on buffering the inputs For sure a buffer stage adds complexity
But aside buffering and attenuation i understand now better that some voltage gain is almost unvoidable to be sure to be able to drive any power amp around
I mean deciding for just a passive attenuator followed by a buffer circuit can be a viable option only with high sensitivity power amps but hardly a solution for all situations
Unfortunately because it is so much easier to design a buffer with extremely low THD than a line preamp with some voltage gain with very low THD
Gain corrupts
