Electrical characteristics of Sansui preamp in/out RCA ports = ?

[audio_ca]

Has anybody seen or measured the actual impedance of the input and output RCA ports on any of the 70's Sansui gear? Specifically I wonder about the I/O port characteristics of the AU-9900 and TU-9900 but any info/thoughts on the subject would be interesting.

I did some searching but just seem to find manuals describing the preamp options. That said, I am new to thinking about the internals of Sansui gear but am now up to two AU-9900's and a TU-9900 tuner. We don't listen to radio but is too iconic to pass on. Still looking for the right Technics turntable. Cheers.

 

[muovimies]

For what it's worth, the AU-317 service manual specifies 47k input impedance for the power amp input. Have you checked the AU-9900 service manual, or just a regular one meant for users?

 

[audio_ca]

For what it's worth, the AU-317 service manual specifies 47k input impedance for the power amp input. Have you checked the AU-9900 service manual, or just a regular one meant for users?

Thanks! I am new enough to thinking about cracking one open so as to not expect to find a readily available service manual.

Inputs are around 50k Ohms which is what I was hoping for-- not in the realm of high output impedance.

 

[muovimies]

Hifiengine.com is the best place for manuals and service manuals, downloads need a registration and there are limits these days how many you can load in a day, but it's free.

 

[chazix]

I'm not sure how interesting this is, but it seems like output impedances are much less likely to be specified.

I'm not sure why that should be so for fixed-level ports like tape outputs. In many cases, I suspect it's because the output impedance depends on the source that's selected. There doesn't seem to be a spec for tape output impedance for the AU-9900, and I'm too lazy to try to trace through the schematic to see if it depends on the source.

For preamp outputs, it seems like it would be desirable for impedance to be specified, but I can see that this might not be straightforward either. Preamp output impedance doesn't seem to be specified for the AU-9900, though it looks like it would be largely defined by a 100 ohm resistor at the output of a buffer amplifier.

It just so happens that I was puzzling over this question for a different model, AU-717, where the preamp output impedance is specified (as 47K Ohms), but when I measured DC resistance of those outputs it was very near zero. It was a little while before it dawned on me that the preamp output comes right off of the wiper of the volume control, so if the volume is turned all the way down, impedance will indeed be near-zero. Moments like that help me to control my hubris .

 

[Hyperion]

I thought that there was a standard adopted, back in the (now famous) day, for output and input impedances based on what the respective output or input was used for.

 

[audio_ca]

I thought that there was a standard adopted, back in the (now famous) day, for output and input impedances based on what the respective output or input was used for.

Exactly, Hi-Z input and low-Z output vs. balanced in/out at ~600 Ohms which essentially makes your cable a tuned component of the circuit. I wasn't sure when the transition from the latter occurred, but it was definitely before these mid-70's amps. I just hadn't found the correct spec sheet yet :O

As a corollary, and for better or worse, finding the schematic and maint. guides pushed me over the edge to rebuild these things myself vs. sending out which will no doubt take a while; i.e. one switch at a time.

 

[bubhardy]

I'm not sure how interesting this is, but it seems like output impedances are much less likely to be specified.

I'm not sure why that should be so for fixed-level ports like tape outputs. In many cases, I suspect it's because the output impedance depends on the source that's selected. There doesn't seem to be a spec for tape output impedance for the AU-9900, and I'm too lazy to try to trace through the schematic to see if it depends on the source.

For preamp outputs, it seems like it would be desirable for impedance to be specified, but I can see that this might not be straightforward either. Preamp output impedance doesn't seem to be specified for the AU-9900, though it looks like it would be largely defined by a 100 ohm resistor at the output of a buffer amplifier.

It just so happens that I was puzzling over this question for a different model, AU-717, where the preamp output impedance is specified (as 47K Ohms), but when I measured DC resistance of those outputs it was very near zero. It was a little while before it dawned on me that the preamp output comes right off of the wiper of the volume control, so if the volume is turned all the way down, impedance will indeed be near-zero. Moments like that help me to control my hubris .

Impedance is an AC spec, at a specific frequency, or an average over a specified frequency range. DC resistance is strictly at DC, or 0Hz.
When you were measuring with a meter, you were only reading the resistance of the pot, from the wiper to common.

 

[audio_ca]

Impedance is an AC spec, at a specific frequency, or an average over a specified frequency range. DC resistance is strictly at DC, or 0Hz.
When you were measuring with a meter, you were only reading the resistance of the pot, from the wiper to common.

Your comment gets to the heart of what I was asking- I wonder if anybody has perhaps collected or seen any frequency-dependent data regarding the I/O ports. I may set something up to make the measurements if the data are not already out there in the public domain.

 

[chazix]

Impedance is an AC spec, at a specific frequency, or an average over a specified frequency range. DC resistance is strictly at DC, or 0Hz.
When you were measuring with a meter, you were only reading the resistance of the pot, from the wiper to common.

Oh, for sure. I was just being honest about how I measured, which I know was not ideal. Point well taken.

Still, I'm very sure that an AC impedance measurement would still have been near-zero, for any frequency in what's usually considered the audio band, and probably for much higher frequencies. Whether there is any doubt about that or not, I imagine it might be agreed that the output impedance for this amp (AU-717) would be expected to vary with the volume control position - the key thing that I goofed about.

I imagine that this kind of variation is not uncommon, so even when preamp output impedances are specified, the specs might depend on operating conditions that might not be specified.

Cheers

 

[bubhardy]

Oh, for sure. I was just being honest about how I measured, which I know was not ideal. Point well taken.

Still, I'm very sure that an AC impedance measurement would still have been near-zero, for any frequency in what's usually considered the audio band, and probably for much higher frequencies. Whether there is any doubt about that or not, I imagine it might be agreed that the output impedance for this amp (AU-717) would be expected to vary with the volume control position - the key thing that I goofed about.

I imagine that this kind of variation is not uncommon, so even when preamp output impedances are specified, the specs might depend on operating conditions that might not be specified.

Cheers

It seems that much of what “goes on” between the inputs and outputs of audio gear is unspecified. I’ve never seen the output impedance a preamp, or input/output impedance of a power amp in an integrated amp/receiver specified, at least for Sansui gear. I believe this is due to the manufacturers not intending their gear to be used in this way.
The output impedance on a power amp has to be pretty low, to be able to source the current demands of speakers, since their impedances can vary all over the place, over frequency.

 

[audio_ca]

It seems like there are a lot of technical people around here. I bet somebody, sometime, and outside of Sansui, must have taken the schematic for one of the Sansui 1970's amps and created a node list for SPICE analysis. I have physical models of speakers I could hook that up to for fun, especially if coupled with room geometry. But what I think might be more useful would be troubleshooting and more specifically tweaking/tuning the layout with a modern eye. The most time consuming part about that is collecting the correct properties for the old components, which is why I haven't done it yet.