One preamp driving two amplifiers, how to calculate the effect of 2nd amp? - Page 1
Forum section: electronics / beginners

author: audiotubes
author_rank: Regular Contributor
timestamp: February 09, 2020, 11:09:44 am
content: Hi,Years ago I studied electronics and built several projects. Fast forward to now and I have forgotten most of that. I have seen some great posts here and I hope somebody will help me with understanding my situation.I have an audio preamp which has two pairs of outputs. It is not possible to select one or the other. When the preamp is running both pairs of outputs are driven. I have two amplifiers that I would like to use and since I have two pairs of outputs I run two sets of cables between the preamp and the two amps. I don't use both amps at the same time but it is easier to leave the cables connected. According to the manufacturer of the preamp this is ok.I'm trying to understand the effect of this situation. It seems that the preamp will be driving two loads and therefore the input to any one amp will be less than it would if only one amp were driven. I don't know how to determine the exact numbers to know whether this is really ok or not.Thanks for any light you can shed on this situation!

author: magic
author_rank: Super Contributor
timestamp: February 09, 2020, 11:47:17 am
content: Quote from: audiotubes on February 09, 2020, 11:09:44 amEither that or the output of the preamp will be more than if only one amp were drivenMore in terms of output current, at any rate.It's a simple division between the output impedance of the preamp (a few hundred Ω) and the input impedance of the amps (many kΩ). More amps in parallel - less effective impedance in the lower branch of the divider. The division ratio is still close to 100%, so the difference isn't big, but current draw is doubled. Moreover, output impedance of the preamp is likely in >99% dominated by protection resistors and if there are two sets of outputs built-in, chances are that each has separate protection resistors and there are effectively two dividers in parallel, completely independent of each other.Quote from: audiotubes on February 09, 2020, 11:09:44 am

author: audiotubes
author_rank: Regular Contributor
timestamp: February 09, 2020, 01:13:21 pm
content: Hi and thanks.Part of the problem is I don't know how to determine the input impedance of the amps. Thank you for information about the current requirement. I guess and hope there is internal protection to stop the preamp from self-destructing. I also don't know whether the input impedance of the amps changes depending whether it is powered on or not.I have two preamps both with two sets of outputs. One manufacturer told me that there are separate output stages. But the other manufacturer told me they share the same output stage. Unfortunately neither has the option of selecting a specific output.I would like to know some way I could determine whether the signal is degraded by doing this. I mean degraded measurably even if not audibly.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 09, 2020, 05:49:15 pm
content: Usually it is ok.If you want to know input impedance - it is one of the main amp characteristics, so it can be described by manufacturer in manual.

author: audiotubes
author_rank: Regular Contributor
timestamp: February 09, 2020, 07:15:05 pm
content: Thank you. Yes, but in this case the manuals don't include the information. I asked the manufacturer but it is like pulling teeth. After many emails the support guy is not very supportive